Torque vs Horsepower

Sports cars usually have high horse power and high speed and seem to require a lot of thrashing of the gearlever. Utility vehicles (trucks tractors etc) that need a lot of pulling power have engines with a lot of torque and can pull away from low revs. Just what exactly is so different between engines with lots of horse power and lots of torque anyway?

Not an expert here, but I will give my 2c. The first thing I can think of is displacement. “There is no replacement for displacement” as the saying goes. Most cars that you have to rev the snot out of to get power are say, less than 3.5L or so. Trucks on average start at 4L and go up from there. The larger the engine (as a general rule) the more torque it will produce, and at a lower RPM. Length of stroke will get into play as well, the longer the more torque but it won’t want to rev as much… a trade off.

Any other ones?


But, assuming the engine displacements are equal, the things that decide if an engine is going to be high-horsepower or high-torque are going to be:

  1. Crankshaft throw (and thus, stroke).
  2. Valve overlap and timing.
  3. Tuning of intake and exhaust runners.
  4. Use of and tuning of the wastegate on a turbo, or the setpoints of pressure on a supercharger.

In general, you want to have a larger stroke/bore ratio, combined with a higher low-engine-speed volumetric efficiency.

Of course, these things all affect horsepower too. In general, high-horsepower engines develop their power at high engine speeds. Thus, having a smaller stroke/bore ratio, different valve timing and lift, shorter length intake and exhaust runners, etc. will tend to shift the power band of the engine towards higer speeds.

My 0.02: The rpm an engine produces depend on the stroke of engine cylinder(s), the shorter it is, the faster it will turn the flyweel, in general. The rpm need to be lowered, though, in order for 4 whells to turn (or at least one). So, to accomplish this, a [torque] converter is used. In a sports car, the ratio is such, that a light weight can be pushed at a great speed. In a truck, a great weight can be pushed at a low speed.
Think of a battery operated motor. Normally, it can’t pull a car. But if its speed (rpm) is reduced by a converter, it will be able to pull a truck from a ditch. This effect is widely used.
Go to a Radio Shack and by a tiny 3v motor. Power it from a 3v battery and try to stop it with your fingers. It’s very easy. Build a converter (with Lego wheels), so it rotates 1 revolution per hour or per day. You wont be able to stop it with your fingers. As a matter of fact, your fingers will be crashed if you insert it between two last hogweels of your converter. Be carefull!

I think that the question is intended to be with respect to torque and horsepower from the engine itself, not the result of going through a gearbox. The rpm’s of an engine by themselves tell us nothing about the torque or the horsepower produced.

      • Sports cars with tiny motors is a relatively recent phenomenon. I don’t know how old you are or what country you’re from, but 30 years ago in the US, medium and small “sporty” cars with 400 cu. inch engines (and larger) and 4-speed automatic transmissions weren’t all that uncommon. And lemme tell ya, those cars just ruled…(sigh)
        -A Ferrari may get more respect, but a Pinto with a big block is way, way more fun to drive.
  • Trucks started out with more gears: decades back (like, six or eight, , , ), cars and trucks often used the same transmissions, but trucks would usually have 2 or 3-speed axles, that effectively ‘multiplied’ the gears. And for that purpose they work just fine; many large trucks still have them today. Sports car drivers wanted something that was easy to shift during brisk accelleration though, and didn’t want to mess with the double-shift-lever arrangement of transmission and axle, so they built transmissions that had more internal gears. Sports car gears started multiplying before sports car engines started shrinking.
  • Tiny sports car engines came from Europe, mostly. And more recently Japan. European countries placed heavier financial penalties on cars with high fuel consumption, which is why it’s darn difficult to find any big-engined cars made there. That’s the only reason in Europe, and it is (currently) the only reason in the US. Which is sad. If you think a 5-‘litre’ Ferrari is fast, imagine what a 7 or 8-‘litre’ would do. - MC

OK, here are some things I typed up today that I gathered from various resources I have about torque and horsepower. And I know there are plenty of people here who don’t agree with me on this, but I’ll post it anyways.

First, note that there is no such thing as a “torque” engine. Any engine that produces 300 ft-lbf of torque at 3000 rpm has 171 hp at that speed. Regardless of it’s stroke, bore, displacement, supercharger, etc. Also, any engine that makes 300 ft-lbf of torque at 5000 rpm will produce 286 hp. This is due to the simple relationship between power and torque:

power = (torque * rpm) / 5250

where power is in horsepower, and torque is in ft-lbf. Also note - at 5250 rpm, hp is equal to torque numerically.

Racing engines are designed to produce a lot of torque at high engine speeds, because this will lead to increased horsepower. This engine can then be geared for whatever range of speeds it will normally be driven in.

The biggest problem with production cars is they spend the vast majority of their time under 4000 rpm, and larger engines (like my Mustang’s 4.6 l) spend most of their time below 2500 rpm. Another related problem is that due to the direct relationship between power and rpm, the power produced at these low engine speeds is low. So to get more power at common driving engine speeds, you must therefore increase the torque at these lower engine speeds.

