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Old 06-07-2006, 08:00 PM   #1
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HP Vs. TQ

I am a little confused on this. Say someone has a car that has 200hp and 200tq then another car has 220hp and 180tq. Which would be the quicker car? What are the advantages to more hp than tq or more tq than hp?


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Old 06-07-2006, 08:22 PM   #2
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This is a more complicated topic than it seems. I could talk for a long, long time about it and end up not really answering the question.

Torque, measured in ft-lbs, is the twisting force transmitted from the crankshaft to the wheels. The amount of work this twist does at a certain engine speed is the Horsepower. Torque can be directly measured on a dyno, and HP is calculated from the torque, using the equation (HP) = (Torque * RPM) / 5252. This is the reason torque and HP lines always cross on dyno's at 5,252 RPM.

The net power, the sum-total of power an engine develops is NOT fully described by any peak HP number, but by the Area of the HP line of the dyno. This is much easier to describe graphically, and I'm not going to go into Paint and draw anything right now. Ordinarilly now I'd talk about how that crank power hits drivetrain loss coming into the transaxle and then is multiplied by the axle ratio (gear ratio * final drive) before it gets to the ground. Your tire height comes into that too, there is too much brain fog to remember the extact math, its been a while. Basically, our SVT's, stock, put down in excess of 1000ft-lbs to the pavement beneath the front contact patch at the peak of 1st gear. Thinking about Honda tranny's with 25% tighter gearing makes it easy to understand how they get like 1.9s 60' times while we struggle to make 2.2 with way more power.

ALSO, in general, an engine will feel more powerful the lower in the RPM band peak torque and HP occur, for the same reason an aggressivly cammed car feels gutless below 4k, just the other way around.

So, that's all clear now, right?
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Old 06-07-2006, 08:37 PM   #3
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This question might be beyond my comprehension level.
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Old 06-07-2006, 08:44 PM   #4
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Fine response, Robzon. This thread can be closed.
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Old 06-07-2006, 08:53 PM   #5
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isn't
+tq = +acceleration
+hp = +top speed
?
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Old 06-07-2006, 09:35 PM   #6
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Quote:
Originally posted by xp0s3d
isn't
+tq = +acceleration
+hp = +top speed
?
That's a FAIRLY good rule of thumb, but the reality is more complex. Example, VW TDI, very high torque at a low RPM, very low HP. The high peak torque makes the car FEEL quick at low speeds, but when you wring the car out on the strip, because it's no good past 4k RPM, the area under the curve is small and it still ends up being an 18 second car. On the highway, they can get past 100mph but not by much, which is the ~75whp coming thru, and in that your rule is a good one. A car with a theoretical, say, 400hp @ 9000rpm and 200ft-lbs @ 4000rpm would have a high top speed if geared properly, because of the high HP when the engine's in its powerband.

Counterpoint 1: I've never driven a TDI, tho I'd like to, so I cant test the statements.

Counterpoint 2: The VW TDI diesel happens to be amazingly tunable, a chip and an intercooler net you like 250ft-lbs of torque and make it a 14s car for stupidly cheap. So perhaps this is a somewhat flawed example.
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Old 06-07-2006, 09:43 PM   #7
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Torque and horse power and connected but different. HP takes time into effect, torque is a static measurement at an exact minute.
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Old 06-07-2006, 09:53 PM   #8
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Indeed, but even knowing that and understanding (somewhat) the numbers, its still real hard to wrap my brain around, cause you'd think it'd be simple. It just doesn't seem to be.

Thinking about it, torque is useful to tell some things about an engine, but I'm betting the engine's behavior can be pretty fully described with just the HP graph.

I'll think about this and post back if I have some revelation
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Old 06-07-2006, 10:47 PM   #9
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The 200/200 vs the 220/180 is to close and would realy depend on where in the RPM range it all happened

I build/tune for TQ and let the HP take care of it self this is why most of my Dynos have more TQ than HP and why my Foci ar some of the fastest around

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Old 06-08-2006, 02:04 AM   #10
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I often tell people. Torque is what pushes you back in your seat, or spins the tires under the car.. That is really over simplifing, but it gets the idea across.

Good explaination above by Robzor
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