Re: TS = more power?

[christian147@domain.elided]

Greg, I think I have the thermodynamics down pretty good.  What I
intended to imply is that high compression hemis require big piston
domes.  That implies large surface area, which implies large heat loss
and slow flame propagation.  I believe that in the Am engine the use of a
tighter valve angle makes a shallower combustions chamber so no dome was
required to reach the desired compression ratio.  With no dome, the
surface area is reduced, so is heat loss to that surface is reduced and
an increase in flame propagation.  The faster flame speed will partially
negate the benefits of the twin spark.  I don't believe that the narrow
angle heads qualify as "true" or "pure" hemis, the term simi-hemi seems
better. The combustion chamber on these engines is much more open then on
the 90 degree heads. 

The best engine minds that my money can buy all agree that the fastest
burning combustion chambers are well designed pent-roofs with concave
piston crowns.  It would seem that the more the combustion chamber
approaches a sphere, the lower the surface to volume ratio the less heat
loss, the faster the flame and the lower the octane demand.  Just the
opposite of a high comp hemi.

Not said, but also implied is that a good pent-roof burns so fast that it
would benefit less from 3 plugs than a high comp hemi will from 2. 
Cutting the hemis long burn time in half will have more effect then
cutting the pent-roofs time by 2/3s.  Those two statements put me into
the area of dangerous conjecture, but I have empirical data to support
it.  As long as I am in such a poorly supported place, I will do
something really crazy and make flame speed implication from total
ignition advance.  A 2L single spark ALFA with 9:1 comp makes power with
something like 41 degrees of advance by 5000 RPM.  If you jack the comp
to 11.5-12 and pick your fuel carefully that might come down to 35.  I
recently watched a dyno session at my mentors shop.  A jap bike
engine,slightly larger bore (88 vs. 84), so flame travel distance
shouldn't skew results to badly.  single spark, pent-roof, piston crown
unknown, 12:1 comp VP red fuel (burns slower then any pump gas I have
compared it to).  On pulls from 10,000 to 14,000 RPM timing never
exceeded 28 degrees.  Apples and oranges I will agree, but which do you
thing would benefit more from a second spark plug.

Chris Hall
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