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<x-charset iso-8859-1>a semi hemi cut simply increases chamber volume. This lowers the CR, so you can have a narrower engine with less deck height. it was a good idea in it's time, but the effect on combustion quality wasn't known immediately. After 20 years now we DO know what it does to combustion quality, but some people are keeping their head in the sand. John Aircooled.Net Inc. ----- Original Message ----- From: "Craig Smith" <ijetski@iprimus.com.au> To: <type3@vwtype3.org> Sent: Friday, June 21, 2002 4:25 AM Subject: Re: [T3] Compression Ratio Recommendations > 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/ > ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/ </x-charset>