DIY_EFI Digest V5 #970

[David Hunt]

Two replies for this one.


> Oops, my example was flawed.
>
> The extreme example is a 1.5 inch rod with a 3 inch stroke.  The piston
would be at the bottom of the bore from 90 degrees to 270 degrees.
>

Actually, the previous example of 3" rod and 3" stroke is too short, but it
does illustrate your point that piston speed is faster at BDC than at TDC.

Theoretically, the wrist pin throw has to clear the crank which would give a
rod length of 1/2 stroke, + crank diameter/2.  So for a 3" stroke with a 2"
main bearing (actually you have to clear the main bearing cap, but in
reality you have to clear the counterweights which is more difficult) the
minimum stroke is on the order of 2.5"  On the small block Chevy short
strokes are difficult because of the necessity for the counterweights to
clear the bottom of the piston.  The Chevy tradeoffs started with the need
for a 90 degree engine to fit in the engine compartment (I'm guessing) of a
tri-five chevy so they had to get as short a deck height as they thought
reasonable at the time.  It was them darn overhead valves as an extension of
the piston centerline making the engine wider.

The truck engines have taller blocks because there is more room in the
engine compartment.

I believe in long rods, I just don't believe that the pistons that go on
those rods are designed with the same care as pistons that go on their 5.7"
rod counterparts.  Long rod pistons are designed for short term power and
stock rod pistons are designed for longetivity.

Can anyone give a formula for piston height as a function of crank degrees
with a particular rod length?

dh


>
> It's for piston dwell time at TDC and BDC--affects cylinder fill...and
does
> affect HP/torque curve. However--it's more reciprocating mass.
>

You are quite right!  When you balance an engine (I haven't done this since
1969 so its from memory) you take 100% of the big rod end weight and 50% of
the small rod end weight (a higher % for an overbalance...etc) to make up
the bob weight.  A reciprocating engine is in balance only at 45, 135, 225
and 315 degrees.  The Rod throw mass is too small for a perfect balance at
TDC and BDC and too large at 90 and 270.  So anything that affects small end
rod weight, such as lengthening the rod increases the inherent derivative
balance of the engine.  Granted this distinction in on the minor side, but
is yet another consideration.

dh


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