153 | Rod to Stroke Ratio Explained
The rod to stroke ratio is an aspect of the engine’s mechanical design that has a significant impact on the engine’s performance and in particular the shape of the torque curve. In this webinar we’ll find out what the rod to stroke ratio is, investigate its effect on engine operation, and find out how we can modify it when selecting parts for a new engine project.
- It's Andre from the High Performance Academy, I'd like to welcome you along to this webinar, where we're going to be discussing one of the engine building topics that a lot of people are confused about or don't really understand. Although it's a term that you're likely to hear quite frequently. This is the term rod to stroke ratio. So the rod to stroke ratio is a fundamental aspect of the basic mechanical design of your engine. And what we're going to do today is we're going to find out what exactly it is, what does it mean? We're going to also find out how the rod to stroke ratio can affect the operation of your engine.
And then we're going to talk about some of the ways that we can influence or alter the rod to stroke ratio. Now that can be a little bit tricky because as I said, it is a fundamental aspect of the basic design of a particular engine. So in a lot of ways there's not a lot of room to actually adjust the rod to stroke ratio. Let's start by defining what the rod to stroke ratio is. And as it's name implies it is simply the ratio between the length of the connecting rod to the stroke of the crankshaft.
So if we take a connecting rod here, what we are looking at is the centre to centre length. So the length between the centre of the big end of the rod and the small end or the pin end of the connecting rod. So we take that length and then we divide it by the stroke of the crankshaft, and this is going to give us our rod to stroke ratio. So for today I'm going to be doing a few examples based around the Mitsubishi 4G63 and 4G64 range. Just because they give us a few variations that we'll be able to see how we can affect that rod to stroke ratio.
But if we think about the standard 4G63, a two litre, four cylinder engine, these come out with a 150 millimetre long connecting rod and the crankshaft provides an 88 millimetre stroke. Important to note I will also be talking in metric measurements here. Of course these rod to stroke ratios because they are simply a ratio, they'll work in metric or imperial, it doesn't matter. So if we take the conrod length, 150 millimetres, and we divide that by our stroke of 88 millimetres for the 4G63, this is going to give us a rod to stroke ratio for that engine of 1.70 So that ratio as it is is fixed, it is dependent on the length of the conrod that the manufacturer provides and also obviously the stroke of the crankshaft. So as I say in it's raw form it doesn't appear that there's a lot of room for us to actually manipulate or modify that rod to stroke ratio but we'll see what is able to be done about that shortly.
OK so it's also worth knowing what sort of rod to stroke ratios we're likely to see with a range of different engines. We've just talked about the 4G63, the rod to stroke ratio is 1.70:1, is that good, is it bad, is that high or is that low? We are going to see quite a wide range of rod to stroke ratios across different engine designs. And a lot of this will come down to the intended rev range of the engine. We'll find out more about how the rod to stroke ratio influences that shortly. But essentially for a street engine, the rod to stroke ratio may vary in a range of maybe somewhere as low as 1.4 through to somewhere as high as maybe 1.8 But of course that's not the full extreme, but we'd like to expect that most of our normal road car engines are going to fall somewhere in that range.
Now if we're talking about more specific engines or unusual engines, maybe if we look at the sport bike market where we quite often see rev limits in the region of 12000 to even 15000 RPM. With those sorts of engines we're more likely to see higher rod to stroke ratios in the region of 2.0 or even above. OK so we've talked about what the rod to stroke ratio is. That's great but at this point we still may not actually understand what this means. I'll point out here as well if you do have any questions during this webinar, please ask those in the comments and in the chat, and we'll have a question and answer session at the end of the webinar.
OK so essentially the rod to stroke ratio is going to affect the angulation between the connecting rod and the crankshaft for a given crankshaft rotation. So let's just jump across to my laptop screen, and we'll be able to see exactly how this works. So here we've got our example with a short connecting rod. Now what we're going to do is have a look at two examples here where we're just varying the length of the connecting rod. And what we've got drawn in here is the angle.
