Quote:
Originally Posted by kuhlka
Don't you mean torque + redline RPM? An engine with 10 ft lb could theoretically reach thousands of horsepower if it had a 100,000 RPM redline.
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Not sure what he means but it is important to be definite and throw some exact numbers in. How he explains the specific torque output of, say a Porsche 911 at around 86 lb ft per litre is an interesting speculation.
It is odd that this misconception about the importance of power to weight is so widespread.
Torque is not a function of displacement other than bigger engines generally produce more torque but it is almost always non-linear and entirely unpredictable except that it is more difficult to extract the same specific torque from a larger engine. The reason for this is the fact that gasoline and air combust at a fairly fixed rate. The smaller the combustion chamber the higher compression and more advanced timing will be achieved before detonation sets in. It is, frankly laughable to imply that a F1 engine develops the same specific torque as a BRZ.
So, specific torque is dependent primarily upon combustion efficiency: which in turn is dependent upon volumetric efficiency and resistance to detonation. Indeed, the only reason to bench test any given engine on a dyno is to measure specific torque. If all you want is more torque (or power if you prefer) regardless of efficiency then of course just fit a bigger engine. That's one of many bone headed ideas that put the American car industry behind the 8 ball. You will note that specific torque and power declined precipitously in particularly American engines during the implementation of emissions regulation. Specific torque improvements have been driven entirely by fuel economy considerations or limited displacement racing formulae.
What's missing from the peak power to weight ratio is the curve. Only torque can explain that. Two cars of the same weight and same power can have radically different 1/4 mile times depending upon specific torque, whether the gearing is optimized for acceleration rather than fuel economy (the two transmissions in the BRZ rather neatly prove that point) and most importantly where in the rpm range that torque is delivered. This works for all forms of intake supply and varies according to the type of fuel used, I.e. flame front speed and octane rating.
If power to weight mattered American muscle cars should have been bog slow.
I concede I have never seen a direct to engine dynamometer in operation but my understanding is that it measures the force required to brake the engine to a constant speed, revealing the net useable torque generated by the engine at that speed. This force is plotted for the useful rpm range of the engine and bhp derived from that. If instead it measures the power consumed by the brake in order to maintain the engine at the desired rpm then that is a distinction without a difference, the object of the exercise is to derive the available force to accelerate anything attached to the engine. Bhp and torque at any given rpm are related by a constant for the simple reason that Watt wanted a constant unit of measurement.