168 | Understanding Camshaft Specifications
When it comes to selecting a camshaft for your particular engine, there may be a daunting list of available options. In this webinar we’ll discuss what each of the specifications on a camshaft spec card mean, how they will affect the engine operation, and show you that just choosing the biggest cam available might not give you the best results.
- Hey guys it's Andrew from High Performance Academy, welcome along to today's webinar, where we're going to be looking at some of the specifications, numbers and terminology that we're going to find on a typical cam spec card. So this is really important information if you're going to be fitting a set of aftermarket cams to any engine. This is the information that the cam manufacturer is going to provide you and it's essential information to both correctly install the cam in the first place, as well as to correctly degree the cam. And if you don't do those two jobs properly, first of all you risk potentially damaging your engine. And on top of that you also are unlikely to be getting the optimum performance out of that camshaft.
So really important to understand that and I know there is a bit of confusion out there in the industry, particularly with novice engine builders, maybe degreeing a cam for the first time. So this webinar is designed to take out some of that mystery. As usual we will be having questions and answers at the end of the webinar, so if there's anything that you would like me to explain in more detail or any questions generally relating to the topic today, please ask those in the comments and in the chat, and the guys will transfer those through to me. Now in terms of that room for confusion, one of the aspects with this, is that we can get cam spec cards in both metric and imperial units. Now I know for a lot of you guys, particularly based in the U.S.
you're not probably too familiar with working in metric units. For those of us in the metric world, the exact opposite is obviously the case. So as for anyone who has followed a few of our webinars on engine building topics, or taken our engine building courses, you'll know that I'm a big advocate of being able to work in either units. Regardless where abouts you are in the world, you are going to need to be able to deal with both metric and imperial components when you're assembling engine, degreeing cams, so it's really important to be able to swap between those units. And it is really easy if we simply remember that there are 25.4 millimetres in one inch.
So that's our conversion factor, that's all we really need to keep in mind. But when it comes to camshafts, as we'll find our shortly, there are a few specifications that actually alter quite draatically depending on whether or not the cam card is in metric or imperial units. The other aspect that can add a little bit of confusion into matters is whether or not you're dealing with a hydraulic camshaft or a solid lifter camshaft. So this really is dependent on the valve train in the engine that you're going to be fitting the camshaft to. But there are some really dramatic differences between how you will install and set up a solid lifter cam versus a hydraulic lifter cam.
So again if you don't have that right you don't understand those aspects, then you're gonna be setting yourself up for some potential problems. So today what we're going to do is have look at camshaft specifications on two cam cards. Both have been delivered via Kelford Cams here. So this is set up in Kelford's usual manner. The way it is displayed is possibly slightly different between different camshaft manufacturers.
But generally we're going to find pretty much identical information between different cam manufacturers. So it all should be fairly similar, fairly easy to follow along with. One of these camshaft profiles is solid and the other is mechanical. One, the solid cam is for a Honda B18C, and the hydraulic profile is for a GM LSV8. Alright so what we'll do is we'll start with the Honda cam card, let's just jump across to our overhead camera for a moment here.
So for a start we'll start up the top of the cam card and we've got some information about what the cam is manufactured for. So this one is as we can see for a Honda B16A or B18C VTEC engine. We also have the cam manufacturer's part number. So in this case 176T and a spec card number. So all of that information there is relatively simple, relatively bland.
But this is actually a pretty important place to start. While it is pretty rare, you do wanna check this information, and simply start by making sure that you've got the right cam. You've actually got the cam that you've ordered, and it is for the correct engine. So this is the first place to avoid wasting a bunch of time and potentially ending up with some major catastrophes on your hands. Now we're gonna move into one of the areas where we end up with differences between a hydraulic and a solid lifter.
So we've got our valve clearance here. This may also be referred to as valve lash. So what we can see here is that we're specified on the intake to have 0.3 millimetres, obviously we're dealing in metric, clearance or lash. This needs to be set hot and it needs to be set at the valve. Likewise we're got exactly the same here on our exhaust.