So you can see that angulation between the connecting rod and the piston and the crankshaft there. So let's have a look at what happens if we keep the crankshaft rotation the same and we simply fit a longer connecting rod to that same crankshaft. And what we can see is when we do this, obviously our piston has now jumped up much higher so we need to factor that in. It's not as simple as just fitting a longer connecting rod. But we can see that the angulation there has reduced quite greatly.
So this is the effect we see when we increase the rod to stroke ratio. That angulation is reduced. I'll just head back across to my notes for a second. OK so as the rod to stroke ratio increases and that angulation decreases, there are a few effects from that. First of all what we find is that the thrust loading between the thrust face of the piston and the side of the bore, the cylinder wall, that's reduced.
So this has the effect of reducing frictional losses, and it also has the effect of reducing the wear between the piston and the cylinder wall. Reducing the rod to stroke ratio of course has the opposite effect. It increases that thrust loading, it increases the frictional loses. Now this is certainly not desirable, particularly in a very high RPM operating engine, so that's one factor we'll need to keep in mind. Now what we've looked at there is the effect on that rod angulation.
What we want to do is also look at the effect of how the rod length or the rod to stroke ratio affects the location of the piston in the engine cycle. So let's just jump back across to my laptop screen. And we've got a diagram here of our engine cycle, and we're looking at the position of the piston versus the crankshaft rotation, so we've got two lines here, two graphs drawn. We've got one for a 100 millimetre long conrod, and then, while it's probably a little bit unrealistic, we've also got one for a 500 millimetre long conrod. So we've chosen that, we've gone with the 500 millimetre conrod.
Obviously we'd struggle to fit that inside a real engine, but this is just to really highlight the differences here. So what we can see, our red line for our longer rod to stroke ratio, our very long connecting rod, we can see that particularly around top dead centre, the piston stays near top dead centre for longer, compared to our shorter connecting rod. This is referred to as dwell. So the dwell time around top dead centre is going to increase as we increase the rod to stroke ratio. And this has some important knock on effects.
Before we talk about those though, let's also have a look now at how the rod to stroke ratio affects the piston acceleration. OK so we've got three lines drawn on this particular graph. First of all on our x axis we have where abouts in the engine cycle we are. Then on our vertical axis here we've got our piston acceleration. There are no numbers on this, it's just a case of being able to see the magnitude of these effects.
So we've got three rod lengths here, we've got our orange graph which shows our 100 millimetre long connecting rod, we've got our red which shows a 200 millimetre long connecting rod, and then we've also put in one there for a 5000 millimetre long connecting rod. Again obviously unrealistic and impossible but this sort of shows the effect of the acceleration, how this is altered as we move towards a rod to stroke ratio of infinity. So the important point to note here is if we look at what's happening to the acceleration as the piston comes around top dead centre, we can see that as the rod to stroke length increases, that acceleration is quite dramatically reduced, and that really goes hand in hand with what we're looking at here. We've got a longer dwell around top dead centre for the longer rod to stroke ratio, and hence the piston is going to accelerate more slowly away from top dead centre. It's always a good idea just to get a little bit of a graphical view of exactly what's going on.
I find that this helps really reinforce what's happening and gives you a bit of understanding of why. I'll head back across to my notes now. OK so there are a few effects that we need to understand here. First of all as we've talked about, the longer rod to stroke ratio results in slower acceleration away from top dead centre. Now the result of this is we can end up with more pressure acting on the top of the piston as the piston begins moving away from top dead centre.
Now particularly when we are trying to make peak torque out of our engine, what we're really interested in is the cylinder pressure that's occurring around about 16 to 18 degrees after top dead centre, that's where we can take maximum mechanical advantage from that pressure acting down on the top of the piston, being transferred through the connecting rod and then finally into the crankshaft. If we achieve that we're going to end up with a peak torque and in turn for that given RPM range, we'll end up with maximum power. It looks like we've just got a little bit of a technical glitch guys so we're just going to reset the stream so just bear with us please, and we'll be back up and running really shortly. OK guys I think we're back up and running. We're not quite sure exactly where that dropped out for people so I'll just back up, just a little bit.
