Torque vs Horsepower
 

Was just discussing this with someone another day..

Let consider 2 identical cars with just different engines, both traveling at 60km/h.
Car A's Engine is running at 3000 RPM / producing 300 lb-ft(Torque) = 171HP
Car B's Engine is running at 8000 RPM / Producing 200 lb-ft (Torque) = 305HP

If they both gun it at the same time, is Car A gonna accelerate much faster than car B? (For simplicity purpose, just assume the acceleration only last a few seconds, and the torque of both remains unchanged.)

Car B seems like it'd be faster since it has so much more HP, but at 8000 wouldn't it need to change gears soon?
Is car A 171hp @ 3000RPM or is that the peak HP?
The writing is kind of confusing...

I would think Car A would accelerate faster due to higher torque?
Plus 3000 rpm is pretty low, it still has alot of rpm range to go through unless all of its power is low-mid end?

I know reality gets a lot more complicated than this, that's why i said for simplicity purpose, let's just assume it only last a few seconds, meaning no big changes in RPM, and torque remains constant. (Horsepower will be slightly higher at the end because of the higher RPM) No gear change, no redline.

This is more of a theoretical question only

i would say no, should run fairly even if not faster for B


why? lets put

s2k vs 350z for example

240hp 155 pounds of torque

vs 280hp 260 pounds of torque


Car A's Engine is running at 3000 RPM / producing 300 lb-ft(Torque) = 171HP
Car B's Engine is running at 8000 RPM / Producing 200 lb-ft (Torque) = 305HP

Car A produces max. torque at 3000rpm which gives it the advantage to have the first jump due to higher torque numbers as well as earlier rpm range. but keep in mind it DOES NOT take forever in one gear to go from 3000rpm to 8000rpm, takes like 2.5 seconds. Therefore Car B will catch up since medium to top end is all about hp.

hence why s2k and 350z runs fairly even

For your original question,

if the gear ratio are identical, plus

if car A has a redline of 200hp@ 6500rpm ( but with greater torque) [ It runs out of power sooner]

and if car B has a redline of 300hp@9500rpm ( but with smaller torque)

Car B would be quicker during a 60kph pull. ( assuming they stay in the same gear during the entire "race")

then you start to factor in weight, traction, etc.....

what i am really interested in knowing is - does torque alone drive acceleration? or horsepower? or a combination of both?

I was always under the impression torque alone is what responsible for how much a car can accelerate, but some of the response here is making me question otherwise.

And I am very surprised car B would be quicker in cococly's examples

lets say


A car

300hp, 150 pounds of torque in a 1200kg shell
redlines at 8500rpm



vs

B car

320 pounds of torque with 180 pounds of torque in a 1300kg shell
redlines at 6500rpm

Which one will be faster? i would say A

I know everyone gonna bash
OMGGG HP sells cars, torque wins races. IMO thats completely bs

you need torque at the begining when you are trying to move the load. Once you get moving the momentum needs hp to keep it up. If you are racing around the track, you only stop once! otherwise ur rpm is constantly up there.
besides, coming from a start you are not going to start at 1500rpm to launch. You will be launching somewhere around 6000rpm

why are we comparing torque with power? torque is a form of force and power is force over a period of time. 2 diff. units.....

The torque you guys have been talking about is the torque straight off the crank. Torque at the wheels can be multiplied using gearbox's mechanical advantage. so a bau tec s2k can make same torque as a mustang v8 by changing out the final gear. But does that mean the AP1 with 9000rpm is faster than before in straight line? Not necessarily..because the horsepower remains the same despite of that. The only advantage now is you can more easiliy keep or hit tec in any speed. But if you are a slow shifter, don't bother.....

conclusion, car with same weight, same gearing ratio, but car 1 with 350hp and 160lb.ft and car 2 with 200hp and 400lb.ft, car 1 will win as long as he keep TEC during the launch.


if you want to see which car is faster, both at the same weight and gear ratio, overlay a HP curve onto another one. HP curve is derived from torque curve anyways, so no point of looking at torque curve again.

This calls for a drag race at No. 3 Road during rush hour!

