As I've always stated the dyno is simply a tuning tool that is you will perform a pre tune power run then a post tune power and the dyno will simply measure the increase in power from your selected modifications (this is only provided we do both power runs on the exact same dyno)

Now this is were it gets tricky most people will try to convert the rwhp to crankshaft power with the simple belief that there is a mere 20% loss in the power due to the drivetrain (I do not believe it is this simple as different dyno's will yield different results the variables are to high)

Now back to the subject are we happy saying that a xk that measures 250whp on the dyno drops 50hp through the driveline and a xkr that measures 500whp is expected to drop double being 100hp ?

Wouldn't it be safe to say due to the same components in the driveline the xkr should only lose 50hp why is there this belief that the more power a vehicle makes the more horsepower it loses through the drivetrain

This percentage formula is flawed In so many ways the only way to measure crankshaft power to put the motor on an actual engine dyno

 

Very interesting posit on the percentage loss theory vs. specific loss theory; and I hope someone here with extensive dyno experience will respond and enlighten us. Clearly a before and after run is the only way to measure the result of any tuning effort.


Why do people feel the need to convert rwhp to crank hp? When the baseline and post tune measurements are done the same way; the results should clearly measure the benefit of the work done, or lack thereof. If the dyno shows I got value for my money isn't it time to go smoke some rear tire and enjoy?

 

Because the latter number is always bigger than the former and big numbers are what people like to see.

 

The "mine is bigger than yours" approach?

 

You are absolutely right. If the drive train loses 50 hp then that will be the same loss whether it is a 200 hp, 500 hp or 1000 hp engine. Here is a link to an interested article explaining in detail drive train losses and why they cant be compared to crankshaft hp. Tech Page

 

It is funny when people start comparing wiener sizes. Of course the RWHP to Crank HP depends on a LOT of variables, down to tire pressure and even tread pattern. Manual transmissions ALWAYS have more parasitic loss, even with lockup torque converters. Some vehicles may have a maladjusted throttle sensor which never fully opens the plates. Temperature. Barometric pressure. Altitude (which involves Barometric Pressure), Humidity.
Heck, even the time of day and the Coriolis Effect can affect results.
One vehicle's RWHP results can only be compared the the same vehicle's RWHP results on the same dyno.
As stated above, parasitic loss "Should" always be the same power number and not a percentage, theoretically.

 

I'm thinking a few before and after 1/4 mile times might be more enlightening. Same driver/track; hopefully similar weather conditions.

 

Rob
I'm a big fan of testing mods out on the 1/4 ...lol I guess any excuse to 'legally' smoke up the treat !

 

You've hit the nail on the head , well said Cee Jay

 

So let me interject with an off the wall question: Has Jaguar ever released RWHP as a specification?

 

No manufacturer to my knowledge have ever released power figures from the wheels it's always power at the motor