Engine power increases with rpm because power is a function of time. Torque, on the other hand, is relative to the force exerted on the piston by the expanding gases in the cylinder. If you had perfect, equal cylinder filling on each stroke, then this force would be proportional to the bore, or displacement of the cylinder, and this force would be equal per stroke regardless of rpm. You can think of power as being determined therefore by how often this cylinder fills and fires in a given amount of time. One of the reasons two-stroke engines produce so much more power than a comparable four-stroke engine - although it has a poorer volumetric efficiency (the efficiency of filling the cylinder with fuel-air mixture), it fires twice as many times in the same amount of time.

So to increase low-end torque, we must actually also see what increases low-end power. And vice-versa. Engine torque will derive from the pressure exerted on the top of the piston by the exhaust gases. The average pressure exerted on the piston through the length of the stroke is known as the mean effective pressure (mep), and is given in terms of psi. Another measure, imep or indicated mean effective pressure, is determined by using some fancy equipment to actually measure on a running engine what the pressure is. Engineers commonly use mep as an indication of power plant performance in internal combustion (IC) engines.

Because it’s pretty hard to measure the imep, another measure, brake mean effective pressure, or bmep, is often calculated in it’s stead. This equation is:

bmep = (hp * 33,000) / (Length * Area * N)

where hp is power in horsepower, Length is the length of the stroke in ft, Area is the piston top surface area in square inches, and N is the power strokes per minute (or rpm / 2 for a four-stroke engine).

Now, what are the things that affect this bmep? Well, the first is volumetric efficiency. This of course is the efficiency of filling the displacement of the engine with fuel and air mixture on each stroke. You could also look at it this way - if a cylinder only fills with 85% efficiency on each stroke, this is similar to reducing your engine displacement (size) by 15%. Due to frictional losses, valve overlap, etc. this will not ever be 100% in a naturally aspirated engine. In fact, this efficiency falls off steadily as engine speed increases. One way to get back these losses is to use a turbo or supercharger, which will compress air into the cylinder, and thus give a volumetric efficiency relative to a naturally aspirated engine of more than 100% (but it’s not actually more than 100%, because you can’t look at it that way…)

What else increases volumetric efficiency? Well, tuning of the exhaust and intake runners will achieve some level of “pressure wave supercharging”, to help move the intake gases in and the exhaust gases out. A lower restriction air cleaner, fuel injection system, intake manifold, larger and more valves, etc. will also increase airflow into the engine. Likewise, lower restriction exhaust, removal of the catalyst, larger and more exhaust valves, etc. will allow the exhaust gases to flow out of the cylinder more easily. Also note that for both the intake and exhaust cycles, the valve timing and lift are also key to not only achieving greater volumetric efficiency, but also to tuning where the maximum possible flow through the valves will occur.

And when you’ve looked at what you can do with respect to volumetric efficiency, there is now friction horsepower loss to contend with. This tends to increase as a function of the square of the rpm of the engine, and can be very substantial at high speeds. One reference I have here gives the friction horsepower of a “stock” 350 Chevy as being 12 hp at 2000 rpm, 25 hp at 3000 rpm, 47 hp at 4000 rpm, 72 hp at 5000 rpm, and 110 hp at 6000 rpm. This friction loss can be reduced by careful engine building and lower-friction bearings and components, and by synthetic oils. But the trend of increase remains the same shape.

Now let’s look at bore and stroke. Increasing the stroke of an engine by increasing the crankshaft throw not only increases the displacement of the engine, but also increases the mechanical advantage on the crankshaft. So increasing the stroke increases the torque both from increased displacement and from greater mechanical advantage. Thus, even for two engines of the same displacement, the one with the greater stroke should have the greater torque. However, there are some other things to consider here:

  1. As stroke length increases, thus does the piston speed at the same rpm, and therefore the frictional power loss will increase as well.

  2. As this piston speed increases, the volumetric efficiency will fall off as well. A larger bore engine will also allow for larger and/or more valves, with less shrouding needed. Thus, if the two engines have the same displacement, then the shorter stroke one will have the greater volumetric efficiency.

  3. You also have to consider inertia effects of the longer connecting rod, and increased side-forces on the rings and piston due to the increased rod angularity with respect to the bore centerline.

Of course, there are those that will argue that a larger bore engine may result in an increased chance of detonation at high compression ratios than a smaller bore engine, due to an increased distance that the flame front must travel across. But this will vary greatly due to combustion chamber design, and I don’t know if it can be analyzed so generally. Overall, many claim that the small bore/long stroke engines are much better suited to the higher compression ratios, and if in our example of the two equal sized engines we also have a higher compression ratio than the larger bore one, then the larger stroke engine will see even more torque improvement.

