Regular engines: why do the HP and Torque numbers are almost always the same (or similar)?

Turbo or supercharge engines: the Torque # is always higher than the horsepower. Why?

Rotary engines: if I remember correctly, the HP is much greater than the torque. Why?

 

Horsepower is a measurement of work performed over time. It is expressed as 1HP = force needed to lift 550 pounds 1 foot in 1 second. Torque is a twisting force which is constant.

Generally speaking, torque increases along with increases in displacement. This is why larger displacement engines tend to produce more torque. There are other factors to be sure such as piston travel, cylinder alignment ('V' vs 'I'), and state of tune (this doesn't mean spark plugs). Blown engines (turbocharged and supercharged) simulate greater displacement because volumetric efficiency can exceed 100%. The fuel/air mix is literally forced into the cylinders under pressure which is considerably greater than ambient.

Incidently, most all the time it is not horsepower you feel when driving your car but rather torque, so it is best to chose cars with engines which produce good torque numbers. When you go WOT, that is when horsepower comes into play.

 

/thread

btw hello fellow northern virginian

 

Its because the way HP is calculated (i.e. from torque).

HP is a measure of power.
Torque is a measure of force.

HP = torque (lbft) x rpm / 63025

As you can see in the formula, HP is calculated by the torque value.

The peak HP and peak torque numbers may be close, but where they occur on the RPM range of a certain engine is likely to be far apart...ie since RPM is on the top in the formula, if an engine makes very little torque but can spin at a high RPM, it can still generate high HP numbers relative to the amount of torque it is making at that RPM (ie S2000 makes very little torque overall, but the gearing gets it high HP numbers). Key word here is peak.

Think of torque as you pushing on a wall in your office. The wall's not moving anywhere, hence ZERO power is being generated (since power requires a displacement/movement in its calculation...ie "RPM" in engines). If you're pushing a box across the floor, you're applying a force and displacing it, hence performing work (i.e. torque is the force that spins the engine, the faster it spins the more work is being performed).

But to answer your question more simply, the torque curve and HP curve of an engine crosses each other at 5252 rpm. That's why they are close numbers wise (since they are different units, they really can't be compared straight up)...but above 5252 or below 5252, the number/value is not the same or close.

 

Hello to you, sir. While technically, I am outside of Northern Virginia, in reality, you would never know it. The "borders" have become diminished.

 

Good post, but I think you me HP = torque x rpm / 5252 (otherwise the torque and HP curves would cross at 63025 RPM).

The net result is that you get high horsepower by making lots of torque at higher engine speed.

Engines that have higher torque than HP numbers will have a torque curve that drops off at high engine speed. I have a 305 V8 Camaro, 285 ft. lbs. or torque at 2800 RPM, but only makes 190-ish HP. If I could push that torque peak up past 5000 RPM it'd have far better peak HP.

Edit: I think http://www.vettenet.org/torquehp.html is the most sensible torque vs. horsepower page I've seen, I always try to link it in these discussions

 

I think my dyno comparison is a perfect way to illustrate what was posted above.



Notice the torque curve for the Audi (FI engine) drops off dramatically starting at about 4500 rpm. The TSX (NA engine) torque number is low, however the curve stays relatively flat. Because of this the HP number continues to climb, whereas the Audi HP number does not.

 

1) This is just how the numbers end up based on the units of measure. If we use "elephantpower" instead of horsepower, then we'd get different numbers.

2) Turbo/Blowers are usually setup to increase mid-range RPM performance and tail off more or less sharply at high RPM's, where normally aspirated engines usually get their peak HP. So blown engines have less peak HP than you'd expect from their mid-range performance.

3) Rotaries have low torque because, as SouthernBoy pointed out, there is only 1.3 litres of displacement in the classic 13B. But torque at the crankshaft is in reality a meaningless value. What matters is force on the ashphalt and that takes engine RPM range plus all the gearing into account. Put the wrong gears on a blown big block hemi Cuda and it'll feel as wimpy as a K Car.

 

[QUOTE=Fishy;11135863]1) This is just how the numbers end up based on the units of measure. If we use "elephantpower" instead of horsepower, then we'd get different numbers.

2) Turbo/Blowers are usually setup to increase mid-range RPM performance and tail off more or less sharply at high RPM's, where normally aspirated engines usually get their peak HP. So blown engines have less peak HP than you'd expect from their mid-range performance.

3) Rotaries have low torque because, as SouthernBoy pointed out, there is only 1.3 litres of displacement in the classic 13B. But torque at the crankshaft is in reality a meaningless value. What matters is force on the ashphalt and that takes engine RPM range plus all the gearing into account. Put the wrong gears on a blown big block hemi Cuda and it'll feel as wimpy as a K Car.[/QUOTE]


They'd have to be pretty darned high for that to happen, but your point is well made. Gearing, both transmission and final drive, play a huge roll in how an engine will accelerate vehicle. The general rule of thumb is to go with lower gears for engines capable of higher RPMs and do the reverse for those limited in their RPM range.

One of the best ways to describe the difference between horsepower and torque is this. Suppose you are on a road trip driving along a nice two-lane road. This road has some hills. You are using your cruise control and you notice that as you ascend a hill (not very steep), there is no appreciable drop in speed - your cruise maintains a constant speed regardless of the hill. This is a factor of torque. Now let's say as you're driving along, you fall behind a slower moving car so you look for a chance to pass him. As you move out into the opposite lane for the pass, a car pulls out from a driveway up ahead and starts heading towards you. So you nail the throttle and the engine responds in kind, accelerating your car enough to safely pull back in your lane. This is a factor of horsepower - work performed over time.

 

Others have already answered most of your questions. However, it seems like no one has really answered your second questions, "Turbo or supercharge engines: the Torque # is always higher than the horsepower. Why?"

From what I know, the torque # isn't always higher; for instance, 911 turbo, GTR, 335i. I think you are mostly referring to family cars or cars that are not as sporty, such as some VW and Audi products, like GTi, A4 2.0T, etc. These cars have more torque than hp, and not only that, the peak torque occurs very early, as low as 1500rpm. Why do these engines make so much torque at such low rpm? It's because most people don't rev the engine higher than 4000rpm for city driving. It also means you don't have to downshift to get going, the torque will get you moving very quickly even if you are in a high gear.

 

To go even further on what everyone else says, like iforyou said the VW 2.0T engines usually have higher peak torque than horsepower. This is because they use a small turbo which spools up much faster, at the slight expense of top end performance.

For cars that have similar hp/torque numbers its usually due to engine design. More bore = more horsepower, more stroke = more torque. An F1 engine is a great example of this, the pistons and rods travel relatively little in comparison to the bore, especially moreso when you compare it to a regular passenger car. The most recent F1 engines make around 700 horsepower and 200 lb/ft.