Quote:
Originally Posted by Wilson
HP per litre is the only way to measure one engines greater output over another, that is engineering 101.
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"The only way"? The above
isn't engineering 101, at best it's flawed middle school arithmetic. Engineering
is comparing things to figure out which is better, and hp per liter is one of the simplest,
least useful metrics.
There are advantages to "hp per liter" as a comparison tool. It's easy, because the data is readily available. It's also important in many race series (F1, etc), which limit displacement to equalize the field. In a street car, however, we basically care less, because there
are no displacement rules.
What
do we care about in an engine? Power output, obviously, because we want our car to achieve a certain performance. What next? Weight, for a sports car. The engine is the single heaviest thing in the car, so it dramatically effects the weight, balance, polar moment and hence handling of the car. An engine might make 200 hp per liter, but so what if it's
heavy?
Imagine you're designing an Arial Atom, you'd like to put over 500 hp into it, and you've got a couple engines to chose from:
Nissan GT-R VR38 3.8L TT V6, 600 hp (Nismo)
Ariel Atom/ Hartley 3.0L V8, 500 hp
On paper both of these engines look comparable, 156 hp/ liter for the Nissan vs 167 hp/ liter for the Ariel seems pretty close. Until you consider: the Nissan engine weighs 608 lbs. The Atom engine weighs 198 lbs.
The production V8 Atom weighed 1213 lbs total. If they had used the GT-R engine it would have weighed
410 lbs more. Even with the 100 extra hp of the Nissan the
car would have had a significantly worse power to weight ratio, and with an extra 410 lbs hanging behind the back axle handling would have been
terrible.
This is an obviously extreme example to illustrate the point: HP per liter is virtually useless, either to a car designer or us as drivers. Hp per
pound is far more important. And you
can use it to compare engines of different displacements and cylinder counts, ie the 414 hp V8 from the M3 at 444 lbs vs the 335 hp, 429 lbs I6 from the 1M. It's not the only metric that's important, obviously, but its
far better than HP per liter.
And on this far more important metric, high end normally aspirated motors trump turbos. At least in street cars. Follow?