We've looked at the graphs of the piston position versus rod to stroke ratio, and we've also looked at the concept of piston acceleration versus rod to stroke ratio. And it's important to understand how these can affect the operation of the engine. So first of all, as we've discussed, the longer rod to stroke ratio is going to result in slower acceleration of the piston away from top dead centre. The key point here is what we're trying to do is achieve maximum cylinder pressure at around about 16 to 18 degrees after top dead centre. If we can achieve that we're going to end up with maximum mechanical advantage from that pressure acting on the top of the piston, being transferred down through the connecting rod and then finally into the crankshaft.
So this will give us maximum torque, and in turn for a given engine speed, it's also going to result in maximum engine power. So by accelerating slower away from top dead centre, this can give an advantage in the pressure acting on the top of the piston. In turn obviously this can create more torque as we've just discussed. The longer rod to stroke ratio also has the effect of aiding cylinder filling, particularly at very high RPM, so this is one area where the longer rod to stroke ratio becomes a bigger advantage as our engine rev limit increases. Now there are also some compromises here to understand, so it's not all positives with a longer rod to stroke ratio.
So if we go with a longer rod to stroke ratio, what that's actually going to also do, is it's going to hurt our low RPM torque. And this is because the air velocity into the engine is actually reduced with a longer rod to stroke ratio. This is because the piston accelerates away from top dead centre slower. So if we look at a short rod to stroke ratio where we know the piston is going to accelerate away from top dead centre faster, what this means is for each degree of crankshaft rotation past top dead centre, the piston is moved further down the bore. What this in turn means is that for each degree of crankshaft rotation, we've created more volume in the cylinder and this creates a higher vacuum inside the cylinder which promotes air flow through through the inlet port, past the valve and into the cylinder.
So the shorter rod to stroke ratio can actually improve cylinder cylinder filling at low RPM. In turn as well with it's higher air velocity at low RPM that we're likely to see, this can also promote better mixture preparation, or in other words a more homogenous mixture of the fuel and air. So hopefully what you've been able to take away from this is that when it comes to selecting the rod to stroke ratio, there is no perfect answer. It's all a compromise, and what we really need to take into account is the rev range that we want our engine to be operating in. And essentially what we're looking at here is that a long rod to stroke ratio, or a high rod to stroke ratio is better suited for high RPM engine operation and a short rod to stroke ratio is better suited to an engine that's going to operate predominantly at low RPM.
There are a few rules of thumb in terms of the sort of ballpark rod to stroke ratios we may expect to see for a given engine. So for example for an engine that is likely to be revving to maybe 7000 to 8000 RPM, we're likely to see a rod to stroke ratio somewhere in the region of maybe 1.65 to 1.75 For an engine that's going to rev a little bit lower, maybe with a rev limit of 5000 to 6000 RPM, we're probably more likely to see a rod to stroke ratio in the region of 1.4 through to 1.6 Now as I've mentioned a little bit earlier in the webinar, sport bikes which are routinely revved to 12000 to maybe even 15000 RPM, we're likely to see rod to stroke ratios of 2.0 or even higher. And while it's really difficult to get any hard facts on formula one engines, it is thought that the formula one engines in the era where they were running to 18000 RPM, were using rod to stroke ratios in the region of 2.8 So it gives you some idea of the sort of magnitude of rod to stroke ratios that we're likely to see for a given application. OK we will be moving into some questions and answers reasonably shortly so this is again a good time, if you've got anything that you'd like me to explain in a little bit more detail, or anything generally about this topic, please ask those in the comments and the chat, and I'll get to those shortly. OK so the last topic we're going to look at here is how we can actually make changes to the rod to stroke ratio.
Now on face value this might seem pretty hard. We've got some engine components that have been developed and designed by the engine manufacturer, and if we're dealing with a factory engine, then it's pretty difficult to make wholesale changes to these. However we may still have some options to affect the rod to stroke ratio. In some instances we may be able to get a slight adjustment. In some instances we can actually make quite dramatic changes by mixing and matching engine componentry, as well as changing the rod length.
