Rod/stroke ratio, piston speeds, etc. (was: Hey my Chevy has Ford Rods !!!!!!)

[elcamino73]

I do not understand how dwell time can be different at BDC than TDC!
Enlighten me please.
And we all have too much time on our hands.

----- Original Message -----
From: "The Dupuis" <dupuis10 at telusplanet.net>
To: <diy_efi at diy-efi.org>
Sent: Wednesday, January 09, 2002 9:04 PM
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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