Horsepower versus torque

[scooby_dooby]

So, horsepower isn't a good metric for fluid driving, 0-60 and 1/4 mile aren't for the same reason, and while torque is a good metric for that, it isn't a good one for comparing maximum values. So what is a good metric?

Horsepower to weight ratio is the best measure of perfomance if you have to choose a single number. For example if a car is 2000lb, with 250hp, unless it has a very abnormal powercurve it's gonna be a beast, you don't need to know much more.

Basically you're looking for a way to quantify low end torque, which is not really possible without analyzing the dyno chart, and weight of the car (gearing is integral too, but is always overlooked). Low end torque doesn't impact any of the specs such as 0-60, or 1/4 mile, all it does is make the car more driveable at low RPM's.

No number is going to tell you how a car "feels", but by comparing HP and TQ you might get some idea of the feel, but even then that can be very misleading, especially when comparing different manufacturers.

 

[scooby_dooby]

You will also launch from traffic lights faster than anybody else...
(They might keep up or be faster, right untill the moment they have to shift gears. And then you'll seem them in your rearview mirror)

Depends on the car. Automatic Honda's are dogs off the line, the guy dropping clutch at 4000rpm's is going to blow him away.

 

[Moloch]

Horsepower to weight ratio is the best measure of perfomance if you have to choose a single number.

Basically you're looking for a way to quantify low end torque, which is not really possible without analyzing the dyno chart. Low end torque doesn't impact any of the specs such as 0-60, or 1/4 mile, all it does is make the car more driveable at low RPM's.

No number is going to tell you how a car "feels", but by comparing HP and TQ you might get some idea of the feel, but even then that can be very misleading, especially when comparing different manufacturers.

So you don't think a car that didn't "come on" untill 5500 rpm would be slower in a 0-60 test compared to a car with similar weight and hp but "comes on" at 3000 rpm?
Low end torque definally effects low speed acceration when you're not in the hp curve of your engine, but once you get there and continue to shift gears it really isn't important.
Although a car with low end torque wouldn't need as many gears to keep it in the power band- check out the audi R10 TDI racecar- only a 5 speed because with the turbo diesel torque it doesn't need close gearing so it can get away with a 5 speed.

 

[Frank]

No number is going to tell you how a car "feels", but by comparing HP and TQ you might get some idea of the feel, but even then that can be very misleading, especially when comparing different manufacturers.

Yes, I know. The overall feeling is often quite different, even with the same specs. Steering, how you sit, the feel of the clutch, break and shift, etc. Some cars feel much smoother and faster even when driving slow, while some really fast cars take some serious guts to launch and use.

 

[scooby_dooby]

So you don't think a car that didn't "come on" untill 5500 rpm would be slower in a 0-60 test compared to a car with similar weight and hp but "comes on" at 3000 rpm?
Low end torque definally effects low speed acceration when you're not in the hp curve of your engine, but once you get there and continue to shift gears it really isn't important.
Although a car with low end torque wouldn't need as many gears to keep it in the power band- check out the audi R10 TDI racecar- only a 5 speed because with the turbo diesel torque it doesn't need close gearing so it can get away with a 5 speed.

Yes it certainly affects acceleration when "you're not in the hp curve of your engine" which is why it's hard to quantiy, and isn't really represented in any peak numbers, or the performance stats like 0-60 or 1/4.

Now, you could argue it does have some effect in how long it takes the car top reach it's powerband in 1st gear, and thus IS represented in 0-60 times etc, but that's usually pretty negligible. The impact would be greater on an automatic than a standard, someone driving standard should be able to launch fairly close to their powerband and stay in it.

 

[scooby_dooby]

I think it all comes down to what you like, do you like the scream of an engine at 8000rpms, or getting kicked back into your seat at 2000rpms? That will determine how much you value low end torque.

 

[Frank]

But shifting stalls you for about a second in elapsed time.

 

[KimB]

But shifting stalls you for about a second in elapsed time.

Only if you're slow

 

[Frank]

All cars that have monstrous 0-60 acceleration (~3-4 s) do all that in first gear.

 

[Frank]

Only if you're slow

No, it's not the time you take to push that stick in the other gear, but the time it takes the engine and car to recover from the stall you induce while doing so.

 

[KimB]

No, it's not the time you take to push that stick in the other gear, but the time it takes the engine and car to recover from the stall you induce while doing so.

Somewhat, but that can be mitigated by not letting all the way off the gas, or by pressing on the gas an instant before letting the clutch out. Hard as hell to get right, but it's certainly possible.

 

[Frank]

Somewhat, but that can be mitigated by not letting all the way off the gas, or by pressing on the gas an instant before letting the clutch out. Hard as hell to get right, but it's certainly possible.

True. That's what professional racers do. Although they have automated sequential shifts nowadays.

