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Quote, originally posted by jhord »</TD></TR><TR><TD CLASS="quote">
Brad- Agreed, but if you "soften" the domes of the hc pistons that really seems to help prevent detonation. Knock the edges off everything and make 'em smooth. My carb'd 750/800SS is nearly 13:1 (Pistals) and runs fine on 92 pump gas. The timing plate is retarded 2 degrees. No, I don't daily commute on it but it seems happy enough in town.
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How much did you end up softening the domes? Assume the Pistals were 13:1 before the softening, did you happen to measure the CR after wards? Was it a true rounded edge or a chamfer? I would imagine the lack of detonation has more to do with lowering the CR than changing the actual dome shape? Can you elaborate how dome shape may effect detonation outside of it changing the CR?
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Quote, originally posted by brad black »</TD></TR><TR><TD CLASS="quote">the problem with comp on a long manifold carb engine with the std ignition system is that you have no way of achieving the tapering advance curve you need. we have lots of problems with hi comp bikes pinging on the best premium fuel we have, especially the ram/dp kits that have something like 12.5:1. even 11:1 can be an issue.
injected bikes with the shorter intake and tapering advance curve have no problems. i run my st2 on our std unleaded and it has around 10.8:1.
you don't need much retard really, but getting the 4 to 6 degrees you need under 5,000 rpm without generating idle issues is impossible.</TD></TR></TABLE>
Brad interesting bit of info regarding the long/short intake runners effect on timing and the importance of short runners on hi-compression engines. Out of curiosity have you tried using flow matched long runners to negate the sub-5K pinging? You mentioned the short runners wont ping with a 4-6 degree retard, in your opinion would chopping an inch more off the shorts runners and having them flow balanced further decrease the need for retardation?