Applying Torque vs Horsepower

[TexasRebel]

Out of context in this conversation. An electric motor can produce max torque at zero rpm = zero hp.

We're talking ICEs here. When are you at 1500 rpm in an ICE? For about half a second in first gear, if that, and certainly not when racing.

Not really out of context in a conversation about torque and horsepower. 1,500 RPM is arbitrary. You are at your torque peak about once per upshift. How beneficial would it be to not overshift your torque peak?

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[Norm Peterson]

Not really out of context in a conversation about torque and horsepower. 1,500 RPM is arbitrary. You are at your torque peak about once per upshift. How beneficial would it be to not overshift your torque peak?

I think you're confusing maximum acceleration in some given gear with maximum acceleration at any given speed.

For maximum overall vehicle acceleration, you want to maximize area under the torque curve at rpms up around the HP peak, which is subject to gear spacing and rpm limits. You intentionally "give up" running at max torque rpm because the advantage of lower gearing that requires higher rpm is worth more than the torque dropoff at the crankshaft costs.



FWIW, I was an engineer for most of a 42 year career starting in 1970, and I've been playing around with this particular mathematical simulation off and on since a couple of years before that (I still have the original article - by one of the then-editors of Hot Rod Magazine - that put me onto it all).

Arguing in generalities isn't good enough for what you're trying to prove or convince people of; you need to do some real up-through-the-gears acceleration calcs, from idle to redline, or else you're never going to stop spinning your wheels in this rut you've been digging.


Norm

 

[Norm Peterson]

FWIW, you can do an acceleration sim from a HP curve, but it's generally more cumbersome to do it that way.


Norm

 

[GT Pony]

FWIW, you can do an acceleration sim from a HP curve, but it's generally more cumbersome to do it that way.


Norm

Just needs a simple HP to T conversion, then you have the T vs RPM curve.

T = (HP x 5252)/RPM

 

[Norm Peterson]

Just needs a simple HP to T conversion, then you have the T vs RPM curve.

T = (HP x 5252)/RPM

Working with torque is "easy". Try working with HP directly, without converting it to torque. It is possible.


Norm

 

[GT Pony]

Working with torque is "easy". Try working with HP directly, without converting it to torque. It is possible.


Norm

I'd simply convert it to T with the formula, then I'd have T vs RPM. Why go through the pain of trying to use HP when you can have T with a simple conversion?

 

[TexasRebel]

I think you're confusing maximum acceleration in some given gear with maximum acceleration at any given speed.

For maximum overall vehicle acceleration, you want to maximize area under the torque curve at rpms up around the HP peak, which is subject to gear spacing and rpm limits. You intentionally "give up" running at max torque rpm because the advantage of lower gearing that requires higher rpm is worth more than the torque dropoff at the crankshaft costs.



FWIW, I was an engineer for most of a 42 year career starting in 1970, and I've been playing around with this particular mathematical simulation off and on since a couple of years before that (I still have the original article - by one of the then-editors of Hot Rod Magazine - that put me onto it all).

Arguing in generalities isn't good enough for what you're trying to prove or convince people of; you need to do some real up-through-the-gears acceleration calcs, from idle to redline, or else you're never going to stop spinning your wheels in this rut you've been digging.


Norm

What?

the torque drop off is in the higher RPM.

Gearing has nothing to do with engine torque.

For maximum vehicle acceleration you want to maximize engine acceleration. To maximize engine acceleration you want horsepower... also known as torque at speed. When the rotational speed is low, you want more torque to make the rotational speed higher faster. If torque was a flat line, you would have infinite horsepower.

The real numbers exist for ICE, electric, steam, any type of engine or motor you want to deal with. It's not simply an academic exercise. I've no intention of convincing anyone, just disseminating accurate information.

My experience started in high school with an engine (& more) machine shop and continued into competition tractor pulling. Getting a car moving is easy. When your goal is to move an increasing load as quickly as possible, low end torque gets you to high RPM horsepower... Cars are a fixed load with a huge power:weight ratio.

Respectfully, I'm in no rut.

 

[GT Pony]

If torque was a flat line, you would have infinite horsepower.

Linear HP ... but not infinite.

 

[TexasRebel]

Linear HP ... but not infinite.

yes linear, and also infinite.

The limit as RPM approaches infinity of TQ*RPM/5252 (where TQ is constant) is infinity.

 

[Grintch]

They aren't meaningless.

Low end torque gets you into high end power faster. The game is getting that torque to the ground. If you have more torque than your tires can transfer to the ground it's a waste, but as long as you can find traction there is no substitution for low end torque.

There is a substitute - more gear.

Compare two 500hp engines, one with a 10,000 rpm power peak and one with a 5,000 rpm power peak. The torque at peak power works out to 263 & 525 ft-lbs, so the low rev engine wins, right? Wrong, there is a reason race motors rev higher than stock, and it's not to lose.

With proper gearing (2x shorter) the high rpm engine produces exactly the same performance and torque to the wheels as the low rpm motor with more torque. But with real engines pushing the rev's up usually provides more power at the expense of peak torque, which with proper gearing will be faster.

But as others have mentioned, power under the curve is the real secret. A peaky engine will under perform an engine with similar peak hp but a fatter power curve (often with a lower peak torque rpm & higher peak torque value). But gearing still matters, as a 6-10 speed tranny will chop off a lot of the curve compared to a 2 speed powerglide, so a narrow power band can be somewhat compensated for as well.

 

[TexasRebel]

There is a substitute - more gear.

Compare two 500hp engines, one with a 10,000 rpm power peak and one with a 5,000 rpm power peak. The torque at peak power works out to 263 & 525 ft-lbs, so the low rev engine wins, right?

Wins what?
A drag race? yes.
24 Hours of Le Mans? no.

But gearing still matters, as a 6-10 speed tranny will chop off a lot of the curve compared to a 2 speed powerglide, so a narrow power band can be somewhat compensated for as well.

Gearing comes at a price... weight. Make an engine with a completely flat Horsepower curve and you have no reason for a gearbox. A completely flat horsepower curve has peak torque at zero RPM.

 

[Grintch]

Multiply the torque x2 (which is what gears do) and you get exactly the same output at exactly the same speed in gear with the 10,000 rpm engine verses the 5000 rpm engine. So how do you get performance differences in a drag strip or LeMans (and why are they different?).

PS apparently, Top Fuel motors are limited to around 7900 rpm. Why limit them if low rpm and high torque is faster? Why don't high torque diesels win anything until they get major rules advantages? When they won LeMans, they were allowed more displacement, more boost, AND bigger restrictors. My grandma can win a race with that many advantages.

 

[Grintch]

Multiply the torque x2 (which is what gears do) and you get exactly the same output at exactly the same speed in gear with the 10,000 rpm engine verses the 5000 rpm engine. So how do you get performance differences in a drag strip or LeMans (and why are they different?).

PS apparently, Top Fuel motors are limited to around 7900 rpm. Why limit them if low rpm and high torque is faster? Why don't high torque diesels win anything until they get major rules advantages? When they won LeMans, they were allowed more displacement, more boost, AND bigger restrictors. My grandma can win a race with that many advantages.

 

[bluebeastsrt]

I like these threads. It's basically a look at me. I took a physics or match class way back in highschool thread.

 

[Grintch]

No it's answering a question with facts, rather than rumours and feelings.

Don't ask math questions if you don't like math answers. And performance really comes down to math.

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