What is the difference between an engine built for speed, and an engine built for power

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I’m thinking of a sports car vs. tow truck. An engine built for speed, and an engine built for power (torque). How do the engines react differently under extreme conditions? I.e being pushed to the max. What’s built different? Etc.

In: 3108

[deleted]

Consider two engines with the same displacement. Displacement is calculated by multiplying the stroke length of the pistons by the cross-sectional area of the cylinder. A sports engine will be built to make high engine speed which is more easily achieved by a short stroke, so it will have a larger bore. Low engine speeds seeking more torque have longer piston stroke because the greater distance the connecting rod is from the crankshaft acid, the greater the torque output. These engines will have smaller bores.

[removed]

The chief difference is in the amount of power you get per pound of engine. We determine engine power by multiplying engine speed by the amount of torque it produces.

An engine designed for a fast vehicle will optimize for less weight. Typically it will burn gasoline because the engine can be made less robustly. These engines can shed weight further (and still make power) by spinning faster. A working engine on the other hand will optimized for things like durability, efficiency, or drivability. These engines will spin more slowly because it makes sense for the use case. When they spin more slowly they need to make more torque to make the same power vs a low torque but high revving engine.

An example of this: the engine in a BMW super bike weighs 150lbs and makes 180hp. It makes that power at 14000 RPM. A 6BT Cummins might also make 180hp but it weighs closer to 1100lbs. That diesel will redline closer to 2900 RPM. The bike engine is meant for racing at the cost of efficiency and longevity, the diesel is meant to be more efficient and easier to drive.

Dear God, the answers here are terrible.

The short answer is that there’s not a difference.

Powerful engines have high power output, meaning horsepower. Torque is a measure of angular force, which means force of twist, or how hard it twists.

Horsepower is, mathematically, torque multiplied by revolutions per minute divided by 5252 (just trust the constant, the explanation isn’t relevant for this).

So how to you get high horsepower? You use high torque or you have high rpms. High rpms mean more movement, which means more friction and less efficiency.

Higher torque means the engine has to be able to take higher internal forces (since torque IS rotational force), which means it’s heavier. Heavier is often bad for a race car.

So the answers about diesel being heavier, but making more torque are right then, right? I’m mad over nothing!

Well, no.

Let’s look at the most prolific engine family of all time, the Chevy small block. Specifically, let’s look at the 3rd and 4th gen, the LS family.

The same engines were routinely used BOTH in trucks and Corvettes. Sure, they’d get tweaks to push the horsepower a bit higher in the Corvette, or a bit more low end torque for the LQ (truck designated) engines, but they’re ALMOST identical. So what gives?

The definition of torque. That’s what.

Torque is force at some distance from the centerline. Basically, if you put a 1lb weight on an arm 1 ft long, you have 1 ft-lb of twist. If you double the length? The same 1 lb is now doing 2ft-lb of twist.

So how’s the Corvette, a high performance sports car, using a truck motor that doesn’t rev very high? One word: gearing.

ANY engine can have ANY torque value at the tires, which is where it’s measured. As Archimedes said, give me a long enough lever, and a fulcrum on which to place it, and I will move the world.

Any engine can be a race car engine. Any engine can be a truck engine. The energy output per second, or power, is all that matters. With gears, you can reshape your power to be used however you need, speed or grunt.

So why do semis use diesel? Because it’s cheaper. That’s it. Diesel is cheaper per used power because diesel is more power dense than gasoline. The higher compression ratio is more efficient. Glow plugs last longer than spark plugs. Diesel engines last longer and are simpler to repair when they do fail.

The claims on shorter piston movements meaning more power but less efficiency? True, because of friction losses in longer strokes with higher RPM. That higher RPM, we we saw in the equation, means more power.

TL;DR: there’s virtually no difference in the engines. The transmissions are where the differences are. Diesels are used because they’re efficient and reliable.

Edit: promptly a complaint about not using metric, so here you go: Power (kW) = Torque (N.m) x Speed (RPM) / 9.5488

If you’d rather use a multiplier of 1/9.5488 instead of 1/5252, be my guest. You can similarly use 1N of force on a 1m and 2m bar, etc. I don’t care. After a decade as an aerospace engineer, I don’t particularly care what anyone’s using.

It’s really about the gearbox.

Same engine, let’s say petrol 4L V8, in a sports car it has a gearbox that allows for high rotation on the wheels, if you want a truck with the same engine it needs a gearbox that outputs lower speed but higher torque

Y’all I don’t know what a compression-extraterrestrial-supercalifragalisticexpialidocious-ratio is😭 You guys are a supposed to explain like I’m 5, not like I’m Einstein😭. Then again I could just be dumb but so are 5 years olds so🤷

Power is torque times speed, to make it short. 10 torque x 1 speed = 10 speed x 1 torque, so to speak. And with gearing you can exchange one for the other. That is what a drivetrain and gears do as you shift from 1 to 6.

