Viper Driveline Loss Calculations...

[Sean Roe]

Hi All,

I've been thinking about this and am curious to see what others think. Instead of using a percentage for driveline loss, how about we just use the actual loss? Otherwise, based on the way most people do their calculations, the actual driveline loss in the same car would vary between two different HP engines when it shouldn't.

Example; say we take a Viper that makes 400 RWHP and figure it has a driveline loss that would require adding 15% to that # to equal crank HP, which would equate to 60 HP at 15%. Take the same car, same engine, but now modified to yield 500 RWHP. That same 15% figure becomes 75 HP. How can the driveline drag go from taking 60HP to turn, to now 75HP to turn? It doesn't. Maybe we should just settle on an actual driveline loss figure for a stock tranny, gear set and tires. Just a thought.

 

[Bill 96 GTS]

Bump. I have thought the same thing for a long time. Driveline loss should be constant figure for a given vehicle, not a % of observed HP. OK all you dyno experts, what are your thoughts?

 

[Sean Roe]

Here's my experience:

Caldwell built motor on their engine dyno with exhaust and PCM from the car:
572HP, 600 Ft Lbs. Torque

Put said engine with same exhaust, PCM, fuel and fuel pressure in '97 GTS with stock flywheel, clutch, transmission, gears and tires:
513 RWHP, 539 Ft Lbs Torque

The stock driveline ate up 59 HP and 61 Ft Lbs Torque.

I just don't see where we can always use a "fixed" percentage. The math just doesn't work out.

513 RWHP x 11.5% = 58.99 + = 572 (rounded up by .01)
450 RWHP x 13.1% = 58.95 + = 509 (rounded up by .05)
400 RWHP x 14.75% = 59.0 + = 459

If I used the standard percentage that most use on my old Caldwell engine, my 513 RWHP x 15% = 76.95 + = 590 (rounded up by .05). It might sound nice, but that's not what it made.

How about comparing to Fred Kinder's car? I think his was dynoed on Caldwells' engine dyno and then in his car? How about it Dan or Fred?

 

[treynor]

Consider:

Friction in the drivetrain consumes some of the power between engine and flywheel. Power consumed is proportional to frictional force multiplied by distance. Distance in this case is related to RPM and gear choice, and thus can be set aside. Friction force is the normal force between the two mating surfaces, multiplied by the coeffecient of friction between them. Ignoring second-order effects of changing coeff as a function of force, we thus see that:

Friction loss ~= constant * constant * force level (torque) in drivetrain.

Thus for a given drivetrain we would expect frictional loss to remain a constant percentage of torque regardless of the power level of the engine.

If it helps, consider for a moment the "driveline loss should be a constant figure for a given vehicle" statement. Now ask yourself what's happening at light throttle, say at 70 mph cruise. Is the drivetrain STILL consuming 40 HP? Obviously not.

 

[GTS Dean]

OK Ben, I think I hear your argument that it varies with rpm. However, engine dynos give us corrected FWHP. Chassis dynos give us corrected RWHP by accelerating a known mass over time up to the rev limit. The DynoJet doesn't have to worry about wind resistance or varying road friction on accel/coast-down runs.

It would seem to me that stock drivelines eat up the same AVERAGE amount of HP (or Torque, as you correctly state) regardless of engine power. Isn't the driveline intertia and friction constant? The only *easy* tricks to play with are wheel and tire weights.

Please correct me if I'm off track here.

 

[RoyV101621]

I like the idea but who's gon’na help.
I know of no one who has done an engine dyno and driveline dyno on the SAME car and motor. The Chrysler guys may have but no one's talking.
What's your loss number?

 

[treynor]

Actually, I'm arguing that drivelines consume a constant fraction of the total FORCE being transmitted through them -- force in this case being torque. The logic is that the coefficient of friction between various driveline components (gears, U joints, etc.) remains constant, so the force of friction in the driveline is this aggregate coefficient multiplied by the torque being carried by the driveline.

At low loads (i.e. cruising) the load on the driveline is quite low, and thus the friction force (and attendent power consumption) is also quite low. After all, if the driveline were consuming 50 HP at 70 MPH regardless of acceleration (or lack thereof) we'd get about 3 MPG. At WOT, the driveline load is high, and so is the friction drag. Thus, in general,

Power lost to the driveline will be proportional to:

SPEED OF DRIVELINE ROTATION
multiplied by
FORCE (TQ) BEING TRANSMITTED BY DRIVELINE
multiplied by
AGGREGATE COEFFICIENT OF FRICTION IN DRIVELINE

 

[Tom Welch]

Gentlemen,

Not being an Engineer and having the vocabulary of such, but considering that I deal with acceleration, forces of nature, lift and drag, among other things I think that we are missing the most important point in drivetrain loss. That is the forces acting upon the vehicle as it accelerates.

Chassis dynos are an ok tool for testing modifications. But they are far from accurate in terms of the forces of nature acting upon the chassis during acceleration. In 20+ years of drag racing, my viper was the first car to sit on a chassis dyno!

Drag strip MPH is the true test of drivetrain loss. I have found that a Manual transmission car with average aerodynamics and a solid style rear end with only U-joints in the driveshaft has an average drivetrain loss of around 11-12% Automatic transmission cars of the same tend to loose approx 14%. Add more u-joints and suffer more power loss.

As a vehicle accelerates more power is required to maintain acceleration as the forces of nature exponentionally increase. This is why drivetrain loss remains fairly constant throughout a given powerband(rockets excluded.....LOL)

If you know the weight of your car with the driver and the vehicles 1/4 mile trap speed in MPH you can calculate RWHP and check that against an engine stand dyno pull. If anyone is intrested I will post or email the formulas for weight to power ratios. They are in the back of the Mopar Chassis Tuners Manual. I have found them to be accurate to within 1-2% on every engine/chassis combination that I have checked.

Tom
Http://btrviper.com

 

[treynor]

Standards, standards

The normal calculation is driveline loss from flywheel to driving wheels, which in the case you cite is 15%: 600 BHP * (1-.15) = 510 RWHP

 

[Fred(DrivingSchool)Kinder]

On the Caldwell engine dyno a motor produced 600 hp 600 torque with headers and cats (no cat back). DynoJet(Apex) produced 507 hp 515 torque. this car had a TNT tunable muffler system added to the cat back so the numbers are not exact for drive line loss but close. 18% is the number for this car.

 

[Sean Roe]

So, regarding the conversion of peak rear wheel inertia dyno results to engine load dyno approximations, should we add a percent, or a fixed value to the HP and Torque peaks on an otherwise stock Viper?

 

[Fred(DrivingSchool)Kinder]

15%-15.5% looks like a good (drive line loss) factor range for a motor with 600hp at the crank.
Crank HP - .15% + RWHP

For my example 507 RWHP=600 Crank HP, RWHP + approx plus 18.34% of RWHP = crank hp.

 

[RoyV101621]

And being that all Vipers use the same drivetrain and produce max power at the same approximate RPM that leaves us with a constant +/- 2% right? Lets call that parasitic lose constant the Roe Factor

Crank HP = Rear wheel DynoJet hp + Roe sub hp
Crank TRQ = Rear wheel DynoJet torque + Roe sub trq

Roe sub hp = 59 hp
Roe sub trq = 61 ft lbs

Crank HP = Rear wheel DynoJet hp + 59 hp
Crank TRQ = Rear wheel DynoJet torque + 61 ft lbs