Listers,
Here are some excerpts from a magazine article discussing rod length that I
think are noteworthy, either because they seem particularly pertinent, or
particularly incorrect. The author is a fellow San Diegan, Doug Baker, who
published a series of three articles in Super Ford magazine. You can
probably get reprints if you're interested by e-mailing your request to
powermech@aol.com.
In the Sept, '96 issue he presents vector diagrams and equations that
describe the rod and piston motion and goes on to comment on some of the
consequences. He points out that typical changes in rod length only cause a
few thousands of an inch difference in piston position versus crank angle.
Among his other comments: "Increasing rod length creates three
opportunities to reduce friction and increase power: a reduction in the
rod-to-bore angle, shorter distance from the centerline of the pin to the
top of the piston reducing piston cock in the bore, and the resulting
shorter piston has less skirt area to create drag." and "The shorter piston
also weighs significantly less, and even though in our example where the
400M rod is heavier than the 351W rod, the net reciprocating weight is
reduced from 1027 grams to 841 grams - - - the reduction in reciprocating
weight translates to a power increase, since less energy is required to
start and stop the pistons." Doug, you were doing pretty well till that
last "start and stop the pistons" faux pax. I've seen this same statement
made numerous other places and it seems to have become one of the standard
myths in this business. The fact is, the only energy lost in starting and
stopping the pistons is incidental friction. Consider a swing; it starts
and stops too, but we don't have to use any energy to make this happen.
Same thing with the pistons. They go up and down just like the swing
exhibiting more-or-less the same sinusoidal motion. But, in all fairness,
most of what precedes this gaff seems correct, at least to some degree. And
in the last paragraph he says: "This combination (3.5 stroke, 5.4 rod)is
possible to package in the 302 block instead of the 351W. Even lighter
reciprocating parts, and very short piston height make this a more valid
combination than the same thing in the 351W. So why not compare a 351-inch
302 to a long-rod 351W? Differences in block deck height change the
relationship and length of the intake runners, even using the same type of
intake manifold. These changes in intake dimensions will more profoundly
affect the power characteristics of the engine than the rod ratio effect,
again masking what we are looking for. As another example, it is a
published fact that Pro Stock drag racers deliberately reduce rod length
and block deck height to create more advantageous relationships of
carburetor venturi, manifold runner positioning and port alignment. The
"mountain motors" with rod ratios down in the 1.35 range run well anyway!"
Well, these all seem like important and valid points and I have to accept
on faith Doug's facts concerning short rod "mountain motors". But, in this
case aren't the shorter rods favoring the higher rpm range?
And the debate rages quietly on, and on, and on - - -
Bob
Bob Palmer
UCSD, AMES Dept.
rpalmer@ames.ucsd.edu
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