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Registered
Join Date: Sep 2005
Location: Winnipeg, MB Canada
Posts: 221
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Deck Height & Detonation
I have read that the optimal deck height for a 3.6L is 1mm, and that as you exceed this it leads to detonation.
Why is this? Why is the 3.6L more prone to this than the earlier smaller motors? because of the larger bore size? Longer time for flame propagation? How is the deck height limit affected by varying CRs? Thanx Kirk
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1989 911 Carrera 4 (964) |
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03-23-2008, 08:58 AM
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Registered
Join Date: Jul 2001
Location: Portland Oregon
Posts: 7,007
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Kirk,
Thats true,....  The answer is somewhat complicated but the quick version is that excess deck height leaves more air-fuel mixture at the circumference of the piston crown that spontaneously ignites before the flame front arrives after ignition. This affect is exacerbated as the bore size increases and the squish bands are further away from the spark plugs. The things you can get away with on a 2.2 to 2.7 litre engine create major grief when the bore size exceeds 95mm and the bigger ones are even more sensitive. Juggling CR & deck height is a compromise when using OEM pistons due to the fixed dome sizes. In certain race engines, we will reduce deck height to lessen the propensity to detonate at very high CR's but one must be VERY careful as the pistons get very close to the heads at TDC when hot. One uses a custom piston (where you select the dome configuration for a specific CR) to enable the ability to maintain strict deck height clearances while keeping the desired CR.
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Steve Weiner Rennsport Systems Portland Oregon (503) 244-0990 porsche@rennsportsystems.com www.rennsportsystems.com |
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03-23-2008, 09:14 AM
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Registered
Join Date: Sep 2005
Location: Winnipeg, MB Canada
Posts: 221
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Thanx steve,
You are an encylopedia of knowledge on these motors! Kirk
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1989 911 Carrera 4 (964) |
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03-23-2008, 11:46 AM
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