What are performance camshafts? And should you upgrade yours? Find this out and more in our guide to performance cams…
Unless you own a rotary powered car, you can be assured camshafts affect you. Your car may have one, or as many as four, but no matter the number they are a vital part of your engine’s performance.
Camshafts are a hugely complex subject that verge on top-level physics if you delve deeply into it, but even at a basic level they are probably the most misunderstood subject in UK tuning, leading to very common and pretty major mistakes when choosing cams for your engine.
This feature should give you a better understanding of performance cams, and what is good for your engine.
WHAT ARE CAMSHAFTS AND WHAT DO THEY DO?
At the most basic level, camshafts are cylindrical rods of metal in piston engines with lobes protruding from them. It’s their job to open and close the inlet and exhaust valves the right amount at the right moment.
The amount, length of time, and point at which the cams open the valves all play a big part in dictating how much power and torque an engine develops, where in the rev range it does this, and if the engine will even run at all. As you can imagine, with the right changes a camshaft swap can seriously improve performance. However, a mistake can have disastrous consequences, not just for performance, but for reliability too.
WHY WOULD I WANT TO CHANGE MY CAMS?
Whatever you do, don’t just rush into changing cams straight away, as a cam swap is not always a good thing! You need to be sure it will improve your car before you do so, as we have seen some swaps that have dropped huge amounts of power. The thing with camshafts is they generally remove performance from somewhere in the rev range to add it in another, and once you add forced induction to the mix things can get even more complex.
Don’t think for one second that the wilder cam you go for the faster your car will be, even if it produces higher peak power numbers, as this is often not the case. From the factory, a car’s cams are generally very mild, and are usually made in such a way for economy, reliability, and, most of all, driveability. This means that there’s almost always room for improvement, but with mild engine specs, especially on turbocharged engines, these standard cams can often be the best overall choice.
NUMBER AND LOCATION OF CAMS
It doesn’t hugely affect performance, but different engines have different positions for cams, as well as different amounts of them. The vast majority of modern engines have four valves per cylinder, and these generally have separate cams for the inlet and exhaust valves. So twin cams on straight engines, and quad cams on V and flat engines.
Many older engines have two valves per cylinder and a single cam operating them all. These engines are getting much rarer, but are still produced today, most notably the awesome LS engines seen in Corvettes, VXR8s, and so on. The LS engines, along with many older engines, also have the camshaft mounted in the block, but the vast majority of engines now have the camshafts at the very top of the engines, mounted in the head.
To complicate things a little, some engines – most commonly from Honda – have a single cam operating all 16 valves, and there are engines (a few older Ford and Fiat units in particular), with twin cams but only eight valves. Some engines can have three or five valves per cylinder, too. But on the whole, engines tend to be either single cam and two valves per cylinder, or twin cams with four valves per cylinder.
SO HOW MUCH CAM IS TOO MUCH CAM?
This is the age-old problem with cam swaps because, as mentioned earlier, what a cam gives in one area of the rev range, it usually takes away from another. Depending on how suited it is to the rest of the engine spec, it can give more than it takes away, or produce far less than it removes. Be it N/A or forced induction, going with too big a cam without the rest of the engine being suited to give power at similar rpm often gives you a very small top end bhp increase at the expense of massive losses at low rpm. Overall, the car would end up slower.
Very wild cams generally make the powerband smaller regardless of spec, with race cars often having a powerband of around 2000rpm, right at the top of the rev range. Not only can this make the car a chore to drive quickly on the road, but unless your gearing is correct you could actually drop out of the powerband every time you change up a gear – which would make acceleration slow no matter what the peak power number is.
They often call cams ‘Fast road’ and ‘Race’ specification, and this is for good reason. Although it might give you a few more peak horsepower, a race cam is not much fun on the road and can actually make you slower overall.
WHAT SORT OF CAMS ARE RIGHT FOR MY CAR?
Cams are all about getting the air in and out of your engine, but depending on how your engine receives its air in the first place can change what sort of cams are right for you. All engines are different – as are people’s opinions of what’s too wild – and you should always do research into the effects of different cams in your chosen engine before making a final decision. Now you’ve been warned, here’s a basic guide…
With no air forced in to your car, you have little choice but to run fairly wild cams and, unfortunately, live with a smaller and higher rpm powerband when you go for increased power. Exactly how far you wish to go is up to you, but often up to 280-290 degree cams give a substantial power increase, along with suitable supporting changes, while still being acceptably drivable on the road.
These engines, even from the factory, run far milder cams than their normally aspirated equivalents because they have the air forced in, meaning they don’t have to sacrifice so much low down power by using particularly wild cams. For maximum gains while retaining driveability, high lift but low duration cams are the suggested route to take, and these are often marketed as ‘Turbo cams’. Long duration cams have a double disadvantage on turbo engines because the low down power loss you get on all engines is usually compounded by a slower turbo spool, so you’ve got to be careful how far you go.
With your average four valve per cylinder engine, you can usually get well over 200bhp per litre on pump fuel with relatively mild cams around 260 duration, which keeps the car tractable and responsive low down. On race turbo engines running high boost, we’ve seen around 400bhp per litre running similarly mild cams. That’s not to say wilder cams are not useful, because if you’re building an engine for all-out power or top speed use then super long duration cams very similar to an N/A engine will get you the power at much lower boost; albeit at the expense of a lot of torque and low down power.
Another common trick on turbo engines are unequal cams, where the inlet cam is, for example, 265 duration, and the exhaust cam is standard, or the inlet is 285 duration but the exhaust is a milder 265. This, as mentioned earlier in the unequal cams section, is a good way of increasing power by letting the turbo push in more air without losing too much low down power.
Overlap is a debatable subject on turbo cars, and often can depend on the tuner’s preference and engine spec. While overlap leads to the fuel/ air mix still being burned as it exits into the turbo and potentially helping spool, if the engine has more backpressure than boost pressure the effect is often reversed and performance is lowered with very little gains anywhere.
Supercharged engines respond very similarly to turbo engines with regard to cam choice, aside from one factor: overlap. Apart from a few maximum-effort high-rpm only applications, overlap will do nothing for a supercharged engine aside from hugely lower its performance at some parts, if not all, of the rev range.
The reason is that the pressurised inlet air and the fuel that is added along with it, would just be thrown out of the exhaust, increasing emissions, exhaust temps and back pressure, but wasting a huge amount of power at the same time.
Thanks to the way VTEC engines use one cam profile for low rpm use and another at high rpm, wild cams are less of an issue for driveability in a VTEC-equipped car as they can be in other engines. Unfortunately, Honda fit wild cams as standard in their performance VTEC engines and, for most situations, to take them any wilder would make the car undriveable. However, on milder VTEC engines this gives a great opportunity to increase top end power without ruining drivability.