So this is an essential aspect of the operation of a mechanical or solid lifter cam. It's essential that we have some level of clearance between the base circle of the cam, and the roller, rocker, the lifter, or the valve, depending on exactly where we are going to be measuring that clearance. In this case as we can see in our spec card, that clearance of 0.3 millimetres, needs to be set at the valve. So this is a fundamental difference here. With a mechanical lifter cam, we always need some positive clearance or lash between the base circle of the cam and the valve actuation.
Whereas with a hydraulic lifter, we're got a lifter that actually pumps up with oil pressure as the engine is running. So that'll always expand to take up that clearance. So it runs essentially with zero lash. And this also brings about an important difference between the cam profiles. These aren't interchangeable.
A cam profile needs to be ground differently for a mechanical actuation, mechanical lifter compared to a hydraulic lifter. So what I'll do is just temporarily jump across to our diagram here of the cam lobe. And the difference here is in this aspect of the cam, which is our clearance ramps. You can see these on both sides. So we've got our base circle of the cam.
So this is what runs against our lifter, or valve actuation when the valve is actually closed. So with a mechanical cam profile, because we have some positive clearance around here, as the cam rotates, and starts to open, we move onto these clearance ramps. So we've got a clearance ramp on the opening side and also on the closing side of the cam lobe. And this is designed to really gently take up that clearance, before we move onto the actual opening ramp of the cam, which is where the valve is aggressively open. So that clearance ramp is different between a hydraulic and a mechanical cam profile.
With a mechanical cam it's essential, otherwise we're going to really beat up on the valve train. With a hydraulic cam because there is no lash, because the cam base circle is always in contact with the valve actuation mechanism, we don't need that clearance ramp. OK we'll get rid of that. And we've looked at our Honda cam there, just wanna quickly jump across to our cam spec card here for our GM LSV8. So the important point that I just wanted to show you as we work down this cam card here, is our valve clearance for this camshaft as you can see is completely blank.
So this is simply because it is a hydraulic profile cam, so we have no valve clearance. So hence there are no numbers in there. Alright we'll jump back to our Honda cam card for a moment. So I've covered this here but it is really important to understand the position in the valve train where we will be setting our valve lash. So in some instances this valve lash will be set at the valve, as in the case with this Honda engine.
In other instances we will actually set the valve lash between the base circle of the cam and a rocker mechanism for example. So it's important to just take note of that. And also we see there that we are told that this lash needs to be set when the engine is hot. Now obviously as everything heats up, as the engine comes up to operating temperature, a lot of the components in the cylinder head, the engine block are aluminium, so everything moves around and expands with heat. So the clearance that we will see with the engine at room temperature, will be different to what we're actually seeing when the engine is running.
This brings us to the point where obviously when we are assembling our engine for the first time, it's not gonna be possible for us to get that engine up to operating temperature. It's sufficient for us to set the valve lash cold for the purposes of degreeing our cam. But of course once the engine is actually fitted to the car, is run and brought up to operating temperature, we would then need to recheck our valve lash and adjust it. In the case of the Honda B18 and B16 cylinder heads, this is really easy because we have screw adjustment for our valve lash right on the end of the rocker. OK so we'll move down now and we're going to have a look at our cam lift here.
So looking here we've got our cam lift listed at 7.45 millimetres on our intake cam and 7.29 millimetres on our exhaust cam. However there's a really important consideration to take in here. This is again going to depend on the type of engine. And in this case both the B18C, as well as our GM LSV8, run a rocker valve actuation mechanism. So this rocker actually has the effect of magnifying the lift provided by the cam lobe, and increasing this when it actually comes to measuring the lift at the valve itself.
And it's of course the valve itself that really defines the airflow into the engine. So let's have a look at how that works here. We've got our rocker ratio. So we can see that the rocker ratio on the B18 is 1.55, it's the same on the intake and the exhaust. So then we have our net valve lift.
