139 | Everything you need to know about Choosing Pistons
When it comes to building a performance engine the piston material and design is critical to achieving good engine performance, quiet operation, and long term reliability. In this webinar we’ll discuss the options when it comes to factory and aftermarket pistons and find out why a certain design may be suitable for your application.
- It's Andre from the High Performance Academy, welcome to this webinar where we're going to be discussing some of the fundamental aspects of piston design. Now this is obviously an important aspect for anyone who is building a performance engine. We're no longer restrained by the design of the factory pistons and this gives us a huge range of freedom in terms of what we can choose for our pistons. So this obviously also means that there's a little bit that we need to understand in order to make sure that we're making the choice that's suitable for our particular application. Now particularly when we compare what's available in the aftermarket compared to what's generally being fitted to factory production engines.
One of the key differences is that we'll find the majority of factory pistons fitted to road going cars are manufactured via a casting process, while in the aftermarket we're much more likely to be fitting a forged piston. Now we're going to start by discussing the differences between those two types of piston. So we'll start with a cast piston, and really this is relatively simple. What we have is a situation where molten aluminium or more to the point, an alloy of aluminium is simply poured, it's melted and then it's poured into a piston shaped mould. Now this gives the OE manufacturers a number of advantages.
First of all and probably one of the main drivers behind the technique is that casting is incredibly quick and it's also very cost effective, and this is obviously one of the key priorities for any OE manufacturer who is into large scale production. Another aspect, when the casting process is finished is it does give a lot of control over the finished shape of the piston. And this means that there is limited machining required after the casting process has been completed. So again this is just one more aspect that helps improve the speed of the process of manufacturing a cast piston and of course this also brings down the cost. Now one of the aspects, we'll talk about the alloy of the material shortly, but one of the aspects with a cast piston is due to the alloy and in particular the silicon content in that alloy, it allows very tight piston to bore clearances to be maintained.
This is simply because the cast piston material doesn't tend to expand as much as it heats up when compared to an aftermarket forged piston. Now maintaining those tight clearances between the piston and the bore is really critical for an OE manufacturer. In particular this reduces the piston noise. We're not going to be hearing the piston essentially rock backwards and forwards as it goes up and down the bore. So this reduces the piston noise.
It also reduces blow by, it help stabilize the rings on the bore walls. And it also helps reduce oil consumption. It's not all good news though when it comes to the cast pistons obviously, otherwise we'd be using them in all of our performance engines. And there are some significant disadvantages. The first of these is that the cast piston material tends to be quite brittle.
And this is one of the most common problems we see with cast pistons in a performance application. If we happen to run the engine into knock or detonation then it is very easy for a cast piston to end up breaking. We find that one of the most common areas we'll end up with a cast piston failing is where the ring lands break due to the pressure spikes caused by knock. The other aspect is because with a cast piston we are literally pouring molten aluminium alloy into a mould. There's no grain structure to do with that finished piston so this doesn't help give any strength to the completed piston.
So again this just adds up to a finished piston that is weaker than a comparable forging. So when we look at the differences now with a forged piston, rather than starting with molten aluminium and pouring it into a mould, what we end up with is a slug of aluminium alloy, they alloy that's going to be used for our forged piston. And under immense heat and pressure, this is then forced into the shape of a piston in a dye. What this does is it creates a grain structure inside of the aluminium piston. This also creates a higher density of material and it creates a lot more strength than we can expect from a cast piston.
On the downsides though with the forging process, is it is a much more intensive process, hence it's more expensive and this is why we don't tend to see it being used for OE pistons. Now I said I'd talk a little bit about the alloy or the material used for making pistons. This is another area where there are some significant differences between forged aftermarket pistons and what we may see with a factory cast piston. So the material used to cast or forge our pistons out of is an alloy which means that it has a variety of different elements that make it up, it's not pure aluminium. And in particular it may include a component such as nickel, copper, magnesium and silicon.
Now the silicon content's one of the areas that we really need to understand and focus on and that's what we're going to be talking about here. The silicon that's included in the piston helps to reduce its thermal expansion. In other words reduces how much that aluminium piston will expand or grow as it comes up to operating temperature. Understandably it's exposed to a lot of heat with the heat of combustion going on inside the engine. And aluminium tends to expand at quite a high rate as it heats up anyway and what this requires, if we've got a material that expands a lot as it heats up is it requires us to provide additional clearance when the engine is cold.
And in particular I'm talking here about the piston to bore clearance. OK so by adding silicon to the alloy, this reduces the thermal expansion of the finished material, the finished piston. It doesn't grow as much and this hence allows us to set tighter piston to bore clearances when we're machining the engine. The silicon also is a very hard component and this improves the wear resistance of the piston. It makes the piston physically harder.
We're less likely to have the piston skirt wear or show scuffing. It also improves the ring groove wear. Now this brings us to how much silicon should a piston have? What we find is that most of the pistons that are used in OE applications tend to have somewhere in the range of about 9% to 15% or 16% silicon content. I just wanna mention a few of the terms that you may have already heard or read about, And just to explain them really briefly, these are the terms, hypoeutectic, eutectic, and hypereutectic. Now below around about 12% silicon content what we find is that all of the silicon in the alloy is dissolved in the alloy.
