Performance Camshafts

The two important aspects of a camshaft, in terms of engine performance, are camshaft duration, or cam duration, and valve lift. Both cam duration and valve lift are determined by the camshaft lobe. Cam duration is the time that at least one valve of a cylinder remains open, i.e., off its valve seat, measured in degrees rotation of the crankshaft, while valve lift is the maximum distance the valve head travels from the valve seat.

Valve Lift

Valve lift is determined by two things: the camshaft lobe lift, and the rocker arm ratio. The camshaft lobe lift is the difference between the distance between the heel of the cam lobe (when the valve is closed) and rotating center of the camshaft, and the distance between the toe of the lobe and the center of the camshaft. This difference is multiplied by the rocker arm ratio so that if the camshaft lobe lift is 0.325in. and the rocker arm ratio is 1.5, the effective valve lift is 0.4875in.

The valve lift you would require is somewhat related to intake valve head diameter. An engine with an intake valve head diameter of 1.400in. to 1.500in. will generally perform best with a valve lift of 0.395in. to 0.475in.; an engine with a larger intake valve head diameter of 1.750in. to 1.875in. will generally perform best with a valve lift of 0.425in. to 0.550in.; and an engine with a large intake valve head diameter of 2.000in. to 2.250in. will generally perform best with a valve lift of 0.475in. to 0.650in. But these are just rough guidelines; ultimately you will need to take some gas flow readings on a flow bench to determine the best valve lift for your particular engine.

Camshaft lobe diagram

The Camshaft Lobe

A number of factors inhibit valve lift. The most important being the gap and the angle between the intake and exhaust valves, the piston to valve clearance and the intake charge pressure. These factors also influence cam duration. Another factor influencing valve lift is valve spring compression. Obviously, once the valve springs are fully compressed, it cannot give any more and the valve cannot be pushed further down into the combustion chamber.

Cam Duration

As we've mentioned earlier, cam duration is measured in degrees rotation of the crankshaft, rather than the camshaft, and the crankshaft completes two full rotations for every rotation of the camshaft. In other words, with a 310 degree camshaft, the valves are open for only 155 degrees of actual camshaft rotation.

A performance camshaft for a naturally aspirated engine will have a duration in the range of 270 degrees to 310 degrees or more, with a 270 degree camshaft described as a 'mild' camshaft and a 310 or more degree camshaft being described as a 'wild' race camshaft. A stock camshaft usually has a duration of around 270 degrees but what differentiates a 270 degree performance camshaft from a stock camshaft is increased valve lift and a much faster rate of valve lift. With a faster valve lift rate, the valve reaches full lift quicker and remains at full lift for longer. This increases Volumetric Efficiency (VE) as more air flow in and out of the engine is possible.

A determining factor, when choosing camshaft duration is the purpose of the vehicle. The longer the duration of the camshaft, the further up the rev range the power band shifts, and the rougher the idle. Obviously, as the power band moves higher up the rev range, bottom end power is lost. Also, as cam duration and valve overlap increases, torque is lost. Fuel efficiency also decreases and exhaust emissions increase as valve overlap increases. This is because the intake valve opens sooner while the exhaust valve is still open. Thus some of the intake charge that enters through the intake valve escapes through the exhaust valve.

High performance camshafts start at 280 degrees of duration. These camshafts have increased valve overlap but not too much so emissions and fuel economy are not severely affected. These are generally good camshafts for modified street cars and produce good power from 2,500 RPM up to 7,000 RPM but they do not have a smooth idle because of the increased valve overlap.

A 290 degree camshaft requires more cylinder head work in terms of cylinder head porting and gas flowing as they work better when the engine's Volumetric Efficiency (VE) is improved. As you'd expect, these camshafts produce a fairly rough idle. These camshafts are generally good for rally cars and produce power from 3,000 RPM up to 7,500 RPM. A 300 degree camshaft requires even higher levels of VE, reaching the physical gas flowing limitations of a two valve cylinder head with a single camshaft. These camshafts are good for modified race cars and produce good power from 4,000 RPM up to 8,000 RPM. However, they have a very rough idle.

A camshaft with a duration of more than 300 degrees is an out and out race camshaft with a power band in the 4,500 RPM to 9,000 RPM rev range. To make effective use of a 300 degree camshaft, you need to ensure that the engine has a very high VE. You also need to ensure that the engine can rev beyond the red line of most stock engines.

Valve Overlap

Valve overlap diagram

Valve Overlap Diagram

The limit for opening the exhaust valve is approximately 80° before bottom dead center (BBDC). Opening the exhaust valve any sooner tends not to increase power production but will shift the power band higher up the rev range and will reduce low end torque as downward pressure on the piston during the power stroke is released. The same applies to closing the intake valve where 80° after bottom dead center (ABDC) is the limit for increased power production.

Effect on Cylinder Pressure

The use of a performance camshaft can reduce the cylinder pressure when the engine is not operating at the optimal power band. This is because the late closing of the intake valve allows some of the intake charge to be forced back out of the combustion chamber. This will rob the engine of torque at lower rpm. As a result, the compression ratio should not be too low and should be increased if possible. We discuss increasing the compression ratio later on.

Turbo Cams

The limit for opening the exhaust valve is approximately 80° before bottom dead center (BBDC). Opening the exhaust valve any sooner tends not to increase power production but will shift the power band higher up the rev range and will reduce low end torque as downward pressure on the piston during the power stroke is released. The same applies to closing the intake valve where 80° after bottom dead center (ABDC) is the limit for increased power production.

EFI and Performance Camshafts

Any type of power modification, affects other areas of the car and this is no different when installing an aftermarket camshaft on a engine that uses Electronic Fuel Injection (EFI). If you are to retain the stock Electronic Control Unit (ECU) without having to re-flash the ECU, the camshaft needs to be of a short duration with a maximum of 220-222 degrees at 0.050in lift and have a wide lobe separation of at least 112 to 114 degrees. A longer duration camshaft or one with a narrower lobe separation will not create a good vacuum signal for the EFI system and would necessitate re-programming of the ECU. If it is possible, we would recommend that you have your ECU re-programmed when you change the camshaft because while the camshaft might work with the stock fuel and timing curve mapping on your ECU, it will not be working at its optimum level and will thus be down on performance and fuel consumption.