So this needs to be understood as well. Our net valve lift is our cam lift multiplied by our rocker ratio. And then we also take out our valve clearance. So in this case this is the actual peak valve lift that we should be measuring if we have a dial indicator sitting on the back of the retainer for our intake valve. We should be seeing 11.25 millimeters of lift on the intake, and we should be seeing 11.0 on the exhaust.
So again that net valve lift on a mechanical versus a hydraulic lifter, you need to take into account that the valve clearance or valve lash will be subtracted to get our net valve lift. OK we're going to move further down now and the next section of our cam card includes our duration. And this is one of the areas where we do see a lot of confusion. It's also one of the aspects that is used quite frequently to compare cam profiles. So it's really important to understand the implications of how the duration is provided or is expressed on our cam card.
So first of all here you see that we have advertised duration at 0.1 millimetre lift and we see on the intake we have 284 degrees and on the exhaust we have 274 degrees. So what we're talking about there is the duration of the crankshaft rotation. So the number of degrees of crankshaft rotation that the valve is actually open. So advertised duration, essentially what we're looking at is the total number of degrees of crankshaft rotation when the valve is off its seat. So as soon as it's cracked off its seat, in this case we're looking at 0.1 millimetres so it's just a very small amount of lift, until the time that it closes back to that 0.1 milimetres, that is our valve duration, our cam duration, or advertised duration there.
Actually let's just also while we're talking about this, we'll jump across to our little diagram here of our cam operation. So from this diagram, hopefully I've got this in a position where we can see it. That looks like it's probably a little bit better. So we've got the four engine cycles here, we've got our power stroke, our exhaust stroke, our intake stroke, and our compression stroke. We've listed TDC here, bottom dead centre, TDC, BDC, and TDC again.
So this is the point here where we're at TDC on overlap. The red line here is our exhaust and the blue line here is our intake. So if we're looking at our duration what we're looking at is a point essentially for our advertised duration the point where the valve first opens and the point where the valve first closes and our advertised duration is simply that entire duration in terms of crankshaft degrees. Hopefully you guys will be able to see that. OK the problem with our advertised duration is it's not actually overly useful.
The problem is that our advertised duration can be dramatically affected by the way the cam lobe first starts to open and close the valve. So particularly with our mechanical profile, the clearance ramp that we've already talked about, that gently opens and closes the valve, so that can have a really dramatic effect over our advertised duration, which can really skew our results. So in terms of giving us a representation that's a little bit more realistic, we generally use another way of representing our duration, which is our duration at either one milimetre lift or our duration at 50 thou. So again if we look at our overhead camera here, we've got our duration at one millimetre lift so for a metric cam card we would be looking at duration at one millimetre. On an equivalent cam card we'll look at in a second, we would be looking at duration at 50 thou.
So this is essentially the number of crankshaft degrees of rotation when the intake valve or exhaust valve is open above one millimetre of lift. So it takes out the effect of that clearance ramp which can skew our advertised duration. So of course because we're looking at a shorter duration, our numbers are smaller, so we've got 254 degrees on our intake and 246 on our exhaust. So if we're looking at a cam, often these will be expressed as their advertised duration as well. So we may see a cam expressed as a 284/274 so that's talking about the advertised duration.
We'll also have a look here at our imperial cam card just for a moment here. And in this case our advertised duration is at 6000ths of an inch lift. So this is the imperial representation of what we just looked at for our metric cam card at 0.1 millimetre. And then we've also got our duration at 50 thou and we can see that that's represented at 236 on the inlet and 242 on the exhaust. Now the other aspect we need to consider here is duration at our cam versus our valve lift.
So it's important to note here that the advertised duration here on our LS cam, you'll see that it is at cam lift rather than valve lift. So that's a really important distinction that's easy to overlook. Remembering again that we do have a rocker ratio. In this case with our LSV8 we have our rocker ratio listed at 1.7 So the duration that the cam lobe is at 6 thou or 50 thou lift is going to be different to the duration that the valve is open at 6 thou or 50 thou. So it's really important to make that distinction.
