Applying Torque vs Horsepower

[GT Pony]

In short, you need both torque and the rate at which the torque is applied to know anything about performance.

Also known as horsepower.

Or, you need torque and the associated gearing. I can take that 1/4 HP RC airplane engine and gear it to make 1000 ft-lbs, but it will still accelerate a car like a 1/4 HP engine.

+1 ... true in the basic sense, and that's why HP (along with proper gearing) - not torque - will ultimately determine the performance.

As mentioned along the way in this discussion, the torque (and therefore the HP curve) of certain engines are more suited for specific kinds of use. For instance, cars with lots of low end torque and long big flat torque curves give a near linear HP curve (the 5.0 Coyote is somewhat like that), making them easier to drive and go fast for instance on a very twisty road coarse where the engine might have to go through a large RPM range. Cars like F1 or Indy cars are designed more along the lines of living at redline all day long, and have pretty low torque and very high RPM (and gearing) in order to get the HP needed to perform like they do.

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[TexasRebel]

I like to ask questions along these lines to engineering candidates I interview. Here are some ares to ponder:

Let's say I have a shaft spinning at 10 RPM producing 10 lb-ft of torque. Now put a 10 tooth gear on that shaft and connect it to another shaft that has a 20 tooth gear. How fast does that second shaft spin? How much torque does it produce? How much power does the first shaft make? Assuming 100% efficiency, how much power does the second shaft make?

Power is equal. Size is not.

a 1/4" mild steel shaft can transmit 500 HP if it spins fast enough.

1/4 HP can twist a 6" chrome-vanadium steel shaft if the torque is high enough...:gossip:

 

[bigjohns97]

Also known as horsepower.

Actually if you quoted "both torque and the rate at which the torque is applied"
that would be horsepower.

Just quoting the rate part is actually just a reference to RPM

 

[GT Pony]

Actually if you quoted "both torque and the rate at which the torque is applied"
that would be horsepower.

Just quoting the rate part is actually just a reference to RPM

Agreed ... when I said "Also known as horsepower" I was referring to the whole quote ... as I've said many times HP = T x RPM/5252.

The "T x RPM" term is the same as saying "both torque and the rate at which the torque is applied".

"torque" = T
"rate at which the torque is applied" = RPM

 

[EricSMG]

Power is equal. Size is not.

a 1/4" mild steel shaft can transmit 500 HP if it spins fast enough.

1/4 HP can twist a 6" chrome-vanadium steel shaft if the torque is high enough...:gossip:

Exactly. And which one is going to move down the road faster?

Hint - the 1/4" mild steel shaft will, since, the 1/4 hp motor would need to be geared down so excessively that the final drive would barely be rotating.

Throw that same excessively short gearing on the 1/4" high speed shaft and viola! You've got torque to the ground at a high rate of speed = fast.

Very simple.

 

[EricSMG]

Arguably the fastest car in the 1/4 during the muscle car era was the Buick GSX. It made 510 ft-lbs of peak torque at 2800 rpm and 360 peak HP at 4600 rpm, redline was 5500 rpm. It had the highest torque and lowest HP of all the big dog muscle cars yet is generally recognized as the fastest and is still winning in Pure Stock drag racing.

I guess you haven't seen a full size diesel pickup making 900 or more ft-lbs of torque in the 2000 rpm range and about 500 HP at less than 4000 rpm turning low 10s in the 1/4.

Your generalization gets proven wrong all the time.

Sigh. No. You're missing the entire point.

If you took the same GSX and shifted it's torque up a 1000rpm it would make a shitload more horsepower, same torque, and be faster because you could either A) keep the same gearing and travel at a faster rate of speed per gear at peak hp, or B) lower the gearing even more and apply more force to the ground at the same road speeds as before per gear. Either way the car is faster.

Low rpm torque is slower than high rpm torque, all else equal, is the point.

 

[dgc333]

Sigh. No. You're missing the entire point.

If you took the same GSX and shifted it's torque up a 1000rpm it would make a shitload more horsepower, same torque, and be faster because you could either A) keep the same gearing and travel at a faster rate of speed per gear at peak hp, or B) lower the gearing even more and apply more force to the ground at the same road speeds as before per gear. Either way the car is faster.

Low rpm torque is slower than high rpm torque, all else equal, is the point.

But it's torque peak wasn't a 1000 rpm higher. The GSX made the highest peak torque at the lowest rpm with the lowest peak HP at a very low rpm compared to the other top motor muscle cars of the era. It was also very heavy compared to its contemporaries if not the heaviest yet it was as fast or faster in the 1/4.

Having been an engineer and hot rodder for the past 40 years I fully understand the relationship between torque, rpm and HP. What folks have been missing in this discussion is weight, gear ratios, and how wide the power band is (rpms between peak torque and peak HP). My only point was you can make a car very fast that has and engine with an abundance of low rpm power.

