P/W ratio - crank or wheels?

[kstar]

edit: Oooops! - this post was meant to be a part of the 1/4 mile/performance thread!

I have a question which I think is germane to this discussion:

Is power-to-weight ratio based on crank horsepower vs. weight or rear wheel horsepower vs. weight.

If I recall corectly, it seems that most numbers I have seen rely on crank horsepower.

So, let's say that I have a motor that makes 300hp at the crank and the car weighs 2400 pounds - is P/W ratio based on 300/2400 or should I discount 12 to 15% of the crank horsepower to get to a rear wheel horsepower . . . than introduce weight into the equation?

For instance, if I enter 300hp (crank) and a 2400 weight into the following "calculator" I get a 1/4 mile time of 11.65 seconds at 117 mph. Should I be entering the rear wheel hp?

http://www.geocities.com/MotorCity/4601/hpcalc.html

Thanks and best to all,

Kurt

[kstar]

"Is power-to-weight ratio based on crank horsepower vs. weight or rear wheel horsepower vs. weight."

Was this question so silly as to not get a response?

Surely someone here knows the answer otherwise . . .

Regards,

Kurt

[Lucky]

Like you said, most published calculations of lb/hp use crank hp.

The calculator you linked to uses rear wheel horsepower. The last of the calculators on that page will take vehicle weight and 1/4 mile time and calculate both rear wheel and crank hp -- however, it assumes 25-30% of crank hp is lost between the flywheel and the wheels. On 911's that is way off... most measurements I've seen are about 15%.

[CamB]

The most appropriate measure of power to weight seems to me to be RWHP/weight. Otherwise there is no way of comparing between cars.

For instance, my 69T has about 11% transmission loss as measured recently by a Bosch dyno (measures loss on deceleration).

Richard Nelson's 915 box had about 14% on what sounds like a similar dyno (albeit 10,000km away from me in the UK).

And that is for a gearbox attached to the diff - no propshaft. So the rule of thumb (20-30% mentioned) will only apply to rule of thumb cars - not 911s.

So... my theory (I don't know any of this! - all guessing) is that 200 Porsche horsepower is better used (less wasted) than, say, Corvette hp. So the valid comparison is rwhp/weight. In my case, an uncontrollable 108hp.

I figure that gives me just 20-21lb/rwhp. Wow

[Jack Olsen]

Generally, it's done with crank figures. But it's the sort of thing that should always be taken with a grain of salt. Different dynamometers, under different circumstances, will produce different results. And guys apply widely varying standards to the method for determining what their cars weigh, too (how much fuel, with or without driver). Published stock weights aren't reliable, and even 'ceritified' public scales often have trouble with sub-2500 pound weight measurements, in my experience.

[widebody911]

I have a table, using FWHP, at http://www.vintagebus.com/3.6/perf.html