Tranmission correction factor for dyno readings
 

As I am sure almost everyone reading this knows, chassis dynos measure rear wheel horsepower (RWHP) rather than flywheel horsepower (FHP). RWHP is lower due to the frictional losses getting from the flywheel to the wheels. FHP can be estimated from RWHP by assuming what this transmission loss is.

As FHP is a bigger number, it is the one everyone likes to quote. Unfortunately, from what I have read this causes certain tuners throughout the world to use "optimistic" assumptions of transmission losses.

For example, if a car makes 200 RWHP you could assume anywhere from 5% transmission loss to 30%. This would give a FHP estimate range of 211-286 - a 75hp difference!

So, I would like to initiate a bit of a discussion on what people have seen as an appropriate transmission loss (or correction factor) for 911s (or 930s).

My thoughts are:

- it might be different between dynos (my dyno tuning was done on a Dynapack which attaches directly to the hubs (wheels removed)) and this may cause a difference

- it might (should) be different between transmissions (ie a 901 transmission is lower than a 915 which might be lower than a G50 or a 930 fits)

I would also expect that a transaxle transmission would have much lower losses than a front engine - prop shaft - rear diff drivetrain.

Finally (long post, huh), the dyno operator I used sticks to 1.1x (or 9.1% losses) for a 911. Bearing in mind this is on a hub dyno (which may measure lower losses than one which has wheels and tyres in the measuring path), this is still way down at the conservative end of the calculations I have seen. Gearbox is a 915 from an SC.

I have also seen 11-12% losses on the same car on the same shops other Dyno (a Bosch rolling road). It estimated the losses using coast-down. This was with the old 901 gearbox.

Useful (but not working today) links are at:

http://www.pumaracing.co.uk/power3.htm
http://www.pumaracing.co.uk/trans.htm
http://www.pumaracing.co.uk/coastdwn.htm

I would certainly agree that the 911 gearboxs should have less loss that a FERD vehicle but should be the same or close to FWD vehicles. Chassis dynos using the tires also need to account for wheel slippage which is not the case with hub dynos. In the end only true comparisions can be made using the same test equipment and conditions. Too many factors can through off a chassis dyno such that they should never be compared, I feel the only true measure of HP is an engine dyno with controlled conditions and standard test methods. It is not uncommon for tuners to brag about RWHP only to have seemingly lessor powered vehicles outperform them, this is due to the way in which the graph is read. In real world applications it is not uncommon for a lesser-powered vehilce to out perform a higher powered vehicle if those figures are derived from peak HP, the more important measurement is the area under the HP curve and in this case a car with a lower peak but flatter curve makes more usable HP than one with a sharp but higher peak. The only exception to this would be if the car was geared to keep the engine very close to peak HP but that's really only possible on true race vehicles with different gears for different tracks. A dual purpose (street/track) vehicle or one with only one gearset to choose from would have less peak torque and HP but a much flatter curve for both such that it has more available power outside of the peak area on the race only motor. To answer your question go with the conservative number, 1.1x seems about perfect to me.

Ok Great,so who can do the Math for me?according to the dyno sheet it read 505.8rwhp so what is my total hp? I been told that you have to add a 15 % loss?is it 15%,12%,9%
What is it???:confused:

Why not just use the RWHP numbers and be done with it?

It's the only number that really matters anyway. FWHP is really just an acedemic excercise, the number is useful mostly for chest beating and boasts of manliness.

When it comes to racing, drivetrain and frictional losses count. Transaxles and gearing are just as important as the engine.

The only usable HP is that which makes it to the tires, "when the rubber meets the road".

Haha - yes - why not just use RWHP and be done with it.

That would be nice, but I just get wary when I see big numbers bandied around. Also most people don't understand RWHP.

Juan. 15% losses implies the RWHP is 85% of the FWHP, which is the same as RWHP / 0.85.

In your case, 505/.85 would be 594hp.

If you used the conservative figure I have been told (9.1%) then you would have 505 / .909 = 555hp.

Call it 550 - 600hp at the flywheel. Geez, imagine it not really mattering if you actually have that last 50 hp.

The interesting thing to me is that a 911 with 250hp should be quicker than a FERD with the same hp/torque.

Interesting...