'Crank Throws' and Torque
Thread Starter
11 Second Club
Joined: Jan 2007
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From: Pusan, ROK
'Crank Throws' and Torque This is an idea that just swam through my head. It is one of those bore vs. stroke theories.
Everyone knows that you can increase torque on something based on how far away the force is being applied (a wrench with a cheater bar on a bolt for instance).
So with that in mind, do stroker engines or larger engines have more torque, outside of the added torque from displacement, just based on the pure fact that the throws and bearings are farther away from the crank centerline?
Or would the way that pistons/rods/crank connect and operate make it not work like this?
I would imagine that there would be some gains in that, but if your RPM is limited by valvetrain instead of bottom end it would make me believe that in at least this regard, more stroke would be better than more bore.
What do you guys think?
Everyone knows that you can increase torque on something based on how far away the force is being applied (a wrench with a cheater bar on a bolt for instance).
So with that in mind, do stroker engines or larger engines have more torque, outside of the added torque from displacement, just based on the pure fact that the throws and bearings are farther away from the crank centerline?
Or would the way that pistons/rods/crank connect and operate make it not work like this?
I would imagine that there would be some gains in that, but if your RPM is limited by valvetrain instead of bottom end it would make me believe that in at least this regard, more stroke would be better than more bore.
What do you guys think?
FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
They are one and the same. The actual torque increase is from the better leverage. The leverage chage at the crank also means more displacement.
Oldsstroker is also correct that more bore or more stroke means more tq. Displacement and efficiency basically give you your tq.
A bigger bore piston cause more force at the same stroke and a bigger stroke causes more force even with the same bore. a bigger bore and stroke cause even moreforce or tq.
Oldsstroker is also correct that more bore or more stroke means more tq. Displacement and efficiency basically give you your tq.
A bigger bore piston cause more force at the same stroke and a bigger stroke causes more force even with the same bore. a bigger bore and stroke cause even moreforce or tq.
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
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From: Upstate NY
maybe consider this, torque is force x distance
where distance is the length from the fulcrum point or in this case the crank throw distance which is half of the stroke distance.
And it's a linear measurement, so if you double the distance you double the torque.
A LS1 stock with bore of 3.9" and stroke of 3.62" has 350 lbft of torque.
A stroked LS1 using same bore but a 4" stroke generally put out what, 400-450 lb-ft of torque?
And that would be apples to apples, same octane fuel, nearly same compression ratio?
a stroke of 3.62" has a fulcrum distance of half which is 1.81". The stroke of 4" has a distance of 2", which is 10.5% greater. So based on that alone you'd expect to see only an increase of 10.5% of torque over 350 lbft, which is only 36.7 lb-ft. But a stroked LS1 doesn't put out only 387 lb-ft of torque, if that was it then there wouldn't be a market for stroker kits. The added torque is primarily coming from displacement, and/or increase in compression ratio.
I see dyno numbers generally over 400 lb-ft for rear wheel torque for a stroked ls1, and my stock ls1 put down 340 lb-ft rwtq. So say it's a 100 lb-ft of difference between a 4" stroker and a 3.62" oem stroke: 10.5% is only 10.5 lb-ft of torque which, mathematically, is all that can be coming from the increase in fulcrum (crank throw due to increased stroke) length.
where distance is the length from the fulcrum point or in this case the crank throw distance which is half of the stroke distance.
And it's a linear measurement, so if you double the distance you double the torque.
A LS1 stock with bore of 3.9" and stroke of 3.62" has 350 lbft of torque.
A stroked LS1 using same bore but a 4" stroke generally put out what, 400-450 lb-ft of torque?
And that would be apples to apples, same octane fuel, nearly same compression ratio?
a stroke of 3.62" has a fulcrum distance of half which is 1.81". The stroke of 4" has a distance of 2", which is 10.5% greater. So based on that alone you'd expect to see only an increase of 10.5% of torque over 350 lbft, which is only 36.7 lb-ft. But a stroked LS1 doesn't put out only 387 lb-ft of torque, if that was it then there wouldn't be a market for stroker kits. The added torque is primarily coming from displacement, and/or increase in compression ratio.
