torque vs. HP

[porschenut]

I've noticed that the 928's spec torque numbers (in lb-ft) are higher than the HP numbers for every year of production. Is this common for a large-displacement V8?

In contrast, the 911's numbers are reversed, i.e. the HP number is bigger than the torque number.

I need an education on exactly what HP and torque are. What do they feel like when driving? How would a higher torque/lower HP car compare to a lower torque/higher HP car in terms of their acceleration characteristics and throttle response?

I have a zillion questions. Better just tell me everything you know about torque and HP

[emcon5]

Read Chuck's tech article.

http://www.elephantracing.com/techtopic/hpvstorque.htm

Tom

[divezic]

Simply put:

HP=Torque*RPM/5250

or

Horse Power is the output of an engine's torque at a specific RPM.

Take a truck for instance where you have 200hp. The engine puts out 233 ft-lbs of torque at 4500 rpm. Go above this and the torque will drop off.

Now take the Porsche rated at 200hp. It will only put out about 140hp at 4500rpm (163 ft-lbs of torque). Go to 6000 rpm and the torque will continue to build: 175 ft-lbs gives 200 hp.

Note that both engines give 200 hp but the truck is designed to give low-rpm power (for hauling and pulling) and the Porsche is designed for high-rpm power (for racing).

[porschenut]

Dave, thanks for the explanation. Sounds pretty simple.

Tom, thanks for the link.

[Mojo]

Horsepower is what gets you going fast enough to hit the wall
Torque is how far you go through the wall.

[klaucke]

Some muscle car guys will tell you "torque is everything!" I don't buy this one bit. Simply put, HP is the rate at which you can make torque. You can make any torque you like with a transmission, thats all a tranny does, change gears for varying amounts of torque. Rather, I think what most people interpret as "torque" is really the area under the power curve, which they feel as "torquey" or that it has some pick up at any RPM. The area under the power curve is important in feeling how fast a car is, however just judging torque numbers is irrelavent because the torque is altered through the tranny anyway. You can make any torque you want (slightly less from frictional losses), however, you can never make more HP, through a tranny.

The fact that the 928's torque #'s are higher than the HP #'s just reflects when the engine makes it's max torque (higher torque than HP means it makes it's peak torque below 5252 RPM, and vice versa). This is mostly a function of camshaft. The engine could easily be tuned and cammed for torque #'s that are lesser than it's max HP, with more max HP. 911's are known to be "peaky" engines so your observation does not surprise me, ask anyone with an early S.

[livi]

Being a doctor I´m a technical moron. The torque concept has allways been an enigma to me. Just can´t grasp it.

My brother, on the contrary, is an engineer working on cargo ships and rebuilds Harley Davidson bikes in his sparetime.

He ones explained it to me like this. Imagine trying to stop a spinning bicycle wheel as opposed to a spinning car wheel with your bare hands. Stopping the car wheel will take a lot more strength since it represent a much higher torque value.

It makes a lot of sence to me allthough I´m still not sure I REALLY understand the concept.

What do you think ? Is my brothers simplified explanation valid ?

Markus
Carrera 85

[beepbeep]

Yes.

Torque is how hard you press on your bicycle pedals...power is how hard you press on pedals times how fast you spin it.

As we all now, just pressing on pedals without moving them won't make you sweaty. Pressing on them and turning them fast will make you soaked in no time.

And also: 5250 RPM thing only works when using imperial units and have torque/power plotted in same graph . I suggest you forget about it.

Power is measured in Joules per second (= Watts) and torque is measured in Newton per meter. Even britts are using those units nowadays. Canadians too. NASA too.

[jluetjen]

Really, the only reason that the 928's peak torque number is higher then a 911's is because the engine is bigger. Torque is directly related to how much air (and fuel mixture, but since it's 14 parts and air only 1 part fuel, we'll call it air as a simplification) your engine can consume in 1 revolution.

