Much has been written
about rod/stroke ratios in the past and it seems to be discussed on a
regular basis on the hardcore50 message boards.
The consensus of some so-called experts is that when it comes to
connecting rods, “longer is better”. Well, longer IS better with many
things, connecting rods not necessarily being one of them. There are
many “ratios” pertaining to engine design, including compression ratios,
rocker arm ratios, bore to stroke ratios, horsepower to cubic inch
ratios, and on and on. Exactly where rod to stroke ratios fit in to the
equation depends on who you talk to and how smart they really are.
Much of the discussion over the last 20-years on the subject was
centered around a theory first written about by Smokey Yunick, a more
than reputable engine builder. Smokey wrote that a longer connecting rod
length “FOR A GIVEN STROKE, MIGHT be an advantage under CERTAIN SPECIFIC
applications.” Smokey never said, “a longer rod was always better”.
And yet, for years, some engine builders have decided (and shouted from
the mountain tops) that a “better rod stroke ratio” was the most
important aspect of a high horsepower engine, regardless of other, much
more important factors of the engine design.
Even Smokey, in his later life, admitted that most of this “rod/stroke
stuff” was either taken out of context or mostly malarkey.
But in the interest of malarkey, I have compiled a
chart showing various
American production V-8 engine specifications, and listing their bore,
stroke, connecting rod length (center to center) and, their rod/stroke
ratios.
Let us assume that we are looking to install a V-8 engine into our 1932
Ford 5-window coupe. We want both power and reliability, since we plan
on doing a little drag racing and also some cruising. Those of you who
think that the higher the rod/stroke ratio, the better, should ponder
this question. For our coupe, would you rather select a 239 inch Ford
flathead (1.86:1 rod/stroke ratio) or a 351 inch Cleveland (1.65:1
ratio)? Does anyone think that the “better” rod/stroke ratio of the
flathead is going to outweigh the advantage of the better design and 112
cubic inch displacement increase of the Cleveland?
On the Ford V-8 side, the “best” rod/stroke ratio listed on the chart
was on the very first overhead valve V-8 produced by Ford, in 1954. This
was the 239 cubic inch Y-Block engine that replaced the flathead V-8,
and had a 2.03:1 rod/stroke ratio. A “great” rod/stroke ratio, but a
pretty crappy engine in every other respect.
Anyway, a very wise man once said, regarding the “rod/stroke ratio
controversy”, that the optimal rod/stroke ratio for a V-8 engine is
“8-rods to one crank”.
Have fun perusing this chart. Some of the engines listed may bring back
some great memories for you older guys. And for some of you younger
guys, you don’t know what you missed.
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