Re: [T3] Compression Ratio Recommendations

["Shad Laws" <shad@lnengineering.com>]

<x-charset iso-8859-1>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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