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Hi
In order to get this amount of power/boost without detonation you need a piston head design with a lot of squish. a 2 valve hemisperical head (911) has almost no squish and is therefore limited in this respect. The squish area cools the parts of the combustion chamber that could initiate detonation, The downside of squish is unburnt fuel and therefore emmisions and this is probably the reason for tighter clearances. If you look at the heads of these engines, they are usually 16 valve which is not for aspiration (They are turbo motors) but allow a design with a large squish area (959). As for 0.040", yes this seems to be a std for V8's and having seen one revved to 10K hit the head then its about right, for type 4 VW ive been to 0.060" is required, It really depends on the engine design, rev limit and how much flex you get, but remember the most force on a conrod is at the top of the exhaust stroke so it is 'stretched' significantly
Neven
Neven
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