So in particular some of these techniques that we'll talk about here, are quite commonly used to negate the negative effects of fitting a longer stroke crankshaft to a given engine. So when we fit a longer stroke crankshaft, this increases the stroke, so all things being equal, our rod to stroke ratio is decreased. So we may in some instances be able to fit a longer conrod into the engine in order to help negate the increased stroke. But obviously all things being equal, if we just fitted a longer connecting rod, I've got two examples here. If we fitted a longer connecting rod, and everything else was equal, we'd obviously end up with our piston just simply coming out of the top of the block at top dead centre.
Obviously that's not going to work. There are a few ways that we can deal with this, depending on exactly what piston design we're dealing with and the sort of magnitude of change we're wanting to make to our connecting rod. So let's just jump across to my remote camera for a moment. And this has a standard compression height. A compression height is simply the height from the centre of the wrist pin hole, or wrist pin bore, to the crown of the piston.
So this will work nicely with a stock crankshaft stroke, and a stock 150 mil long connecting rod. What we can do however is raise the wrist pin bore up in the piston, so we'll actually move the wrist pin bore higher in the piston, and in turn we'll move this ring pack higher on the piston as well. Now in some instances we may actually get a situation where the wrist pin actually intersects through the oil control ring. And in this case we can still get away with that quite happily. What we'll end up doing is fitting a rail support to support that oil control ring.
So this allows us to raise the wrist pin up in the piston, in other words we're reducing the compression height of that piston and that allows us to fit in that longer connecting rod without the downside of ending up with the piston sitting proud of the engine block at top dead centre. OK the other option we can go with here as well, in a number of applications there are blocks which have a taller deck height. So the deck height of the block refers to the distance between the crankshaft centreline, and the deck surface of the block. So by using a taller deck block, this gives us the ability to fit a longer connecting rod, again without the piston protruding from the top of the block at top dead centre. Just to put some numbers around this, I'm just going to give you some examples here from the Mitsubishi 4G63 range.
So as we've already talked about, the stock two litre Mitsubishi 4G63 uses a 150 millimetre long connecting rod with an 88 millimetre stroke to give us a rod to stroke ratio of 1.70 Now a common modification with the 4G63 engine is to use the stroker crankshaft from the larger capacity 4G64, the 4G64 is a 2.4 litre engine. So that crankshaft provides a 100 millimetre stroke. So up from the stock 88 millimetre stroke. However if we use the stock connecting rod along with that 100 millimetre stroke crankshaft, that's going to take our rod to stroke ratio down to 1.50 So not ideally suited if we still want to run that engine to high RPM, particularly if we're building a race engine and we want to actually extend the factory rev limit and maybe rev it to 8000 or 9000 RPM. Certainly we may be able to achieve that, but a rod to stroke ratio of 1.50 is really not ideal for those higher RPM ranges.
So a common way of addressing that is to use the 4G64 block. So this is the factory block used for the 2.4 litre Mitsubishi engine. And one of the modifications to that block is that the deck height of the block is six millimetres taller than the 4G63. So this allows us, all things being equal to now fit a longer connecting rod into that engine. We can fit a 156 millimetre connecting rod into the engine, coupled with a modified piston with a reduced compression height.
And if we do that we go from 1.50 rod to stroke ratio, to a 1.56 Again not really ideal but certainly it's an improvement and for a minimal amount of effort and extra cost it's certainly something that is very achievable. Now the other option which is used quite often in the 4G63, 4G64 tuning world, is to use the taller deck 4G64 block, but to couple that with the stock two litre stroke crankshaft. So in this case we can actually fit a 162 millimetre long connecting rod into that engine. And if we look at the rod to stroke ratio there we get 162 divided by 88 which gives us a rod to stroke ratio of 1.84 And this obviously is an ideal solution or a better solution I should say at least for those people who are wanting to run these engines to very very high RPM. This is the sort of combination we often see in the drag applications for the 4G63 where these engines are run to 10000 or even 11000 RPM.
OK so we'll move into some questions and answers, again if you do have any more, please ask them. Hopefully this has given you a little bit more understanding of what that rod to stroke ratio is. We'll see what we've got in here. So our first question comes from Justin who's asked, what do you think about the 4G64 revving to 8K with a 1.56 rod to stroke? It's not ideal but I've actually done worse than that. One of the 4G64s that we built as a drag engine, we ran that to 9500 RPM with a 1.56 rod to stroke ratio.
