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Old September 28th 06, 08:03 PM posted to rec.autos.misc
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Default Stroke vs Bore for a Given Displacement?

On 9/28/2006 12:02 PM, wrote:
>
wrote:
>> 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)?

>
> The longer the stroke, the higher the torque because the offset on the
> crank will need to be greater to accommodate the longer travel - like
> having a longer wrench or adding a cheater bar to break loose a stuck
> nut. The greater the bore the higher the torque because there's more
> piston face for the gas to push on.
>
> Now, whether these two numbers have any bearing on how two given
> engines will compare is improbable. Engine design is so much involved
> than bore & stroke that this isn't even a preface to getting a desired
> output torque. What if I had a single piston with a 24" of stroke and
> 18" bore but connected that to the crank with a paperclip for a wrist
> pin? What kind of power could I expect from this engine?
>
> Matthew
>


What I have learned...

Oversquare (i.e. bore/stroke > 1) engines produce max torque at higher
rpm vs an undersquare engine that produces max torque at low rmp. For a
given displacement and only changing bore/stroke an oversquare engine
will the same max torque as an undersquare engine. But oversquare
engines can rev much higher because the pistons move slower for a given
rpm. This an advantage at high speeds because higher rpm means the
transaxle can be stepped down to produce massive torque at the wheel.
That's my understanding...

Dan
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