 

[EricSMG]

But it's torque peak wasn't a 1000 rpm higher. The GSX made the highest peak torque at the lowest rpm with the lowest peak HP at a very low rpm compared to the other top motor muscle cars of the era. It was also very heavy compared to its contemporaries if not the heaviest yet it was as fast or faster in the 1/4.

Having been an engineer and hot rodder for the past 40 years I fully understand the relationship between torque, rpm and HP. What folks have been missing in this discussion is weight, gear ratios, and how wide the power band is (rpms between peak torque and peak HP). My only point was you can make a car very fast that has and engine with an abundance of low rpm power.

Good, let's redirect this thread into an intelligent debate.

First, it should be clear that I understand the concepts of gearing - one can't understand the benefits of high rpm power/low torque without first understanding gearing.

Second, you're misunderstanding me in thinking that I'm suggesting that a car with lots of low rpm torque can't be fast. Not at all what I'm saying. What I'm saying is that lots of low rpm grunt without a reasonable amount of rpm (power) is not fast. Very different things. If you limited the shift point of the GSX to only 3000rpm, as another poster suggested, it would be a slow car.

A stock '87-'93 Fox body 5.0 makes 300lbft of torque @ 3200rpm and 220hp @ 4200rpm and weighs about 3100lbs.

An E92 M3 makes the same torque @ 3900rpm and 414hp @ 8300 rpm and weighs 3650lbs.

The M3 will absolutely obliterate the Fox body. As the speed increases it will get worse, faster. Above 100mph? Embarrassing. Above 150mph? What Mustang?

Low end torque without the power is slow.
Small amounts of low end torque with lots of power is fast.
Lots of low end torque with lots of power is fast.

Therefore, low end torque by itself is not fast, and it gets slower, quicker, the higher the road speeds (gearing).

Edit: why is the GSX considerably slower than the E92 M3 in the 1/4 mile (105mph vs 110mph, respectively), stock for stock, if it weighs the same and has shitloads more torque?

 

[mustang_guy]

Been watching this thread for awhile now. It never ceases to amaze me how many people comment on something they think they know what their talking about, but really dont :lol:

Its oddly fun watching most people try and explain this

 

[TexasRebel]

Exactly. And which one is going to move down the road faster?

Hint - the 1/4" mild steel shaft will, since, the 1/4 hp motor would need to be geared down so excessively that the final drive would barely be rotating.

The one with more horsepower, no matter what the RPM.

 

[Hack]

The important information when trying to determine how quickly a vehicle will accelerate is how much torque is available at the driven wheels.

Engine horsepower is the key number because it tells you how much torque you can produce at the wheels through gearing. Engine torque doesn't give you this information.

 

[airfuel]

The simple explanation.

 

[EricSMG]

The one with more horsepower, no matter what the RPM.

You keep proving my point and disproving yours.

LOW rpm torque is not fast, all else equal. If you take a stock Coyote but only increase torque at 1500-2500rpm and nowhere else it won't make one damn bit of a difference in a race.

Horsepower matters. Rpm matters.

Low rpm torque only matters/helps when lugging the motor in higher gears - and it does help in a BIG way here, no argument. In all other situations it's pretty much not used. So, I agree with you that it does have benefits, just not when max performance is needed or wanted.

 

[Dominant1]

so guys what will be the end of this discussion?????

 

[TexasRebel]

You keep proving my point and disproving yours.

LOW rpm torque is not fast, all else equal. If you take a stock Coyote but only increase torque at 1500-2500rpm and nowhere else it won't make one damn bit of a difference in a race.

Horsepower matters. Rpm matters.

Low rpm torque only matters/helps when lugging the motor in higher gears - and it does help in a BIG way here, no argument. In all other situations it's pretty much not used. So, I agree with you that it does have benefits, just not when max performance is needed or wanted.

Not really. A high horse engine won't do anything without low end torque.
Low end torque gets you moving. Remember, when you are stopped RPM = 0... HP=0.

When RPM = .0001 you want torque to be high enough to get moving. The higher the torque as RPM approaches 0 from the positive side, the faster you get moving. (Remember this is at the wheels, and I'll bet you hated limits in math class).

The alternative is to store energy in a massive flywheel so you don't stall before the engine generates enough torque to maintain rotation and accelerate.

An engine that produces 1,000 HP at 1 RPM can do the exact same amount of work as one that produces 1,000 HP at 10,000 RPM. Mechanical inefficiencies in the drivetrain are the determining factor. Plus when talking about a mobile application power to weight ratio matters. If an engine is only working to move itself and the power transmission machinery... it's useless.

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