Now we need to take into account though, this was destined as a drag engine only, it wasn't a street driven engine. And drag engines generally tend to get pulled down and inspected and refreshed a little bit more frequently than a road car engine. So in that particular application it actually worked exceptionally well. The car that we put that particular engine in ran quite deep into the eights. So it was a pretty quick car for the time that it was running and the engine proved to be really reliable.
What we do know though is if we could improve that rod to stroke ratio from 1.56 to extend it, we'd probably see, all things being equal, that the amount of power that the engine produced at high RPM would improve. So it's quite common when we're running these very low rod to stroke ratio engines and we're trying to really extend them in the rev range, we might find that the power and torque actually tends to nose over quite dramatically. But to answer your specific question, even in a road going application to 8000 RPM, while it might not on paper be ideal, the 1.56 rod to stroke ratio in the 4G64 has been well and truely proven. B Bop has asked, do you guys have any experience with the 6G75 MIVEC? I'm sorry no that's one engine that I don't have any experience with there. Barry G has asked, how does the piston speed approaching TDC affect the ignition required for the motor? E.g.
when you're having longer dwell at top dead centre.
OK so while yes there's going to be an effect there, what we're looking at is the combustion speed, that's really one of the aspects we need to take into account. So when we're talking about our ignition timing, what we're trying to do is start the spark event so that we can achieve peak cylinder pressure at 16 to 18 degrees after top dead centre. So right now I couldn't give you an in depth discussion on exactly how the ideal spark timing will be affected if we simply make one change which is the rod to stroke ratio. However when we're on the dyno tuning as well, we don't necessarily need to know exactly what's going to happen, we're going to use the dyno, we're going to test, and we're going to find out. This is one of the common questions I get in terms of tuning is what timing should I run for this particular combination? And even with all of the experience that I've got over the years with tuning, I still can't sit here and say you need to run this specific air fuel ratio and this specific ignition timing.
Every engine I put on the dyno, I'm simply going to be testing and finding out what the ideal combination is for that particular engine. Our next question comes from Cypher Mark who's asked, what do you think would be a safe rev limit for a built engine with an 87 millimetre bore and a 94 millimetre stroke? OK so what it comes down to here a lot is the quality of the components. So another one of the 4G64s, obviously you're not talking here about a 4G64 necessarily but I'll just give you an example because this actually is really really close. One of the engines we built was a 2.2 litre stroked 4G63. So this still used the 4G63 block and it used a 94 millimetre stroke crankshaft and it used an 85.5 millimetre bore.
So on paper that doesn't give us an ideal rod to stroke ratio, a little bit better than the 100 millimetre stroke crankshaft but still not ideal. And that particular engine, we ran it to 10500 RPM and it was still very very reliable. Again it was purely for a drag application. So really it comes down to the quality of the componentry. What would be a complete disaster for a factory conrod or a factory piston or a factory crankshaft, when we're using quality aftermarket components, often these can put up with things that on paper may not be ideal.
The point that you've missed out of there though as well, is you've listed the bore and the stroke, but of course we need the rod length as well to be able to calculate the rod to stroke ratio. B Bop has asked what is the formula for finding the rod to stroke ratio? Hopefully I explained that really clearly earlier in the webinar but if you missed that out, we simply take the centre to centre length of the connecting rod and divide that by the stroke of the crankshaft. So again we're looking at the length between the centre of the wrist pin boss and the centre of the big end journal of the crankshaft, that is the conrod length, and we're dividing that by the stroke of the crankshaft. Brad has asked, I'm using a 2.3 with a 7.2 pounds lighter rotating assembly, how much of an improvement do you think that will allow us to push the rev limit out to safely? So again reducing the mass of the components is definitely an advantage but it comes down to where have you reduced the rotating assembly weight? Have you taken that weight primarily out of the crankshaft? Or have you taken the mass out of the pistons? One of the worst parts about it, a high RPM engine, is the forces being applied through the connecting rod, when the piston needs to change direction at top dead centre. And it's really there, the piston speed or acceleration and the mass of the piston that's the biggest concern.
