Hi, Thanks. I'm thinking that the best way to compare
one car to another is to compare to-the-wheel power,
not the usually guesstimated at-the-crank power. Let's
say the Enzo has 660 crank HP, and so does some other
car, and everything else is equal except the Enzo has a
more efficient transmission than the other car. The Enzo
will win any real power contest so the at-the-crank
comparison is misleading.
You can always compare cars by what power they actually
deliver to the wheels (not forgetting to consider their
weights, weight and power distribution etc). I think the
only value there is to at-the-crank measurements are if
you are considering/measuring differences/changes to the
transmission itself. For instance, bolting an engine to a TIP
tranny to test, then bolting it to a manual. Also, a typical
gear box is not equally efficient in each gear at transmitting
power. Usually a box is designed to be (or is inevitably) most
efficient at one of the upper gears. This is maybe the gear
that the care will be driven in most of the time and/or where
the car will be measured for gas mileage etc. So let's say you
get 500 at-the-wheel HP in 4th. You might get 490 HP in all
the other gears. That is more complex (but real) than any
guesstimated crank number, and the real number(s) will help
explain real world results at the strip etc. If your lower gears
are much less efficient than 4th gear, then you might be
scratching your head about why your quarter-mile times
were lower than your dyno curve would seem to predict.
Joe

 

This is what a standard turbo dyno looks like

 

I guess that puts me at 854hp ....

 

Hi Stephen. I have always wondered if the rollback
measurement of drivetrain losses + measured at-the-wheel
power, has ever been compared to a real at-the-crank dyno
room measurement to see how close the real running loss is
to the spin-down measurement. The reason I ask is because
some of the engine power drain is due to the compression of
turbocharged cylinder charges, and the piston sidewall friction
during the combustion downstroke. During a spin-down measurement the motor is off-the-gas (maybe it should even
be turned off?), so the pistoms are not having to compress as
much of a cylinder charge as when they are at max HP, and
the sidewall friction is also lower, so I'm thinking that the spin-
down resistance is less than the real loss when the motor is at
work.
On the other hand (I'm thinking while typing...) if we want
the *drivetrain* loss (everything beyond the crank), we would
want the spin-down to have as little resistance as possible
from the motor itself. Ideally we'd want the dyno momentum to
turn everything from the wheels to the crankshaft, but nothing
more. Ideally the motor should be severed from the crankshaft!
The internal motor friction should be taken out of the equation
because it has it's effect before the power is delivered to the
crank anyway. Maybe that's why it's good to do the spin-down
with the car still getting enough gas to idle, so it adds enough
power to mostly overcome the internal engine friction. I guess
the most accurate spin-down test would be to first map how
much throttle is needed to keep the motor spinning in neutral
at every RPM in the dyno test range, and then have that amount
of gas delivered during the spin-down so at no place was the
motor itself slowing the drum down.
Joe

 

I agree at the wheel horsepower is the only thing that really matters but how do you compare 586 at all four wheels to someone who has 640 rwhp?

 

640 > 586

If you stick with the practical, you would then also compare
the weights of the cars (lighter is better), and you would
consider what you want to do with the power. In a road-race
test, more power is simply better. In a drag race, up to a
point, AWD can have an advantage over RWD etc. In low-traction
conditions, AWD is better. The question is really "How do you
compare different cars?". It depends on your criteria. My
daughter would have to know the colors.
Joe

 

run em on the street. That will tell you who's car is quicker.