sweet said:
yeah Tuan knows what he's talking about. read some of the material on high C/R and different cams such as Toda/JUN etc.
the more aggressive, high overlap cams need a higher C/R to make lots of power because they bleed some off. details are explained elsewhere so just research a bit.
later
http://www.superhonda.com/forum/showthread.php?threadid=53070
"A mild cam with an early intake valve closing point will work well at low RPM. But at high RPM the intake valve will close before the maximum amount of air/fuel mixture has been drawn into the cylinder. When this happens performance at high RPM will suffer. If a high static compression ratio is used with a mild cam (i.e. and early intake valve closing point) then the mixture may end up being "over-compressed". This will lead to excessive compression losses, detonation and could even lead to head gasket or piston failure."
The Itr cam has an early intake valve closing point...that is how it gets its great midrange power but lacks the top-end to that of a wildercam. Compared to the characteristics of a more wild cam ex; Skunk2 stage II you'll compromise low end power because of the late valve closing point...but during high rpms more power is being made because more air/fuel is allowed....look at example
"On the other hand, an aggressive cam with a late intake valve closing point will work well at high RPM. But at low RPM the intake valve will close too late for sufficient compression of the intake charge to occur.
As a result torque and performance will suffer. If a low static compression ratio is used with an aggressive cam (i.e. a late intake valve closing point) then the mixture may end up being "under-compressed". Thus a high performance cam with long duration should ideally be combined with a higher static compression ratio. That way the engine can benefit at high RPM from the maximized amount of intake charge afforded by the late intake valve closing, and still achieve sufficient compression of the mixture as a by-product of the dynamic compression ratio."
The picture above is a good example of late-valve caused by the use of an agressive cam. With the use of a higher c/r piston the compromise of low-end power will be greatly reduced and on the other hand performance gained up-top will be substantially increased.
11:1 - 11:5 is the best c/r for toda spec B bumpsticks...of course raising the c/r of any engine will increase poer, but dyno tuning results have showed the best gains on pump gas with spec b's is 11:1 - 11:5.
MrMike said:
when u have high overlap essentially u have the intake and exhasut valves open at the same time so the intake charge can go straight out the exhasut valve with out even being compressed and burned. This gives u lower vacuum at idle, resulting in lower gas millage, and a loppy idle. With those types of cams ur probably still gonna benafit from them with stock compression, its just u can benafit even more with a higher compression ratio.
Very true, a nice simple explanation
