Detonation under no load
Detonation under no load I have an l76 with tsp cnc ported ls3 heads, Tsp 229/244 .615/.629 cam and holley terminator x max. Heads were not milled, but block was decked slightly. Using sunoco 93. It's a fresh rebuild and has about 5-10 mins of run time. I heard a noise (rattle) and pulled off the heads. It has signs of detonation on cylinder 8. The engine only has idle/free rev time, no driving, so no load on the engine. The noise started when the holley went into learn mode. What would cause detonation with bo load?
I sent out the injectors and had then cleaned. They came back good with no issues. Waiting to get the engine back together to check for vacuum issues. I'm running ngk tr5gp plugs, I'm thinking a colder plug might be the next. Could the plugs be too hot for this mild setup?
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There isn’t enough density to cause detonation under these conditions. Can you describe too us what you are seeing? If it’s audible, there is a rod bearing going away and the piston is starting to bounce off the head. Could also be something going away and wearing in the valvetrain very quickly. Detonation will be seen as pitting and usually at the outer edges and usually at the exhaust valve reliefs.
#8 is a corner and corners are where lack of piston to head clearance will show its ugly head first. If you see carbon missing on both the major and minor thrust, the piston can hit the head enough to collapse a piston’s skirt. Then the problem quickly exaggerates itself as the piston skirt collapses more and hits the head harder with every revolution. We’ve seen piston skirts collapsed equally across all 8 cylinders as much as .030” after piston to head contact. Crazy stuff. Not saying it’s your issue, but food for thought. .036 is our bogey for tight piston to head clearance accounting for head gasket thickness.
#8 is a corner and corners are where lack of piston to head clearance will show its ugly head first. If you see carbon missing on both the major and minor thrust, the piston can hit the head enough to collapse a piston’s skirt. Then the problem quickly exaggerates itself as the piston skirt collapses more and hits the head harder with every revolution. We’ve seen piston skirts collapsed equally across all 8 cylinders as much as .030” after piston to head contact. Crazy stuff. Not saying it’s your issue, but food for thought. .036 is our bogey for tight piston to head clearance accounting for head gasket thickness.
Last edited by Summitracing; Feb 22, 2020 at 12:47 AM.
There isn’t enough density to cause detonation under these conditions. Can you describe too us what you are seeing? If it’s audible, there is a rod bearing going away and the piston is starting to bounce off the head. Could also be something going away and wearing in the valvetrain very quickly. Detonation will be seen as pitting and usually at the outer edges and usually at the exhaust valve reliefs.
#8 is a corner and corners are where lack of piston to head clearance will show its ugly head first. If you see carbon missing on both the major and minor thrust, the piston can hit the head enough to collapse a piston’s skirt. Then the problem quickly exaggerates itself as the piston skirt collapses more and hits the head harder with every revolution. We’ve seen piston skirts collapsed equally across all 8 cylinders as much as .030” after piston to head contact. Crazy stuff. Not saying it’s your issue, but food for thought. .036 is our bogey for tight piston to head clearance accounting for head gasket thickness.
#8 is a corner and corners are where lack of piston to head clearance will show its ugly head first. If you see carbon missing on both the major and minor thrust, the piston can hit the head enough to collapse a piston’s skirt. Then the problem quickly exaggerates itself as the piston skirt collapses more and hits the head harder with every revolution. We’ve seen piston skirts collapsed equally across all 8 cylinders as much as .030” after piston to head contact. Crazy stuff. Not saying it’s your issue, but food for thought. .036 is our bogey for tight piston to head clearance accounting for head gasket thickness.
Thank you for the response, believe me, I was confused how detonation would happen under no load. I had the engine torn down by my engine builder and all the bearings look great, there is no scoring or marks on the head or piston. The engine builder said he sees detonation marks on the top ring lands of 2 pistons, unsure which cylinder he found the second issue. Maybe we are missing something, but there is nothing obvious in the engine.
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Joined: Jan 2008
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From: Ohio, Georgia, Nevada, Texas
Definitely an odd one. The outsides is the top piston land can take on a light brown fuzzy appearance over time with prolonged detonation that is usually the result of lower octane. Pitting in the top is more severe and at the outer perimeter and usually more in the exhaust side. Keep us posted!
