detonation marks on pistons

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Daddy
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detonation marks on pistons

Post by Daddy »

So long story short my car was smoking and my friend and I thought it was the valve seals. He went to change them and dropped a valve in the cylinder. While trying to get it out he bent it and had to take the head off. When he did he saw detonation marks on all four pistons BUT ONLY ON THE INTAKE SIDES. The no. 1 piston was by far the worst and 3-4 weren't too bad but of course we are going to change them any. Luckily I have everything to fix the problem but I want to find out what caused it before I start driving it again. Just glad we found the problem before it got worse.

The motor:
Rebuilt it 3 years ago with super tech pistons, arp head studs, cosworth hg, all new gaskets, polished crank, bored cylinders .20, enthalpy tune ( z32 maf, t28, 91 octane, 255 lph pump, sr injectors), boost was only at 10 psi

I'm sure I am forgetting something. My friend said he saw the same problem on an sr before but couldn't remember what caused it. If there is anyone who can help i would greatly appreciate it. Thanks!
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Re: detonation marks on pistons

Post by Daddy »

heres a pic of the no.1 piston
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airman
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Re: detonation marks on pistons

Post by airman »

I've experienced similar, but a little worse - it was knock induced. (Cylinder temperature/pressure far too high) Too much timing. Your a/f ratio may have looked okay which may have masked the issue.

Is it possible your base timing (distributor) was set too far advanced? Enthalpy tunes are usually conservative enough to avoid this, but I can't speak from experience.
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Re: detonation marks on pistons

Post by p00t »

airman wrote:I've experienced similar, but a little worse - it was knock induced. (Cylinder temperature/pressure far too high) Too much timing. Your a/f ratio may have looked okay which may have masked the issue.

Is it possible your base timing (distributor) was set too far advanced? Enthalpy tunes are usually conservative enough to avoid this, but I can't speak from experience.
Airman were your detonation marks heaviest on the intake side?

I agree with Airman that you are too far advanced on the igntion timing. Enthalpy's tunes wont be perfect for every setup. You can be too far advanced on the dizzy or your setup runs hotter... whatever the reason. Honestly I would leave the timing the same and use some sort of knock listening device to see if its always knocking at some point in the RPM range. You can then tell where its knocking and adjust the tune or dizzy to remove the knock.

My concern would be that you retard the dizzy by some amount and just assume it will be cured.
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airman
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Re: detonation marks on pistons

Post by airman »

p00t wrote: Airman were your detonation marks heaviest on the intake side?
If my memory serves correctly, the pistons were pitted on both intake and exhaust sides but I do not recall one being more affected than the other.
R.I.P. 1990 Red Hatch - DOHC 5 speed // CP 9:1 - Eagle Rods - Clevite - ARP // Holset HX35 - 20psi daily - MSPNP2
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Re: detonation marks on pistons

Post by R34SR »

p00t wrote:
airman wrote:I've experienced similar, but a little worse - it was knock induced. (Cylinder temperature/pressure far too high) Too much timing. Your a/f ratio may have looked okay which may have masked the issue.

Is it possible your base timing (distributor) was set too far advanced? Enthalpy tunes are usually conservative enough to avoid this, but I can't speak from experience.
Airman were your detonation marks heaviest on the intake side?

I agree with Airman that you are too far advanced on the igntion timing. Enthalpy's tunes wont be perfect for every setup. You can be too far advanced on the dizzy or your setup runs hotter... whatever the reason. Honestly I would leave the timing the same and use some sort of knock listening device to see if its always knocking at some point in the RPM range. You can then tell where its knocking and adjust the tune or dizzy to remove the knock.

My concern would be that you retard the dizzy by some amount and just assume it will be cured.
A simple set of homemade DET cans will help you out tremendously, i can tell you from personal experience, i never tune without it and it saves motors....and costs less than $10 to make.
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