Cranking compression vs. knock

[jpwalt1987]

I'm still working on this local guys car trying to get the tune right. I can't get this thing to stop knocking at 15psi on 93 octane. I have switched O2 sensors on the AEM wide band with another known good one. I have leaned it out to 11.2-11.5 from 10.8 thinking it could have rich knock. No luck. Timing has been between 16 and 19. I remember when I was checking the compression on this car it was right at 180-185psi That's with the throttle locked open and cranking it about 5-6 times. My car is built to the max and I only have about 150 psi cranking compression. The car I'm working on has an unknown history except for what I have done to it. Someone has been into it before because it has rjc head gaskets and studs. It also has really good oil pressure. Too good to be an original untouched bottom end. I am at a loss here. I even turned off the wideband controller to see how it would react. Not much difference. I am baffled on this car. Anyone have some suggestions?


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[jpwalt1987]

I put a set of NGK ur5 plugs in it this afternoon. Looking at the old ac43ts plugs I don't see much difference in them except one. On has a bit more black speckling around where the electrode meets the porcelain. I am starting to think that maybe I have one injector not contributing the same amount as the others. Possibly a faulty injector harness maybe? If I have 5 compensating for the 1 that is lagging behind then I think that the overall afr average would be correct but have that one cylinder leaner and causing the detonation. I still don't know why I have 30 psi more on the cranking compression. Just thinking out loud.


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[Chuck Leeper]

Engine has been opened.. I wonder where the cam is in at???

 

[TurboBuRick]

Are you doing third gear pulls?

What MPH are you seeing knock? Is the low o2 MPH the same as knock MPH?

Rick

 

[TurboTdr3.8]

Wonder if the valve springs is weak or not set right . , Im going through the same thing with mine knock no mater what you do low boost and good racing gas does not help . mine is leaning toward valve problem or floating or cam timing , I know how you feel in trying to figure it out its a bummer . Boy I wish these cars had OBDII could make life a lot easier some time in finding problems maybe !!!

 

[750H.P.V6]

If it has a flat tappet cam check to make sure the #3 ehxaust lobe is still there.

Neal

 

[bison]

Cranking compression doesn't tell you anything about detonation tendency. It's possible to have very low cranking compression and still have extremely high cylinder pressure.

 

[jpwalt1987]

Well I pulled the NGK plugs after a few blasts and could not tell if there was one cylinder that was not contributing. I also turned the waste gate rod out almost all the way. It made 10psi and didn't detonate. Fuel correction was off because of the low boost I imagine. I slowly cranked it back up and got to 13.9psi and 19*timing. It had no detonation except at the shift. <3 knock retard. It started raining so play time was over. I am starting to think that the boost gauge isn't accurate also. I will swap my 3 bar map sensor onto it tomorrow and see.


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