Tim,
The chief Tiger owner "Strocker" [sic] or Stoker motor exponent in S.
Calif. is Ken Mattice. His 347 motor doesn't seem to have any more cooling
problems than average, although his cooling system isn't typical either.
There are intermediate stages of stroking between 302 and 347 that might be
a better compromise. For example, a 330-incher with 5.135-inch rods. The
problem I see with this and other discussions of engine performance options
though is the lack of a specific goal. Just what are we trying to achieve
and what are the priorities? BTW, the only replacement for displacement is
cubic bucks!! More displacement may be a cheaper way to get more power, but
higher rpms works too. If you build a 289/302 you can safely run to, say,
8,500 rpm AND let it breath adequately, then you can equal or beat the peak
power of a 347 stroker with it's inherently lower rev limit. On the other
hand, the 347 will be a lot more tractable for the street since it will
work better in the low rpm range. Using a roller cam helps both motors run
good over a broader rpm range too. I'm looking at an article describing
building a 302 with the goal of 400 HP. They achieved 388@6,500 with
188@3,500 to give you and idea of the overall curve. That was with the
Edelbrock Torker manifold. With the performer manifold it got a peak of
346@6000 and it was 190@3,500. Another point of interest, comparing 1&5/8
versus 1&3/4 headers, the 1&5/8 made 25 more horses at 4,500 rpm and the
1&3/4 only got 10 more horses at 6,000 rpm.
The Rod-Length-to-Stroke-Ratio subject was a long running thread on the
List about a year and a half ago. DrMayf, the Sunbeam land speed record
aspirant and retired aerospace engineer modeled the rod length problem and
came up with the these five conclusions:
1. Longer rods have lower piston/cylinder side loads, hence less cylinder
and piston wear.
2. Short rods have a higher axial loading, hence potentially more bearing wear.
3. Short rods produce higher torque early, less torque later (in the cycle).
4. Short rods produce a higher peak torque (in the cycle).
5. No matter what the rod length, the area under the torque curve is
EXACTLY the same.
I would add, as a footnote to conclusion #2, that the increased axial
loading is going to translate into rods breaking at lower rpm, which might
be a little counter-intuitive. On the other hand, as DrMayf points out, the
longer rod is heavier which creates more inertial forces. My bet would be
on the longer rod for high rpm though; somewhere between 1.7 and 1.9 would
be good. BTW, the SVO 302 uses the longer 5.155 rod versus 5.092 standard
rod, shades of the old 302 and BOSS 302 motors. (Hard to believe a silly
1/16" added length could be that big a deal. Sure would like to know just
what those engineers were thinking!)
Theory aside, there is a lot of anecdotal evidence by individual engine
builders that longer rods make more horsepower. (Although, on the other
hand, there's a lot of contrary evidence too.) I suppose, as you suggest,
breathing differences could be a factor. However, it seems like better
filling of the cylinders would be better served by more time at BDC since
that's when the sucking is happening. But the situation is way too
complicated and dynamic for simplistic reasoning. Take for example, the
effect of bigger header tubes. What about longer dwell at TDC? It's
possible, but the actual differences in time at both TDC and BTC, comparing
say 1.5 versus 1.7 rod/stroke ratios are very, very slight. I'd put all the
concerns about power differences aside and stick with stress advantages of
the longer rods as the main point.
Hope you're having a great day,
Bob
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