[Brian M]

I see recomended compression ratio for different cams, why do you need so much compression for a cam, what is the problems with running lower compression than the cam asks for?

 

[nosmatt]

the short easy version?
well cyl pressure is what builds torque, a cam with a narrow lsa and long duration will already be sacrificing cyl pressure, so using this cam on a low compression motor will have even less cyl pressure, will not have enough vacuum to idle at a low rpm, you have to jack the rpm up to nascar levels to even drive around....lol

now the opposite is also true, a very wide lsa, short duration cam coupled with a higher compression motor will build alot of cyl pressure, which is great for torque, but bad for low octane pump gas. you can have a engine with a relativly small cam, a 10.5:1 streetable c/r, and run 93 octane and still ping and have to run alot less timing than someone with a more radical cam.

now, this is how i understand it, and again, this is the short version as im tired of typin

 

[Mach1Morgan]

Because the effective compression stroke starts at the point that the intake valve closes, by adding more cam duration the piston will be farther up the bore at the begining of the compression stroke and the cylinder pressure will be less at TDC. This tends to make the low end torque suffer more than if the compression was increased while installing a bigger cam. It's actually not true that narrow lobe seperation cams bleed off cylinder pressure. What they bleed off is intake manifold vacuum. If you ever take cranking compression readings with a narrow lobe seperation cam and a wide lobe seperation cam of the same duration, and on the very same engine, the engine will have more cranking PSI with the narrow lobe seperation cam installed. This is because with a narrow lobe seperation the intake valve opens sooner and therefore closes sooner, starting the compression stroke while the piston is farther down the cylinder bore.