Subject: Re: [DMVR] Bore vs. Stroke

From: Ric Johnson <[email-masked]>
Date: Tue, 14 Jun 2005 02:54:52 -0500

At 09:39 PM 6/13/05, Karl wrote:
>All other things being equal, will there be an effect on Torque and/or HP
>if 2 engines of same displacement achieve this displacement by different
>combinations of Bore & Stroke?

This is by no means the definitive answer, but I'll take a stab at it based 
on my fuzzy memories...
In my experience generally the shorter stroke engine will develop more 
horsepower at a higher rpm at the expense of torque. An example of this is 
the early pre-S type Mini-Coopers which used two engines of nearly 
identical displacements; 997cc and 998cc. The original Mini-Coopers like my 
'62 had a 997cc BMC A-series engine that was basically just a stroked 850. 
After a couple of years that was replaced with a 998cc A-series which had a 
larger bore and shorter stroke, but essentially the same head and 
carburetion. The 998 would rev more freely and developed a bit more hp but 
didn't have quite the torque of the original 997. Another example, though 
not quite as clear due to different displacements, was the early Chevy 
small block Z-28 engine, which was basically a 327 with a 283 crank to get 
the displacement to 302 c.i., just under the Trans-Am 5 liter/305 c.i. 
limit. Again the engine would produce prodigious hp at high revs, but was 
pretty gutless in terms of low rpm torque.

Ric

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From: John Innis <[email-masked]>
Date: Tue, 14 Jun 2005 07:30:15 -0500

I think Ric has it pretty much dead on.  Again I am not an expert, but
this is what I do know.  Long stroke motors develop more torque, but
sacrifice top end HP.  Short stroke motors produce good top end HP but
sacrifice low end torque.  If you think about it this makes sense.  A
long stroke motor will have more torque because the crank throw is
longer, giving the connecting rod greater mechanical advantage over
the crankshaft.  But the piston has to move a lot Further, and because
of this, at higher RPM, it has to move Faster to achieve the same RPM.
 At some point, the inertia of the piston starts to become a factor,
and the piston actually starts robbing power from the motor when it
has to change direction at the top and bottom of every stroke.  And
you also have the increased friction along the cylinder walls due to a
longer stroke, etc.

John

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