In all of the research and reading I have ever done the idea has always been to
have the clearance between the head and the top of the piston at tdc as small
as possible with no actual contact. This normally results in the top of the
piston under the "quench" or squish area being clean, with no carbon deposits
after running. In an engine with a race prepared stock crank and modified stock
TR-4 rods this distance is .026". At this clearance using a rev limit of 6800
RPM there will be no contact, at .025" the pistons will touch the head. This
is also with Hepolite "Powermax 87mm" liners and pistons. Forged pistons
probably expand more with heat than these pistons, billet rods & a billet
crank probably stretch or flex less. These days I usually shoot for about
.028" clearance when setting up the engine.
The most powerful engine that we have built used HD LA Sleeve 87mm
liners that are thicker then the Hepolite liners, Forged pistons, std. size
valves, & a billet crank. It used Weber 45 DCOE carbs with 42mm chokes &
carefully tuned 4 into 1 headers with a tuned length exhaust pipe with a
megaphone on the end.
This engine made peak power at 7,000 - 7,200 RPM. It had a 13.4:1 CR with a
306 degree duration cam giving and actual .525" lift at the valve. The piston
to head clearance was set up to be .028" under the squish area. This engine
made 189 HP at the flywheel with a very broad torque curve that peaked at 5,000
rpm, if I remember correctly. It was built to run on 110 Octane ERC Racing
gasoline. The total advance at 6,000 RPM was 34 degrees.
The compression ratio is clearly related to the camshaft duration and
both are related to the Octane rating of the gasoline the engine is going to be
run on.
I would certainly not try to run this engine on 102 octane fuel. You would have
to retard the timing way back and the engine would just not run well. To run on
102 Octane fuel, I would think that the compression ratio would have to be
lowered to somewhere around 11:1 to avoid detonation & the engine would loose a
lot of torque which could be only regained by shortening the cam duration.
Maybe the results the Christian Max got have to do with the lower octane
fuel that he has to use. We do not have that issue here in the US.
Greg Solow
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