Look at your cam card carefully and look at what the information there is telling you because this is the information that you're going to need to use when you're degreeing the cam. In this case with the push rod LSV8, we actually don't need to degree the cam at the valve, we can do this, based on the information in this cam card, we would do this at the cam lift value. So it's a really important distinction there that again is often overlooked. If you ignore that and you degree this cam at the valve, you're going to end up with some really confusing results at best. Now another important point there, we've looked at the metric cam where the duration is generally expressed at one millimetre of lift and then we've looked at the imperial cam where the imperial equivalent is to specify the cam duration at 50 thou lift.
The problem is that we can't directly relate those two numbers. So if we're trying to compare a cam with metric values or metric specs versus imperial specs, we can't directly compare the two. And the reason for this is 50 thou is 1.27 millimetres of lift. So all things being equal, if we specify this exact same cam profile in both metric and imperial, we'd find that the duration at one millimetre lift would be quite a lot greater than the duration at 50 thou lift. And that doesn't mean that the cam is all of a sudden grown, it just is a different way of expressing it.
So again just simply understanding those aspects is really important. OK so we're going to move into some questions and answers really shortly. Before we do that though I'm gonna cover a few more important aspects on these cam cards. But if you do have any questions this is the perfect time to ask those and we'll get onto those shortly. OK we'l swap back to our Honda cam card again.
And moving down, we've gone through our duration, we've gone through our lift, all of these pieces of information are important, they give us a lot of information about how the cam is likely to perform. But they aren't strictly that useful to us when it comes to actually getting the cam installed and degreed correctly. As we move down we've now got some information that is what we're going to need when we're degreeing the cam. Here on our Honda engine we see that we have our timing events at one millimetre valve lift. Again really important to just make that distinction.
In the Honda we're looking at valve lift events, in the LS we're looking at cam lift events. So we have IVO which stands for intake valve opens, IVC which stands for intake valve closed, EVO, obviously exhaust valve opens, and EVC exhaust valve closed. So this tells us where the intake valve and exhaust valve should be opening, and in this case we're looking at where they're opening and closing to one millimetre of lift. In this case if we look at simply the intake we should be finding that our intake valve reaches one millimetre of lift, 19 degrees before top dead centre. And it should close down to 55 degrees of lift, 55 degrees after bottom dead centre.
Now even after degreeing numerous cams on a multitude of engines, I still like to start this process by actually drawing out the valve opening and closing events as we've already looked at. And it just helps, in my opinion, avoid any potential for confusion. So once we've drawn this out, this gives us a really good indication of where abouts the intake valve should be opening and closing in the engine cycle. So if we look at our intake valve, remember this is blue, what we're looking at, we've got lift on our vertical axis here. So we're looking for a point, I'm just going to draw this in, and we'll say that we're looking at one millimetre of lift, and let's just assume that that's somewhere around about this point here.
So we're looking for this point here where the intake valve first opens to one millimetre of lift and then we're looking for this point here where the intake valve closes back down to one millimetre of lift. Now the reason why it's a good idea to draw this out is straight away we can see that our intake valve should be opening to one millimetre of lift before we reach top dead centre. So that's what the terminology BTDC stands for. So we should be opening before top dead centre. And then of course we're closing here some way into the compression stroke so this is after we've gone past bottom dead centre, so ABDC.
Same obviously we can apply to our exhaust valve too. So these are the points that we're actually going to be using when it comes to degreeing our cam. These are the points we want to take into account. So we want to make sure that when we're adjusting our cam, our vernier cam gears, we're adjusting them until our intake valve reaches one millimetre of lift, in this case 19 degrees before top dead centre, closing back down to one millimetre of lift, 55 degrees after bottom dead centre. I'm not gonna cover the exhaust cam 'cause essentially it's exactly the same process here.
So this is where we get into the confusion about the different ways of degreeing our cam. This is the technique that I teach and prefer, and the reason for this is it gives us two points that we can check our cam operation. So this allows us to also ensure that our duration at one millimetre of lift matches our cam card. If it doesn't, we're going to end up with our intake valve potentially opening at the correct spot but it's going to end up closing either too soon or too late in the engine cycle. Generally this would be a good way of us confirming that our valve clearance is correct.
