[DMVR] Bore vs. Stroke

[John Innis]

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

On 6/14/05, Ric Johnson <johnsonr@dwx.com> wrote:
> 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.
> 

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