HP per Liter, is it Important?

  • Thread starter Venom800tt
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Venom800tt

Venom800tt

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Knoxville, Tennessee
After several debates over the years with various people on different forums about HP per liter, I never could figure out if there is any real importance to the number other than for bragging. So as the topic says, does it serve any real use?
 
It's only important in terms of bragging rights, but what's more important for actual performance is average hp between shiftpoints... in other words, the average hp you make in 2nd gear when shifting from first till you have to shift to 3rd.

You can have over 100 hp per liter, say, a 200 hp 2 liter engine, but still be slower than a guy with a 180 hp 3 liter engine because you make 200 hp only at 9000 rpm, and make a paltry 90-100 hp from 5000 rpm to 7000 rpm.

Not to mention the effects of power to weight, torque to weight, gearing, etcetera... of course, low actual displacement for the amount of power produced has its benefits in fuel economy... as long as the engine is tuned and geared properly. (No the EVO doesn't count, because it runs so pig-rich, it's drinking enough fuel for a 5 liter engine).
 
well it kind of irritates me that my 1995 Ford Taurus has a 3.8l V-6 a little >150hp while my 2007 Ford Ranger with a 2.3 Inline 4 Produces virtually the same HP with slightly better gas milage, but it's so little (never compared it because I just now started seeing how much mpg the ranger gets) that I would rather drive my Taurus.
 
HP per litre is more important to me atleast as a guide of how much stress a engine is under.
How much a engine is good for with the HP per litre in mind also gives you an idea how much thought went into the engine design when originally designed by engineers, this is why engines that were in cars that were also intended for racing aswell as engines in vehicles also intended for heavy labour usually are good at handling high HP per litre figures (High stress). They are often much higher tolerances and stonger designs (thicker web casting, steel construction etc)

The larger the engine and the more cylinders the less this is of concern, which means the engines can be constructed much more loose, lighter (More alloy less thick casting) and generally cheaper aswell as tuned leaner, this works very well for GM especially. This is why they have a lower hp per litre figure generally but are still powerhouses.



In the end for performance cars is it important? No, you can have great or crappy engines of high and low HP per litre, it all comes down to the individual car and engine not a Hp per litre number.
 
The only thing that matters is 5 cylinders and a honkin turbo. :drool:
 
Doesn't BHP/Liter really just reflect how rev happy the engine is? (Forced induction aside. You can get an engine to do anything with a few turbos and a supercharger attached)

Because, really, what engines are putting out 100 bhp/liter anywhere below, say, 7000-7500 RPM? Compare the M5 which makes 500 hp at 5750 RPM, and around 385 lb-ft of torque to a huge truck engine that produces a thousand or so lb-ft of torque but only a couple hundred horsepower at 2000 RPM.
 
I thought it was the kW/hr/l ratio that was important, basically how much mechanical output you get over a given time for a specific amount of fuel.
 
Another thing I have been wondering about HP/L and engine efficiency is why is a supposedly "inefficient" such as the 8.4L Viper V10 able to produce 600 hp, yet still manages better average fuel consumption than a 4.3L F430 V8, which only makes 500 hp? The only thing I can imagine is because the engine isn't at such a high RPM when cruising, but is there more to it than just that?
 
A buddy of mine has got a one litre car with approx 130 bhp.. When you drive something high-strung as that, you understand what the fuzz is about.. :) Put it this way, it's not the number that decides how the car is, but it's the car that suggests what the number means.
 
Venom800tt
Another thing I have been wondering about HP/L and engine efficiency is why is a supposedly "inefficient" such as the 8.4L Viper V10 able to produce 600 hp, yet still manages better average fuel consumption than a 4.3L F430 V8, which only makes 500 hp? The only thing I can imagine is because the engine isn't at such a high RPM when cruising, but is there more to it than just that?
Click to expand...

Longer gears will certainly play a part there, lower cruising rpm.
 
Venom800tt
Another thing I have been wondering about HP/L and engine efficiency is why is a supposedly "inefficient" such as the 8.4L Viper V10 able to produce 600 hp, yet still manages better average fuel consumption than a 4.3L F430 V8, which only makes 500 hp? The only thing I can imagine is because the engine isn't at such a high RPM when cruising, but is there more to it than just that?
Click to expand...

It's all about the ratio of km/h / mph to rpm. Basically, the lower the rpm you can run at a given speed, the better your economy.

Many sports cars are geared short. Extremely short. They're geared so that you hit top speed at redline in top gear. Which gives you cruising rpms that are ridiculously high.

This is because typically high-strung race engines make more power at higher rpms, and allowing them to drop out of those higher rpms makes them feel sluggish... so while a high strung 100 bhp per liter engine can do 100 mph at just 2000 rpms, it won't feel happy doing it... whereas a large, high-displacement engine will have enough torque to make it feel effortless.

If the engines are both tuned to run stoich (an air-fuel ratio of 14.7:1) as most engines are tuned to do, nowadays, in partial-throttle cruising, and they have the exact same horsepower and gearing, the smaller engine will usually return much better fuel economy at sane cruising speeds.

When looking at EPA numbers, people often complain how some small-engined cars don't get economy as good as big engined cars. The trick is, don't look at the highway numbers... look at the city numbers, as they better reflect real-world consumption... no trick of gearing can get past the fact that a bigger mill will drink more in traffic than a smaller one.
 
HP/L is only important to Japanamaniacs who are absolutely desperate to prove that their favorite cars are better than anything else in the world, evar.

And it's important to racers in classes with displacement limits, or people who pay taxes based on displacement. Other than that, nobody truly cares, though as others have said it is a measure of stress and driving profile for the car.
 
Being an owner of a high strung 200hp 2 liter vehicle, I have to say that I've only found one real advantage to a high hp/liter ratio - and that is duality.

It's easy to drive the car slowly and cruise - and it feels totally different than when you drive it aggressively because the engine is operating in a completely different regime. I imagine a turbo provides a similar experience.

A minor advantage of high hp/liter would be weight, but a better measure of that would be hp/weight.
 
Depends on the turbo. With old turbo tunes, that duality could bite you in the behind. One second you're doing a smooth overtake, the next, you're surging into that 18 wheeler the next lane over.

But modern turbo cars... they're so damn boring... no turbo-lag... no nuthin... :lol:
 
As VIPERGTSR01 and others have said, it's a matter of stress and engineering intent, and is not a direct performance figure but a clue to the character of the car. This is perhaps most true with tuned turbocharged engines, where HP/L can be a clear indicator of the size of the turbo(s), especially if you know what HP/L the engine produced stock.

Many people grossly overestimate and underestimate its value, which is annoying.

Also, like Danoff, I appreciate the duality of a high-strung engine. My Ninja is a perfect example -- low-speed, low-RPM cruising is docile, with ample torque to outpace traffic. But at 6000RPM, the power begins to build, and from 8000RPM onward it really gets on the cams.
 

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