Barry,
You may or may not be aware that about a year ago we had quite a lengthy
(no pun intended) discussion on this subject on the Tiger List. Although no
official winner was declared, contributions by Dr. Mayfield and others gave
strong theoretical support for the conclusion that rod length to stroke
ratio was not a significant factor in engine output, either torque or
horsepower. For one thing, if the torque is integrated over the full
revolution, the answer is independent of the length/stroke ratio. Also, the
experimental evidence is that similar results are obtained from various
motors that cover an extremely wide range of length/stroke ratios. One
point I believe I made early on in the discussion was that there was an
optimum length/stroke ratio in terms of minimizing the stress on the rod. I
believe, if memory serves me, this minimum occurs at length/stroke = 1.73 .
For a 3.5" stroke, this would make the optimum rod length very close to the
6" you now use. However, the basis for calculating this optimum length has
nothing to do with power or efficiency; only minimizing the rod stress
which, of course, does become very important at high rpms. I have never
heard anyone explain why the Boss 302 used the longer 289 basic HiPo rod,
but I suspect it was just easier than producing a special shorter heavy
duty 302 rod rather than any esoteric reasons having to do with output,
high rpm or otherwise. Alternatively, they may have been trying to reduce
the stress on the rod at high rpms by using a rod closer to the optimum
ratio. We may never know for sure.
My question for you, Barry, is what, if any, explanation can you give for
the better high rpm output which you seem to attribute solely to the longer
rod length? Were there any other differences between the two motors you are
comparing other than rod length which might account for the differences?
Being a skeptic, I would say that it is probably harder to build two
engines with identical output than to build two that are the same. Slight
differences in porting, port alignment, chamber volume, etc., etc., all add
up to give a different result. I think I can prove to you that the
difference in friction between the 5.4" and 6" rods is inconsequential
compared to the 400 or so horspower you are producing. Also, the longer rod
VERY slightly extends the time the piston spends near TDC, but again, this
seems too small and effect to be important, although several car magazine
articles on this subject have made a big deal out of extending the dwell
near TDC.
Now the plot thickens with this latest post:
>Ray (or Barry or anyone else with an opinion),
>
>
>Any comment on the trick where you reverse the pistons to relocate the
wrist pin
>and thus improve the rod geometry?
>
>
>Regards,
>
>
>Theo Smit
>tsmit@novatel.ca
>B382002705
>
I first heard about reversing the direction of the pistons in talking to
some MOPAR people. Whether this trick actually works, again I'm skeptical,
but it is believed by a number of the MOPAR crowd. For those that are a
little confused by what is being done here, you have to understand that
there are basically two types of pistons;symmetric tops and asymmetric
tops. Most, if not all, pop-up pistons have asymmetric tops to clear the
valves. This means that the ones on the left side are facing the opposite
way as the ones on the right. Consequently, unless you make special pistons
for the right and left banks, the wirst pins on these pistons have to be
centered on the piston. Flat top pistons, on the other hand, have symmetric
fly cuts and can be installed either way; EXCEPT that in this case, they
are made with the wrist pin off-center. The reason for this is just to
quite them down so they don't slap so hard against the side of the cylinder
as they reverse direction. Again, this has nothing at all to do with power,
just makes the pistons a little quieter (mostly when idling). Getting back
to Theo's question, for reversing the pistons to possibly have any effect,
you would necessarily have to be using flat top pistons with asymmetric
wrist pins. Then the question is, would this really make any difference in
torque/power output. Again, I think not based on the fact that the
integrated torque over the whole revolution is, I believe, still the same.
The problem with these kind of anecdotal stories is they are much easier to
get started than they are to refute. Emotionally, we all want to believe in
magic bullets, quick fixes, and engine builder's secrets. The truth is
usually much less exciting. However, I do have a reasonably open mind to
anyone's suggestions as to how we could account for improvements in output
from just changing the rod length/stroke ratio or similar modifications.
Barry??
Skeptical in San Diego,
Bob
At 08:23 PM 9/29/98 -0500, Ray McCrary wrote:
>>Return-Path: <bkschonb.ucs@smtp.usi.edu>
>>Date: Tue, 29 Sep 1998 14:20:32 -0400
>>From: "Schonberger, Barry" <bkschonb.ucs@smtp.usi.edu>
>>Sender: "Schonberger, Barry" <bkschonb.ucs@smtp.usi.edu>
>>Organization: University of Southern Indiana
>>To: spook01@mindspring.com (Ray McCrary)
>>Subject: Re: Rod length and oil additives
>>Importance: Normal
>>
>>Ray, the standard on our race engines for Team Tiger was 5.4" it is now
>>6". High RPM horsepower is significantly increased.
>>
>>
>>Barry Kent Schonberger
>>Dean of Students
>>University of Southern Indiana
>>8600 University Blvd.
>>Evansville, IN 47712
>>(812) 464-1862
>>(812) 465-7021 FAX
>>BKSCHONB.UCS@SMTP.USI.EDU
>>
>>
>
Robert L. Palmer
Dept. of AMES, Univ. of Calif., San Diego
rpalmer@ames.ucsd.edu
rpalmer@cts.com
|