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<x-charset iso-8859-1>Shad, That's good information, thanks. The modern toyotas use a shallow hemi shape, but the plug is straight and centred in the chamber, and the valves are inclined away from each other. I failed to see how this compares with what people are calling a Semi-Hemi on a VW head. The valves lean the wrong way, the plug is in the wrong spot. or did I misinterpret the implication the SH was being compared to the hemi. Can anyone see how the SH works? I would have though with the valve angle in the VW head, a squish type chamber would be best. Craig, confused. ----- Original Message ----- From: "Shad Laws" <shad@lnengineering.com> To: <type3@vwtype3.org> Sent: Friday, June 21, 2002 7:04 AM Subject: Re: [T3] Compression Ratio Recommendations > Hello- > > > 1) Berg cannot be totally off the wall as he has built alot of very > > powerful and durable engines and Im very happy with the HP output, > gas > > mileage and running temp of my mere 1679. > > Same question as John: Compared to what? > > > > 2) Why does the best Muscle engine ever made (the Chrysler Hemi) use > a hemi > > head with no quench area? > > Well, I kept the previous discussion limited to the wedge chamber only. > > Power is made in a few ways. One of them is thermal efficiency. > Another is volumetric efficiency, i.e. "breathability." The hemi > heads can flow much more air than a wedge. Although thermal > efficiency does drop (lovely gas mileage on those muscle cars :-), the > gain in breathing offsets that for more power. > > > > 3) Why did Toyota use a full hemi head on thier older econoboxes? > they got > > great gas mileage and ran pretty darn well! > > I haven't seen the old Toyota chambers with my own eyes, but I do have > a guess as to what they were probably doing... > > Deep hemis are different than shallow hemis. And, technology changes! > > The old muscle-car deep hemi, the Porsche 547 engine (4-cam), and the > Porsche 911 all use a somewhat deep hemi chamber. If you take a later > normally-aspirated 911 Porsche chamber and piston and put them up to > each other, you'll see that they do kinda-sorta have some sort > of "quench," although the area doesn't look flat like it would be for > a wedge. If you look close enough, you'll also find that more of the > clearance volume between the two is near the exhaust valve instead of > the intake valve, going along the lines of the "half-dome" idea I > presented before. These things do help. The fact that there still is > a large, funky dome isn't too great, but these other improvements do > help to make it better. For cars that were designed with power in > mind, this is fine. For cars with even more power in mind, you could > afford to lose even more. Take for example the 911 Turbo pistons. > Because they need a lower CR, they don't get as much help from the > pistons, and gas mileage goes down. > > Now, let's look at 4-valve heads. The best arrangement for a 4-valve > head is a pentroof, right? One plug in the dead middle with four > canted valves around it (although the two intake valves are often > parallel to one another to simply rocker arm/cam design, as are the > exhausts, the pairs of the valves are canted with respect to the whole > thing... I dunno if I'm being clear here... lemme know if I'm not). > Well, take a look at the pentroof design for a sec. What does it look > like? It's a shallow hemi! And, it has quench pads! The pentroof > combines the excellent breathability of the hemi design with the > excellent thermal efficiency of lots of quench, without the need for a > large dome (shallow hemis have smaller chamber volumes) and while > keeping the spark plug in the dead center of the chamber. Five- and > six-valve heads are variations upon the pentroof design. > > Again, not seeing them with my own eyes, I'm guessing that the old > Toyota heads you refernce are a shallow hemi design. Shallow hemis > rock. > > The only problem with a shallow hemi is that if you only have two > valves, it doesn't breathe all that well. A deeper hemi is needed to > make it breathe better for more power. > > 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/ ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/ </x-charset>