I rebuilt my motor last summer to a 355 reusing the stock crank and rods. I had new KB flat top pistons with 7CC reliefs installed and had the assembly rebalanced. The calculations I ran for CR are around 10.8 and quench of .054". I also sent my heads out to AI and had them ported and larger valves put in. I am running a HOT cam with 1.6 RR's. I bought new 32# injectors, and am running the Z06 mass air meter calibrated for the LT1, and also put a new opti, and a 160 deg thermostat. This setup is running on the stock throttle body, stock exhuast manifolds, and original O2 sensors with 100K miles. Lastly I am running a tune from PCM for less.

THE PROBLEM: When I first installed the tune, the motor detonated under light acceleration (bad enough to hear it). So I had the timming pulled back and tried again using a DATAMASTER to detect for knock , only to find the problem was still there under moderate to heavy accelerations. I then had the timing pulled again (back further than stock timing), and it seemed to fix the problem. I drove it all winter like this, but now it's time to figure out what the problem is.

Does anyone see a problem with my setup? What is the best way to find problems like this? I have run a data logger, but can't make sense of the numbers the sensors are spitting out. What could be the cause of this detonation? And where can I find information about sensor readings.

Thanks

 

Just to add some information... I ran a compression check on all my cylinders tonight. I took all the plugs out, opened the throttle body all the way and ran it for 5 compression cycles. The numbers came out to be between 172 and 177 psi. Which is around 11.9:1. To me this seems quite high from where it should be.

 

Calculating compression ratios using a cranking compression test is very inaccurate, I would estimate your compression to be around 10.5-11:1. Now to your detonation problem-Whats the opperating temps like?

 

The hottest I have seen it run was around 185, but usually it runs 175 or so.

Would excessive fuel cause detonation? When I pulled all my plugs out they were all somewhat wet and smelled like fuel. Seems like most plugs I have seen are usually dry.

 

Yep this is what you gave us......

This is what I see.....

A GM hot cam with 10.8:1 gives a DCR of 8.56. Then you are running this with a quench of .054!!! That's going to give you problems right there. Big quench and a high DCR are going to cause knock.

You can go to a thinner head gasket (look in this forum Denny just asked about them). You can also try Evans NPG+, might want to do a search for Mr Horsepower on Evans NPG and do some reading. Between those two things you should solve most of your problems.

Bret

 

Call me crazy, but wouldnt you want some THICKER head gaskets to lower the CR to reduce knock.

 

This is a old thing that Smokey found out a long time ago..... A thicker head gasket only makes the A/F that's trapped in the dead spots of the chamber cause more detonation or pre ignition problems, while a smaller one will help push all that crap to the center of the cylinder and not do these things.

Bret

 

I'm also not against changing out the cam to fix this... What would be a decent cam here to get the DCR down. Also I thought a DCR of 9 was acceptable, and the quench height was based on a .039" gasket and a .015" piston to deck clearance. So would the impalla gasket help here? (.029")

THanks

 

a DCR of 9 is fine IF your quench and cooling are dead on! the impalla gasket or the mr. gasket #5716(either one) would be ALOT better!

 

He wasn't suggesting a gasket change to reduce compression ratio... he was suggesting a thinner gasket to reduce quench height, which appears to be excessive.

 

Will .010" help enough though? to me it seems that .010" is so small.

Also how did GM compensate for the LT4... I mean they are using quench hieghts higher than .054" right? (maybe .065 and up?) and they bumped compression up to 10.8, (Not sure what the DCR was for that cam) Correct me if I'm wrong, but to me it seems like my setup should be somewhat close to that.

 

Nice answers!

Keeping the quench low squeezes more charge into the chamber where the
combustion temperatures are more uniform.

There is enough temperture variance from the chamber to the quench area
to cause erratic combustion - leading to detonation.

Fel Pro makes a nice steel head gasket with a compressed thickness of 0.015".

When I took my heads off, I mic'd the gasket in several places and got an
average of 0.016" (low 0.015", high 0.017").

My current setup yields about .041" quench.

It doesn't take much; 0.010" less on a 4.0" bore is going to push a couple cc of charge into the chamber.

 

Yup. I read Smokey's book too, and Four Bits says that's your problem right there... Too little quench, by about 20-25 thousandths.

 

OK I have run the calculations and cam up with the intake valve closing at 71deg ABDC. That sound right? Seems high and with those numbers I cam up with a DCR of 8.1. And then we come back to why I am getting a reading of 11.9:1 when I run a compression test. Since the motor is not running, could this be because the lifter are colapsing and closing the valves early (thus increasing CR). If this isn't the case, can someone explain why a compression test is so inacurate?

Also a thought I just had today... The KB pistons I bought had about .25" deep reliefs cut in-line with the valve reliefs on the outer edge of the pistons, to keep the piston metal away from these hot spots and reduce chance of detonaiton. Could this indeed be increasing the quenched volume in these two areas so much that it is detonating in the area that KB designed for the opposite reason? Should these reliefs not exist in my application?

 

Dont think you can just pull the heads off and do this though TransAm-Z
I didnt look but Im assuming that gasket is an MLS (multi layered steel) gasket.
MLS gaskets require a basically mirror finish on the head/deck surface to seal correctly, if your block/heads were cut for composite gaskets they will/should have a very rough texture on them.