DIY_EFI Digest V6 #3

[David Hunt]

Clint,

I think Matt has it about right.  I also certainly appreciate all the time
that he put into the project.

I haven't had the time to go through the numbers yet, but the piston is
slower at top, faster at the bottom and extremely little change in the
vicinity of half way.  But I do agree that the longer rod will probably have
less 'scrub angle' for a poorly designed piston.  That is the essence of my
original question.  What does it do to the longer lived engine.  I know that
the 350 with stock rod length and corresponding piston pin offset is good
for hundreds of thousands of miles.  For example, when I had my 307 bored to
4" I installed the pistons from a '74 350.  No discernable wear to the
pistons.  No wear on the second rebuild either.  When I sleeved the GN motor
and reinstalled the pistons they lasted for nearly 100,000 miles on a
non-intercooled turbo V-6.  Neither engine was a smoker when rebuilt.

I've known two others that put their 350 pistons in backwards (notch toward
the rear) and couldn't notice the expected power increase, but didn't get
10,000 miles before there were blue skies all over town.

I was really hoping that someone would know the 'magic' of the numbers for
piston pin offset as it relates to stroke vs. rod length.

>
> Date: Wed, 9 Jan 2002 19:04:44 -0700
> From: "The Dupuis" <dupuis10 at telusplanet.net>
> Subject: Rod/stroke ratio, piston speeds, etc. (was: Hey my Chevy has Ford
Rods !!!!!!)
>
> Just for kicks, I've drawn two 350 chevs in AutoCAD, accurate to 6 decimal
> places.  One has 5.7" rods and the other has 6.2" rods.  I've broken
> crankshaft rotation into 5 degree chunks.  Also, I found the point that
the
> crank and the rod are at right angles to each other, indicating peak
piston
> speed.  Finally, I've plotted how many degrees it takes for both engines
to
> "dwell" within .010" of TDC and BDC.  I'll give it to you in inches first:
>
> At the bottom of the stroke, a piston on 5.7" rods travels 0.004870" and a
> piston on 6.2" rods moves 0.005043".  At the top of the stroke the piston
on
> a 5.7" rod travels 0.009139" and the piston on a 6.2" rod travels
0.008967"
> Maximum piston speed for the 5.7" rod is at 163.024501 degrees and with
the
> 6.2" rod it happens at 164.323482 degrees.  Maximum piston velocity for
the
> 5.7" rod is 0.158697" in the 5 degrees centered on max velocity, and the
> same value with the 6.2" rods gives 0.157648".  At 90 degrees, both
pistons
> are moving the same speed - 0.151795" in 5 degrees.
>
> Dwell time, as defined by the above, is 10.75982 degrees for the short rod
> motor at TDC and 10.862748 degrees for the long rod.  At BDC, the short
rod
> dwells for 14.737538 degrees and the long rod motor dwells for 14.48436
> degrees.
>
> So, let's take a 6000 rpm motor.  Each revolution takes .01 seconds, and
> 5/360 of that is .000139 seconds.  TDC to 5* ATDC average speed for 5.7"
rod
> = 65.8"/sec and 6.2" rod = 64.6"/sec.  Peak piston velocity +/- 2.5
degrees
> for 5.7" rod = 1142.6"/sec and 6.2" rod = 1135.0"/sec.  Piston velocity
for
> both engines at 90* = 1092.9"/sec.  Average piston velocity for 5* BBDC to
> BDC for 5.7" motor = 35.0"/sec and 6.2" rod = 36.3"/sec.
>
> I'm not trying to be anal, only to indicate that piston velocity is also
> related to crank angle and rod/stroke ratio.
>
> All this proves ONE thing for sure - I've got WAY too much time on my
hands!
> LOL!  Another thing is that in order to drop peak piston velocity from
> 1142.6"/sec to 1135.0"/sec costs LOTS of money, in the form of pistons,
> rods, machining, etc.  Does the extra .102928 degrees at TDC justify the
> long rod motor?  An extra 0.000000286 seconds at TDC???  You decide.  I'm
> too tired.
>
> Matt
>


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