In reply to a message from Wiggles sent Wed 4 May 2011:
Paul,
Also interesting that the ‘‘modern’’ pent roof, four valve engine
(1912) pre dates the hemi, but it got a back seat until the sixties
because of alcohol fuels and supercharging. This quote does not
mention the offset spark plug, which makes the ‘‘orange peel’’
chamber even worse for detonation. You can find many quotes that
the hemi’s central spark plug prevents detonation, but most of them
have offset plugs to fit the largest possible valves. Twin
plugging a hemi is a solution that goes back to the 30’s. On the
Porsche 911 engine, a wide VIA hemi like the XK, it lowers the
octane requirement several points and drops the ideal spark timing
from 35 to 25 BTDC. Jaguar made a few twin plug heads for the D
Type, but claimed they were of little benefit. Probably because
the focus was LeMans and finishing, they kept the compression low
because of fuel quality and the real issue of fuel ‘‘sabotage’’.
Heynes states in his paper they kept the C Type to 8:1 because the
bad gas at Lemans caused piston crown burning at WOT. The
definitive ‘‘hemi vs pent roof’’ study was done by Honda in the 60’s
and makes a good read. It is the inspiration and reason for the
pent roof revival by Ferrari, Coventry Climax, Westlake and
Cosworth in the late 60’s. A lot of data, but it concludes that
the four valve pent roof’s real benefit is lighter valves with a
higher mean effective valve area allowing higher rpm and output.
They tested engines out to 24,000 rpm and, interestingly,
detonation is no longer an issue above 15,000 rpm…
Paul–
The original message included these comments:
In reply to a message from PS sent Wed 4 May 2011:
I thought this passage to be very interesting, and relevant.
''One of the characteristic chambers that people are familiar with
is the Chrysler Hemi. The engine had a chamber that was like a half
of a baseball. Hemispherical in nature and in nomenclature, too.
The two valves were on either side of the chamber with the spark
plug at the very top. The charge burned downward across the
chamber. That approach worked fairly well in passenger car engines
but racing versions of the Hemi had problems. Because the chamber
was so big and the bores were so large, the chamber volume also was
large; it was difficult to get the compression ratio high. Racers
–
PS
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php