 

[Moloch]

All cars that have monstrous 0-60 acceleration (~3-4 s) do all that in first gear.

Only really the corvette z06 since it has a 7k redline and has enough torque to allow for a huge first gear.
A viper btw has to shift and still manages 3.9 seconds.
Besides, with clutchless manuals and DSG trannys there's almost no lag between shifts.. and for DSG there's none at all.

 

[Powderkeg]

See, now this is why the American car buyer is hopelessly idiotic. Absolutely no sense of perspective.

Actually I don't think it's the American car buuyer who is hopelessly idiotic and has absolutely no sense of perspective here.

The point is that low-end power and torque is useful to all vehicles for normal driving.

Normal driving in the US consists of 50-60% of your driving done on the highway at speeds exceeding 60 MPH (nearly 100 KPH) Personally, over 80% of my driving is at those speeds.

So, how usefull is low end power and torque to a person who normally drives at or above 100 KPH? (ie. Most of America)

 

[Powderkeg]

Most cars of a decent size are geared such that even going from 55 to 75, you probably won't ever even cross 3000 rpm.

You don't drive on the highway much, do you?

Most cars of decent size are geared so they will downshift and engine RPM will increase to 3000+ RPM any time you press the gas pedal down more than about a half inch on the highway.

With my old Honda it wouldn't drive right unless you kept the engine over 2500 RPM at all times, and I would almost never shift below 3500 RPM. My wife's '05 Altima also stays in pretty much the same RPM ranges at the same speeds.

The only car I've ever owned where it was actually rare to break 3000 RPM was an old 1980 Chevy Stepside pickup truck with an inline 6.

 

[Moloch]

my dad's 94 s-10 doesn't like to downshift either, pitty with the shitty V6 it has.

 

[KimB]

So, how usefull is low end power and torque to a person who normally drives at or above 100 KPH? (ie. Most of America)

Very useful. In particular, an engine with more low-end power will be more fuel efficient, because it means that the gearbox can be geared higher, such that the car will run at low RPM's at higher speeds. Running at lower RPM's doesn't waste as much energy.

 

[Moloch]

Very useful. In particular, an engine with more low-end power will be more fuel efficient, because it means that the gearbox can be geared higher, such that the car will run at low RPM's at higher speeds.

Chevy corvette
that's how they get 30MPH highway with over 350hp.
I think even the new z06 with 505 gets 25ish highway.

 

[Mintmaster]

It seems the misconceptions are already clarified now, but I'll just reiterate it again.

A car with twice the amount of torque and the same weigth will accelerate roughly twice as fast. And that's nowadays just about the only thing you notice or care about, power wise.

Consider this case:
Car A: 100 Nm peak torque, 100 peak HP
Car B: 200 Nm peak torque, 100 peak HP
Car C: 100 Nm peak torque, 200 peak HP

Car B will not accelerate twice as fast as car A. The only reason B is faster than A is because while revving to the powerband, car A will have an advantage. Once you get there, both will accelerate at the same speed. However, car C will accelerate at twice the peak acceleration of car B (except initially when starting from standstill), even though it has half the peak torque.

Torque is next to useless as a performance figure.

It's more about feel and responsiveness. It's a secondary figure that gives an idea of the dynamics of an engine when it isn't working near optimal RPM. The only reason torque matters is because the engine takes time to rev up. You can take any engine and reduce the gearing to get as much torque at the wheels as you want without affecting the horsepower. There's no "tradeoff" here.

All cars that have monstrous 0-60 acceleration (~3-4 s) do all that in first gear.

Cars with monstrous acceleration don't need any more power going from 0-60 because they're traction limited anyway. Cars without monsterous acceleration won't be doing 0-60 in one gear, and the only advantage of a high torque engine will be for a short time in first gear.

mjtdevries and scooby_dooby have the right idea.

 

[Mintmaster]

In response to the original post:
Yes, it is somewhat of a travesty that torque isn't advertised instead of horsepower. After all, with a well-designed gearing system you can operate at high speeds with a high low-end torque just fine. You just have to have the gearing set so that your engine runs at lower RPM's even at higher speeds.

Advertising the horsepower unfairly gives an advantage to engines that have just a little bit more torque at higher RPM's. Of course, this sort of engine does have the advantage that to give the same sort of feel of power to the driver, you don't need as many gears for it to provide a smooth ride.

Shame on you, Chalnoth! You're a physics major!

You got it backwards. A high horsepower, low torque engine can be made into a high horsepower, high torque engine with the right gearing. Torque is all about gearing. An electric motor from a toy can generate 10,000 Nm of torque with the right gearing. The output shaft will turn really slow, but that's why the relevant metric is torque x RPM, i.e. horsepower.

If you take a high torque engine and gear it so that it runs at the peak torque RPM instead of the peak power RPM, it will accelerate slower, not faster. You'll actually have less torque at the wheels.