And we could liken this to if we had a glass of water and we poured in perhaps a couple of tablespoons of sugar. If we stirred that all around then the sugar becomes completely dissolved in that solution of water. So this is what happens when the silicon content is below 12%. Now 12% is known as the saturation point and if we get above 12% silicon content, what we find is that that excess silicon can no longer be absorbed into the alloy. So you can understand, you liken this again to our glass of water.
If we keep pouring tablespoon after tablespoon of sugar into that water, at some point the water can no longer dissolve that sugar and it will precipitate out or end up with excess sugar at the bottom of the glass. So this is exactly what happens if we add more than about 12% silicon. And what this excess silicon does is it forms a hard precipitate that actually remains separate to the main alloy of the piston. So in terms of the three names that I mentioned, hypoeutectic means that we've got less than 12% silicon in the alloy. The eutectic piston is a silicon content of about 12%, and a hypereutectic piston has a silicon content greater than 12%.
Now obviously we've already talked a little bit about how that silicon content affects the way the piston is going to operate. in particular the hardness of the piston, the wear resistance of the piston, and of course the thermal expansion of the piston. What we find is that the majority of the current cast pistons that are being fitted to performance factory engines are generally quite close to the eutectic point. And contrary to popular belief, hypereutectic pistons where we've got more than 12% silicon content aren't specifically any stronger than a eutectic piston. The advantage of increasing that silicon content is mainly in the strength of the ring grooves.
In particular what we find is that the rings are obviously exposed to quite a lot of heat, and if we fit the rings higher on the piston the rings are going to be exposed to more heat and this can result in micro welding between the ring and the ring land due to that additional heat. So by increasing the silicon content with a hypereutectic piston this does reduce. That allows the manufacturer a little bit more freedom in where they're going to place the rings on the piston. You can place the rings closer to the crown, and they can also place the ring sets closer together. OK so obviously in a performance application we are much more likely to be using a forged piston.
However I think it's just important to understand that cast pistons probably get a pretty bad rap and often it's not really justified. There's a lot of factory cast pistons that are actually exceptionally good and can handle mild to even significant increases in power over what the factory engine was producing. More often than not though what we find with the cast piston because of the brittle nature of the cast alloy and also the lack of grain structure that's present in a cast piston is it reduces the window for our tuning efforts and what I mean by this is that a cast piston, when we're starting to produce very high specific power levels from our engine, the cast piston is not going to be very tolerant of detonation. On the other hand a forged piston is a little bit more malleable which we'll hear about shortly and that allows it to actually survive some mild detonation. Now the point I want to make now here is obviously when we are tuning the engine which is a slightly separate issue.
We don't want the engine to be suffering from detonation hence if we do a good job with the tuning or our tuner does a good job with the tuning, provides a safe tune, that is not suffering from knock, then we can often see maybe double or even more the factory power level on a cast piston. And our Subaru FA20 development engines actually are a pretty good example of that. And stock form on our Mainline dyno, the cast factory pistons produces around about 110 kilowatts at the wheels. We have moved to E85 fuel and added a turbocharger to that factory engine, and the factory cast pistons are still quite happily operating and they're supporting in the region of about 230 kilowatts at the wheels. So that's over double the factory power as measured at the wheels on our Mainline dyno.
And they also don't get an easy life. The car is predominantly either being run hard on our dyno or it's being driven in anger around the local racetrack. So this just indicates that if your tuning is on point and you've taken into account the potential weaknesses with a factory cast piston it's still definitely possible to do a really good job and make a lot of power with those pistons. I will add to that a small caveat though. Obviously not all factory cast pistons are created equal.
And you'll find that there are certain engines where the factory pistons are known to support really massive improvements or increases over factory power. You're also going to find the odd engine where it seems that even mild increases in power over factory are enough to break the piston, even if we do keep control of detonation, make sure the engine is tuned correctly. OK so we'll move onto our forged pistons. Now we're going to have a look at a few of these shortly and look at some of the features on the pistons. And even when we're talking about forged pistons, it's not all cut and dry.
There are actually two alloys that are commonly used for manufacturing forged pistons. And these are referred to as 2618 and 4032. Not particularly inventive names. What you're going to find is that for performance applications 2618 seems to be the alloy of choice and we'll find out why that is. So the main difference really between these two alloys is in the silicon content, we've already discussed that in some length so we already have an understanding of what that's going to mean to our finished forged piston.
With the 4032 piston we're going to be looking at a silicon content sitting around about 12% to 13%. So remember that's around about that eutectic point. 2618 on the other hand has a much lower silicon content of 0.2% or less. So when we consider that silicon content and its effect on the pistons, understandably the 4032 piston with the higher silicon content offers similar advantages to the eutectic or hypereutectic cast pistons. Due to the higher silicon content the piston is harder, it's more wear resistant and it's going to have that lower expansion coefficient as it heats up, which is going to allow us to run tighter piston to bore clearances.
The downside with the 4032 alloy though is in extreme circumstances when we're looking at making very very high specific power levels it is more brittle than the 2618 alloy, and this means that it isn't as suitable for very very high specific power levels. OK so with the 2618 alloy, which we're going to see is the most common alloy used in our forged pistons. Because of that very low silicon content we know that the piston is going to expand more. The other aspect that's really one of the keys to why 2618 is the preferable alloy for performance applications is that the finished piston is slightly softer. And the key point here it's more malleable.
