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<x-charset iso-8859-1>Hello- > > Rescently several have gone to traditional chambers and can't believe how > > easy it is to tune them, drive them and the extra power they have. Theory > > and real world don't agree here. > > You and Shad seem to be agreeing here, but I was under the impression that > he was arguing from the theory side and now you're arguing the opposite. Well, theory DOES agree with practice. But, Berg doesn't agree with theory or practice. The above statement assumed Berg=theory, but dabble around in some _real_ technical books on combustion dynamics, ICE theory, etc. and you'll see that semi-hemi even theoretically is a bad idea. > Both of you are in a completely different situation from me. I have no interest > in the high performance, big buck end of our spectrum. I just want a > dependable, somewhat lively car that I can rely on for 100k+ miles. Gotcha. > Now I'd like you and Shad to tell me what you know, from your experience, > that will work for what I want out of my engines. Speaking about normal aspiration with gasoline... (other things *might* be an exception). I like the following characteristics from my engines: -good gas mileage -low head temperatures -low exhaust gas temperatures -easily tunable -no hesitation - crisp throttle response -good ring life (because unburnt fuel, specifically that used to overcome hesitation, doesn't wash off the oil from the cylinder walls!) -excellent power production -outstanding engine life For every single one of these reasons, semi-hemi is a BAD idea. And, utilizing the very simple idea of quench (something we figured out LONG ago in practice, and in the last few decades have proven in great detail in theory as well), we can accomplish all of these quite a bit better! How many world-class racing teams, regular production car OEMs, etc. EVER used a hemispherical chamber with two valves in the wedge positions on a normally-aspirated gasoline engine in the last 60 years? Zip. There's just gotta be a reason why... > At this point, I suspect that > both of you have opinions, but very little experience. I have never and will never build a semi-hemi engine: you are correct there. I did WAAAAY too much very hard-core studying of detailed modern combustion theory and knew way too many people who wasted their own money to make sluggish engines :-). I have, however, spent much time developing engines that use a stock- like chamber, higher compression, no racing gas (Berg said over 6.6:1 needs racing gas.... right...), and absolutely rock. And, surprise, my practice agrees with the theory I spent so long understanding. > The problem is that the parameter space we're talking about has too many > dimensions for any one of us to have examined all of it. Then hit the books. The amount of experience of everyone on the list combined doesn't match up to the amount of experience that the pioneers of the combustion dyanimics field have. See what they've written :-). > In the meantime, I believe this thread was started by someone who > asked about building a somewhat lively engine which would last 100k miles. > > This is something which I belive I have a lot of experience with. > > OTOH, I have an engine down the basement which I'm building right now. The > heads are done, but I realized at the last minute that I had never gotten > around to semi-hemi-ing them. Now I don't know what I should do.... > > What do you suggest I do to keep the compression ratio down so I don't > have to worry about the gas I buy? Well, measure the chambers. Also, what are the stroke and bore? What's the camshaft and valvetrain like? So long as the valves aren't going to be kissing the pistons (needs a big cam for that :-) and the resulting CR isn't too exorbatant, running a 0.035"-0.060" deck with no semi-hemi is quite preferable. And you can likely leave the stock distributor set at the stock 28 degrees of total centrifugal advance (anytime you "need" 36-38 degrees of advance total you should throw up a big, red flag). And you can leave the racing gas alone, too :-). Rule-of-thumb: the combustion process takes approximately twice as long as your advance, i.e. the timing is set to approximately center the time of the burn at TDC. In other words, the more advance required, the suckier the burn :-). This is also why vacuum advance is needed: with less air/fuel in the chamber, the pressures decrease. This slows the burn, much in the same way that a low CR slows the burn, and more advance is needed. Take care, Shad Laws LN Engineering http://www.LNengineering.com ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/ </x-charset>