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Stroke vs Bore for a Given Displacement?



 
 
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  #1  
Old September 25th 06, 09:59 PM posted to rec.autos.misc
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Default Stroke vs Bore for a Given Displacement?

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
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  #3  
Old September 26th 06, 02:35 PM posted to rec.autos.misc
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Posts: 4
Default Stroke vs Bore for a Given Displacement?

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.
  #5  
Old September 28th 06, 08:03 PM posted to rec.autos.misc
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Posts: 4
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
  #6  
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.

--
Don Bruder -
- If your "From:" address isn't on my whitelist,
or the subject of the message doesn't contain the exact text "PopperAndShadow"
somewhere, any message sent to this address will go in the garbage without my
ever knowing it arrived. Sorry... <http://www.sonic.net/~dakidd> for more info
 




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