From: emory!hpwarq.wal.hp.com!lupienj (John Lupien)
X-Source: The Hotrod Mailing list
Date: Aug 1992
Subject: Re: Weight of a chevy v6

>  I find that most of my calculations are based on conrod/stroke
>  ratios followed by piston speeds. Seem to like short strokes.

I can't help you on the chevy/ford/smallblock/bigblock/whatever
questions, although I know there's a wealth of answers out there
that will probably come back later, I just wanted to comment on
the above bit.

Yes, short strokes are a big win in many ways: lower piston velocity,
less momentum to overcome changing the piston velocity, smaller crankshaft
excursions (allows smaller case), and of course it all adds up to higher
RPM - the key to Big Power. Well, almost the key - things still have to
all work as a system, so you can't just run the revs up and get power,
you need good breath, good balance, and there are a couple of other
short-stroke-disadvantages to deal with. One is that the impulse from
combustion gets transmitted to the rod mediated by the upwards piston
momentum - with lower piston momentum, you get more of an impulse on the
rod, which will tend to hammer on the rod bearings (both of 'em).
Let's see, what was the other one? Oh, yeah, to get higher compression
with a shorter stroke, you also need to leave less volume at TDC, meaning
much tighter clearances for things like valves opening or spark plugs
sticking out. Also this can make it hard for the flame front to reach
all parts of the charge in a timely manner. Good swirl can improve this.


-- 
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John R. Lupien
lupienj@hpwarq.hp.com

From: emory!wal.hp.com!lupienj (John Lupien)
X-Source: The Hotrod Mailing list
Date: Feb 1993
Subject: Re: No Subject Line

> >I have always theorized that the high back pressure is helpful in
> >reducing conrod loading at high revs. Comments?
> >[Well, a 4" dia piston with 12 psi of pressure has about 150 lbs 
> >of pressure on it.  Does that help?  Dunno.  Would be an interesting
> >problem to compute.  JGD]
> Well, its a start :-)

Assuming a 4" stroke and 4000rpm, the piston experiences between 0 and 50g's
of acceleration. How much does a 4" piston weigh? 150lbf is 
"reasonably significant" if the piston is in the region of 3 lb mass...


-- 
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John R. Lupien
lupienj@wal.hp.com

From: emory!wal.hp.com!lupienj (John Lupien)
X-Source: The Hotrod Mailing list
Date: Feb 1993
Subject: oops...

I wrote that for a 4" throw at 4000rpm you get 0 to 50 g's, but that's
wrong, it's actually - to 25 g's. This means that for a piston that weighs
less than about 7 pounds, the 150lb of force from the exhaust pressure will
make quite a significant difference.


-- 
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John R. Lupien
lupienj@wal.hp.com