Technical question

[Scro]

I understand that the difference from one cam to another is the amount and duration of lift. What I don't understand is how this difference changes the power curve of a motor. What's happening in the combustion chamber to give this cam more "bottom end" and that cam more "top-end"? How does a valve staying open longer or opening farther change the power curve?

[ccdhowell]

Dude, I hope I don't mess this up, but this is how I see it. A bigger cam is better on the top end because it's actually allowing more air and fuel in at lower rpms than the engine can burn efficiently. The cylinder burns cold because of all the excess fuel. The excess is released into the exhaust. When the rpms pick up and the engine is under load it will effieciently burn this increased fuel load and smooth out. As the rpms increase the lower lift/duration cams will not flow enough fuel/air to allow the engine to continue to accelerate, they are maxed-out.

It really is about effieciency, kinda like the pipes were on 2stroke engines. The cam controls the amount of fuel in the engine and at what point it enters before combustion and leaves after combustion. A particular cam is more effiecient for a particular range of rpms than another.

I know it works, been dealing with engines of all types for many years but without anny formal training I can't say it in a technical way. Someone will come along and clean this up for me.

[Tommy Warren]

makes perfect sense to me...its not so much how much the cam opens the valve its when and for how long.

[John451]

I think it also might have something to do with backpressure, the more the valve opens the less backpressure there is which makes it able to pump gas and air in and exhaust out as fast as it can. And the opposite for one that opens it less, That's why some quads have 2 intake valves and 2 exhaust...