What people often speak about when an engine has “torque” or “power” is the shaft straight out of it, and at what RPM it delivers said torque.

So the yamaha R1 bike engine has torque of 112nm max and 147kw power, but needs to rev up to 11k rpm for it. The harley davidson 2 liter Milwaukee-Eight 117 has 167 torque but only 78kW power, and max torque is at 3.5k rpm.

Not only it has more torque, but it can deliver it earlier – it is more “torquei”. So it needs to accelerate a lot less to have its peak torque and power.

the yamaha will be delivering that torque at a much faster speed resulting in greater power. Its a much faster bike.

But if you needed to attach it to raise a weight, and you could use gearing, you could achieve almost any lift you needed and the yamaha would have double the power.

The simple answer is that there isn’t really a difference. All other things being equal, speed is directly a function of power. The engines are the same, the differences are in the gearing and the design of the vehicles.

The one area where differences may arise is that there will be a bit more focus on power-to-weight ratio over durability in engines for lighter, faster vehicles.

I’ve seen a few answers saying that it’s mostly in the gearbox, but isn’t there a pretty big difference in the powerband of the engine (how effective it is across different rpms)? Most engines built to handle heavy loads tend to prioritize being extremely efficient in a narrow band, while cars built for speed (well, cars built to accelerate well) tend to be relatively efficient across a wider band of rpms?

Sure, gearbox design has probably changed that a bit (since automatic gearboxes are more efficient these days) but…

in a nutshell, speed high rpms, power low. specific power “torque” bands are dictated by camshaft design/computer management.

By speed do you mean top speed or acceleration?
By power do you mean high torque or high hp/kw?

Imagine small carousel at the playground:

If it’s empty and you (the human engine) push it near the center you’ll generate high speed at the edge with not so much effort.

On the other hand if it’s full of people and you’ll try to push near center it will be really hard to do. But! If you push it far from center it’s so much easier. In this case speed is also lower.

You can do similar thing with taking the engine and applying various transmissions to it.

The final answer is: it’s not the engine, transmission makes the biggest difference.

0 views

I’m thinking of a sports car vs. tow truck. An engine built for speed, and an engine built for power (torque). How do the engines react differently under extreme conditions? I.e being pushed to the max. What’s built different? Etc.

In: 3108

[deleted]

Consider two engines with the same displacement. Displacement is calculated by multiplying the stroke length of the pistons by the cross-sectional area of the cylinder. A sports engine will be built to make high engine speed which is more easily achieved by a short stroke, so it will have a larger bore. Low engine speeds seeking more torque have longer piston stroke because the greater distance the connecting rod is from the crankshaft acid, the greater the torque output. These engines will have smaller bores.

[removed]

The chief difference is in the amount of power you get per pound of engine. We determine engine power by multiplying engine speed by the amount of torque it produces.

An engine designed for a fast vehicle will optimize for less weight. Typically it will burn gasoline because the engine can be made less robustly. These engines can shed weight further (and still make power) by spinning faster. A working engine on the other hand will optimized for things like durability, efficiency, or drivability. These engines will spin more slowly because it makes sense for the use case. When they spin more slowly they need to make more torque to make the same power vs a low torque but high revving engine.

An example of this: the engine in a BMW super bike weighs 150lbs and makes 180hp. It makes that power at 14000 RPM. A 6BT Cummins might also make 180hp but it weighs closer to 1100lbs. That diesel will redline closer to 2900 RPM. The bike engine is meant for racing at the cost of efficiency and longevity, the diesel is meant to be more efficient and easier to drive.

Dear God, the answers here are terrible.

The short answer is that there’s not a difference.

Powerful engines have high power output, meaning horsepower. Torque is a measure of angular force, which means force of twist, or how hard it twists.

Horsepower is, mathematically, torque multiplied by revolutions per minute divided by 5252 (just trust the constant, the explanation isn’t relevant for this).

So how to you get high horsepower? You use high torque or you have high rpms. High rpms mean more movement, which means more friction and less efficiency.

Higher torque means the engine has to be able to take higher internal forces (since torque IS rotational force), which means it’s heavier. Heavier is often bad for a race car.

So the answers about diesel being heavier, but making more torque are right then, right? I’m mad over nothing!

Well, no.

Let’s look at the most prolific engine family of all time, the Chevy small block. Specifically, let’s look at the 3rd and 4th gen, the LS family.

