15% dyno loss...WHY is it a constant 15% over hp ranges?
 

250hp from a 911 comes from 212.5 on a chassis dyno
Yet 500hp (911) comes from 425 on the dyno.

Is there not a point where the losses remain constant - NOT increasing as hp increases? Why/why not?

Craig
RS....Varioram a comin'

It's a bad system, and it's obviously inaccurate. But it's the only one we've got that is simple to apply across the board.

RWHP is a much more meaningful number, and RW torque curves are probably even a better way to evaluate what an engine is going to feel like. But one dyno is rarely going to compare equally to another, in the real world. So all the numbers have to be taken with a grain of salt.

If you can come up with accurate transmission losses for one car, great. But to make meaningful comparisons, you're going to need everyone else to do exactly the same thing, with the same degree of accuracy, which isn't gonna happen. 15% is a crude tool, but it's simple enough for people to use uniformly.

Craig,

In your example, the 250 hp engine loses 37.5 hp going through the drive line. The 500 hp engine loses 75 hp going through the same drive line. Doesn't make any sense to me either. It seems that drive train loss should be a fixed hp number (+ or - a few %).

Like JO says...rwhp and torque is useful info...crank hp is really only good for bragging rights.

There are a zillion products for boosting HP and lowering weight. Has anyone done any R&D to reduce power loss from the drive line? I would think the boys in Stuttgart would be all over this.

Run less tranny fluid. Reduce unsprung weight.

Overall, I have a hard time believing that a theoretical Porsche with 500 HP would have a loss of 75hp, while one with 100 hp would have only 15 hp. Just does not make sense in my engineering mind......given the probable relative constant of driveline losses.

If one put 2 engines on a engine dyno and then installed and ran them on a chassis dyno.....then we'd find out. $$$.

Perhaps weee'lllll never knowww....

Craig
RS

On a steady state basis, drive train losses are due to windage and pumping losses, rolling/sliding element friction and internal material losses when parts are elastically loaded and unloaded. Transient (accelerating) performance is also affected by the inertia of the rotating parts which is a product of weight and geometry. There is only so much one can do with these losses; friction and pumping losses can be reduced with better and less lubricant. Windage is a tough one as one gets into aerodynamic shaping which competes with the ideal structural and economic (to reduce manufacturing costs) shape of drive train parts. Rolling and sliding friction can also be reduced with better manufacturing and more rigid parts but the need to turn the power 90 degrees in the differential requires sliding friction in the ring and pinion gear mesh and this is a persistent loss. Rotating inertia can be reduced by making parts of lighter material and removing material where is is relatively unstressed (like the centers of axles). Internal material losses can only be addressed by changing materials. Some improvement in these losses can be achieved but most of the required changes would be costly. Jim

Like "Rainman" (*) Jack Olsen says, run your tranny 10-20mm low on oil for a measurable RWHP gain. Or so they say.

(*) This question was my first post ever on Pelican. :cool:

yea but..... he makes a good point, and I've never gotten a good answer. If my 911 looses 20 Hp with my stock 2.4E motor, why would it loose 75 hp when I pull the 2.4 and drop in a turbo 3.6 with 500 Hp output??? Same drivetrain. (Which wouldn't last long!)

I don't have much to contribute here except that in physics, the coefficient of friction is a fixed factor, and friction is determined by (force) x (coefficient of friction). It would therefore stand that as hp increases, and if the coefficient of friction remains constant, in this case 15%, then the hp loss increases proportionally.

Frictional losses are a direct ratio of the applied force (say 3% or 3 lbs per 100 lbs contact force) on the surfaces in contact (like gear teeth). More horsepower means greater forces; gear geometry hasn't changed in the example case (of 2.4 to 3.6) so the only way torque (force times distance) can increase is to increase the tooth loads (force). More force means more frictional losses. Also, I do believe windage losses go up something like the cube of the rpm. Cheers, Jim

In a steady state situation where an unvarying amount of power is being transmitted the loss will be equal. i.e. at a steady 60mph a 150 and a 300hp engine w/ the same gearing and aero packages will have the same absolute driveline loss.

Dyno runs are not steady state, there are accelerations involved which cause the absolute power dissipated uselessly to increase non linearly

"at a steady 60mph a 150 and a 300hp engine w/ the same gearing and aero packages will have the same absolute driveline loss."

Yes, but this is because both engines are sending the same power through the drive train (the power required to produce 60 mph on the road or on the dynometer - say 50 horsepower); as soon as the load requires more horsepower the engine outputs will have to go up as will the losses. Jim

I would think the 911 with a rear engine, CV joints, and no drive drain would be better then a front engined car.

Isn't that what I said?

It all depends upon drivetrain RPM too. Losses will be higher when the drivetrain is spinning faster (churning more fluid, etc.). It's a very complicated model.

-Wayne