What does engine horsepower depend on?

[dezso3]

Generally, as an engine gets bigger, its horsepower increases. Horsepower doesn't just depend on the size of the engine, but also whether it is turbocharged or supercharged. But are these the only things that horsepower depends on, or are there more variables? For example, the Lincoln Town Car has a 4.6 Liter V8, but it only has 239 horsepower, whereas the Lexus LS 460 also has a 4.6 Liter V8, but has 380 horsepower. The Toyota Camry XLE also has a regular 3.5 Liter V6, and has 268 horsepower, which is still more powerful than the Lincoln Town Car. The Chevy Impala also has a 3.5 Liter V6, but only has 207 horsepower. The Subaru Impreza WRX STi has a 2.5 Liter turbocharged engine with 305 horsepower, which is more powerful than an engine that is almost twice as large, the 4.6 Liter V8. So what exactly accounts for the huge differences in horsepower between engines of the same size that are NOT turbocharged?

[akboss]

Originally Posted by dezso3

Generally, as an engine gets bigger, its horsepower increases. Horsepower doesn't just depend on the size of the engine, but also whether it is turbocharged or supercharged. But are these the only things that horsepower depends on, or are there more variables? For example, the Lincoln Town Car has a 4.6 Liter V8, but it only has 239 horsepower, whereas the Lexus LS 460 also has a 4.6 Liter V8, but has 380 horsepower. The Toyota Camry XLE also has a regular 3.5 Liter V6, and has 268 horsepower, which is still more powerful than the Lincoln Town Car. The Chevy Impala also has a 3.5 Liter V6, but only has 207 horsepower. The Subaru Impreza WRX STi has a 2.5 Liter turbocharged engine with 305 horsepower, which is more powerful than an engine that is almost twice as large, the 4.6 Liter V8. So what exactly accounts for the huge differences in horsepower between engines of the same size that are NOT turbocharged?

Typically it is the type of engineering and reduction of power loss that creates higher horsepower engines in smaller packages, even while naturally aspirated (non-turbo). Newer engines are almost always more powerful than outgoing engines, and the 4.6L V8 in the Lincoln Town Car has remained largely unchanged in about 2 decades - hence a horsepower figure that may have stood out in the late 80's, but now looks comparable to engines half the size. BTW, 239 horsepower is the newer Town Car - going back to the early 90's, the same 4.6L V8 made only 190 horsepower!

As CAD and other software increases the mechanical precision of engine manufacturing, and as all components from intake to exhaust are improved and maximized in efficiency, horsepower increases.

But one thing to note is that horsepower isn't the only figure that is important - torque is equally significant, and is responsible for the seat-of-the-pants feeling you get when accelerating. That old Lincoln motor was short on horsepower, but the same car that made 190 ponies was also making almost 280 lb-ft torque. To put that into perspective, a 2010 Honda Civic SI has 200 horsepower, but only 139 lb-ft of torque. So in order to keep a Honda Civic SI accelerating quickly, you need to keep the RPM's of the engine sky-high. That's why driving around in a Town Car feels powerful because from a stop light, you've got a massive amount of torque in the engine that can push you off. It sort of feels like a boat accelerating, powerful and linear. A screaming, screeching 4-cylinder will probably outrun you, but you might wish you were driving the Town Car...

[Polygon]

This is really quite simple. An engine is really just an air pump. The more air it moves the more power it will make. It's really is that simple.

Also, horse power is just a derivative of torque. In reality, horse power is very arbitrary number.

[akboss]

Originally Posted by Polygon

This is really quite simple. An engine is really just an air pump. The more air it moves the more power it will make. It's really is that simple.

Also, horse power is just a derivative of torque. In reality, horse power is very arbitrary number.

Well, yes, in its very simplest terms you are correct. But the OP was asking how a smaller displacement engine can sometimes produce more power than a larger displacement motor, if the common school of thought is that larger is more powerful. So how do you make a smaller air pump that pumps more air than a larger one, without forced induction? I'd say 'breathing' is the most important - efficient intake and exhaust systems.

What do you mean 'horsepower is a derivative of torque? I'm not trying to be a smartass, I just would like to understand. To me it was torque that was arbitrary, but interested to know what you mean.

Cheers,
A.

[dizzle1]

A small engine will make more power but needs to rev high. So the civic si K20 makes 200hp and 139tq NA that as much as you can get as of power per liter. So on the torque side for a 2.0L 139tq is good. Still its not as great as the S2000 but is close enough. The M5 makes 500hp out of a V10 and 384tq ands thats really good for a 5.0L even the new M3 4.0L V8 is good with 414hp and 295tq.

Thats 100hp per liter and all NA. No domestic engine that I remember has matched the power per liter output of these imports. The new mustang gt 5.0L V8 makes 412hp and 390tq. The best effort I've seen from an american automaker in V8's. The V6's are getting closer too 3.6L 306hp still 54hp shy.

I think the breathing of the cylinder head is where all the power comes from, A good cams, Intake and Exhaust VVT's, roller rockers, Intake manifold, big throttle body, 4 valves per cylinder, good sized valves, individual ignition coils and balance of the engine air fuel will have a significant affect on HP and TQ.

[fredjacksonsan]

Originally Posted by akboss

Well, yes, in its very simplest terms you are correct. But the OP was asking how a smaller displacement engine can sometimes produce more power than a larger displacement motor, if the common school of thought is that larger is more powerful. So how do you make a smaller air pump that pumps more air than a larger one, without forced induction? I'd say 'breathing' is the most important - efficient intake and exhaust systems.

What do you mean 'horsepower is a derivative of torque? I'm not trying to be a smartass, I just would like to understand. To me it was torque that was arbitrary, but interested to know what you mean.

Cheers,
A.

HP is calculated from torque. There's a pretty good explanation/discussion here.

For the OP, power produced by an engine is variable depending on how it is put together, for example the 4.6L in the Town Car can be upgraded to produce more than 500HP. A similar thing is going on for the smaller motors that make more power - technology, upgrades, forced induction, etc, all which contribute in some small [or large] way to how much air the engine can pump.