So if you've taken the weight out of the crankshaft, but the pistons are heavier than stock then that's not going to really be a huge help to you. Again just like for my previous answer really, a lot of this comes down to the quality of the componentry you're using. And a lot of the aftermarket forged pistons, connecting rods, and crankshafts that we do have access to these days, will allow you to do things that on paper, probably wouldn't be advisable. OK our next question comes from Baha GTs who's asked, so the longer rods are only better for lower revving engines? No actually it's the complete opposite way there. If we are increasing the rod to stroke length which we would normally do by increasing the length of the connecting rod, this actually improves the engine's breathing and improves the engine's performance at high RPM, and we actually may end up giving away a little bit of bottom end performance due to the reduced acceleration of the piston away from top dead centre.
So a short rod to stroke ratio is more suited to low RPM performance, a long rod to stroke ratio is more suited to high RPM performance. Barry has asked, going to a longer rod, how would that impact the rod weight? Is this something that needs to be taken into consideration especially if you're looking for high RPM? Obviously if we are increasing the length of the connecting rod, by virtue of that, we are adding material into the connecting rod, so yes there's likely to be an increase in the mass of the connecting rod. This is why in some specific applications where we're really pushing things to the limit, we may end up using something like an aluminium conrod. These are very very application specific. You're only likely to find these in drag engines, because the aluminium material, while it is lighter than an equivalent steel connecting rod, despite this conrod being a much longer connecting rod, and as we can see there's a lot more material there, because of the lower material density, this is actually lighter than the steel conrod here.
But the aluminium material does fatigue. So it has a fatigue life and generally we're going to need to replace that quite regularly, so not ideal for a road going application where we're going to see a high mileage. I guess the answer there is yes, if you're going to increase the rod length, there's going to be an effect on the conrod weight. You need to take all of those factors into account when you're deciding on what you're trying to do and as you correctly say, there's gonna become more of an effect at very high RPM. Cypher Mark's asked, my engine has a 1.58 rod to stroke ratio, I'm tuning it to 7300 RPM, does that sound reasonable? Yeah absolutely that would be absolutely safe.
Obviously if we could improve that rod to stroke ratio, there may be some improvement to that high RPM performance but certainly that's not outside of the realms of normal rod to stroke ratios. Raisin has asked, do you have different considerations for a selected rod to stroke ratio based on the application? I.e. road race versus drag where the motor will dynamically move through the rev range differently. Yes this actually is a consideration and that's probably something I should've touched on there. As we've hopefully remembered from the webinar, the longer rod to stroke ratio can have the effect of reducing the low RPM torque due to the reduction in air velocity and that reduces our cylinder filling.
So this is a consideration for any application where we are going to be moving through the rev range dynamically quite a lot. So rally would be a good example of this, where we may be wanting good power and torque from very low RPM. Maybe a little bit less concerned about our performance at high RPM so that's something we'd take into account. Drag racing on the other hand, we're generally focusing very very specifically and very narrowly on a high RPM rev range. So our engine shouldn't be dropping down into the lower rev range.
So obviously that sort of application, the longer rod to stroke ratio is going to be definitely an improvement there. Barry has asked for motors that have a low rod to stroke geometry with high thrust loads on the cylinder, do you set or adjust the piston to wall clearance with this in mind? Really the piston to wall clearance is more an aspect of the piston design, so the material that the piston is made from, really what this comes down to is the thermal expansion coefficient of the material that the piston is made from and the amount of heat this piston's going to be subjected to. So that's not really something we're going to factor in with the piston to wall clearance. That's really set around those two parameters. However there are some other factors that we may want to consider there.
If we have got a very short rod to stroke ratio, we know that we're going to get a lot more thrust loading, we probably would want to stay away from some of the forged side relief type pistons where the skirt area of the piston that's going to contact the cylinder wall is reduced. So that's one consideration there. We could also be more likely to use a coating on our piston skirt to help reduce the frictional losses there. Jason has asked, does the composition of the connecting rods have any applicability in determining rod length? For example aluminium conrods. No not specifically.