Another theory related to this is the “long rod” theory, which says that the longer the connecting rod is, the more time or “dwell” the piston will have at top dead center (TDC). And therefore, there will be more time allowed for the gases to burn completely at the highest engine pressure. Thus, you will get even more force out of each firing with the same amount of fuel as the rod descends through it’s stroke. However, what really happens is that the long rod has a very small mechanical advantage due to it’s angularity at these high pressures, and thus this effect is negated. A short rod, attached to a crankshaft with a long throw, will move towards a higher angularity faster than a long rod with a short crank throw.

Another advantage of a long throw/short rod engine is that cam timing is not nearly so important as in a short throw/long rod engine. This is due to the fact that the piston spends less time at or near TDC, and thus allows for an earlier valve lift start and longer duration. Also, the exhaust valve can stay open longer as the piston ascends, increasing the amount of exhaust gas that is pushed out of the cylinder, and thus increasing the volumetric efficiency.

And so on, and so on. There are a great many things I have left out or glossed over, because I am tired of typing. Hope this helps somewhat more.

Ms. Una Persson

Quiet…guess I wrote too much, huh? :wink:


To get around the problem of relatively large volumes of fuel/air mixtures not burning fast enough in higher revving engines such as V-twin race-rep bikes they will use two plugs to prvides the spark, in fact the Aprilia RSV has a third plug that only come in when a certain engine speed is reached.The brand shiny new Honda 1800 v-twin cruiser does not rev at all high for a bike, less than 5k but it too uses two plugs per cylinder but I think this is to reduce emissions more than anything.

I would be interested in knowing what compression ratio does to torque, from what I can tell it seems to make enignes more peaky the higher one goes.

You are quite correct, multi-plug engines do solve some of the problems of a large bore/stroke ratio.

Well, I thought I knew some of the answers, but I had to do a little research in my book “Internal Combustion Engine Fundamentals”, by John B. Heywood, Copyright 1988 by McGraw-Hill Inc, New York. (and why the hell is a doctoral-level engineering class given a book that says “Fundamentals”? Oh well…)

According to Heywood, the bmep increases on a curve as compression ratio increases, finally leveling off at a maximum of about 17:1 for a gasoline engine (assuming the specific heat ratio, gamma, is equal to 1.3 for a gasoline/air mixture). Of course, most IC engines burning gasoline will not so easily make it to a 17:1 ratio, due to knock and detonation problems.

However, according to him the effect of higher compression ratio is pretty much linear across the rpm band of the engine - it increases low-speed power by the same ratio as high-speed power, all things being equal. Of course, at higher speeds cylinder wall cooling may become an issue, and thus knock may be the limiting factor once again at high engine speeds.

Fuel efficiency also increases as compression ratio increases, thus you get a double benefit. How large is this power and efficiency increase? According to the works he quotes, between 9:1 and 11:1 with all other things being equal, one can expect up to a 6% increase in both power and fuel economy (at normal motoring loads, of course. You don’t get the full benefit of the power AND the economy at the same time).

However, he also claims that the effect on NOx emissions is small, something I find difficult to believe and understand. If you look at the Gibbs Free Energy relations of NOx formation, the higher cylinder temperatures you get from a higher compression ratio should, in my mind, turn it into a high-NOx engine. He also claims higher HC emissions as compression ratio increases, another thing that seems counter-intuitive to me. He attributes this due to “increased importance of crevice volumes…lower gas temperatures during the latter part of the exhaust stroke…lower exhaust temperatures…”

YMMV. :slight_smile:

Thanks Anthracite for that. :slight_smile:

Anthracite, all true and interesting but I think comparing some graphs (torque vs rpm, power vs rpm) for different engines would illustrate this topic well. some engines have flatter graphs while other have a pronounced peak.

I used to have a 500cc Ducatti motorcycle which felt strange because it was not specially powerful at normal rpm and when you were going to overtake and started revving up, when you got to the point where other motors start to lose power, this one suddenly hit the sweet spot and took off like a bullet. It just had very little power at low rpms. It was designed to be revved way up.

Yes, sailor, that would be useful. But I have no web presence and no desire to create one and put graphs on it.

It is amazing just how driving two different cars with nearly identical peak powers, but very different torque curves, feels. My Mustang GT has 300 lb-ft of torque (reputedly, according to Ford) and 225 hp. From 800 rpm to 3000 rpm it feels strong. Above 4000 rpm you can feel it gasping for breath - there is a definite perception of a loss of power, although it still has not reached it’s peak output.

Compared to a Turbo-Intercooled Shadow I rode in (225 hp also, but only about 200 lb-ft of torque IIRC) the cars are very different. The Mustang feels much stronger at most driving speeds, but does not have the head-snapping acceleration at high engine speeds as the turbo reaches full boost.

All things considered, I want both qualities. Perhaps it is time to finally put the SVO Blower with the 14 psi pulley on my Mustang. YMMV, but I’ve seen dyno slips of the same car as mine with this setup at 385 hp / 445 lb-ft of torque. :eek:

Look at this,
I think you might find it interesting.