This means that it tends to deform rather than breaking. This makes it stronger in those extreme applications and in particular if we do suffer detonation briefly while we're tuning or for example if some of the tuning parameters move outside of the bounds of what we were expecting to see, then the 2618 alloy is much more likely to survive that. Again I want to say that doesn't matter what piston material you are using, if you have't got the engine tuned correctly, if it is continually suffering from detonation, even the strongest piston in the world is still ultimately going to be destroyed. So just because you have a set of forged pistons, this doesn't give you a get out of jail free card to go and do a really shoddy job of your tuning. Now in terms of the piston to bore clearance, this is probably one of the key aspects that we really need to understand, as it differs between a factory cast piston and a 2618 forged piston.
So it's not uncommon, obviously the piston to bore clearance will change based on the bore diameter as well. But it's not uncommon for a factory cast piston which may quote a piston to bore clearance of perhaps 0.9 to may 1.1 thousandths of an inch. It's not uncommon to fit an aftermarket forged piston manufactured from 2618 and require to increase that piston to bore clearance up to somewhere in the region of maybe 3.5 to even 5 thousandths of an inch. So this gives one of the characteristics of a forged piston in that during cold start performance the piston to bore clearance is loose and this will result often in a little bit of noise from the piston. Now once the piston has actually reached operating temperature though, remember that additional clearance is there in order to provide the correct clearance when the engine is up to normal temperature.
So when the engine is up to normal temperature that noise should all go away if we've got our piston to bore clearances set correctly. OK so now we're going to focus on some of the piston features. Now there are a huge range of features that we can choose when we're looking at after market pistons for some of the more popular applications. And some of these can be a little bit confusing and a little bit daunting so I'm gonna do my best now to go over some of the most common. I'm probably not going to be able to do justice to every single one but certainly the ones that you're going to want to understand.
First one I want to talk about is a term, it's known as anti detonation grooves, it's also referred to as contact reduction grooves. What we'll do is we'll just switch to our overhead camera here. And hopefully we're going to be able to get a good look at this on a forged JE piston. This is the little ridges that are visible on the top ring land, so above the top ring. Now these perform two tasks.
As their name sort of suggests, contact reduction grooves, what they do is when the piston rocks over at top dead centre, as it goes from the compression stroke for example to the power stroke, those contact reduction grooves reduce the contact between the piston and the cylinder wall. So this helps in a very minor way, reduce the frictional losses inside of our engine. Now the term anti detonation grooves as they're often referred to, I find a little bit confusing and maybe a little bit misleading. They're not specifically there to reduce our chances of detonation. The explanation from JE is that those grooves can help break up the detonation pressure waves and help reduce the chance of those detonation pressure waves damaging the top ring.
I've pulled engines apart that have these anti detonation grooves, or contact reduction grooves fitted to them, and what we find often is that if the engine has been suffering from detonation, this one hasn't, but we will actually find because the sharper edges that we get with these contact reduction grooves, they will actually show a peppered affect from detonation. So again these aren't a get out of jail free card, we still have to tune the engine correctly. But they are a feature that you'll find available on a lot of aftermarket pistons. The next one I'm going to talk about is a technology that I employed on a lot of the drag engines that I built. And that is a gas porting of the piston.
So let's just swap to our top camera again. And we'll see from this, this is a used piston, this is a flat top piston out of one of our 4G63 drag engines. An we can see around the outside of the crown of the piston there's a number of small holes drilled vertically down through the piston. And these are called vertical gas ports. And the idea behind these vertical gas ports, is that they allow combustion pressure to go down and get directly behind the top ring.
Really when it comes to our engine's performance, it's the ring seal that's one of the most critical aspects that we need to get on top of. So using those gas ports allows that combustion pressure to get directly behind the ring forcing it out against the cylinder wall and ensuring a really good ring seal. There is another technology that's often employed, and again if we could just have a look on our top camera, this is gonna probably be a little bit harder to see. if we look carefully just above the top ring, We've also got horizontal gas ports drilled into this piston. So these are the tiny little holes that we can see, they're like a half moon, just above the top ring.
And these essentially have the same effect as our vertical gas ports in the crown. They again are just another way of that combustion pressure making its way down past in behind that ring, allowing that pressure to get in behind the ring and force it out. Now when it comes to vertical and horizontal gas ports we do need to be a little bit careful of the application we're going to be using these for. In particular vertical gas ports will tend to get blocked by carbon in a street application or applications that are running on gasoline based fuels. This particular piston that we were looking at was from a methanol fuelled engine and methanol tends to run much cleaner than a gasoline based fuel.
So this isn't going to be a technology or a technique that's particularly useful in the most part for a street car. Horizontal gas ports do tend to be a little bit less prone to becoming clogged with carbon. We don't tend to see the carbon buildup make its way down to that top ring quite so dramatically. And for this reason we do see horizontal gas ports being used more for track racing. So the vertical gas ports we would use predominantly for drag racing and the horizontal would be more for a circuit based application.
The other thing I wanted to talk about here is the actual design of our piston. The piston is obviously one of the components where we really want to get a fine balance between the strength of the piston, we don't want the piston breaking but we also want to minimise the mass That piston is being swung around on the end of the connecting rod and every time the engine goes through its cycle the crankshaft and the connecting rod have to accelerate that piston, slow it back down, change direction, and do it all over again. So anything we can do to reduce the mass of the piston has to be beneficial. Now we are starting here a little bit behind the eight ball with a forged piston. As I mentioned the material density is higher with a forged piston, simply due to that forging process, than what we would expect with a cast piston.
