>===== Original Message From "Foster, Stan" <stan.foster@hp.com> =====
>Thanks Derek, that is quite a formidable spreadsheet !.
>
>And that thickness is consistent with Roger Williams who says in both TR6
>books that the US head will need about .080 removed to bring the compression
>back up to 9.5:1
>
>According to Richard (zero decked, standard pistons) 3.5 - 3.416 = .084 for
>9.5:1 CR
>
>Less than I was expecting.
The best/most mechanically efficient method for increasing compression is to
begine with a zero deck height. That way more of the combustion chamber shape
can be saved. When you keep shrinking that chamber shape you completely change
the burn characteristics. On race engines the pistons will actually stand
proud of the blocks deck to maintain as much of the chamber as possible before
cutting the head down. My pistons stand proud of the block by .013". I was
originally looking for zero deck clearance but my machinist (who buildd
primarily circle track motors) went a little too far. Luckily, the compressed
thickness of the head gasket is still enough to not cause any problems.
Though Richard Good did tell me to be carefull of rod stretch with the stock
rods above 6,000RPM. He grenaded an engine once where the pistons had about
that same clearance as mine. He missed a shift and there was a short burst
close to 6,300RPM. It was enough for the stock rods to stretch and close the
distance bewteen piston head and valve. OUCH! That is quite a good argument
for a rev limiter.
The downside to a zero deck clearance is that there is a little more money
involved, but if you are totally rebuilding an engine and the block is being
machined anyway... the difference isn't a whole lot. However, if you are just
skimming the head to boost the compression and not tearing down the whole
block then it is less expensive to just do the head. In real world driving,
the difference between a 9.5:1 engine achieved through a zero deck clearance
and shaving the head only @ .080" vs a 9.5:1 engine with just shaving the head
@ .120" is probably only really measurable while analyzing the exhaust.
Shawn J. Loseke
Fort Collins, CO
http://www.triumphowners.com/79 (1972 TR6)
http://www.triumphowners.com/690 (1978 TR8)
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