Guys,
Coincidentally, I ran across yet another article on rod ratio in the
September '96 issue of Super Ford magazine. This is the second of a
two-part article by Douglass Baker here in San Diego (makes him suspect
already!). I am trying to get a copy of the first installment. There's a
bit more science in this article than most; at least he shows a force
diagram and gives the equation for piston velocity. He points out the
extremely small differences in the piston motion due to typical rod ratio
differences and attributes measured changes to the effects of friction,
length of intake runners, piston pin-to-deck distance, etc. rather than rod
ratio per se. His closing statements are:
"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!
In short, it is not so much a particular magic rod ratio that's important,
but the rod length and ratio in relationship to an engine combination and
rpm range."
I consider the last sentence to be a pure smoke screen. Sounds good - says
nothing. In general, he is saying longer is better because (1) it reduces
side force on the cylinder wall which reduces piston/wall friction and (2)
it moves the piston pin closer to the top of the piston which, he claims)
reduces piston rocking and the need for longer piston skirts which also
reduces friction and weight. Maybe this is all true to some degree, maybe
not. There are at least a few specific statements in the article I think
are incorrect, a few are questionable, but at least all the hard numbers
seem correct.
Still more to come.
Bob
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