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Unit 5302 · 10-05-2001, 06:31 PM

I'm suprised I didn't notice this topic earlier.

Here's the deal. Torque is a product of cylinder pressure and displacement. Those are the two components that go into making torque. Bore is a larger component of displacement than stroke.

The equation for determining cylinder displacement is (pi)(r^2)(h). Take a V-8 with a 3.5"x3.5" cylinder. It would displace 269ci. Now make it's bore 4", so 4"x3.5". It now displaces 352ci. Now make the stroke 4" so 3.5"x4". It now displaces 308ci.

While the long rod guys would like to argue the longer the rod, the more force exerted on the crankshaft, and thus, the more torque produced. That theory is lacking regard for the actual cylinder pressure component, and the speed at which that force is being exerted. A long rod engine will be exerting maximum force where the actual amount leverage on the crankshaft is the greatest, however the time spent in that position is greater, all the while cylinder pressure is falling. The long rod engine will also have a great deal of time spent at low force positions where it's angle of force is less, furthermore, the energy taken to move that longer rod with more weight while not on a power stroke would be detremental.

For an example of a classic low torque engine I will use the 5.0HO. With a 4.000"x3.000" cylinder configuration and modest 9.0:1 compression, on paper with stroke theory guys, it should be a kitten. It's rated at 300lb/ft at a modest 3200rpm.

Now for an example of what should be a much torqier engine. The Chevy 350 used in the early 90's Z28. 4.000"x3.480". With it's much longer stroke, and significantly higher 9.75:1 compression ratio it should blow the doors off the 5.0HO (302) Ford's torque numbers. Even if the stroke was the same, the much higher 9.75:1 compression should create more torque than the Ford.

The result is 225hp@4200rpm and 300lb/ft@3200rpm for the Ford. 240hp@4400rpm and 345lb/ft@3200rpm for the Chevy. Numbers are very similar when the 302 is pitted against the 351 in similar configurations. The 302 defies the engine builder long stroke/small bore torque theory.

Horsepower is the amount of work done by the force (torque). The equation as noted above for figuring out hp is (torque x rpm) / 5250.

As far as a diesel vs a gasoline powered car the results can be very much skewed by altered gear ratios which allow a car to increase the actual torque at the wheels by torque multiplication factors, and the pure torque available at a given rpm. Giving that GTI the gearing necessary to run at 7000rpm where the Diesel was running 3000 would have resulted in a different ending I think.

Then again, all of this is very much based in theory as well.

10-05-2001, 06:31 PM	#16
Unit 5302 Registered Member Join Date: May 1999 Posts: 5,246	I'm suprised I didn't notice this topic earlier. Here's the deal. Torque is a product of cylinder pressure and displacement. Those are the two components that go into making torque. Bore is a larger component of displacement than stroke. The equation for determining cylinder displacement is (pi)(r^2)(h). Take a V-8 with a 3.5"x3.5" cylinder. It would displace 269ci. Now make it's bore 4", so 4"x3.5". It now displaces 352ci. Now make the stroke 4" so 3.5"x4". It now displaces 308ci. While the long rod guys would like to argue the longer the rod, the more force exerted on the crankshaft, and thus, the more torque produced. That theory is lacking regard for the actual cylinder pressure component, and the speed at which that force is being exerted. A long rod engine will be exerting maximum force where the actual amount leverage on the crankshaft is the greatest, however the time spent in that position is greater, all the while cylinder pressure is falling. The long rod engine will also have a great deal of time spent at low force positions where it's angle of force is less, furthermore, the energy taken to move that longer rod with more weight while not on a power stroke would be detremental. For an example of a classic low torque engine I will use the 5.0HO. With a 4.000"x3.000" cylinder configuration and modest 9.0:1 compression, on paper with stroke theory guys, it should be a kitten. It's rated at 300lb/ft at a modest 3200rpm. Now for an example of what should be a much torqier engine. The Chevy 350 used in the early 90's Z28. 4.000"x3.480". With it's much longer stroke, and significantly higher 9.75:1 compression ratio it should blow the doors off the 5.0HO (302) Ford's torque numbers. Even if the stroke was the same, the much higher 9.75:1 compression should create more torque than the Ford. The result is 225hp@4200rpm and 300lb/ft@3200rpm for the Ford. 240hp@4400rpm and 345lb/ft@3200rpm for the Chevy. Numbers are very similar when the 302 is pitted against the 351 in similar configurations. The 302 defies the engine builder long stroke/small bore torque theory. Horsepower is the amount of work done by the force (torque). The equation as noted above for figuring out hp is (torque x rpm) / 5250. As far as a diesel vs a gasoline powered car the results can be very much skewed by altered gear ratios which allow a car to increase the actual torque at the wheels by torque multiplication factors, and the pure torque available at a given rpm. Giving that GTI the gearing necessary to run at 7000rpm where the Diesel was running 3000 would have resulted in a different ending I think. Then again, all of this is very much based in theory as well.