In the second half of the valve train discussion, we’re discussing the “brain” of the valvetrain – the cam. I’ll also cover lifters since a cam and its lifters need to be – sometimes quite literally – a matched pair.
What is the camshaft, and why do I care?
Simply put, the cam(shaft) does exactly one thing – it tells the valves what to do. In some engines there can be a multitude of cams – a DOHC engine, or “Dual OverHead Cam”, has two cams overtop of each bank of cylinders. If that DOHC engine is a “V” type, there will be 4 camshafts in total – one for intake and one for exhaust, for both sides of the V. In a pushrod engine like mine, there’s only one cam, and it controls everything. For a Big Block that means controlling a total of 16 valves – 8 intake and 8 exhaust – and every aspect of their operation. That means when each valve opens, when it closes (and therefore how long it stays open), and also how much it opens. Critically, it’s not just these three values that dictate what will happen – but the overall curve that links them all. In the picture below we can see two potential cam profiles – i.e. how much the valve is lifted (the Y axis), and for how long (the X axis). Even though their opening point, closing point, and lift are all the same – they are two radically different profiles. The blue curve ramps up extremely fast, and then holds close to maximum lift for as long as possible, before ramping down extremely fast again. The red, on the other hand, is much slower to ramp up, barely hits max lift, and then slowly ramps down again. In this case the blue is a much more aggressive profile, and will have a (much) higher performance potential. If you remember that an engine is really just a glorified air pump, the amount of air it can pump is directly related to how much “open area” the valve gets – i.e. the area under the curve. In this case, the red curve only gets the darker grey area, while the blue gets the addition of the lighter grey. Of course, the blue curve requires better springs to close the cam twice as quickly, and better pushrods and rockers to withstand the increased force that comes from the increase in acceleration, etc etc.
In most pushrod engines, all of the intake valves will follow the “same” timing structure relative to their piston, as will all of the exhaust valves – but the intake and exhaust will (likely) have a different profile. In a real life scenario, a given cam will show two curves that can potentially be quite different – the exhaust, and then the intake. In a 4 stroke engine, the “strokes” are intake, compression, ignition, and exhaust…but it’s the “intake” and “exhaust” strokes that have the valves open, as they’re generally closed for most of the compression and ignition strokes.
The reason a camshaft is so important is because how the valves operate dictates nearly everything else about the engine. The “Big Block Chevy” – let’s say the 454 – powers everything from Uhauls and RVs to corvettes, drag cars, jet boats, you name it. The reason that the same 454 cubic inch block is just as happy putting along for millions of miles in a Uhaul as it is doing a burnout in a drag car is because of two different cams telling the engine to do those two different things. Yes, other components need to be properly spec’d so that they can support the requests of the camshaft – your crank needs to be strong enough to handle the power, your heads need to flow enough air to make more valve lift worth it, your carburetor or throttle body need to provide enough fuel and air to feed the increase in power. But you could spend all the money in the world on the best of everything else, stick an “RV” cam in it (you know, the cams that make the engine behave the way it would in a motorhome)…and it won’t perform any better than the bone stock RV engine would. In short, all of the other components allow an engine to make power – the cam makes the engine make power.
How a camshaft works / what does it all mean
Now that you know what a cam does (tell the valves how to open / close), and why that’s so important (Do you want a drag motor, or a Uhaul motor?), you’re probably wondering what specifically makes the engine behave differently in a given application. It all comes down to the lift and duration of the cam – or more importantly, the aggressiveness and timing. Very generally, more aggressive / high lift profiles make more power. And higher duration / bigger timing values favor higher RPM operation. So an RV cam will have low lift, and short duration – good for endless miles of “down low torque”, while a drag cam will have high lift and long duration – good for max power at max RPM. To shed some more light on the situation, let’s walk through a “real life” cam profile, and what everything means.