Now this has the effect of, all things being equal, a forged piston is going to weigh slightly more. The material that is used in the piston simply weighs more for the same volume of material. Obviously the tradeoff here is the additional strength that comes from that. So we've got two designs here that I wanted to show you, let's just jump to our overhead camera again. This again is a JE piston.
This is out of a small block Chev. And if we turn the piston over we can see this is what's referred to as a full round piston design. And as its name implies we can see that the skirt is just a complete 360 degree circle. Now this does provide good stability for the piston in the engine. But of course it also adds a significant amount of weight particularly through this section here on either side of the wrist pin boss.
We've got material there that strictly isn't doing much for our piston in terms of supporting it. Generally when we look at the way the piston operates in the engine, it's going to be making contact on the major and minor thrust faces of the piston and that' s where we really need the strength. So a more modern design that we tend to see is, actually I'll change to this one first, again look at our overhead camera. This essentially is another full round piston, although we see this time some of that material that is really not doing anything has been removed. So this goes some way towards reducing that mass.
But if we take things one step further, we've got the piston that's going in our Subaru FA20 engine. And this is again a JE forged piston, this is referred to as an FSR or forged side relief piston. So what you can see is that all of the material around the sides of the piston where it's really not necessary to support and stabilise the piston in the bores, has simply been removed. Now this has a range of advantages. First of all it removes a lot of mass where it's not needed, where it's not doing anything.
So this helps to lighten the piston. What we can also see though is that the pin boss where the wrist pin is supported tends to move closer to the centre of the piston. Now this has a knock on effect that these pistons will generally use a shorter wrist pin. And the wrist pin being that it's made out of a steel material does add a significant amount of weight. So by using a shorter wrist pin we're achieving again a reduction in the weight of the piston.
What we can see in this design, again looking at our overhead camera, we can see that there are struts machined or forged into the base forging that this piston has been machined out of, that support the underside of the oil control ring. So this tends to support the crown of the piston and make sure that the piston isn't going to distort because some of that material has been removed from the sides there. So it is a careful balance between getting the weight of the piston down and also ensuring that the piston isn't going to suffer from a reduction in strength. Now the advantage of doing all of this beyond the weight advantage is that it's also reducing the skirt contact or the skirt diameter. So this is the area of the piston that's actually going to be contacting on the cylinder wall.
So anything we can do to reduce the amount of piston that is contacting the cylinder wall is going to help reduce our frictional losses. So this is another way we can recoup some of the lost performance or lost power from our engine. And again that needs to be weighed up carefully, we obviously need to have sufficient contact, sufficient skirt to stabilise the piston in the bore, but there are advantages in reducing that skirt contact and reducing the frictional losses. If we take that one step further, there is another design which is referred to as an asymmetric piston skirt design. And that's where we've got basically the same forged side relief style of piston, but the skirt on the different sides of the piston is different widths.
Now this takes advantage of the fact that when the engine is operating we have what's referred to as a major thrust face and a minor thrust face. And, I'll Just actually bring this up, bear with me for a second. Right if we just have a quick look at my laptop screen. This is a image of the piston operating on the power stroke so we can see that the piston is moving down the bore as indicated by our arrow there. And we can see if we take a line through the centre of the connecting rod, that line, the angulation of the connecting rod, is pointing across to what's referred to as the major thrust side.
So this is the side of the piston skirt over here that's going to actually cop the most abuse. This is the side of the piston that is going to be supporting most of the load. There's a range of different factors that will influence exactly how much load is being distributed between the major and the minor thrust faces of the piston. But that can be as significant as 10 times more loading on the major thrust face, when compared to the minor thrust face. So the idea behind an asymmetric piston design is simply that the major thrust face is wider than the minor thrust face, and again this has a two fold advantage.
First of all it reduces the material in the piston, so it hence reduces the weight of the piston, and beyond that it also reduces further the frictional losses of the piston operating in the cylinder. Moving on, another technology that we'll often find, and this is again from OE manufacturers, is that the wrist pin may be slightly offset in the piston towards the major thrust side. What this does is it reduces the noise from the piston rocking, it's referred to as piston slap. And this helps reduce that noise, just results in a slightly quieter operation from a piston. And what it does is it just slows the dwell across TDC and helps distribute, essentially the rocking that the piston will go through as it moves across the top of the bore, top of the stroke, and goes from moving up to moving down, it helps distribute that change in direction over a few more degrees of crankshaft rotation.
And that's how it reduces the noise that is produced. Now one key point here is that if you are using pistons with offset wrist pins, then it is critical that the piston is installed in the engine in the correct orientation. We'll just go to our overhead camera and hopefully we'll be able to see this on the screen here. On the top of our Subaru FA20 piston, we have two comments on here, first of all we have this note that says left, and then we've also got an arrow and another note that says front. So this is just giving us the orientational location of that piston in the engine, and it's really critical that we take that into account, make sure that we install the pistons obviously in the correct location in the engine.
