mycarisfasterthanyours

I have a high compression 383 (specs listed in sig). I don't know exactly how high the CR is though, as there are a number of unknown variables. It should be 12:1 give or take maybe .5. I conducted a cranking compression test the other day and most cylinders were in the 210psi range (cold, 4-5 cycles, all spark plugs removed, and throttle wide open).

When I log the PCM while driving, it almost always shows knock retard. I've tried using some toluone (sp?) in it before with no noticable difference in knock retard. I believe the KR is causing poor drivability. The car has a pcmforless tune, but still drives like crap and shows KR. The valvetrain is also very noisy and the longtubes bang from time to time.

My question is this. Making an educated guess, do you guys think my compression ratio is just too darn high? Is is possible to get a safe, drivable tune with pressures like this? Do I keep trying to chase down this knock by eliminating a rattle here, a rattle there, or do I just give up and buy some different heads with larger chambers?

Dave89IROC

how high do you figure you got your octane?

try race gas(110 octane unleaded) see what happens, if it is still showing knock, it is not from spark knock

mycarisfasterthanyours

Well, they sell 91 octane in these parts, and if I remember correctly, I put a gallon of toloune in 1/2-3/4 tank (10 gallons maybe). So, whatever that comes out to, not sure.

I guess what I was looking for is some sort of rule of thumb for cranking compression in relationship to tunability/streetability. I saw a chart in a magazine once upon a time, but now I can't find it. I know there are many other factors, but I'm just looking for a rough idea, if possible.

1racerdude

Ya can order the 101 unleaded in 5 gal lots and have it delivered. That will tell ya if it's false knock or not.
If it's false knock ya are not going to get rid of it, without the resistor in the KM circuit or getting it tuned out of your computer.The sounds of headers,RR's and a built engine is not what it was tuned for. The LT4 KM won't help,I have never got it to work or remove the retard.

Your compression is fine,my shop car on pump gas is a lot higher than 210PSI.
Yea when it pulls 5/10* of timing it kills performance.

amean94ta

they have race cars there go ask the speed/machine shops they will tell ya where to get race gas