The same engines were routinely used BOTH in trucks and Corvettes. Sure, they’d get tweaks to push the horsepower a bit higher in the Corvette, or a bit more low end torque for the LQ (truck designated) engines, but they’re ALMOST identical. So what gives?

The definition of torque. That’s what.

Torque is force at some distance from the centerline. Basically, if you put a 1lb weight on an arm 1 ft long, you have 1 ft-lb of twist. If you double the length? The same 1 lb is now doing 2ft-lb of twist.

So how’s the Corvette, a high performance sports car, using a truck motor that doesn’t rev very high? One word: gearing.

ANY engine can have ANY torque value at the tires, which is where it’s measured. As Archimedes said, give me a long enough lever, and a fulcrum on which to place it, and I will move the world.

Any engine can be a race car engine. Any engine can be a truck engine. The energy output per second, or power, is all that matters. With gears, you can reshape your power to be used however you need, speed or grunt.

So why do semis use diesel? Because it’s cheaper. That’s it. Diesel is cheaper per used power because diesel is more power dense than gasoline. The higher compression ratio is more efficient. Glow plugs last longer than spark plugs. Diesel engines last longer and are simpler to repair when they do fail.

The claims on shorter piston movements meaning more power but less efficiency? True, because of friction losses in longer strokes with higher RPM. That higher RPM, we we saw in the equation, means more power.

TL;DR: there’s virtually no difference in the engines. The transmissions are where the differences are. Diesels are used because they’re efficient and reliable.

Edit: promptly a complaint about not using metric, so here you go: Power (kW) = Torque (N.m) x Speed (RPM) / 9.5488

If you’d rather use a multiplier of 1/9.5488 instead of 1/5252, be my guest. You can similarly use 1N of force on a 1m and 2m bar, etc. I don’t care. After a decade as an aerospace engineer, I don’t particularly care what anyone’s using.

It’s really about the gearbox.

Same engine, let’s say petrol 4L V8, in a sports car it has a gearbox that allows for high rotation on the wheels, if you want a truck with the same engine it needs a gearbox that outputs lower speed but higher torque

Y’all I don’t know what a compression-extraterrestrial-supercalifragalisticexpialidocious-ratio is😭 You guys are a supposed to explain like I’m 5, not like I’m Einstein😭. Then again I could just be dumb but so are 5 years olds so🤷

Power is torque times speed, to make it short. 10 torque x 1 speed = 10 speed x 1 torque, so to speak. And with gearing you can exchange one for the other. That is what a drivetrain and gears do as you shift from 1 to 6.

What people often speak about when an engine has “torque” or “power” is the shaft straight out of it, and at what RPM it delivers said torque.

So the yamaha R1 bike engine has torque of 112nm max and 147kw power, but needs to rev up to 11k rpm for it. The harley davidson 2 liter Milwaukee-Eight 117 has 167 torque but only 78kW power, and max torque is at 3.5k rpm.

Not only it has more torque, but it can deliver it earlier – it is more “torquei”. So it needs to accelerate a lot less to have its peak torque and power.

the yamaha will be delivering that torque at a much faster speed resulting in greater power. Its a much faster bike.

But if you needed to attach it to raise a weight, and you could use gearing, you could achieve almost any lift you needed and the yamaha would have double the power.

The simple answer is that there isn’t really a difference. All other things being equal, speed is directly a function of power. The engines are the same, the differences are in the gearing and the design of the vehicles.

The one area where differences may arise is that there will be a bit more focus on power-to-weight ratio over durability in engines for lighter, faster vehicles.

I’ve seen a few answers saying that it’s mostly in the gearbox, but isn’t there a pretty big difference in the powerband of the engine (how effective it is across different rpms)? Most engines built to handle heavy loads tend to prioritize being extremely efficient in a narrow band, while cars built for speed (well, cars built to accelerate well) tend to be relatively efficient across a wider band of rpms?

Sure, gearbox design has probably changed that a bit (since automatic gearboxes are more efficient these days) but…

in a nutshell, speed high rpms, power low. specific power “torque” bands are dictated by camshaft design/computer management.

By speed do you mean top speed or acceleration?
By power do you mean high torque or high hp/kw?

Imagine small carousel at the playground:

If it’s empty and you (the human engine) push it near the center you’ll generate high speed at the edge with not so much effort.

On the other hand if it’s full of people and you’ll try to push near center it will be really hard to do. But! If you push it far from center it’s so much easier. In this case speed is also lower.

You can do similar thing with taking the engine and applying various transmissions to it.

The final answer is: it’s not the engine, transmission makes the biggest difference.