Really the material that the conrods are made from is almost irrespective of the rod length. We can make long connecting rods out of steel or aluminium. As I've discussed in a previous question though, the aluminium conrods obviously are going to result in a lighter weight component which is ideal for a high RPM application. We do have the downsides of the fatigue life of that component to consider though. Jordan James has asked, if I put a silver top head on my 4AGE black top block, will it change the rod to stroke ratio? No it will not, so the only way we can change the rod to stroke ratio is to either adjust the stroke of the crankshaft or the length of the connecting rod.
So it's completely irrespective of the cylinder head, that will have no effect. Terry has asked, how well will a titanium conrod work in a street engine? Not actually a material that I've personally used in any of the engines we've built. Obviously they are pretty pricey so that does put them out of the reach of a lot of applications. You do need to be pretty particular with the material, the actual alloy of titanium that's used for titanium conrods. But they are well proven if you've got the budget to suit, then there's no reason not to.
They don't suffer from the fatigue life problems that we've just discussed with the aluminium conrods, so they provide a lighter weight conrod than an equivalent steel rod but still with a good fatigue life. Peter has asked, what typically dictates the RPM limit? OK there is a variety of different aspects that will dictate the RPM limit. Really when we're talking about RPM limit, this is sort of a separate factor to our rod to stroke ratio. So as our rod to stroke ratio, we've already found, will help improve the performance of the engine either at low RPM or high RPM, so that's one factor. We can, if we still have strong enough engine components, use a rod to stroke ratio that's ideally suited to low RPM, and run it much higher than it really is optimal for.
And the engine may not fall to pieces, it's not going to do that, we just end up with increased wear, and the performance we'll see at high RPM isn't what it should be. So the factors that really dictate the RPM limit are going to come down to the engine component strength. As we increase the RPM that the engine's running to, we're putting a massive increase in the forces that are going into the connecting rod, particularly as the piston goes past top dead centre, on the exhaust stroke, the connecting rod is being stretched it's trying to be split in two, and it has to turn that piston and pull it back down the bore. So as we increase the RPM, the forces on these internal components dramatically increase. The other factors we need to consider though are that the head design and the cam design are really all optimised around a certain RPM range.
So if we take a factory engine for example that has a rev limit of 6000 RPM, and we somehow manage to increase that rev limit to 8000 RPM and the engine doesn't fall apart, we'll actually find that there is no power and torque to be had by exceeding that factory rev limit, because that's where everything's designed to be working fundamentally as the manufacturer intended. B Pop has asked, would a 1.69 ratio be safe closer to 8000 RPM? Yeah absolutely, again, that's actually quite a reasonable ratio for 8000 RPM anyway. But just again to reiterate, I've run 1.7 rod to stroke ratios to 10000 plus RPM in a Mitsubishi 4G63. So beyond that, we also need to consider the strength of the engine components. But these days with modern components, even a lot of factory components, 8000 RPM is absolutely no stretch.
Barry has asked, what's your preference when going to a longer conrod, H-beam or I-beam? OK there are two schools of thought on this, we've actually done a previous webinar that our members can find in the archive talking about the pros and cons of H-beam versus I-beam. A lot of this I think these days honestly now is media hype. There are really good quality conrods in both H-beam design and I-beam design. And probably one of the more important aspects to consider rather than the actual beam design, is the quality of the material that has been used. So that's one factor to consider.
In really general terms though, the generally accepted principals are, that an H-beam design is better for high power, high torque engines. So ideally suited to our very high specific power level turbo charged engines. Whereas an I-beam design tends to be a little bit lighter and is favoured more for a very high RPM naturally aspirated application. These days though there is a lot of interchangeability so it's very difficult to put in a strong rule of thumb as to which type of conrod design you should be going for. OK guys that brings us to the end of our questions.
So again hopefully that's given you a bit more insight and understanding into what the rod to stroke ratio is, what it means, how it's going to affect your engine performance, and a few aspects that we can use to help adjust or modify the rod to stroke ratio from what the factory gave us. Now for our members if you do have any further questions, please ask those in the forum, and I'll be happy to answer them there.