So this week we thought we’d post a quick guide to common car camshaft terminology and how they can affect your car.
1. CAMSHAFT LIFT
Lift describes (usually in millimetres) how far the valves open – the higher the lift, the taller the camshaft lobes have to be to achieve this. More lift gives more flow, which, in turn, usually gives more power, and with only very minor ill-effects to performance elsewhere in the rev range.
Unfortunately, there are lift limits due to reliability and general engine design. To have incredibly high lift without long duration would mean the cam lobes would be almost vertical, and that would put huge strain on the cams and the entire valvetrain. You either need to seriously beef up your valvetrain to cope, or keep the ramp angle of the lobes sensible by limiting lift or increasing the duration at the same time, which also affects the powerband.
The shape of the lobes is not the only issue, as lift is also limited by the length of the valve stems and how far the valves can protrude into the combustion chamber before they collide with the pistons, the cylinder walls, or even each other!
2. CAMSHAFT DURATION
Duration is how long the valve is open for, and this is dictated by the width of the cam lobe, described in degrees. Duration doesn’t describe how long it’s fully open, though. It can either mean the distance the valve is lifted at all off its seat, or in some circumstances how long it’s lifted a certain amount, often 1mm or 0.050in.
This second indication of duration is much more useful, because below that the flow past the valves is not enough to noticeably affect performance anyway. Extra duration increases flow in and out of the engine, which generally gives extra peak power, often to a larger extend than increased lift. Unlike lift, it usually also affects the car’s powerband. On the whole, the longer duration the cams are, the less low down power is made and the more high-end power is made.
This can be a hugely complicated matter, as even if the cam itself gives huge top end potential performance, any other part that restricts the car at that point in the rev range may well mean the car loses the low end power without adding much extra power up top. This is most common with turbo engines where the turbo is not big enough to make use of the increased flow the longer duration cams give, giving losses right through the rev range. Increased duration will increase (or cause if there isn’t any as standard) overlap too, but we will cover that in a separate section.
3. CAMSHAFT TIMING
Cam timing is basically when the valves open and close in comparison to the piston position. As well as needing to be within certain parameters for the car to run – or at least run without the valves colliding with other engine parts – the point in the combustion cycle that the valves begin to open or close has a big effect on performance and the powerband.
Cam timing can be used to increase or decrease overlap to affect power and, as we will talk about in a separate section, with the use of adjustable cam pulleys the cam timing can be changed by an expert as the car is tuned to get maximum gains.
4. ADJUSTABLE CAMSHAFT PULLEYS
To tweak an engine’s cam timing for maximum performance when tuning the engine on a dyno or rolling road, you need to have adjustable cam pulleys fitted. This enables you to undo a few bolts and advance or retard the timing on each cam without needing to remove the cambelt. As all engines and engine specs are different, this is the ideal way to make an off the shelf cam perfectly suit your application.
This is when both the inlet and exhaust valves are open at the same time, and can have good and bad effects on a car’s performance, depending on the amount of overlap and the application it’s being used in. Most engines have some overlap as standard, and it can be beneficial to performance as the exhaust gas leaving the engine can suck in more inlet air, but just like all other variations, if you go too over the top for your engine spec it can harm performance rather than improve it.
A lot of overlap can be particularly bad on supercharged engines due to the pressurised air getting blown straight out of the exhaust before it gets combusted, and on some turbocharged engines the situation can be similar, depending on the engine spec and what is expected of the engine. Having said that, it’s also true that certain specifications of turbo engine can benefit from increased overlap. It’s a complex subject!
6. VALVE SPRINGS
When changing cams, you often need to upgrade the valve springs, too. These are the items that force your valves to shut once the cam lobe allows them to. Higher lift cams can open too far for the standard springs to handle, and many wilder cams need stronger springs to force the valves to shut fast enough to avoid colliding with anything.
Another reason for stronger valve springs is that high boost or back pressure can actually overcome spring pressure, causing them to not close properly. This is called valve float, and can either seriously limit engine power or actually cause the valves to collide with a rising piston.
7. UNEQUAL CAMSHAFTS
Camshafts generally have equal lift and duration on both the inlet and exhaust sides, but that doesn’t always mean it’s the best way to do it. On turbo cars – especially ones with smaller turbos running high boost – a lower duration exhaust cam, compared to the inlet cam, is beneficial because the pre-turbo backpressure causes exhaust gases to struggle to leave the engine if you leave the valves open too long, causing you to lose performance.
8. AREA UNDER THE CURVE
You sometimes hear the phrase ‘Area under the curve’ and this relates to the power and torque curves on an engine’s rolling road or dyno read-out. For race use, you mostly want maximum peak power that still enables the car to stay in the powerband with the gearing the car needs, so the area under the curve is less important.
However, for fast road use and general driveability, the ‘Area under the curve’ phrase comes into its own. Take two engines, for example. One has 350bhp and one has 400bhp, but the 350bhp engine has a really wide spread of power and torque right through the revs, while the 400bhp one only has good power and torque in the last 2000rpm of its rev range. In most conditions, especially on the road where you aren’t always near the rev limiter, the less powerful car would be quicker. This is a common symptom of an engine with too much cam for road use, and makes for a frustrating car to drive.
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