Often this is going to be pretty straight forward because we'll have different valve pockets in the piston, but particularly on a V8 engine, we do want to just make sure that we've got all of those pistons in the correct orientation and around the correct way. OK another aspect we'll talk about now is the accumulator groove. We've got a couple of piston designs here. And I think all of these have accumulator grooves so let's go to our overhead camera and I'll just try and get, hopefully you'll be able to see this. On our second ring land what we can hopefully see is that there is actually an accumulator groove or a recess machined into that ring land.
now the idea behind the accumulator groove is that is helps to insure that our rings remain stabilised. Now what we're going to find is that during engine operation no matter how well we've gapped our piston rings, we are going to end up with some amount of blow by making its way past the top ring through the end gap, and it's going to end up moving down through the piston and finally past our second ring as well. What we don't want to do is end up with pressure building up between the top ring and the second ring. This, in extreme circumstances can destabilise the top ring and effect our ring seal. So the accumulator groove simply provides a larger volume for any blow by gasses to accumulate in before they can actually go about building up any pressure and destabilizing the rings.
So that's the idea behind an accumulator groove. I will mention that you're going to find that with a lot of piston manufacturers, a lot of these aspects that I've talked about, will be provided as standard features on their forged aftermarket pistons, but it's always a good idea to understand what these features are, why they're included. And then in some circumstances these may be additional features that you may need to actually ask for. We're going to move into some questions and answers shortly so if you do have any questions that you'd like to ask, please punch those into the comments and Colin will transfer those through to me. Now we'll move on and talk about piston coatings.
So when it comes to piston coatings, there are a huge range of these out there. I have definitely not used all of them so I can't speak about every single coating out there, but I have tried a fair few of them, and I've sort of formed what I like as my own personal favourites, of course everyone's experience may be somewhat different to mine. I feel like with my background in building high horsepower drag engines, we did get a chance to test some of these piston aspects to their extremes. So in particular one of the coatings that I do really like is the anti friction coating. Now again if we can just jump to our overhead camera.
I'll just give you the stark comparison between a piston that has been coated which is the piston in my left hand here for our FA20. You can see the sort of blackish grey coating on the skirt of the piston. And in my right hand we can see a bare forged aluminium piston which has been uncoated. So the idea behind the friciton, or anti friction, or friction reducing coating is simply as its name implies to help reduce the friction between the piston skirt and they cylinder wall. For me this does make a lot of sense and it's a relatively cheap coating to add to the piston skirt so this is one that I use quite frequently.
You do need to be aware that there are a variety of these anti friction coatings though and they're not all made equal. So the coating that I've just shown you there is a hard coating that is designed to really not wear away. There are other coatings that are applied to the piston skirt that are actually designed to wear and you'll find that if you pull the piston out after it's done maybe a season of racing, a lot of the anti friction coating will have actually worn completely away. So I've never really seen the point in those particular coatings. One way they are often used, and I find this is probably a little bit of a cheap way out of properly fixing the issue, is if we do have a slightly excessive piston to bore clearance, these coatings can actually be used to build up a very small amount of material, essentially onto the piston skirt and take up some of that clearance.
Of course if it's going to wear away though that's not going to be a long term solution and you're going to be back to excessive piston to bore clearance. The correct solution is really to machine your engine and fit a set of oversized pistons. Now the other coating that is really common and well documented is thermal barrier coatings which are applied to the crown of the piston. The idea here is to reflect heat back into the combustion chamber. This can also have the advantage, technically can have the advantage of improving the engine's efficiency, often or supposedly adding up to a little bit more power.
Now again I've had my own experiences with thermal barrier coatings and this time I'm being less convinced of the results. Now I'll quantify that as well though. I've never been in a situation where I've been building two engines and the only single change between those two engines is the addition of a thermal barrier coating. So I can't say with 100% certainty that they aren't effective. However in my own applications I didn't really find any obvious benefits from them.
In particular I think a lot of people go about having these thermal barrier coatings applied thinking that again it gives them a little bit more leniency if there is a problem with the tune of the engine, perhaps runs a little bit lean, their combustion temperature gets too hot. And yes there may be some truth in that but my own thinking here is that the additional window that it gives you is so tiny that I dont' think it's really going to get you out of too much trouble, You're probably still gonna end up with a melted piston, only it'll be a melted piston with a thermal barrier coating applied to the crown. Another common coating that is applied to this time the underside of pistons is an oil shedding coating. These are a coating that basicallly make the piston very very slippery for oil to stay on so the oil doesn't tend to stick on the coating on the piston, and it returns to the crank case, returns to the sump quicker. The idea behind this is twofold.
First of all if we've got less oil clinging to the underside of our piston, this will reduce the weight of the piston that's reciprocating so that's one bonus there. Also can help to get heat out of the piston crown a little bit better because the oil mist that is collectiing on the underside of the piston is simply running off. Another aspect and this is something I haven't personally tried myself, so I can't talk about this from personal experience, this is something we saw when we actually did a shop tour at JE pistons. They do a lot of pistons for some very high end race applications. And in particular the ones we saw were being manufactured for Honda Performance Development for their IndyCar program.
