Is the drivetrain loss a fix # or is it always a % of the total power?

Thanks
Jeremy

 

This comes up regularly.

Here's part of a thread from Feb:

Some of the losses are proportional to rpm and some are proportional to power transmitted, and some are proportional to acceleration of the components.

Rpm: bearing friction is the best example, but it's only a small % of the loss. Windage, or flailing around of the oil in the trans and rear end should also be rpm dependant. Tire deformation at the road/dyno roll is mostly this, but maybe some power related. So would higher tire pressures give higher RWHP? Probably.

power: gear friction. Hypoid bevel gears as found in the RWD rear end have lots of sliding friction between the meshing teeth. The more power they transmit, the more fricton and heat they generate. Winston Cup cars have rear end coolers just for this reason. RWHP increases can often be measured when changing to synthetic rear end lube. 9 in. Ford with it's high amount of offset of pinion and ring gear axes (compared to 10 or 12 bolt GM) is a big offender, but it's also very strong. Trans gears have the same effect, but to a lesser extent. Running in 1:1 ratio (direct) or 4 th in a M6 is probably least loss.

acceleration: every component with mass (which means most of them) have inertia which resists being accelerated. The faster the acceleration (on the road or dyno) the more power is consumed accelerating components. Because inertia is rpm related, those things turning the fastest have the greatest effect.
Also, inertial is higher when the mass is farther from center.

Lightening a flywheel is at about 3 times as effective as lightening a driveshaft the same amount when running in direct. If you are in the lower gears, the lighter flyweel is many times more effective.

If you are going to lighten things to get more RWHP, the biggest offenders are engine speed related items, especially flywheels. Wheels and tires are big inertial hogs, but they are only turning a fraction of engine rpm.

Factory power and torque ratings are taken at steady state or on a "step" dyno test, where inertia doesn't matter, so acceleration power numbers on a chassis dyno will always be less. Fortunately aftermarket engine dyno tests are most often run ar some acceleration like 300 rpm/sec. It's a lot easier on the engine also.

Most (but not all) chassis dynos can only measure power/torque when the vehicle is accelerating. They are "Inertial Dynos" because what you are doing is accelerating a large drum with a known inertia. A vehicle putting down 450 RWHP accelerates the dyno drum much faster than a ricer putting down 100 (FWHP?).
Most are rated at 1000 hp. I've never seen a 1000 RWHP run but the test wouldn't last very long!

I won't even get into torque converters. If you had a lockup converter that would take all your engine's power, you'd get higher RWHP numbers running the test with it locked and in 3rd gear[1:1] .

Here's the whole thread:
http://web.camaross.com/forums/showt...threadid=81279

My $.02

 

I have...... it didn't!!

That was a dyno pull on a 1st Gen Ford Lightnin' with a twin-turbo 351W...... and an honest street car...... errrrr... street "truck".

I attempted to use my comparable engine and chassis dyno pulls to break down the "fixed" and "variable" components of drive train loss. The "fixed" - primarily inertial in my mind - were only in the magnitude of 16-18HP. The "variable" - primarily friction on the manual tranny data - was the major part of the losses.

 

Yeah, I figured I'd ask since I didnt think it would have been something asked before and so therefor didnt do a seach, sorry. Though thanks for the explain OldSStroker. I asked bcuz someone from ARE over on LS1tech made the statement that the 6sp losses about 45-55hp and then there was a little discussion about if it was a % or a fix #. I thought I ask here since this is the only place anyone seems to know the details about how anything works.