G
Guest
·This is a compilation of things to keep in mind when selecting a camshaft. I did this for a member a while back and inspired by another thread I've decided to go ahead and post it on the boards. This was compiled using personal experience, talks with other engine builders, as well as multiple published written materials.
Before we get to it, know that engines perform best when the system is optimized as a whole! EVERY component must be considered when selecting other upgrades. Don't expect a miracle cam to make you the ultimate engine. The system has to work together. A cam that works well for engine A may not for engine B.
(This is just a list of main considerations. I know this is redundant but it needs said again... Cam choice needs to be made on many engine characteristics and components so the things below may not always hold true for your application.)
OK, you've waited long enough...
The cam is the brain of an engine. It helps determine the engines characteristics at every rpm and can be optimized for the power delivery you wish to attain.
If you open the intake valve sooner then you give the incoming charge a head start in filling which can help high rpm power. This also increases the valve overlap period and the exhaust gases can be used to help pull in new charge. The low rpm power can suffer from a greater overlap duration though. Make sure you check valve-to-valve and valve-to-piston clearance when opening the intake sooner.
The most important part of a four stroke is the intake valve closing because it defines when the power producing period begins. To improve high rpm volumetric efficiency hold the valve open longer. It does this by using the inertia of the intake charge to help ramming before closing. The downfall is the low rpm torque will drop off because of low rpm port velocities being low and allowing the flow to reverse.
The exhaust valve opening point is also important because it determines when the pressure from the combustion begins to release. Opening the valve to late will increase pumping losses but to quick will kill cylinder pressure to soon. Early opening of the exhaust valve is generally used for high rpm and compression. High compression lets the charge burn quicker and it builds pressure sooner in the stroke. Early opening can be hard on the exhaust valve(s) and guide(s). An efficient exhaust port and system can allow for a later valve opening.
The exhaust valve closing point determines the end of duration and exhaust stroke. At high rpms a late closing lets the engine scavenge the exhaust better. At low rpms though, gas velocity is low and reversion occurs.
The optimum overlap is defined by various engine characteristics, including, combustion chamber design, intake and exhaust flow characteristics, and even piston dome. A tall dome may require more duration to allow for better scavenging. A lot of overlap works well with an open free flowing exhaust. If you are restricted to a poor exhaust system reduce overlap to improve volumetric efficiency. Also use less overlap when low lift flow is high and ports are not lazy.
The most common mistake with choosing a cam is to much duration.
Increase valve event duration or lift aggressiveness when rpm and/ or displacement are bumped up.
Shorter duration increases torque but more duration helps build horsepower. A long stroke engine tends to respond better to increased duration. Increased duration also moves the volumetric efficiency curve up the rpm range... along with torque and hp curves.
If the head you are using has good low lift flow then the engine won't require as much duration. At the same time, if the ports are large and lazy use less duration and overlap to increase cylinder pressure and volumetric efficiency.
If the exhaust-to-intake flow is low then you can increase exhaust timing to compensate.
Restrictive exhausts usually benefit from having more exhaust duration.
The better the exhaust port and system the later the exhaust valve can be opened. You may have to shorten duration to keep overlap at a good level though.
The lower the compression ratio the earlier the intake valve should close. Same goes for the other direction, higher compression needs a later closing intake. 12:1 mechanical compression can give you 10:1 corrected compression but at high rpms the dynamic compression could be 17:1. This is because volumetric efficiency is high from intake charge inertia.
Match valve lift to port flow and valve head diameter.
An engine with a high rod-to-stroke-ratio needs max airflow at a later degree because piston speed occurs later.
The faster the valve is lifted the more exposed area throughout the intake event. If you lift a valve quicker in the same duration you don't have the negative effects of increased duration. At the same time, it requires better valve train components. More often then not, a cam with more aggressive rate of lift will accelerate the bike faster.
Before we get to it, know that engines perform best when the system is optimized as a whole! EVERY component must be considered when selecting other upgrades. Don't expect a miracle cam to make you the ultimate engine. The system has to work together. A cam that works well for engine A may not for engine B.
(This is just a list of main considerations. I know this is redundant but it needs said again... Cam choice needs to be made on many engine characteristics and components so the things below may not always hold true for your application.)
OK, you've waited long enough...
The cam is the brain of an engine. It helps determine the engines characteristics at every rpm and can be optimized for the power delivery you wish to attain.
If you open the intake valve sooner then you give the incoming charge a head start in filling which can help high rpm power. This also increases the valve overlap period and the exhaust gases can be used to help pull in new charge. The low rpm power can suffer from a greater overlap duration though. Make sure you check valve-to-valve and valve-to-piston clearance when opening the intake sooner.
The most important part of a four stroke is the intake valve closing because it defines when the power producing period begins. To improve high rpm volumetric efficiency hold the valve open longer. It does this by using the inertia of the intake charge to help ramming before closing. The downfall is the low rpm torque will drop off because of low rpm port velocities being low and allowing the flow to reverse.
The exhaust valve opening point is also important because it determines when the pressure from the combustion begins to release. Opening the valve to late will increase pumping losses but to quick will kill cylinder pressure to soon. Early opening of the exhaust valve is generally used for high rpm and compression. High compression lets the charge burn quicker and it builds pressure sooner in the stroke. Early opening can be hard on the exhaust valve(s) and guide(s). An efficient exhaust port and system can allow for a later valve opening.
The exhaust valve closing point determines the end of duration and exhaust stroke. At high rpms a late closing lets the engine scavenge the exhaust better. At low rpms though, gas velocity is low and reversion occurs.
The optimum overlap is defined by various engine characteristics, including, combustion chamber design, intake and exhaust flow characteristics, and even piston dome. A tall dome may require more duration to allow for better scavenging. A lot of overlap works well with an open free flowing exhaust. If you are restricted to a poor exhaust system reduce overlap to improve volumetric efficiency. Also use less overlap when low lift flow is high and ports are not lazy.
The most common mistake with choosing a cam is to much duration.
Increase valve event duration or lift aggressiveness when rpm and/ or displacement are bumped up.
Shorter duration increases torque but more duration helps build horsepower. A long stroke engine tends to respond better to increased duration. Increased duration also moves the volumetric efficiency curve up the rpm range... along with torque and hp curves.
If the head you are using has good low lift flow then the engine won't require as much duration. At the same time, if the ports are large and lazy use less duration and overlap to increase cylinder pressure and volumetric efficiency.
If the exhaust-to-intake flow is low then you can increase exhaust timing to compensate.
Restrictive exhausts usually benefit from having more exhaust duration.
The better the exhaust port and system the later the exhaust valve can be opened. You may have to shorten duration to keep overlap at a good level though.
The lower the compression ratio the earlier the intake valve should close. Same goes for the other direction, higher compression needs a later closing intake. 12:1 mechanical compression can give you 10:1 corrected compression but at high rpms the dynamic compression could be 17:1. This is because volumetric efficiency is high from intake charge inertia.
Match valve lift to port flow and valve head diameter.
An engine with a high rod-to-stroke-ratio needs max airflow at a later degree because piston speed occurs later.
The faster the valve is lifted the more exposed area throughout the intake event. If you lift a valve quicker in the same duration you don't have the negative effects of increased duration. At the same time, it requires better valve train components. More often then not, a cam with more aggressive rate of lift will accelerate the bike faster.
