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Hello- > > I'd recommend _not_ semi-hemi'ing your heads, no matter what. It's one of > > the worst ideas in the ACVW world... for one, it'll make you MORE likely to > > detonate, especially under boost. > > I've never had a problem with detonation on semi-hemi cut heads. If you > have detonation problems with your engine, it it most likely due other > factors such as using the wrong type of and improperly jetted carburators, > too high compression ratio, and many other mistakes in improperly matched > engine components. I've never had detonation problems... I've also never run semi-hemi heads. And, I never will. And, zero of our customers use them, too. It's a BAD idea. Anyone with a real background in ICE engineering will agree wholeheartedly. > If you are going to make engine modifications, you should do so with great > care, and stick to the recommendations from professionals. I think I am a professional... :-) > I don't > consider myself a professional, so the recommendations I make on engine > mods are second hand from Gene Berg (not the only professional out there, > but was certainly one of the loudest!), and first hand in that I have > sucessfully tried them myself. You're right: Gene Berg is the loudest. He is a marketing expert. But, he has no engineering or technical background, and made several BAD suggestions. IMHO, his combustion chamber suggestions (semi-hemi and ultra-low CR) may be only a couple of them, but they are the worst of them. It 100% eliminates quench, leading to a slow burn. This kills thermal efficiency, making engine temps higher, gas mileage lower, power output lower, and ring life lower. And, it makes detonation _more_ likely to happen. Rule-of-thumb: the amount of time for fuel to burn is twice the amount of the ideal amount of ignition advance. The simplest proof of the slower burn is the amount of advance needed: a good chamber with excellent quench usually only needs 26-30 degrees of timing at WOT. A Semi-hemi head needs 36-40 at WOT. Hang on just one minute: didn't Berg say that 6.6:1 is the max CR for a stock chamber, but semi-hemi raises the margin to 6.9:1, which the opposite of what I just said? Yes, he did. Like I said, he has no technical background. And recommending _all_ engines to stay below 7:1 is ludicrous. Take care, Shad Laws LN Engineering - Aircooled Precision Performance http://www.LNengineering.com ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/