And the pistons can be what's called hard anodized coated in the ring grooves. Now what this does, we've already talked briefly about the aspect that if we get a lot of heat into the rings this can result in micro welding between the ring and the ring groove, and this affects the quality of the ring seal. And obviously as soon as our ring seal starts to go downhill everything sort of goes backwards from there. So by hard anodizing the ring grooves, this allows much much tighter clearances betweent he rings and the grooves, helping to stabilise the rings and also reduces the chance of that micro welding. So this allows the ring to groove clearance to be tighter as mentioned and it also allows them to use incredibly thin rings so this is technology that we're seeing in the top end of racing where they're really fighting for every last horsepower.
Last thing before we move into questions, is I just wanted to mention our wrist pins and how we can attach those into the pistons. There again is a little bit to understand here, and quite often if you're dealing with an off the shelf piston performance forged piston from the likes of JE, then understandably it's going to come with a wrist pin that's up to the task for whatever that piston was designed for. Now again with the wrist pins if we could go to our overhead camera just for a second, we've got a couple of wrist pins here. They obviously come in a range of different diameters, different lengths and also different thicknesses. They also come in a range of different materials which can be specced for specific applications.
So for my example here is that I've done a lot with building high performance Mitsubishi 4G63s. Doesn't matter if you're building LSV8s or RB26s, 2JZs or 4G63s, the kind of aspects I'm going to be talking about here still cross over quite nicely. And the average JE forged piston for that application is probably going to be quite happy up to maybe 600 or 700 horsepower, there's probably gonna be no issue with the off the shelf fitted components. Now while we see these wrist pins and they look like they're pretty stiff and they're not gonna move around a little bit. It's also hard to understand the amount of force that is being applied to the pistons when we really start producing very high power levels and we run the engines to very high RPM.
And when we start to really lean on the engine we'll actually find that these wrist pins will start to flex quite dramatically. Now that's obviously not good for anything inside our engine. We'll start to notice this because if we inspect the pin bosses in the piston we're going to see signs of galling where the wrist pin has actually been wearing against the piston material, the aluminum. We'll also see this potentially in the small end bush in our connecting rod. And basically what's happening is everything's flexing and moving and we're getting metal to metal contact and wear.
So this is something we need to stay away from if we want reliability in really high horsepower applications. So one of the aspects that we went to with our forged piston design for our Mistubishi 4G63 drag engines, again that's one of those pistons that I've got there. We went to a larger diameter wrist pin, so the factory wrist pin in the Mitsubishi 4G63 is 22 millimeter, we went to 23 millimeter. We also increased the wall thickness. So this is simply the diameter of the wrist pin.
So all of this adds up to improved rigidity of the wrist pin. We also moved to a superior material. One of the upgraded materials that JE provide is referred to as 93C. So this is just a better grade of material, it's stronger. Now again the flipside of all of this is as we got to a larger diameter wrist pin and we go to a thicker wrist pin, this is all adding more weight to our wrist pin, to our piston assembly.
So this is detrimental, we need to weigh all these aspects up. But by making those changes we found that instead of pulling the engine apart and finding that our piston was essentially destroyed, the pin bores were damaged, we were finding that these parts were actually surviving quite nicely. And it can be quite surprising when you pull a wrist pin out of an engine that's run under high load and actually check it for straightness, quite often you're going to find that it is showing quite a significant bend. This will often show up when you try and disassemble the engine because it can be very difficult to physically remove the wrist pin out of the piston. Another technology with the wrist pins that is quite common is a tapered wrist pin, where the ends of the wrist pin are physically machined a little bit smaller, this is the internal diameter of course, the outside diameter of the wrist pin remains the same.
So the inside of the bore on each side is a little bit thinner and then as we get through to the centre which is supporting the connecting rod, the material is thicker. So again just another little trick that can be used to help reduce the weight of the wrist pin while ensuring it has strength where it's required. OK in terms of locating our wrist pins in our pistons obviously we don't want the wrist pin coming out of the piston. In probably the majority of the aftermarket forged pistons that I deal with, the wrist pins are located using wire locks. We'll just go to our overhead camera and have a look at those.
So the wire locks are simply a C shape that's made out of steel. And we just simply locate those wire locks into the bore diameter in the side of the piston, we'll just have a quick look at that now as well, hopefully we can get in there. So there's a groove machined into the wrist pin boss in the side of the piston. And that's where our wire locks go. A little bit fiddly to install.
We do have a full module in our practical engine building course showing you how to correctly install and remove wire locks. Another technique that is a little bit less common certainly in the import market is spiral locks. So these are a little metal spiral as their name implies. So I'll just stretch one out. If you don't know what you're doing with them these can be a little bit trickier to install.
So we'll just have a look at that on our overhead camera. So we need to expand them as I've just done by pulling them apart. And then these are simply spiralled into the groove, the receiver groove, in the piston to retain the wrist pin. Quite often with spiral locks we'll find that the piston actually gets to be fitted with two spiral locks on each side of the wrist pin. Just to ensure that that wrist pin isn't going anywhere.
One other technique which I unfortunately don't have any examples here is a circlip. And these are relatively straight forward. These are often used in engines where we can't actually assemble the piston onto the connecting rod during the engine assembly process. This will be for the likes of Subaru EJ Series engines where we have to assemble the two halves of the crank case together and then physically fit the pistons onto the connecting rods once the block is bolted together. So these can be installed using circlip pliers, they are a little bit fiddly and we do also need to be careful because these circlips are pressed out of a piece of steel strap or strip and what you'll find is during the process that they're manufactured one side of the circlip has a sharp edge and the other side is slightly more rounded.