HP is a function of how much air your engine can consume in a given amount of time.

An example:
2 Liter Ford Pinto engine in a Sports 2000.
1 Liter motorcycle engine in a D-Sports Racer.

Both engines make about 150-180 HP depending on the level of tune. It's just that the motorcycle engine does it at 10000 RPM while the Pinto motor does it at 5000 RPM. The Pinto engine will make roughly 2X the torque of the motorcycle engine though at any given engine speed. It all winds up pretty similar at the rear wheels though since the motorcycle engines are geared 2x the Pinto motor. Lap times (given similar tire and aerodynamics packages) are pretty similar.

Another example: F1 engine versus an unrestricted NASCAR engine. Both put out about 800-900 HP. The 5.5 liter NASCAR engines do it at about 8000-9000 RPM, while the 3 litere F1 engine does it at 18,000 - 19,000 RPM. Obviously the NASCAR engine has roughly 2x the torque of the F1 engine, but their doing roughly the same amount of work.

So why use one versus the other? Which is better? That's the fun part. It all comes down to...

packaging
weight
drive-line loss
fuel consumption
reliability
longevity
etc.

[IROC]

One simple way to look at it is torque is the ability to move the load - horsepower is how fast you want to move it.

I am designing a system to deploy a 3200lb turret out of the bottom of a C-130 (don't ask) and I only have about 3 hp @ 11000 rpm to do it (very low torque, in other words). The result is that you can move 3200 lbs using 10 in-lbs of torque, but it's going to move very slowly. If I had more horsepower available, I could lift the load faster.

Another way to look at it is horsepower is sort of a measure of how much torque you have available at a given rpm. Raw hp and torque numbers don't mean much outside the context of rpm. 500 ft-lbs of torque sounds impressive, but if it's at 200 rpm, that ain't much horsepower.

Mike

[Por_sha911]

Originally posted by klaucke
Some muscle car guys will tell you "torque is everything!" I don't buy this one bit.

Well just hold on a second there partner. IF you are driving down mainstreet looking for a quarter mile drag race then torque IS everything. The difference is that our cars were designed to handle the higher speeds (and take the curves doing it). Any basic muscle car will blow the doors off a 911 at the stop light. Torque is what snaps your neck back and plasters you to the seat when the light turns green. Pure torque has its place but if that's what you are looking for then you need a Corvette.

[Bill Verburg]

What you want is torque at higher RPM, then you can take advatage of gears.

[beepbeep]

You can always trade torque for speed with gearing...


Power is everything, torque is meaningsless...you can power a big rig with (properly geared) F1 engine and it will move just as quick as one powered by huge diesel. You'll have to rev it and it will be impractical but it would work.

[Early_S_Man]

Beep ... did you snooze through a physics course or two???

Power is measured in Joules per second (= Watts) and torque is measured in Newton per meter. Even britts are using those units nowadays. Canadians too. NASA too.

Torque isn't measured in ANYTHING per unit OTHERTHING!!! It is a product of force x distance!

HP is simply a rate of making torque!

NASA hasn't converted to exclusive use of the metric system ... some offices use metric, others use English units! It has been that way since 1958, and always will be! It is the age old 'Engineers vs Physicists' battle over who is right ... or, actually, how students are trained in college classes. For some reason ... engineering schools were Anglicized and used the illogical thinking of Benjamin Franklin with regards to electron flow, and everything went downhill from there! It is actually an 'English vs continental thinking' battle, and nobody has won, yet!

[porschenut]

I think I got it. The 911's HP value is higher than the torque value because its peak torque occurs at high RPMs (above 5252). The 928 produces peak torque somewhere in the 4000 range, so its torque value is higher than HP. This also explains why the 928 is quick off the line and the 911 isn't.

And if I want to brag to my friends about an engine's power, I'll say something like "it'll smoke your Mustang at a traffic light, cuz it makes 300 lb-ft of torque at only 4,200 RPM" - right?