Re: Relationship between Compression, Cylinder pressure and cam ?
In my experience, increased compression will increase idle vacuum a bit, which makes me think that "suction" is being increased somewhat.
I do know, that what your talking about is not a 1:1 relationship. With a bigger cam, you will regain *some* of the lost low end torque by increasing the compression, but never all of it. With the intake valve closing later, your compression stroke is shorter...meaning that your taking a smaller volume of air and fuel and squeezing it harder. You could have the same cranking compression readings, and the same dynamic compression ratio, but with all else the same, the smaller cam (or the earlier closing intake valve) will generally trap more air/fuel at lower rpms and create more low end torque. Once things get spinning, dynamic compression becomes less important and airflow and cam timing become more important for making power.
Here is a real world example for you. 2 similar weight pickups, both nearly identical in all aspects except 1 is a 8.5-1 compression 350 with a 260 duration comp cams high energy cam (212@.050, 110 lsa), the other has about 9.5-1 compression and a 350hp 327 GM -151 cam in it (222@.050, 114 lsa). Both crank 150-160psi. In a 1/4 mile, the smaller cammed truck beats the other truck by 1 to 1 1/2 car lengths...but in the first 200 feet the smaller cammed truck is more like 4-5 car lengths ahead. Once the bigger cammed truck gets wound up, it is reeling in the smaller cammed truck the whole way down the track.
Of course, if the bigger cammed truck would have put a 2400 rpm or so stall convertor in there to make up for the reduced off idle output, he would have won.
'86 chevy 3/4 ton...9.4-1 355, vortec heads, vortec performer intake, 600 edelbrock, xe268, headers, duals w/turbo mufflers, BTO 700R4, 3.23 gears
'77 nova, 388, 200cc pro-toplines, holley 300-36, 750 edelbrock, comp 294s, 10" ATI, 3.73's
'83 ford F-150 XLT...351w, C-6 and 3.55's.