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Combustion chamber thermodynamics


RUNAMUCK

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HI, I am in the throws of building another engine for my superchaged datto. A bloke I know reckoned that if the piston crown height is too low, (@TDC) this can cause detonation. My old motor had a decompression shim to drop the CR, but for one reason or another detenation hurt my old motor. This time around, since I am starting from scratch, I really want to get it right from the outset. Has anyone else heard of low piston height leading to detonation?

Bart.

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Guest Jamezuu

Ive heard concave pistons help solve detonation problems when decompressing as they give the combustion chamber a more rounded structure, they also help to decompress aswell. But it will never solve it as the combustion chamber will still be a different shape to normal. Thinking about it repositioning the spark plug to suit the new chamber may help tosolve it, but thats almost impossible for the adverage joe. Could try differnt types of spark plugs like different heatranges, tips etc.

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what comp ratio are u running and how much boost? maybe the comp was a bit high/to much boost on your old engine. do u have an intercooler? is it a good one? i asked a mate whose mucked around with turbo then supercharged then turboand supercharged a14 he reckons they have a well shaped comp chamber from standard. he said you need around 7.5/1 comp if you run any more than 10lbs of boost and it helps to have a good cooler eg vr4/evo and a good boost retard ignition he has run up to 20lbs boost on his super/turbo mill and had no probs. though hes been through a few motors to find that out. ps his is also injected . somtimes running a small blower too fast produces alot of heat resulting in detonation he also said watch your mixture if u are running a carb on top of the blower too lean = bang

dave

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I am running a drawthrough set up, with a 32/36. It is difficult to intercool with a drawthrough, especially as I have limited room.

My CR was 7.9:1. I was running an A15 head, which has a smaller chamber.

It didn't melt a piston, I blew a head gasket, notched out the head face, and cracked the head.

I had been running 11psi of boost, driving the blower at a ratio of 1:1.

After talking to CHS, I think I may munch out a bigger dish in the piston, using the lathe.

Bart.

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are u still using thr decompression plate? these proved to b a bit unreliable on early holden 6 turbo kits . they basicly used 2 head gaskets with a steel plate between them they were only rated for about 6-8 lbs boost is yours a similar set up? do u have a rising rate fuel pressure regulator? u really need one on a forced induction system with a carb. it would b advisable to use a custom forged blower/turbo piston for any hi performance engine its just better insurance against unreliability denco engineering down there do these i think DAVE

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HI, I am in the throws of building another engine for my superchaged datto. A bloke I know reckoned that if the piston crown height is too low, (@TDC) this can cause detonation. My old motor had a decompression shim to drop the CR, but for one reason or another detenation hurt my old motor. This time around, since I am starting from scratch, I really want to get it right from the outset. Has anyone else heard of low piston height leading to detonation?

Bart.

The question I'd be answering if I was you is why did it detonate?

Personally I'd be surprised if it was the piston height.

What dizzy are you running?

Is it just the N/A dizzy?

Has it been recurved or are you running a boost retard on it?

If it is the N/A dizzy I'd bet the ignition advance is to much and it's lighting the fire too early and the engine is trying to compress a burning charge causing the cylinder pressures to sky rocket and blow yor head gaskit etc.

On boosted engines I always pull at least 5 - 8 degrees of timing as it comes on boost

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