Admittedly, this is a lot – so ignore most of it for now. The key specs that people talk about when they talk about cams are as follows: a) what kind of cam is it? (Solid or hydraulic, flat or roller, gives 4 possible kinds). b) What’s the duration, both advertised and at 50 (more on that in a second!), and what’s the ICL c) what’s the lift d) Here’s what each of those means, how they relate to the chart above, and how they affect your engine.
Cam / Lifter type
This mostly relates to the physical construction of the cam and lifters, and is frequently not included in conversations based around performance – but it should be. First up is hydraulic vs solid. Basically, this deals with the lifters exclusively. A lifter – which we haven’t yet covered – is the interface between the pushrod and the cam. They follow the cam, and transfer it’s motion to the pushrods. The lifters are quite short and stout to handle the somewhat difficult job of translating the rotational motion of the cam into vertical motion, whereas the pushrods are as thin and light as possible to reduce weight. Traditional rollers are solid slabs of metal, and that mostly works great – in theory. In practice, as an engine heats up, it expands, and the distance between all of the valve train components changes. With solid lifters, you have to calculate this and factor it in, and the engine will operate very losely – with a lot of noise and poor performance – until it heats up and the distances settle. Hydraulic rollers use some clever engineering to automatically adapt for this changing distance in real time, but are a tiny bit compressible because of it. So for an all out, max effort race engine…solid rollers will give an edge. For everyone else, hydraulic is far easier to work with. The other consideration is roller vs. flat tappet. This basically dictates how the lifters interact with the cam. It can either have a flat edge that quite literally scrapes around the cam (on a film of oil, of course!), or it can have a rolling bearing on the bottom that, well, rolls around the cam lobe. Rollers are better in every way – they allow more aggressive lobe profiles (never bad), and are also far more durable and reliable than a flat tappet. Not many engines still use flat tappets these days. Here’s a picture of how they look, to make it a clear picture:
Lift / Aggressiveness
The quoted statistic is “lift”, but really what we’re getting at here is how much air the valve can flow – which is a bit more complicated. This one is really, really easy – more flow = more power, so a cam with higher lift will produce more power. Note that cams cannot open or close valves instantly – they have to ramp up, and then ramp back down. So lift is largely governed by duration, and cam profile. But for a given duration and profile, more lift will make more power with no downside. Weirdly, this is where duration at “advertised” and at “50” come into play. If you look at the red exhaust curve above, you’ll see an advertised duration of 270, with a 224 degree duration @ 0.050″. What this means is that the amount of time the cam spends open “at all” is 270 degrees, but the amount that it’s open more than 0.050″ is only 224 degrees. These two numbers give you a really good idea of how aggressive the cam shape is – the closer they are, the faster the cam is opening. In other words if they were only a few degrees apart, the cam would open from 0 to 50 thou of lift really quickly, suggesting significant area under the curve (blue curve in my picture above). But if they’re very far apart, the cam is taking a long time to open up – reducing the total area under the curve (red in my picture above). Since it’s the total area under the curve that’s important for power, lift doesn’t tell the whole story – but comparing advertised to @ 50 duration helps fill in the picture.
