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Car Forum / Driving, Maintenance, Tuning / General Car Topics / September 2006Stroke 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 > Hi, > [quoted text clipped - 9 lines] > 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.... >> Hi, >> [quoted text clipped - 14 lines] > 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. > 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 >> 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 [quoted text clipped - 16 lines] > > 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 > >> Hi, > >> [quoted text clipped - 24 lines] > 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.  SignatureDon Bruder - dakidd@sonic.net - 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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