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Premium Member
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Discussion Starter · #1 ·
If two engines had the same displacement but one had a longer stroke , wouldn't the longer stroked engine have more torque at lower rpms ???
 

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Iron Butt, SS2000
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Yes, that would by my guess. It would also likely have a lower redline and therefore lower HP, think truck engine VS GSXR (sure, lots of other differences in that example but you get the idea). As always, your results may vary.
 

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natural-born world-shaker
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Yep, technically it would and at any rpm - but it depends on the shape of the crankshaft and other stuff.

Torque is a measurement of force, rotational force if you will - just like the force you apply with a torque wrench is used to rotate something, maybe a nut.

Pressure force (caused by burning fuel/air in the cylinder) moves the piston in a linear direction in the engine because of the shape of the cylinder.

The linear pressure force of the piston moving is transferred by wrist pins, connecting rods, etc., to the crankshaft as rotational torque force.

Looking down the crank axially, more rotational torque force can be applied as you move the connecting rod contact farther from the crank center - just like getting a longer wrench and moving your hand farther from the nut. But longer wrenches bend more.

You can have problems with longer stroke and bigger cranks because stuff bends (oscillates) back and forth as the forces shift. Higher rpms and ya risk more oscillation, beef the parts up and ya add weight and increase friction. Speed it up too much and stuff goes everywhere.

Horsepower is a measurement of the amount of work being done by the torque force. It's all about transferring energy from the burning fuel/air to the crank. Some of that energy is transferred to the bearing points as heat (bigger, heavier stuff requires more work and energy and generates more friction and heat), some of it's lost as gas escapes around the rings, rings generate friction, too, improperly timed valves lose energy ... anyhow, any energy not transferred is less working power (horsepower) available.

whew
 

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Banned
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TOMPAGLIA2 said:
If two engines had the same displacement but one had a longer stroke , wouldn't the longer stroked engine have more torque at lower rpms ???
Yes...The longer arm can be expressed as a ratio change...increased mechanical advantage, just as you you would get by lowering your gearing, but without the downside (not counting increased piston speed) :cheers:
 

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killer sperm
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Yeah, the stroker has a longer arm to push against, but the big bore engine pushes harder against it's shorter arm.
 

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Hellbound Train
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Milehog said:
Yeah, the stroker has a longer arm to push against, but the big bore engine pushes harder against it's shorter arm.
That's why you should always combine the two.
 

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Premium Member
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1,389 Posts
hot rod mag. did a long comprehensive article/test.. on this exact subject.
Basically their findings were that cubic inches are what matters. The stroke motor and the bore motor for all practical purposes... made the same power and torque.
 

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Premium Member
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Discussion Starter · #9 ·
Thanks

Appreciate everyone's input and info.
would there be any additonal advantages related to power stoke - the amount of push on each stroke relative to combustion should also be increased???
 

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natural-born world-shaker
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TOMPAGLIA2 said:
Appreciate everyone's input and info.
would there be any additonal advantages related to power stoke - the amount of push on each stroke relative to combustion should also be increased???
Depends .. and others here have better answers .. but anything you do to increase power affects other things - advantages vs disadvantages.

Advantages depend on how you use the extra power - more speed, moving more weight, or getting a woody just knowing it's there. Gear ratios help direct the extra power the way you want to use it.

Disadvantages - as the amount of push on the piston goes up, so does the amount of push on the cylinder, rings, head, valves, and everything that holds them together. Also going up will be the amount of stress, vibration and friction to wrist pins, connecting rods, crankshafts and their bearing points. Sometimes the disadvantages aren't obvious, "Uh oh .. I didn't think about that." Farther down the line affected are parts that transfer the power to - well, where the rubber meets the road. Prepare to spend more money on maintenance, fuel and lube products.
 

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The law of the long arm?

Lets start with the basics.
Same bore same stroke = square
bigger bore than stroke = oversquare
bigger stroke than bore = undersquare
The basic problem with a undersquare engine is piston speed.Caculated this way piston speed= stroke times rpm divided by 6.
As a result two engines of equal displacement one oversquare one undersquare the oversquare engine will make more torque and Hp due to the fact that that is has a slower piston thus more time to fill the cylinder with air.Remember oversquare or undersquare the piston will still arrive at tdc and bbc at the same time. Not to mention lower friction and usually more valve area.What we would think of as a mechanical advantage with a longer stroke is defeeted with friction and poorer air flow.The oversquare engine will also pull rpm without loosing as much tq. and power.
The undersquare engine will still make good torque. Notice as H.D. builds new engines they get closer to square?
IT is the torgue that you feel when you get on the gas. Horsepower is just a product of math.Torque is a measure of the twisting action against an object.
So, torque times rpm divided by 5252 = horsepower. Check your dyno sheets torque and H.P. allways meet at 5252!
1 final word.Years ago I built a oversquare H.D. for a person @89 inch.The bike ran like a scalded dog. But to quote owner IT RUNS LIKE A *** BIKE!
So folks there still ant nutin like a stroker!
`
 

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MaxHeadflow
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1,344 Posts
Stroking a motor adds displacment which adds torque. A big bore will do better as the losses to friction are less and the rod to stroke ratio is slightly better. If you model the pressure to tq curve for both styles you'll find the outputs identical if you use the same rod to stroke ratios. Bruce
 

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A motor with a longer stroke will make more power at low rpms, and a motor with a bigger bore will make more power at high rpms. The longer stroke has mor leverage on the crank, whereas the bigger piston (in the bigger bore) has more power against the crank. The bigger bore will also alow for the use of larger valves with less shrouding (not sure if that is really an issue with H-D motors but for many V8's it is). I saw a build of two identicle small block chevy's the only difference was one had a larger bore, 4.125 and a 3.48 bore and stroke, and the other had a 4 inch bore and a 3.77 inch stroke (i think). anyway they were both around 380 inches total and the smaller bore motor made better power untill 5000 rpm and after that the big bore motor took over.
 
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