Peter Kent wrote....
>A rod/stroke ratio below 1.5 is not ideal and a rod/stroke ratio is very
>good, however, many successful engines have a rod/stroke ratio between
>1.5 and 1.7. I read an article once that ran a Chev 383 strocker on a
>dyno with 2 kits using different rod lengths. The engine produced around
>400bhp, and the difference was 15hp max (started on come in above 4000
>rpm). There was a correponding small increase in torque (but still in the
>2% to 3% range). Worth considerating but not of overwhelming importance.
>
Agreed. A low rod/stroke ratio increases the side loading (thrust) of the
piston on the cylinder wall during the power stroke, leading to accelerated
wear in this area. This may not be all that important in automotive engines
with their relatively infrequent use of high power levels.
In aircraft piston and in industrial(stationary) engines, where long life
between overhauls is critical, and the engines routinely operate at full
rated output, higher rod/stroke ratios are common practice.
I too saw an article (Hot Rod Magazine, about two yrs ago) in which they
built a 350 (5.7L) SB Chev from a "327" crankshaft with 3.25-in stroke in a
400 block overbored .030. This gave an actual 353 cu.in. Custom made long
rods were used - unfortunately I forget the length! and deck clearance was
minimized to enhance the squish effect in the chambers. Compression ratio
was rather high - 10.5:1 or something close to that.
They used 87-octane regular unleaded and reported no detonation or undue
pre-ignition tendencies.
The engine gave close to 400 BHP with relatively mild cam timing. I felt
the authors gave too much credit to the long rods, when it was obvious that
the cylinder heads had been quite thoroughly reworked, and other tolerances
optimized.
Chevrolet have reportedly just released new design SBC cylinder heads for
retrofit which supposedly give some 30 or 40 more BHP over the already good
LT-1 castings. This serves to re-emphasize where the serious gains in
engine performance are to be had.
Bob W
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