Power to weight ratio or High performance

They most definitely are related. Torque = NM (Newton Meters) = KG x M^2/sec^2 Watt = KG x M^2/sec^3 = NM/sec KW = 1000 Watts.

As is torque and power or NM and KW

This is likely the largest reason.

This is more a matter of gear selection than location of torque. Put the engine in the RPM range of the meat of the HP curve, and torque doesn't matter. On an engine with low torque and high HP, this means having a shorter first gear.

Completely true. Gear ratios combined with the HP curve do a much better job than the HP curve alone. The key to performance is to keep the car in the meat of the HP curve at all times.

As a person with both a "high torque" engine and a "low torque" engine in my garage, I can certainly appreciate the differences in characteristics of the vehicles. But in both cases, for maximum performance, I need to maximize HP at the transmission input, and use the gears to maximize the torque at the wheels.

What the heck is "rotational energy"? The inertial mass of the crankshaft is too small to act as a flywheel. Torque alone doesn't tell you squat about how much energy is being produced by the engine.

There are all sorts of engines. Whenever you talk about the "torque" of an engine at the crankshaft, the term is meaningless unless you know the revs. Once you have that you know the power output. If engine A produces twice the torque at 2000 RPM than engine B, that means it's producing twice the power at 2000 RPM than engine B. My biggest beef is those who think a single peak power or torque figure is adequate to describe the acceleration of a car. It's far more complicated than that.

And it's impossible to maintain torque past the power peak. Look at any power/torque curve, and the power always peaks at higher revs than the torque peak.

Knowing the power at the crankshaft (without knowing anything else except the weight) gives a decent indication of acceleration. Knowing the torque at the crankshaft indicates nothing unless you know what revs, what gear, and the weight of the vehicle.

Torque. The engine's power take-off is essentially the end of the crankshaft, which is spinning. Consequently, that is where you measure the output of the engine; torque.

It isn't enough. However, neither is HP. HP varies with RPM as well. To state what the peak HP is still not sufficient to tell you much of anything without knowing what RPM the peak HP is, and what gears it is married to.

Many high performance engines have very flat torque curves and will maintain something close to peak torque well past the peak; ie the torque drops off slowly after the peak.

It does not.

Same for HP.

Agreed.

No speed limiter on the Canadian STi. On a runway, we had mine turning a little less that 6900 in sixth gear. It was limited by drag, and the window seals were screetching from the wind coming off of the side mirrors. My buddy is a highway cop and his static radar recorded a speed of 269 KPH. We then calculated the speed based on the tire diameter, the final drive ratio and 6900 RPMs. It works out to 271.24 KPH or 163.23 MPH. So we got it up to a little less than 163 MPH.

No, no, no. Torque is a "static" variable. It has no time component. All it tells is the amount of work that can be done per unit of angular displacement. In English units (man how I hate them):

1 ft-lb is equivalent to the force of 1 lb of gravity acting on a 1 ft lever. Work (energy produced) from a constant force is defined as:

work = force * distance force is applied.

The distance with that 1 ft lever is the circumference of the circle at the edge of the lever (where the force is applied). So the amount of work done with 1 ft-lb for one revolution is:

work = 1 lb * (2*pi * 1 ft)

You can apply force or torque to something that isn't moving (like a rusty bolt). Until it starts moving, no work is being done even though torque is more than 0.

Sure. torque and power vary in a fixed relationship as long as the engine is moving. What I'm saying is that knowing what the power output at the crankshaft at any given instant (without knowing much else) gives a good first order approximation of how quickly energy is being turned into kinetic energy (E=0.5m*v^2). That's what power is - how much energy is being produced per unit time. Torque at the crankshaft at any given instant doesn't say squat.

I used to own a 1995 Acura Integra GS-R. The sucker redlined at 8000 RPM. There was a power/torque curve in the brochure. The torque curve was flat from about 3000 RPM (~120 ft-lbs) to the torque peak at about 6800 RPM (130 ft-lbs). It tailed off a little and dropped quickly by the time it hit the redline. The power peak was just short of the redline. This kind of high-revving engine has a relatively flat torque up to the peak, which then drops quickly to about 100 ft-lbs at redline.

This guy has some power/torque curves for his 1995 Honda Civic SiR:

No - if you know how much power is being produced at the crankshaft, you have a reasonable indication (there are frictional losses) of the rate at which energy is being converted into kinetic energy.

Power at the crankshaft becomes torque at the wheels which turns into force at the contact patch - more or less. It's not a linear correlation to the F=ma equation, but it better than knowing torque at the crankshaft alone.