If the cam profile is correct, the rocker ratio is correct, and our valve lash is correct, then we should be measuring the same duration that the cam spec card gives us. So this allows us two points where we can check and be very certain that our cam timing is in fact correct. Now the other technique that is commonly used, it's a little bit quicker, is to use the next piece of information we've got down here, which is our suggested centre lines. So you can see in this case, the centre line for the intake camshaft should be 108 degrees after top dead centre. And for our exhaust cam it should be 115 degrees before top dead centre.
So again we'll just swap across to our diagram here so we can see the relevance of that data. So the centre line, as its name implies, is simply the centre line of the cam. So we can see there if we draw a line right through peak lift, we should in this case be having our intake valve centre line 108 degrees, in this case we can see after top dead centre. And our exhaust cam should be 115 degrees, in this case as we can see before top dead centre. So the way this technique of degreeing our cams using centre lines is done, is to measure the valve lift carefully and we're looking at a point where the valve lift is perhaps 50 thou less than peak lift, on each side of that centreline.
Actually I should've probably shown that so let's bring that back here. So it's very difficult, we can't actually accurately measure where we're at peak lift because if we look at that there's going to be quite a lot of dwell as we move past the nose of the cam. So essentially what's going to happen is it's going to look like our valve stays at exactly the same lift for a number of degrees of crankshaft rotation so it makes it impossible to actually degree the cam just based on looking at peak lift. Instead what we do is we choose a nominal point on each side of peak lift. So we might look as I said 50 thou down from peak lift, so we look at that point there, and we look at the same point on the other side of peak lift.
And the theory is that the centre line of the cam should be exactly in between. Now the theory is good however the problem is that that would rely on our cam profiles being symmetrical and most modern cam profiles are somewhat asymmetrical. So depending on how asymmetric your cam profile is, as well as exactly what point you chose to reference down from peak lift, you can introduce an error of perhaps a few degrees in your cam timing. So this is why I don't recommend using centre lines to degree our cams. Now the last technique that I'm going to talk about, the last piece of information we can see down here is we have also measured our lift at top dead centre or TDC.
And this is another way that people do degree their cams. All they do is set up their dial indicator on the valve, the retainer, or the cam depending on the way you're actually degreeing the cam, and they turn the engine over until they're on top dead centre on overlap and look at the lift and adjust the cam timing until in this case we've got 3.07 millimetres of lift on our intake cam and 1.84 millimetres on our exhaust cam. So this is a quick way of degreeing our cam but again just like our centre line, there is some room for error. In this case with our lift to TDC, we only have one piece of information to base our degreeing on. So again if we've got an issue with the valve lash that we've set, if we've incorrectly set that, we can adjust our cam timing, until we have the specified lift at TDC, but that's not gonna mean that our cam timing's correct and in fact it's going to almost guarantee that it's wrong.
There's no sanity check here I guess is the point and if you're degreeing a hydraulic lifter and you hadn't, you were not aware of the fact that hydraulic lifters can't be used during the cam degreeing process, they generally will bleed down and affect your results. This could end up with your cam timing massively wrong if you are only using the lift at TDC method. So I always recommend using the intake valve opening and closing points. It's the most accurate way of ensuring that your cam degreeing is correct. It gives you two points to check.
If you really want to be absolutely certain you can then also check your lift at TDC as a further sanity check just to make sure that everything really does make sense and does align. Now if we jump across to our imperial cam for our LS here, we've got again the same process here, we've got our lift at TDC, we've got our suggested centre lines, we've got our intake valve opening and closing, our exhaust valve opening and closing points. Now remembering again the difference between these two cam cards here is that with the LS we are looking at cam lift. With the Honda we were looking at valve lift. So really really important again I'll just reiterate that.
It is important to make sure that you do understand the differences there. Alright we'll jump into some questions and answers now so if you do have any more questions please feel free to ask them. Daniel has asked, when running big ports and valves is it detrimental to run high duration and lower lift? OK this probably does get a little bit more into cam design. And I'll be the first to put my hand up and say that I am not a cam development specialist so generally what I'd recommend here is that you actually deal with a camshaft specialist. And this can be a really confusing topic.
