Horsepower per liter

[Stutz]

Can someone show me how this is udseful in comparing engines or cars?

And I don't want, well, the LS1 makes 350 horsepower, the LS6 makes 405 horsepower, thus more hp/l.

I know that, but you might as well just say it has more hp. I want to know where hp/l is useful that plain old hp is not.

[YogsVR4]

hp/l means little to nothing in the actual performance of the engine.

[Moppie]

Originally posted by YogsVR4
hp/l means little to nothing in the actual performance of the engine.

Actualy it can be a good indication of how highly tuned the engine is, and therefore how well it behaves at low or high rpm.

Generaly the more hp/L and engine has the less flexiable it will be at low RPM.
It is also a good indication of the engines purpose, is it built for economy, or performance? Or a compromise of the two.

Due to the limitions of useing Cam Shafts to control air flow into and out of an engine it is very hard to make an engine with a high hp/L ratio, and a good flat driveable torque curve.
And so the higher the ratio the closer the engine gets to being usefull for little more than racing.
There are of course exceptions to this:
Engines with variable lift valve set ups like Honda's VTEC allow an engine to have two power curves, one for high rpm and so high hp, and one for low rpm and torque, and so you get a high hp/L engine that is still flexiable.
Engines with variable cam timeing offer a similar advantage.


Engines fitted with forced induction are differnt again. Although a F/I engine with a high hp/l ratio will be less flexiable than one with a low hp/L ratio it will not be as extreme as comparing two Naturaly Asperaited engines.

And of course as enginers learn more, engines get better.
Its possible for an engine design to get a high hp/l ratio over its life time, with out sacrificing any of its flexablity.
The evolution of the LS1 to the LS6 is a good example of this.


Hp/L is also a useful way of comparing the level of technology and complexity in an engine, as well as its efficancy.
Street driveable engines with very high hp/L ratios require more technology, complexity and efficancy to get there. It is a good indication of the engine building skills and attention to detial of a Manufactor.


And finnaly theres bragging rights: Hahaha, my 1.0L 2cyl engine makes more hp than your 5.7L V8 :finger: (if you look at hp/L that is! )



If your looking to buy a car then its a number that I would pay little attention to, although in some cases engines with higher hp/l ratios will be more efficant when driven gently.

[Stutz]

Originally posted by Moppie

Actualy it can be a good indication of how highly tuned the engine is, and therefore how well it behaves at low or high rpm.

So it can only be used when comparing an engine to itself?

Generaly the more hp/L and engine has the less flexiable it will be at low RPM.
It is also a good indication of the engines purpose, is it built for economy, or performance? Or a compromise of the two.

What about the LS1 in the Z28? It had a pretty low hp/l, but blew away most competitiors in performance.

Hp/L is also a useful way of comparing the level of technology and complexity in an engine, as well as its efficancy.
Street driveable engines with very high hp/L ratios require more technology, complexity and efficancy to get there. It is a good indication of the engine building skills and attention to detial of a Manufactor.

I don't think so. If a maker decides he wants a low revving engine, does that mean his engine is less advanced? High revving engines make a higher hp/l, but that doesn't show you that the engine is more advanced, because they are still putting out about the same displacement per minute. A 4 liter engine will put out the same amount of displacement as a 2 liter engine revving twice as high.


Also, higher hp/l doesn't mean more efficiency. The Ford Focus makes more power but less hp/l than the Civic EXs 1.7. The Civics engine, even though it is smaller and less powerful, puts out more displacement at redline than the Focus.

Thus the Civic has to put out more displacement and still makes less power.

Another interesting thing to think about is that if you destroke an engine, your hp/l generally goes up, but your power generally goes down. So if horsepower per liter is good, why not destroke all your engines?

[Moppie]

Originally posted by Stutz


I don't think so. If a maker decides he wants a low revving engine, does that mean his engine is less advanced?

