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Old October 1st 06, 12:58 AM posted to rec.autos.misc
Don Bruder
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Posts: 250
Default Stroke vs Bore for a Given Displacement?

In article >,
wrote:

> On 9/26/2006 7:42 AM, SilverStude wrote:
> >
wrote:
> >> Hi,
> >>
> >> I have a question regarding stroke vs bore for a given displacement.
> >> I think that having a larger bore and shorter stroke will create
> >> larger low end torque vs a smaller bore and longer stroke. The reason
> >> I think is this is that the larger bore will have a bigger
> >> explosion/hammer so torque is bigger. But this engine will not be as
> >> fast as the smaller bore since its reciprocating mass is much bigger
> >> and hence can't rev as high. Can't find much on the net about this or
> >> increasing the # of cylinders for a given displacement.
> >>
> >> TIA,
> >> Dan

> >
> > You always get more torque, when the stroke is longer. The short stroke
> > engines, like the early Ford 260, have to rev up a bit to achieve a
> > reasonable driving torque. Compare that with a vintage straight 8,
> > that can accelerate, smoothly, in 4th gear from 10-12 mph to top speed....

>
> I was thinking along the lines of fluid/hydraulics. If there is more
> surface area then force is increased which would increase torque. Don't
> know if this applies to internal combustion engines though. If it did,
> would this greater force outweigh a marginal increase in the stroke
> (i.e. crank offset)?
>
> I did do a Google search on 'over-square engine' and found some
> interesting reading. And there seems to be a lot of debate. Seems that
> with modern engines rpm and fuel flow are most important.


The throw of the crank (crank offset as you call it), not the area of
the piston, is what's critical for high torque. A long-throw crank will
give you more torque than the same-sized piston connected to a
short-throw crank. Period. Torque is how much "twist" gets put on the
crankshaft. Longer throw on the crank, more twisting force applied per
power stroke. It's possible to "compensate" by adding pistons, and
similar, but at the end of the day, your biggest factor is crank throw.

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