I lifted a head on the dyno......
09-16-2004, 11:02 PM
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I lifted a head on the dyno...... Right at the 600 hp mark on the dyno I lifted a head on my engine. I have a iron block, ARP head studs, Cometic headgaskets, 10.5 compression ratio and use a healthy shot of nitrous. Do anybody know exactly what causes a head to lift and what can be done to prevent it from happening again?
09-16-2004, 11:36 PM
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Everything was machined properly, assembled properly, and torqued properly. The engine has about 12,000 miles on it. I was very surprised to see it happen on such a small shot and at a relatively low horsepower level. That's why I'm asking for help trying to figure out why and how to prevent it from happening again.
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09-16-2004, 11:48 PM
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The heads are the GMPP CNC LS6 heads. Trevor Johnson Cylinder Heads did a little more porting to them before they went on the car. Compression ratio was set at 10.5 to 1.
09-17-2004, 12:44 AM
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I would say you have to much CR. The only way to realy pervent it from ever happening again is to pull the motor. O ring the block and heads. Are u 100% that the heads are not milled at all. I would bet you are closer to 11 to 1.
09-17-2004, 01:35 AM
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Originally Posted by cyphur_traq
yea LS6 heads come stock @ 10.6 : 1. dunno how you're at 10.5?
the GMPP heads also come in two versions; unmilled (70cc) and milled (62cc)
Last edited by jrp; 09-17-2004 at 01:44 AM.
09-17-2004, 01:43 AM
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I had the same issue with Cometics on my 1st dynoing of my SC setup (new motor, no detonation @ 8-9psi).
I swapped to GM MLS.
Cometics suck. This is my 2nd failure using them and I won't touch them again.
P.S. You could blame the deck of my old PP Heads. I switched to AFR's after the issue, but I believe the gaskets were to blame.
I swapped to GM MLS.
Cometics suck. This is my 2nd failure using them and I won't touch them again.
P.S. You could blame the deck of my old PP Heads. I switched to AFR's after the issue, but I believe the gaskets were to blame.
09-17-2004, 02:07 AM
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The heads were desined for 11:1 but we lowered it by using a custom thickness on the Cometic headgaskets. Timing is at 24 degrees...I do agree that we may be using too much timing. The nitrous system is a NX MAF wet kit. Fuel delivery and pressure doesn't seem to be an issue as I have an upgraded fuel system. On the dyno sheet, when the nitrous is activated the a/f ratio spikes to about 14.7 then quickly drops to 11.8 and is linear from there. Fuel was premium unleaded.
09-17-2004, 09:00 AM
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Originally Posted by jrp
the ls6 heads are 10.6 on the stock 3.90 bore. he's using a iron block with a 4.030 bore.
the GMPP heads also come in two versions; unmilled (70cc) and milled (62cc)
the GMPP heads also come in two versions; unmilled (70cc) and milled (62cc)
EDIT: ok should've read his post before posting mine up:
Originally Posted by Gold Z
The heads were desined for 11:1 but we lowered it by using a custom thickness on the Cometic headgaskets.
09-17-2004, 10:29 AM
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By useing a thicker head gasket you killed the quench clearance,thereby making the motor more detonation sensative. You may have to pull more timing than others with the same compression on account of this.
09-17-2004, 10:44 AM
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If you have the 62 cc ls6 heads you need to double check your CR also your cam will affect your cylinder pressure a lot!!The thicker gasket would mess up your quench as mentioned and the thicker gasket will be taken a harder hit then a thinner gasket.Also there has been an issue with the ARP studs needing to be torqued and losened 3 times.This could be a factor.On the cam if you are running a rather small duration cam this could cause problems with detonation,also you mention your af ratio went to 14.7 briefly right before the N.O. shot this could be a problem as well.Just some things you might check.Also do you have a good exhaust system??