I have a question for you guys.. Given the same basic motor, how much of a horsepower increase can you expect from just raising the compression ratio? For example if your current motor runs 10:1 compression on pump gas, and you change pistons and raise the compression to 13:1 and run race gas, what kind of performance increase can you expect?

It really depends on the complete motor for a definite answer. A 30% compression increase would probably require a change of CAM as well so you're now looking at a completely different setup. You could play with different combos in an analyzer program to get a better idea.

Troy.

Just the compression increase alone wont make a big difference..7-10% most likely..The gains come from the fact you can use a much more aggressive cam profile..which can use more carburation...which can use more gear..which...you get the idea

dont mean to butt in, but i was wondering this myself.

i thought i had a 9:1 compression but i checked the numbers and i get about 8.5:1 compression ratio with the Claimer cast pistons, .041 gasket, and 76cc heads. if i change that to .041 gasket and 67cc head i get about 9.1:1 compression ratio. would my cam still work with the compression?

Cam: 206/212 @ .050 lift .425/.440 max lift 112 lobe seperation hydraulic flat tappet.

A 206/212 @ .050 cam will work fine with 8.5 to 1

rule of thumb is supposed to be about 4% per point if nothing else is changed. Take that with a grain of salt im sure it could vary dramatically depending on the situation

Chris, i was actually asking because i was thinking of going to 9.1:1 within the next year or two. i'm planning on adding a pair of Pro Topline Torkers or World SR Torquers (both 67cc).

actually Pat Kelley's daily driver (not his race car) got me thinking about the head swap. our combos are sort of similar (if i do the head/rocker swaps) and what he MPH'd at Pomona is about where i want my car to be.

I'm running 10.59 to 1 compression with a 223/231 @.050,112 LSA cam,I have 205-210 lbs of cranking compression..runs fine on 92-93 octane

Enrique, I think you'd be fine with the higher CR. It should feel much stronger than what you have now. Just the CR change bumps up the DCR from about 7.37 to 8.03. You'll probably need 91 octane.

Is your cam the 12-388-4 (grind #252AH-12)?

One of the magazines did a study on this a couple of years ago. They used an open-chamber Mopar as a test engine because they could stack various thicknesses of copper head gasket on the block, without quench issues.

I'm thinking they found about 4% gain with each full ratio added...but there is also the disclaimer that the biggest gains are the first ones, with diminishing returns thereafter. So a 7 to 1 engine that is increased to 8 to 1 gets a bigger bang for the buck than a 14 to 1 engine that is increased to 15 to 1.

Hi Pat!

i'm not sure what cam it is, on the "build sheet" Dave wrote down 12-206-2(which is the PN for the 260H)and on the final receit it shows COM 12-206-2 but the cam card that came with the engine reads #252AH-12 which seems to be a mix between the 252H and the 260H ground on a 112 LSA.

it would be great if you could run the DCR calc for both cams and both the 76cc and 67cc heads. my Macintosh doesnt speak your calculators language.

Enrique, Comp does a lot of mixing lobes. The 296AH-8 in my bracket car is a mix of two Magnum lobes on a 108 LSA.

Email me with all the specs and I'll run the DCR for you.