It's often we find that when we're looking at a camshaft for a particular engine, the tempting solution to choosing a camshaft can be just to go to the list of cams that are offered for that particular engine, go to the very bottom of that list and pick the biggest, ugliest cam that the manufacturer makes and think that that's going to be the ultimate and give you the most power. And it does get a lot more complex than that because we do need to choose a cam with lift and duration and overlap for that matter, that is suited to our specific application And this is an incredibly complex topic. There is a lot of science that goes into the development of cams, as well as matching cams to your port shape, your port airflow and your valve sizes. Then of course we also have the topic of whether or not your engine is naturally aspirated or forced induction. So there is a huge amount that goes into defining the correct cam profile for your application, then also you need to decide what the usage of that engine is going to be.
So that's something that I can't really advise you on in much more detail, and I do recommend that if you've got a particular application, that you do talk to any one of the camshaft manufacturers out there. Andy's asked, are these numbers always on the low lift side of the VTech type motors, does this at all get different with VVT motors? OK that's actually a really good question there Andy. And at least as far as the Kelford cams for the Honda go, no they are not on the low lobe of the VTech system. And that obviously adds a little bit more complexity because we need some way of locking up that VTech mechanism and forcing the engine onto the high lobe during the process of degreeing the cam. Now the reason, and I would say that this would be common across most manufacturers when dealing with Honda VTech engines, the reason is that it is the high lobe cam that really is gonna define the high RPM performance so that's how Kelford have done it.
When it comes to locking up the VTech mechanism, there are a couple of ways of doing it. You can do this with compressed air directly into the oil gallery that operates the VTech system. In our case for our worked example in our cam degreeing course, we actually disassembled the rockers and we fitted a little packer that made the rocker mechanism physically lock into the high profile high cam system. You've also, Andy's also asked, does this get all different on VVT motors? So yeah it does. With VVT or variable valve timing motors, then the ECU is in control of the cam timing.
So in a way the static cam timing becomes a little bit less relevant because we can advance and retard the camshaft through the ECU when we're tuning it. This does throw another curve ball into the situation though, in that if we are fitting larger cams to a VVT engine, we need to be very careful and check in particular, our valve to piston clearance when we're swinging the cam through its motion. So it's really important to make sure that we still have sufficient clearance. And if not we do want to actually modify the VVT mechanism so that we can limit the mechanical travel of the VVT system. We wanna always make sure that no matter whether the VVT system is fully advanced or retarded, there is absolutely no chance of the valve actually hitting the piston.
Hussain's asked, how can I know which camshaft will be good for off road? Again this comes down to talking to your cam manufacturer. So generally for off road, or this is also suitable for rally, what we want is something that's going to give a broad power band. So generally we're going to want to stay away from anything that's incredibly peaky So you're going to probably for that application want a relatively mild cam profile but again there's not probably enough information in there, I don't even know what engine you're talking about there. Nate has asked, double overhead cam adjusting cam gears changes the lobe separation angle? Yeah so that's something I didn't actually mention there. The lobe separation angle is another important aspect, on a single cam push rod style engine like the LS that I've been talking about there, the lobe separation angle is fixed at the manufacturer of the cam.
With double overhead cams, where we can advance or retard the cams individually, this actually gives us the ability to adjust lobe separation angle and hence affect the overlap. So again there's a lot we can do with the tuning of the cam timing to help optimise the power band of the engine for our particular application. The general trend there as well, just add in with cam timing, is that if we advance the cams, this generally aids performance at low RPM whereas if we retard the cams, this generally will aid performance at high RPM. Alright looks like that's taken us to the end of our questions there, so thanks for everyone who has tuned in there and watched this webinar. Hope you've enjoyed it, hope it's answered some of your questions.
As usual, for any of our HPA members, if you do have further questions after this webinar has aired, please ask those in our forum and I'll be happy to answer them there. Thanks for watching guys.