Is there a general ballpark rule of thumb for power gain for a given percentage increase in ratio? Say, going from a 8.5:1 to 10.5:1 with the same components, just jets and timing.
La Habra, CA
408w SCAT crank, SCAT rods, Probe forged pistons, AFR205's in the works.
[ This Message was edited by: etcetera on 11/3/05 10:19am ]
Cool. I figured there must be a loose approximation out there. Joe probably can guess why I'm asking. I ended up with a very similar combo to one that is in one of the AFR articles, except with a 10.7:1 CR, vs the Turbo friendly CR in the article (they don't list it specifically).
Here's the combo
Scat Crank and Rods
Probe -20cc forged pistons
AED HO 950 (Lightly used)
Comp Cams XR286R (Lightly used)
My 2cents....I believe increasing compression has a diminishing improvement the higher you go. That is the improvement from 8~10 will be greater than 10~12 and so on. In my mind it takes the shine off CR greater than 10.5:1 for a streetable petrol car as the hassles start to greatly outweigh the gains.
A small cam bleeds off less cylinder pressure, so it can use a smaller compression ratio. A big cam bleeds off more compression, so it needs a higher compression ratio to maintain adequate cylinder pressure.
If your compression ratio is low for your combo, you have a lot to gain from an increase. If the engine already has adequate cylinder pressure, further increasing the compression ratio won't produce as large of a gain.
Adding cylinder pressure helps the low-end and midrange more than the top end.