You don't need a time component because you are measuring the potential work. HP is still calculated from torque for a car. Consequently, if torque is varying, so is HP and vice versa.

At any rate, in english it is lb-ft or force * distance. In SI it is NM or force * distance

True

Sure it does. It tells you what the potential work is. HP doesn't tell you any more since the peak HP could be at very high revs which still gives no indication of acceleration unless you know the revs, the gearing, and the weight; same as the torque output.

Then what were you talking about above?

Not if you don't know how fast the engine is turning.

First of all, I never said anything about knowing torque at the crankshaft alone. What I said was, that HP is simply a function of torque and RPM. Dynos do not measure HP, they measure torque at a specific RP and calculate HP. Then engine rotates. As far as the engine is concerned, there is only torque and RPM.

I didn't say anything about knowing peak power. What I said is that as long as you know the power at **any given instance** you know the rate at which energy is being added to the system **at that instant**. If all you know is the power and torque peaks as well as the weight of the car, the power figure gives a closer approximation, as long as the gears are properly chosen for the engine characteristics. Of course this isn't always the case, but it's a very rough approximation.

That it does drop. Not to zero, but definitely less than the fat part of the torque curve.

crankshaft,

Sorry - I worded that kind of strange. It should have said "indication of the rate at which kinetic energy is being added to the system". If you told me that an engine is producing 150 HP at any given instant, I can tell you that it's adding energy to the system at the rate of

111.9 kilijoules per second (minus frictional losses). I don't need to know the gear it's in, the engine speed, the car's speed, or the weight of the car. Of course there are all sorts of frictional losses, such as wind resistance, tire rolling resistance, transmission losses. Much of any engine's crankshaft output goes towards making heat, noise, and vibrations that don't make the car go faster (or that keeps it from slowing down).

crankshaft

Of course. However - torque at the crankshaft (i.e. the published "torque figure" in the specs) is one of those nebulous things that say little about how quickly a car can get up to speed. Power at the crankshaft tells a whole lot more. All I've said is that if **ALL** you know is the power and torque numbers at any given instance, the power gives a considerably better indication of engine performance.

Actually - if I'm given both torque and power at any given instant, I should be able to plug it into the power/torque equation and figure out what the rev rate is.

Did you not read my post completely? The force mentioned is torque for rotating engines/motors but a calc for determining HP (work, energy expended over time and distance) isn't limited to rotational forces.

Mickey

torque

Here's the thing that gets to me. The torque number at the crankshaft (i.e. the number in the spec sheet or on the graphed torque curve) has no direct correlation to any kind of output at the wheels. Power at the crankshaft (minus frictional losses) has a direct correlation with output at the wheels. Power is the rate at which energy is output by the engine. The higher this rate (aka "Power") of producing energy, the faster a car accelerates. It's that simple. Now getting to the point where the most power is available is a different matter.

When anyone says that engine A has 1.5 times the torque of engine B at X RPM, all that means is that engine A also has 1.5 time the power of engine B at X RPM. The so-called "high-torque at lower RPM" engines simply produce relatively more power at lower revs than engines with lower torque at those revs, alhtough this power is going to be less than the peak power output. Crankshaft power and torque are always linked. However - crankshaft power can be directly linked to force at the tire contact patch. Crankshaft torque is meaningless unless one knows the rev rate.

And if you know how much HP the engine is producing at any moment in time, then you also know its RPM at that same moment in time.

But not car performance.

Yup. And if you are given the torque and RPM, you can figure out the HP, and HP as well as REPM, you can figure out the torque; because they are all related.

Which means you know the RPM and, hence, torque... since HP is calculated from torque and RPM.

So, how does that tell you anything about the car's performance, or its potential performance?

Not necessarily. Suppose I have no knowledge of: the car in question, what speed it's currently travelling, or what rev rate it's running at. If you tell me the engine is outputting 150 HP at a given instant, it's still trasferring energy to the car at the rate of 111.9 kilijoules/sec, regardless of engine speed. It could be at 2000/3000/

4000/10,000/100,000/etc RPM, and the rate at which energy is added to increase speed and/or offset parasitic losses would be the same. That's what power is - the rate that energy is made available in one form or another.

I'll keep on saying that knowing torque at the crankshaft, devoid of knowing other factors (rev rate), will not tell how much energy is being transferred from the engine to the rest of the car. Knowing the power output at the crankshaft, devoid of knowing other factors, does indicate the rate of energy being transferred to the rest of the car.

So? It still gives zero indication as to the performance of the car