This is just an aspect of the manufacturing process. we wanna make sure that the sharp edge is facing outwards so that it's going to properly locate in the receiver groove in the piston. And again we don't want those wrist pins coming out in operation. OK let's move into some questions and answers, and again if you do have anything that you'd like me to discuss, please ask those now and I'll do my best. OK first a question comes from F1Jim who's asked interested if you've ever heard of a forged from a cast blank piston? Jim's thinking that it's a hybrid between a forged and a cast piston.
No it's not something that I've personally heard of, I'm just wondering if that's, there are a couple of pistons that were common in some of the Toyota engines, and I'm thinking specifically here of the Toyota 4AGZE. Those were factory fitted with what was referred to as a semi forged piston. So that may be along the lines of what you're talking about there. Jim's also said I've read that 75% of the total engine friction is in the piston to ring and cylinder wall interference and 25% is in the valve train. Do I think those are good approximations? Look I can't say if those are absolutely accurate, I think probably it's fair to assume that there's going to be quite a discrepancy from one engine to another.
Particularly when we look at valve train there's going to be quite a lot in the specific valve train actuation method that is being used by a certain engine. But definitely you only need to assemble an engine short block. And if you assemble the crank shaft into the engine you've got all of your bearing clearances exactly right, the block is machined true and the crankshaft is also machined correctly, you can quite happily rotate that crankshaft in the bearings with a very very minimal amount of effort. As soon as you go and fit the pistons and the rod assemblies into the block then things change dramatically and the engine suddenly gets a whole lot harder to turn over so yeah definitely, we are seeing a lot of frictional loss as a result of our rings and our pistons operating on the bore walls. Candamo has asked, I'm currently running 13:1 engine, I'm thinking of going to 14:1 or more in the future with E85.
How high can I go before I stop seeing considerable gains. OK this is really very dependent on the particular engine design. And what you'll find is that, while technically a higher compression ratio will result or should result in more efficiency and power from the engine. What we get to a situation with is that the dome on the piston that's required to get these very high compression ratios can actually be detrimental to the combustion process, to the flame front propagation. So I can't really give you a specific answer, it is going to be very dependent on your particular engine.
But in most instances I would say that beyond about 14:1 you're probably going to start seeing sort of a plateau. It's probably going to be as you start from a very low compression you're going to see a dramatic rise in the power as the compression ratio comes up. And as you get up to about 14:1, maybe 15:1 you're going to see that flatten off and really you're going to go nowhere. The other aspect is you then understandably, need to be very very careful with all of your clearances between cylinder head and piston, your deck clearance has to be absolutely perfect, as do you piston to valve clearances. Because when you're trying to get such a high compression ratio, you need to really have everything absolutely perfectly working in your favour.
Max Booster's asked compression ratio choice especially for turbo cars, I was told a long time ago that you want low compression ratios but high boost, what are your thoughts? Yeah this is true, so as we increase the boost pressure on a turbo charged engine, then all things being equal we are going to need to reduce the compression ratio if we don't want to run into problems with detonation. And if we get to that point we're going to have a situation where during the tuning process we're going to reach a point where we can't add additional boost without running into detonation so hence we need to reduce timing. And we go round in circles producing more heat, putting more stress into the engine components and making no more power. So the compression ratio does need to be chosen based on our desired outcome and what I mean by this is the entire engine combination needs to be considered, what our power goal are. A really really important on is what fuel are we going to be running on.
So for example if we're going to be limited to a pump fuel, here in New Zealand, we typically have 95 and 98 octane pump gasoline. This isn't a great fuel, and in most instances on a turbo charged car with a compression ratio between maybe 8.5:1 and 9:1, this is going to limit us to perhaps somewhere in the region of 22 to 25, 26 psi of boost. Yes we can run more boost than that but we won't actually end up making any more power. From the other hand you're running it on a really good quality fuel, either a race fuel, so it's for a specific race application, or you've got the advantage of being able to run on a really good fuel such as E85 or an ethanol blended fuel. This opens up your options quite a lot more.
And it's quite common if someone's building a dedicated E85 turbo engine to select a compression ratio that would seem crazy for turbo application normally running on pump gas. So for example in the 4G63 world, I would quite regularly choose compression ratio in the region of about 10.5:1 for ethanol. And in fact the piston that we were looking at before, this is a flat top design from one of our drag engines and this produces a compression ratio of 11:1 give or take a little bit, depending on the thickness of the head gasket et cetera. And yet with these pistons we were running upwards of 50 psi of boost. But the reason we could get away with that was methanol.
So I've now talked a whole to about compression ratio but haven't really given you anything too specific. What I would say is that you're always probably better to err on the side of a slightly lower compression ratio than you are to push the boundaries and go too high. Now the reason for this is when we go with a lower compression ratio, we're going to give away some of our off boost response, our off boost power and response. In particular on a turbo charged engine this can make the engine slightly slower to come up on boost. However once we're actually on boost, this is going to allow us with a lower compression ratio, to optimise our timing and we're going to be able to make more power through our boost pressure.
