[charlie66]

I decided to change the compression ratio in my 4cyl in attempt to help the low end low rpm torque/power to spool the motor sooner. This has been the biggest problem I've had in getting it tuned for drag racing . I had it spooling ok up until I had to tighten the converter do to the power level im looking for. So in turn this is where im at. I bought some custom flat top Ross pistons to replace the Ross 24cc dish tops that was being used. My old compression ratio was 8.4:1 now it will be 9.8:1. I know there will be more objections to this then anyone agreeing with doing this but I think it will help with the characteristics of a 8.4:1 4cyl with 254 .050 duration. Its like stepping into the mud when stepping on the gas until boost.. I have a lot of data with my old pistons. So I know the safe boost levels and a/f ratios as well as timing settings. I have never burnt the motor so I feel good about using them to refer to as I go forth as a guide line for what im trying to do, design a safe tune .. So with this said I have gone an done some calculations to kind of try to get a grip on what to expect with boost and timing setting with the new pistons. So all of my psi setting that I use to use (different tunes for what I was doing in any given day) I compared to the new 9.8:1 static ratio that I now will be using . So first off, the old volume to pressure index of 68 will now be 83. Im hoping this doesn't work against me with detonation, but I think it will help the low rpm torque/power for spooling. The second thing is dynamic compression comparison between the old ratio to the new. The old was 7.15:1 with my present valve closing timing and the effective stroke of 3.12 inches from the 3.75 stroke .The new will be 8.32:1 . basically 1.17 more .
Then I went and figured the effective boost compression ratio, reflecting static c.r., cam timing, altitude for both ratios to see what they are to compare them . The 8.4:1 at 20 PSI = 16.88 :1 . The 9.8:1 in comparison = 19.64 :1. knowing the tune for the old 20 psi I figured out what the new psi will be to match it . So 15 psi at 9.8:1 will yield all most the same at 16.81:1 . So the old 20 is now the new 15psi. This is all theatrical of course but it is all I got and it makes sense to me.. So im thinking ill be able to use my old tunes settings for my high and low boost levels with the lower psi's of the 9.8:1 that matches the higher old psi's.. I made a graph to reference for different levels of boost. I use to drive around with the PSI set at 26 . So if I want to match that for tuning it the same the new psi will be at 20 . And I will set the timing lower and learn where it wants to be..

Any way I just wanted to share this and see what everyone thought. Feel free to give your opinions good or bad I will appreciate it in any form ... its all good...