Measuring HP

[Eric_R_Kissell at email.whirlpool.com]

The thing to remember is that a dyno is best used for measuring relative
improvements resulting from changes made to a given car. Rather than
putting on the K&N air filter that lets all of the dirt through and
claiming it was good for 10 hp, do an experiment using dyno testing to show
whether the K&N filter makes a difference.

The problem enters when we try to take road dyno numbers and use them for
bragging and comparing different cars in terms of engine hp. This is what
the tuners use to sell product. If the cars we try to compare are similar,
then we might assume that a change made to car A resulting in a given
relative improvement will yield a similar improvement for car B.

If you really want to try to go beyond understanding relative improvements
in road hp and torque in order to brag about engine hp and torque, at least
remember that it is incorrect to apply the same drivetrain loss correction
to your measurements before and after the change as a percentage. Think
about it. [Warning: oversimplification follows] If you have a 100 hp engine
with a 10% drivetrain loss, you would see 90 hp at the wheels, which means
you lost 10 hp. If you are a really good tuner and you find a way to get
500 hp out of that same engine in the same car with the same drivetrain, do
you still use a 10% correction for your drivetrain losses? This would mean
that the drivetrain that cost you 10 hp before engine modifications now
cost you 50 hp. You did not modify the drivetrain, so why did the losses
increase? The losses did not increase, you just need to remember that
drivetrain correction as a percentage is not a very useful thing to SWAG
the same way at every road dyno measurement you see.

And yes, I know that if you use an engine putting out 500 hp with a
drivetrain designed for 100 hp you will soon destroy the drivetrain. In
this sense, you drivetrain losses do increase, because you will lose parts
of your drivetrain due to mechanical failure :-)

Does anyone else dream about the fun you could have if you only had your
own dyno? Sure 20-40 or 30-50 mph acceleration times should be almost as
good, but man, those dynos are sexy. It makes me want to go back to college
where we had a dyno with which to experiment.

Food for thought,
Eric Kissell