Duration / Timing
This one is much more difficult, and relatively important. Ultimately what it comes down to is that air has a real mass, and doesn’t just flow effortlessly. And the engine wants to create as much power as possible, but has to do so under hugely different conditions. Most notably, an engine has very different needs at different RPM. At 2000 RPM, air has literally twice as much time to enter and fill a cylinder than it does at 4000 RPM. It also has twice as much time to not do what it’s supposed to do – which will make sense in a second. This means that a given cam duration, which is measured in degrees, has to be matched to a desired RPM. Let’s say you want a max effort race motor that revs to 7000 RPM. At that speed, there’s barely a fraction of a second to completely fill the cylinder with fresh air, and then completely empty all of the exhaust. There’s so little time, in fact, that you have to start opening the intake valve quite early, so that it can be open enough once the piston starts it’s intake stroke, that the air can flow past it. You also want to keep it open pretty late…the air rushing into the cylinder has momentum, so even if it stays open a little after the piston starts its compression stroke, air will still keep filling the cylinder. Similarly you have to open the exhaust valve while the ignition stroke is still happening, so that by the time the piston starts traveling back up, the valve is open enough to get the exhaust out as quickly as possible. And the exhaust also has momentum, so you also want to keep it open a little after the piston starts back down, and let it fully carry itself out. And weirdly enough, because there is so much momentum in the air at those engine speeds, you actually want the exhaust and intake valves to be open at the same time – the exhaust rushing out of the cylinder will actually help suck in fresh air from the inake. At 7000 RPM, you have near zero time, and so even huge measurements of duration – opening the intake valve really early, and closing the exhaust valve really late, mean almost nothing. The important part is to maximize how much time the valve is “fully” open, to let air in and exhaust out. So a big duration – lots of time open, lots of time closed – is important.
Now imagine an RV motor that’s trying to move your Uhaul up a hill at 2000 rpm. Now the air is flowing much more slowly, so if you have the intake and exhaust open at the same time, it’s not good – they’ll start to mix, which is bad for combustion efficiency. Similarly keeping the intake open as long as possible is now bad – without as much momentum, the air won’t be able to fight the piston coming up, and the piston will actually force some of the air you just took in, back out. Similarly opening the exhaust early will just bleed some of the pressure from your ignition stroke out to atmosphere – wasting otherwise freely available power. So in a lower RPM engine, you actually make more power with shorter duration. Every engine has a sweet spot where the intake valve opening will exactly fill the cylinder, and the exhaust valve opening will exactly empty the exhaust. Too little duration won’t fill or empty enough…too much will start reversing the process and waste power. So duration is critical for determining where an engine makes power…shorter duration is better for lower RPM, and higher duration is better for higher RPM.
My cam, and why it is what it is
Now that you’re an expert in cam shafts, here’s some info on my specific cam – a custom Comp Cams hydraulic roller, specifically designed for max power marine applications with wet exhaust. Right off the bat, you know that it’s a) hydraulic – I value ease of use over absolute max power – and b) a roller, because I want the better reliability and power a roller can provide. The specific specs are 286/226/0.591″ intake, and 294/234/0.601″ exhaust (advertised duration / @ 0.050″ duration / lift, respectively). So what does that mean for me? Because my max expected RPM is ~5000-5500, which is actually relatively low, my durations are both on the lower side of “performance” cams – though notably higher than standard consumer or truck / RV cams. Meanwhile, lift is very high for a cam of this duration, and the advertised / @ 0.050″ numbers indicate a fairly aggressive cam profile. Neither are really extreme, but they’re a notable step up from a “normal” ~230 degree cam for this engine. Last but not least, the lobe separation angle of 112 is relatively wide. So why those values? Well, most importantly, I have a “wet” exhaust. That means water is injected into the exhaust, which means my cam duration and lobe separation have to work together and ensure there’s almost no overlap. As mentioned above, bigger duration cams can actually mix exhaust into the intake charge…which in my case would mean mixing water into the intake charge. Not good! My relatively low peak RPM works well with this, but I did have to tune my jet pump to work with a lower peak RPM engine. On the flip side, lift is about as high as I could possibly get it…because I want to make as much power as possible! Last but not least, exhaust has more lift and more duration than intake. That’s because it needs more help than the intake does – it has water vapour in it, which increases its relative mass and momentum – meaning it needs more lift and more duration to adequately “match” the intake properly.
I know that was all a lot, but hopefully it filled you in a bit more on how the cam actually works, how it controls an engine, and why I chose the cam I did. Really it all comes down to getting as much air through the engine as possible – and recognizing that in the real, physical world, the best way to do that changes with engine speed. There’s only one part left in the series where we’ll put everything together and time it to the rest of the engine…but this is probably the most conceptually and technically difficult post in the whole series, so congratulations if you made it through!