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Re: [T3] Compression Ratio Recommendations

<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
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