There are two main factors that contribute to the power of a rotary style engine (internal combustion engine, electric, turbine, etc). These are torque and maximum RPM. These two together determine the horse power of the engine.

One horse power is defined to be the power necessary to lift 33,000 pounds one foot in one minute. This can be done with a REALLY high torque engine lifting the weight directly, or a high RPM engine using a series of gears or pulleys to obtain the power. The formula for horse power is:

hp = (2pi * torque * RPM) / 33000
simplified:
hp = (torque * RPM) / 5252.268

Based on this formula, its easy to see that an engine with high RPMs and low torque can accomplish the same work as an engine with high torque and low RPMs.

Horse power is "work" (work = force * distance). Torque is the leveraged force. If you put a wrench on a rusty bolt and pull on it, but it doesn't break lose, you are applying a force to the bolt. Now, the length of the wrench gives leverage. The longer the wrench, the less force you will need to apply to break the bolt lose. The formula for torque is:

torque = force * radius
The "force" is the amount of force the engine can put on the shaft of the engine. The "radius" is the distance from the center of the shaft where the force is applied.

If you have ever seen (a picture of) a crank shaft, they tend to look like:

 _   _   _   _
- |_| |_| |_| |_- 
This is to place the point where the force is applied farther away from the center of the shaft. Thus, increasing the radius which increases the overall torque, which increases the horse power of the engine.

It would make sense to put the point of force as far out as possible to get the maximum torque you can, but there is point of diminishing returns. Think of a figure skater spinning. If she pulls in her arms, she spins faster. If she lets them out, she slows down. If the point of force is too far out, it takes longer for the engine to reach the higher RPM (more mass, farther out) and ultimately limits the max RPM. So you need to find a balance.

When designing an engine, the engineers can calculate how much force the explosion, or magnetic pull will generate. Then they need to figure out how far away from the center of the shaft the point of force will be. Once they do that, they can (theoretically) calculate the power of the engine at any given RPM.

The max RPM of an engine is a factor of several things. It basically boils down to how well the engine was designed, how well the parts are manufactured, and overall physical limitation. If you think about an internal combustion engine, it needs to take fuel into the cylinder, explode it, release the fumes, and bring in fresh air for the next go around. Springs and valves control all of these functions. If the engine is spinning faster than the springs and values can retract, it is beyond its capabilities and will start to backfire and malfunction.

In terms of engine performance, the horse power determines the top speed of the vehicle (given by horse power divided by the weight and aerodynamic drag of the vehicle). While Torque determines the rate of acceleration.

Brute force torque is usually obtained by having engines that consume large amounts of fuel. Commercial semi-trucks have large diesel engines that have insane amounts of torque. This torque is needed to move the heavy load that they are pulling. Sure, you could pull it if you had less torque and more RPMs (same power), but think of it like the rusty bolt. What would be safer? Breaking it loose with a wrench and a lot of torque, or trying to slam a socket wrench on it that happens to be spinning at 10,000 RPMs?

 

Whew. Man, don't I feel like white trash now. I have a sudden urge to go out and buy a Camaro and start wearing wife beaters soaked in engine oil. Please feel free to vote for this node for the redneck node of the day.

 

If I've made any mistakes or wrong info, let me know and I will correct it. I've been learning about engines so that I may someday know how to fix my motorcycle if it ever breaks down.

This very good node refers only to horsepower. Horsepower, however, is only one particular unit of the more general concept power. Another good example is watts. 1 horsepower = 746 watts. A watt (actually newton-meters per second where horsepower is 33,000 pound-feet per minute) is the metric unit of power. We typically associate watts with electricity, but the laws of work and power hold true throughout physics no matter which units we prefer to use in a particular instance.

"Give me a lever long enough and a fulcrum on which to place it, and I shall move the world."
Archimedes

 

Archimedes didn't say he could move the world quickly. Theoretically, infinite torque can be achieved from any engine, through infinite gearing. You can lift your car with a jack, but imagine how fast you'd have to pump the jack to lift the car quickly.

Many people believe that torque determines acceleration, since torque is angular force and acceleration = force / mass, in an ideal friction-less vacuum, ignoring how much power is required to create and sustain that force. Torque, alone, doesn't get anything done. When you apply force over a distance and get something done, it's called work. Once you divide the amount of work by the time it took to perform it, it's called power

A peak torque of 300 ft-lbs at 3000 RPM (171 hp) is not unreasonable for a Chevy 350. With a 10:1 gear reduction, this becomes 3,000 ft-lbs at the drive axle. You can hook up bicycle pedals to your car and gear it down 30:1 and assuming you create 100 ft-lbs at the pedals, you'll create 3,000 ft-lbs at the axle, but can you pedal at 9,000 RPM? This modified 1.5 liter Porsche engine can. Horsepower is the ultimate goal. Horsepower is determined by Hp= (Tq*RPM)/5252. 1 ft-lb of torque at 5252 RPM is 1 Hp. So is 5252 ft-lb of torque at 1 RPM, and ½ ft-lb at 10,504 RPM. It's even simpler in SI units: One watt is equal to 1 Newton-meter per second (Nm/s).

