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
put a dome on the piston to increase the compression ratio. If you
were to take that solution to the extreme and had a 13:1 or 14:1
compression ratio in the engine pistons had a very tall dome. The
piston dome almost mimicked the shape of the head’s combustion
chamber with the piston at top dead center. One could call the
remaining volume ‘‘the skin of the orange.’’ When ignited the charge
burned very slowly, like the ripples in a pond, covering the
distance to the block cylinder wall. Thus, those engines, as a
result of the chamber design, required a tremendous amount of spark
advance, about 40-45 degrees. With that much spark advance
detonation was a serious possibility if not fed high octane fuel.
Hemis tended to be very sensitive to tuning. As often happened, one
would keep advancing the spark, get more power and all of a sudden
the engine would detonate, Because they were high output engines,
turning at high RPM, things would happen suddenly.
Hemi racing engines would typically knock the ring land off, get
blow by, torch the piston and fall apart. No one then understood
why. We now know that the Hemi design is at the worst end of the
spectrum for a combustion chamber. A nice compact chamber is best;
that’s why the four valve pent roof style chambers are so popular.
The flatter the chamber, the smaller the closed volume of the
chamber, the less dome you need in the piston. We can get
inherently high compression ratios with a flat top piston with a
very nice bum pattern right in the combustion chamber, with very
short distances, with very good mixture motion - a very efficient
Look at a Northstar or most of the 4 valve type engines - all with
flat top pistons, very compact combustion chambers, very narrow
valve angles and there is no need for a dome that impedes the burn
to raise the compression ratio to 10:1.’’–
The original message included these comments:
reality. Many XK engines need more octane than unleaded premium
for ideal timing at WOT, but running regular under light load to
get home will not blow up your engine.
Paul Wigton, steward to a '60 DKW 1000 SP, Tweety, '63 FHC!
Keenesburg, CO, United States
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