No what I said is it requires the aplication of more technology to make a high reving engine produce a high hp/l ratio. Otherwise you end up with an engine with a very limited power curve.
You could fit VTEC to a 5.7L V8 but it would be totaly pointless. The engine does not rev high enough to get outside the operating range of single cam profile. (genneraly 3-4,000rpm on an advanced engine design)
However on a 4cly car with a redline of 7-8,000rpm it is very usefull for extending the engines power curve.
It does not mean the low reving engine is less advanced, just that it is likely to have less technology added, like variable cam timeing systems etc.

Also, higher hp/l doesn't mean more efficiency. The Ford Focus makes more power but less hp/l than the Civic EXs 1.7. The Civics engine, even though it is smaller and less powerful, puts out more displacement at redline than the Focus.

Thus the Civic has to put out more displacement and still makes less power.

Yes and no. Under my deffinition of efficancy the Civic engine pumps more air for less displacement making it more efficant.
And of course the efficancy that any car buyer is interested in is millage and emissions. And the Civic uses less fuel and puts out fewer emissions than the Focus.

Another interesting thing to think about is that if you destroke an engine, your hp/l generally goes up, but your power generally goes down. So if horsepower per liter is good, why not destroke all your engines?

Because high hp at high rpm (high hp/L) generaly equals low Torque.
Its only suitable in some applications, e.g a small light weight FWD Hatchback. Larger cars need an engine will more torque at lower RPM to perform as well so an engine with a longer stroke and more torque but lower hp/L is needed.




As you alluded to in your first post hp/l is not a very usefull value when comparing cars, and 99% of car buyers would not even bother to think about it.
I just offered some suggestions as to how it might be used if you wanted to evaluate every single bit of information you had.
But to choose a car based solely on its hp/L value would be silly.

What about the LS1 in the Z28? It had a pretty low hp/l, but blew away most competitiors in performance.

Not an engine I know a lot about as it was only sold in limited numbers here in NZ.
However Im guesing that the performance was from its high Torque accross a wide rev range. (from what Iv read was its biggest advantage over Ford Australias aging 351windsor)

[FYRHWK1]

hp/liter means nothing, a larger motor will have more pumping losses due to its size and friction, but it still makes more power. a smaller motor can make more specific power but all that requires is the ability to rev high.
since torque does all the work, why doesnt anybody bring up torque/liter? it's just a selling point now, like DOHC people believe it brings up some level of advancement and prestige, import companies are great at advertising.

[Moppie]

Tq/L is about as usefull as hp/L.

What you should be looking at is the at what point say 75% of peak torque is reached, and at what point it drops off.

On a road car the greater the rev range useable Torque is avliable over then the better the engine.
Its one of the things that makes engine with variable cam technology so much better. They often have less peak torque, but the torque they have is avliable over a wider rev range and the engine in any given car is often faster.

A great example of this is when compareding the new Honda CRV with the Nissan Xtrail. The Honda has a smaller engine with less peak hp and torque than the Nissan. and is also heavier, yet in all accleration tests is faster (from 0-100, 80-120 etc). The reason is the CRV engine is able to do more work accross its rev range.

[FYRHWK1]

Originally posted by Moppie
Tq/L is about as usefull as hp/L.

What you should be looking at is the at what point say 75% of peak torque is reached, and at what point it drops off.

On a road car the greater the rev range useable Torque is avliable over then the better the engine.
Its one of the things that makes engine with variable cam technology so much better. They often have less peak torque, but the torque they have is avliable over a wider rev range and the engine in any given car is often faster.

A great example of this is when compareding the new Honda CRV with the Nissan Xtrail. The Honda has a smaller engine with less peak hp and torque than the Nissan. and is also heavier, yet in all accleration tests is faster (from 0-100, 80-120 etc). The reason is the CRV engine is able to do more work accross its rev range.

of course, any kind of specific power output is a useless number, i was just pointing out that it snot even an engineering achievement, its used for car salesman to get ignorant kids to buy the product.