If on the other hand we go with a higher compression ratio, yeah you're going to often be fighting yourself if you're on a low grade of fuel and the boost response may be great, the off boost power and torque of the engine may be great but really where it counts when we're up on boost we're going to be constantly battling detonation and reducing the timing in order to prevent detonation from occuring. So for a street application, in most of the engines that I'm running on pump gasoline, I would be favouring a compression ratio of around about 8.5:1 through to perhaps 9:1. Hopefully that's given you some input there. Max Booster also asked what friction reducing coating do I recommend? I have not used all of them, they are available from a huge range of manufacturers. The coating that I showed on the JE forged piston is a coating that JE offer themselves, so you can tick this basically as an option when you're ordering your pistons from JE.
I have also used a local supplier from New Zealand who supply or provide an aftermarket coating onto your pistons. I do like to use the piston manufacturer's coatings where possible. This just means it's one stop. Also means that the coating is more often than not going to be applied in an absolutely professional manner there's not going to be any sort of chance of that coating peeling or coming off which can happen when we're using an aftermarket supplier. Max Booster's also asked is the oil shedding coating only useful for engines with oil squirters? No absolutely it isn't.
When the engine is in operation just the oil splash from the crankshaft rotating, from oil shedding back through the bearings into the crank case, is creating a huge mist of oil anyway. So with or without oil squirters, we are still actually going to end up with a lot of oil mist inside the crank case so those coatings can be useful regardless. Darius has asked is it a good practice to order slightly oversized, say 0.5 millimetre pistons, even if the engine was fine when disassembled and the cylinder walls looked good. Yeah this is a really important aspect actually which we've touched on in our practical engine building course. So for one of our worked examples there in the practical engine building course, we built a 2JZ Toyota engine which was destined for around about 1000 plus wheel horsepower.
And this engine is fitted with a set of CP forged pistons. Now the customer supplied all of the components and provided us with a brand new short block straight from a Toyota. Also supplied us with a set of stock bore diameter pistons. And as is quite often the case, we found that the piston to bore clearances straight out of the box weren't actually ideal for our application. We were aiming for a piston to bore clearance of around about 3.5 thou to four thou, and what we found is that the piston to bore clearances were already sitting between four and a touch over 4000ths of an inch.
Now that specifically isn't that important. We're still close enough to our target and given the high power application I was Ok with that. But what also becomes a problem is if we want to torque plate hone the block to ensure that the bores are as true as possible under operating conditions, so replicating the distortion in the block when the cylinder head is torqued down. If we're already at or above our target piston to bore clearance this doesn't give us any potential for doing so. So unless there's a reason not to, I'd always prefer to start with a 20 thou or half mil over size set of pistons.
Just so that we can remove, if it's a second hand block, remove any wear, any out of round from that block, and start with completely true bores. And it really again is, to reiterate what I was saying earlier, the piston ring seal is the key aspect that we're interested in. This is our key focus in terms of optimising our engine performance. So in order to do this it requires that the cylinder walls are absolutely perfect or as close to it as we can get. Comet has asked is there an equation for calculating thermal expansion of pistons? I'm 100% sure there is but it's certainly not something that I've got.
Fortunately this isn't something that we need to go through ourselves. So the thermal expansion coefficient of the material will obviously, as we've talked about, depend on the material itself. And this is really an aspect of the manufacturing process, that the piston manufacturer's accounting for. So we don't need to do this ourselves. And when we're purchasing a set of forged aftermarket pistons, these are going to come with the manufacturers recommendations for a piston to bore clearance based on a range of different applications so you can go with their advice.
Robert G's asked when should you use extra clearance rod bearings. OK that's probably a slightly left field topic for today's webinar and one that actually probably is a little more complex than it may seem on the face of it. I'll just briefly touch on this though. So in terms of our bearing clearances, what we're finding is that for the last few decades the general idea has been that when we're taking a factory engine we're modifying it for increased power and increased rev range, that the trend has been to slightly increase our bearing clearances. Now that just allows a little bit more room for flex in the component, the crankshaft and the engine block before we run into problems with metal to metal contact.
If we get that metal to metal contact between the crankshaft and the bearings or the crankshaft and the con rod bearings, this is going to result in almost instant destruction of the bearing, so we must stay away from that. What we're finding is that with modern race engines the actual drive now is to go the opposite way and go to tighter clearances so it's not exactly cut and dry, there isn't one solution for this. The reason that these manufacturers are going to tighter clearances is because this allows them to use a thinner grade of oil and this is all really a drive to reduce the fricitonal losses inside the engine. However there are some things that we need to understand if you're going to reduce the clearances inside the engine, the bearing clearances. In particular we need to be using an engine block and a crankshaft that is incredibly stiff and is not going to flex.
So in our world where we're talking about the enthusiast through to the semi professional engine building market, this isn't normally the case. And generally my rule of thumb is that if I am building an engine that's going to be producing stock power or mildly above stock power, unless I've got a really good reason to change it, I'll generally stick very close to the factory recommendations or specifications for oil clearances. If I'm starting to modify the engine heavily for increased power or increased rev range then I'll tend to increase my bearing clearances maybe in the range of about half a thou. OK that looks like it's brought us to the end of our questions. Now remember if you do want to know more about engine building, check out our website under the courses section, we do have both our engine building fundamentals and practical engine building courses that take what we've just looked at and go a lot further in depth.
In particular you can see the entire engine building process being applied in our practical engine building course, along with learning a 10 step process for applying that to any engine, regardless what make or model. Alright thanks guys, we'll see you all next week.