In the simplest terms, burning fuel faster makes more horsepower. A larger engine burns more fuel, per revolution. All things being equal, a larger engine makes more horsepower. The problem, as has been noted, all things are never equal. Engines with a smaller per cylinder displacement, simply rev higher to achieve the same horsepower, if it's built for it. Torque will be lost, but gearing will correct that. Remember, horsepower is most important. Force (torque) is an instantaneous measurement, while power is a calculation based on how long that force is sustained.

Peak horsepower is dependent on the "top-end" of the engine, or more specifically, how much air flows through it. In real world engines, the Chevy 4" bore small block is a good demonstration of this. It came from the factory in displacements of 350 ci (4"x3.48"), 327 ci (4"x3.25"), 302 ci (4"x3"). A common modification is to stroke it to 383 ci (4.03"x3.75"). The slight bore increase is actually an insignificant result of rebuilding the engine. With identical camshafts, heads, compression ratios, intakes and exhausts all four of these displacements will have only minor differences in peak horsepower, primarily due to friction differences and different rod/stroke ratios. With identical, performance-oriented top-ends:

The 302 will make slightly more horsepower at higher RPMs and considerably less torque that peaks at higher RPMs, and won't be flat or wide. On a racetrack, where you can keep the RPMs up and in the peak power range, the small increase in horsepower is an advantage. Road racers accomplish this with close ratio standard transmissions and changing the rear gear ratios to suit the track. Drag racers use high stall torque converters that slip at low RPMs. Also, with less input torque for any given horsepower level, the transmission and driveshaft can utilize smaller, lighter components to reduce reciprocating weight, which more than compensates for any increased friction that comes from more gear reduction. The all-out racecar will perform better, but you really don't want to drive it to work in traffic everyday. This is why the Chevy 302 was only produced in just enough numbers to satisfy SCCA Trans-Am rules for stock production 5 liter engines. Once de-stroking was allowed, they only produced 350s, in street cars with the 4" small block.

The 383 will make slightly less horsepower at lower RPMs, but it will make considerably more torque in the lower RPMs and have a much wider, flatter torque curve. In a car with a stock drive train, this approach allows you to make more power by increasing flow but keeping the RPMs near stock. Drastic performance gains can be achieved without drastic drive train changes or sacrificing drivability on the street. In a car with a stock automatic transmission you can just mash'n'go! The transmission will downshift appropriately and you'll accelerate with ease. With a stick shift, you'll shift the same and won't have to burn out the clutch on the launch. This is a popular modification for pickup trucks. 

The reason trucks tend to have large, high torque engines is not as simple as most people think. It's about power band and gearing, not torque. Power is still the important factor but large engines have wide power bands and require less gear reduction to produce the massive torque needed to get a heavy vehicle rolling, relative to a small engine that makes the same power. For the sake of comparison, let's say we have two engines. They have comparable power bands but one is from 1,000 to 3,000 RPM and the other is from 7,000 to 9,000 RPM. They are both geared so that the top RPM in first gear is 10 mph. Engine "L" (low revving) must slip the clutch or torque converter until it's going 3.3 mph and engine "H" must slip to 7.7 mph. Second gear is set so that the bottom RPM is 10 mph. L will need to shift to third gear at 30 mph and H will need to shift at 12.9 mph. L will need to shift to fourth gear at 90 mph and H will need to shift at 16.5 mph. At this rate, H will top out tenth gear at 96 mph. These are exaggerated to make a point, of course, but they do reflect the real reason heavy trucks don't come with small high-revving engines. 

 


 

Addendum:

I recently got hold of a dyno simulator. I ran the four engines with identical settings, except stroke of course.

 CI             HP                     FT/LBS             95% peak torque
306       320 @5500       354 @4000      337 @3000 - 351 @4500     27%
332       322 @5000     ≥375 ~3750*     358 @2500 - 365 @4500     40%
355       321 @5000       396 @3500      378 @2000 - 391 @4000     40%
383       323 @4500     ≥418 ~3250*     411 @2000 - 406 @4000     44%
 CI             HP                     FT/LBS             95% peak torque
306       625 @8500        475 @5500     462 @5000 - 463 @6500     18%
332       621 @7500        501 @5500     491 @5000 - 477 @6500     20%
355       614 @7500        520 @5500     499 @4500 - 507 @6000     20%
383     ≥602 ~6750*       540 @5000     530 @4500 - 517 @6000     30%


Heads:                  Wedge/Pocket Porting         Edlebrock Victor Jr.
Valves:                  1.94"/1.50"                       2.08"/1.6"
Compression:          9.0                                  12.5
Induction Flow:       750 cfm                            1000 cfm
Manifold:                Dual Plane                         Tunnel Ram
Exhaust:                Small Tube w/mufflers          Large Step-Tube
Cam:                     Edlebrock Performer            Crane 284-292

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