I've been trying to find that really long thread that compared HP vs. torque - does anyone remember it? If I recall there were some really intelligent posts from one of our members that talked about how the 2 are inter-related, there was an RPM where they both met in the hp/torque band, it was something like 4545 or 5454 or something like that...

Anybody remember it? I have been searching but for the life of me can't find it...

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08 W251 R350
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91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
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79 W123 250
78 W123 280E
75 W114 280

This post has a formula like you mentioned.

Torque/HP

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The rpm is 5252.

The only thing that matters in an engine is power. Power has 2 components, torque and rpm. These combined make up the power number known as horse power.

For example, if you have an engine making 200 ft/lbs of torque at 2000 rpm, the power is 200x(2000/5252) = 76hp

If you have an engine making 100 ft/lbs of torque at 4,000 rpm, the power is 100x(4000/5252) = 76hp

Thus both engines in this scenario are at those moments in rpm are delivering exactly the same power and would deliever exactly the same acceleration results.

Think of riding a bicycle and torque being the tension that you are exerting on the chain. RPM (cadence) is how fast you are turning the pedals. If you are exerting 50 lbs of tension on the chain and suddenly change that to 100 lbs without changing your rpm, you have doubled the power you are delivering. Likewise, if you keep exerting 50 lbs of tension on the chain but double the rpm you turning the pedals, you have also doubled you power.

To see the power of an engine, you must look at the hp numbers, which account for both torque and rpm. If an engine delivers 1,000 ft lbs of torque at 100 rpm, it is making 19 hp. If an engine delivers 50 ft lbs of torque at 4000 rpm, it would be delivering 38 hp and be making twice as much power.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record

Thanks for the excellent explanation - that 5252 was the magic number and helped me find it. The thread mentioned above was great, but this is the one I recall being really impressed by:

Inline vs. V6

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

The important issue to understand is that "power" is energy per unit time. Mechanical energy at the crankshaft is converted into vehicle kinetic energy via drive thrust at the wheels, so the more energy you apply per unit of time (which is power) the faster the car will gain kinetic energy(which means greater acceleration), and 150 HP is the same whether it is from a gasoline engine that makes 150 lb-ft torque at 5252 RPM or a turbo diesel that makes 300 lb-ft torque at 2626 RPM.

Duke

Probably an unnecessary reply I make here given the quality responses above, but my two cents anyway: As said, if you graph torque & horsepower simultaneously you will see a certain rpm where the curves cross. Torque, I believe, can be called "engine twisting power". For example, a 1967 Mopar 440 wedge motor with 375 hp, had very good torque at lower rpms (in most, if not all engines, maximum torque is seen to rise, then drop, sooner than horsepower. This is why the 440 could, if 440 is stock hi-performance, in the same chassis/transmission/gear ratios, etc, would beat a 426 hemi in the quarter mile. The hemi developed it's torque later, but above 3000 rpm the hemi is gaining on that 440. Couldn't beat a 440 in the quarter mile, generally, but if the race were a half mile, the hemi has come into it's own and devastates the 440. This difference in the quarter mile vs the half is because the hemi has more horsepower. Many may differ, perhaps with good reason, but it is a fair intuitive statement to make that "torque determines acceleration, horsepower determines top speed".

i bet with appropriate gearing the hemi would beat the 440 in the quarter.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

No, as I explained previously, it's horsepower that accelerates the car. The shape of the power curve is important as is gearing. The area under the power curve in the rev range determined by gear spacing and shift points is the total energy input to the vehicle. The higher it is, the faster the car will achieve speed and distance.

An engine that produces peak torque and power high in the rev range needs shorter overall gearing and closer spaced ratios to have the same average power input through the gears compared to a high torque low revving engine of the same peak power such as the examples I listed previously.

Duke