Depends on wut kind of race and the torque curve. But to make things simpler

hp = (torque x rpm)/5250

So you say that the acceleration is only for a few seconds. It takes time for the engine to rev up. So if the race is only for a few seconds, then the high torque will be faster because the high hp/low torque car will not be able to utilize all it's power.

For a drag race, it will depend on distance. In short distance race, the high torque car will be faster. As distance increases, the low torque/high hp car will catch up and eventually beat the high torque/low hp car.

This is why cars like RX8, where torque is small, has a hard time accelerating out of corners.

Too many numbers, too many variables..Going crosseyed..

8000RPM, V-TECKKKKKKKKK.

Yes we realized everything is down to TEC.

I don't agree with this, low-torque cars do not mean slow out of the corners. Torque numbers do not mean anything without the RPM numbers.

A boxer can exert a force of 300lb, but he can only punch 2 times per second;whereas Peter Chao can only punch at 150lb, but he can punch 5 times per second. Torque can be analogous to the force the boxer exert and horsepower is how much force he can apply over a period of time.

So who one exerts more energy to the punch bag after a period of time? :rolleyes:

As for cars, the more energy u can apply to the wheels at the instantaneous moment , the faster acceleration u get.

So....comparing cars,

RX8
159 lb-ft at 5500rpm and
GTI
207 lb-ft @ 1700 rpm

So does that mean GTI is faster WOT existing the corner? So the GTI is faster only going at 1700rpm than the RX8 going at 5500rpm? No! you need to translate the entire torque curve to the horsepower curve which is a function of RPM. So straight comparing torque numbers is useless if you want to know which car is faster, but you can judge the streetabilty of the cars by them.

Conclusion: As long as you keep VFAD existing corners, then it's pretty damn fast.

Search, the same question has been asked several times in the past.

I always think torque gets you off the line, and hp keeps you going...

as long as you get one of these

http://img.villagephotos.com/p/2005-9/1080088/2jayZ.jpg

You'll be FLYING!

back on topic...
hp will overall win afterwards for straight line, as long as the gears will keep the car within its powerband

Obviously these cars can't be running the same "gear ratio" between motor and wheels. Whether this gearing difference is because of gear selection, final drive, or wheel diameter, it really makes no difference.

Since the road speed is the same, the motor running at 8000 RPM has a huge mechanical gear advantage. Assuming both motors can produce the torque instantly given identically fast throttle opening (the motor at 8000 RPM would have the advantage), you are *effectively* comparing 300*3000 vs 200*8000. Guess which one is a bigger number. :)

If you want to think about this in more detail, let's assume a tire diameter of 25.2", and both moving at 60 mph. This means the tire is rotating at about 400 RPM. For the low torque car, this means you have an effective gearing of 20:1 (8000 engine RPM / 400 wheel RPM). For the high torque car you have an effective gearing of 7.5:1 (3000 engine RPM / 400 wheel ROM).

So your effective road torque from those motors is:

low torque car : 200 x 20 = 4000 lb*ft torque (from wheel to road)
high torque car : 300 x 7.5 = 2250 lb*ft torque (from wheel to road)

(Yeah I used 60mph not 60 km/h, but I am too lazy to do unit conversion, and this was easier and your results are the same.)

Edit: For the bored, Brian Beckman's "Physics of Racing" is worth a read. Fourth to last paragraph applies here: http://phors.locost7.info/phors03.htm

How much do these cars weigh anyway, and how good is their traction? Perhaps the ~4000 lbs of force through some silly FWD drivetrain will just produce some smoke. ;) We all have AWD cars, right?

me love torque.

Car A is in the wrong gear.

If Car B's redline is at 8000, then the driver of Car B has to shift first.

God damn engrish.

A will accelerate faster than B, but B will pick up and possibly pass it/run even depending on their top end; Also depends on weight, gearing and traction like mentioned above.

what's with all the OT repliess??

let's rethink another scenario: This time, only with ONE car, and assume this car has a COMPLETELY FLAT torque of 300 ft-lbs from 2000rpm up to 7000rpm.

Is the car gonna accelerate
a) much faster
b) bit faster
c) at the same level
when it is running at 6000rpm vs when it is running at 2000rpm? (same gear, no wind drag)

(acceleration measured in increase km/hr per seconds)