I see dyno numbers generally over 400 lb-ft for rear wheel torque for a stroked ls1, and my stock ls1 put down 340 lb-ft rwtq. So say it's a 100 lb-ft of difference between a 4" stroker and a 3.62" oem stroke: 10.5% is only 10.5 lb-ft of torque which, mathematically, is all that can be coming from the increase in fulcrum (crank throw due to increased stroke) length.
Last edited by 1 FMF; Oct 23, 2008 at 02:57 PM.
TECH Fanatic
Joined: May 2005
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From: The 'Burgh, PA
1 FMF is part way there. What he forgets is the effect the longer stroke has on volumetric efficiency and cylinder filling. Typically, the increased volume of the cylinder, combined with the increase in piston speed creates higher air flow velocity into the cylinder, which in turn raises volumetric efficiency. This assumes, of course, that the cylinder heads don't become a restriction. Since the VE goes up, more air is in the cylinder to be compressed, which means more fuel as well, and after ignition more force acts on the piston. All these things combined add up to increase torque beyond the mathmatical equation of TQ=FxD.
Also, as a side note, by definition, if you increase torque you also increase horsepower. HP is simply a time rate value of torque. Mathmatically it is HP=TQ x rpm at which the torque was measured all divided by 5252.
Also, as a side note, by definition, if you increase torque you also increase horsepower. HP is simply a time rate value of torque. Mathmatically it is HP=TQ x rpm at which the torque was measured all divided by 5252.
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the point i was going for with TQ=FxD was regarding what the original post was asking:
just based on the pure fact that the throws and bearings are farther away from the crank centerline
it's easy to do the math and see that the extra leverage on the crank is negligible, it's only 10% increase from a 3.62" to a 4" stroke. All the extra torque of a stroked engine is coming from what you and everybody else have been saying.
just based on the pure fact that the throws and bearings are farther away from the crank centerline
it's easy to do the math and see that the extra leverage on the crank is negligible, it's only 10% increase from a 3.62" to a 4" stroke. All the extra torque of a stroked engine is coming from what you and everybody else have been saying.
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6 & 8 Second Club
Joined: Aug 2006
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From: Illinois, RT 66 dragway area
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I'm no expert. I did test the torque issue a few years ago. 2 engines, same total HP.
The 4" stroke engine made less torque than the 4.375" stroke engine.
Also, we put those 2 engines in the same car & same converter. The 4.375 stroke engine stalled way more.
The car actually ran 9.31 with the 4" stroke & 9.32 with the 4.375.
So I think the HP must be fairly accurate??
.
I'm no expert. I did test the torque issue a few years ago. 2 engines, same total HP.
The 4" stroke engine made less torque than the 4.375" stroke engine.
Also, we put those 2 engines in the same car & same converter. The 4.375 stroke engine stalled way more.
The car actually ran 9.31 with the 4" stroke & 9.32 with the 4.375.
So I think the HP must be fairly accurate??
.
.
I'm no expert. I did test the torque issue a few years ago. 2 engines, same total HP.
The 4" stroke engine made less torque than the 4.375" stroke engine.
Also, we put those 2 engines in the same car & same converter. The 4.375 stroke engine stalled way more.
The car actually ran 9.31 with the 4" stroke & 9.32 with the 4.375.
So I think the HP must be fairly accurate??
.
I'm no expert. I did test the torque issue a few years ago. 2 engines, same total HP.
The 4" stroke engine made less torque than the 4.375" stroke engine.
Also, we put those 2 engines in the same car & same converter. The 4.375 stroke engine stalled way more.
The car actually ran 9.31 with the 4" stroke & 9.32 with the 4.375.
So I think the HP must be fairly accurate??
.
Did you pull the motor at the track and swap it to test it the same day, within a 2-3 hour period to ensure track prep and temp were about the same? Just saying...
Could we have the specs on each motor? Were they the same cubes, or was one a smaller motor revved higher, etc. I think that's all really important information to include when you did a test like this.
