vader86

Not really :D

65Z01

Sounds like a great project for CF!

You can get a good approximation of CHP from trap speed and launch weight.

Last year I had chassis dyno pulls performed right after my last pass down the 1/4mi, calculated CHP and compared with RWHP. Indeed the drive train loss came to about 17% with my stock A4 and gears.

VMAX451

I was trying to stay out of this, but what heck...
The team of engineers would be upset. You are giving them two data points for each case and expect them to come up with a relationship of unknown order. :nono: Gonna need more funding :jester .

GusBustamanteJr

What was the formula for CHP from trap speed and launch weight?

CorvetteZ51Racer

This relationship is even more flawed than trying to calculate crank horsepower from rear wheel. Rear wheel is what gets you down the track to that 1/4 time, but now you've introduced even more variables into the equation than originally. How sticky was the track? How much weight transfer do you get to your rear tires, how wide are the rear tires, what's the effective frictional coefficient of the tires, how fast can the driver shift (if it's a stick), how quickly can the auto shift, is the track truly on a level grade, was there any wind, etc, etc, etc.

Long and short, IF YOU WANT TO ACCURATELY KNOW YOUR CRANK HORSEPOWER, YANK THE MOTOR AND PUT IT ON A WATERBRAKE!!!

Oh, and FWIW, as someone stated earlier, you need a LOT more than 2 data points per transmission to set up a relationship. 2 data points only allows you to fit a linear relationship, which we've already proven doesn't work. Quite honestly, you're looking at taking at least 10 data points per transmission and trying to fit a regression analysis if you want to find a relationship, but with the number of variables involved, you'll likely need a lot more data points than that. In other words, Vette Magazine *wishes* they had the money to properly determine this. I wouldn't be surprised if GM design engineers had the data to determine the losses due to various transmissions and differentials (assuming like-new condition, and the auto trans relationship still wouldn't be worth much), but as far as taking chassis dyno numbers and converting, as Michael said, there's too many variables in the dyno itself because it's an inertial dyno.

For those who aren't aware, most engine dyno testing is done steady state, while all chassis dyno stuff is inertial. When you do steady state dyno tests, you do not see the effects of light weight flywheels, light pistons, crank or rods, etc, etc. You see how much power the motor makes. Period. Chassis dyno numbers reflect ALL of the above variables plus countless others.

No Go

One minor point CorvetteZ51Racer...

Not all chassis dynos are inertia. I use a Mustang 1750 dyno that was run in strictly steady accel rate (300 rpm/sec). It can be made to run inertia or anywhere in between.

I prefer it over the Dyno Jet if for no other reason that it is the only one available within 200 miles...:)

vader86

hp = weight * (speed / 234)^3

hp = weight / (ET / 5.825)^3


[Modified by vader86, 4:34 PM 2/12/2003]

65Z01

It's on my site along with some equations to normalize track results for temp & baro.

LT4BUD

Well the difference between RWHP and motor HP is the HP used in moving the drive train.

So for a given drive train the losses which are a function of the weight of the system would be fixed and independent of the motor HP.

Those losses which are a function of things like friction in the gears will be a function of horsepower, that is when you are transmitting more HP through gear faces the friction will go up as a function of the pressure on the gears. So trying to put more HP through a given drive train will result in higher losses in the drive train.

Some losses are a function of the speed at which the drive train is turning. For example windage losses in the wheels, U-joints, even around the gear teeth in the dif & transmission are a function of speed. So these losses are not constant nor are they a function of engine HP.

While it is customary to dyno with a vehicle in 4th gear, RWHP would be different if you tested in other gears. Should be higher in 1st gear and lower in 6th gear. Why??? Because in 1st gear the drive train is moving slower & in 6th gear it is moving faster. Even changing from 3.45 to 4.10 gears should increase RWHP since the tires and half shafts will be moving slower. Remember we relate HP to motor RPM not Vehicle MPH. Actually probably not this simple because in 1st gear you would be putting more pressure on the differential gears.

Bottom line is RWHP is NOT a percentage of motor HP. Further it is NOT constant, that is it does increase with increasing motor HP.

Only way to do it accurately is dyno testing engine and then engine in vehicle and then comparing!!! I quess experience has shown however that typical results on street cars is around 15% loss.

The relationship between motor HP and RWHP does give some clues how to increase cars performance. Lighter driveshaft, wheels, tires, etc. Minimize friction by the best lubricants. Reduce windage losses both on external items and internal items.

Gee, there just isn't a simple answer to question!!!


[Modified by LT4BUD, 8:22 AM 2/13/2003]

swoosh

HOLD ON GUYS. ONE THESE DAYS I'LL FIGURE OUT THE ANSWER AND LET YOU KNOW, UNTIL THE :banghead:

Kevin93

I'm no physics whiz, but I would think that drivetrain loss would primarily be due to friction... in which case an even coefficient of friction would dictate a percentage loss. I'm sure that there are many people on the forum who know a hell of a lot more than I do though.

CorvetteZ51Racer

There are significant losses due to friction, yes. However, the coefficient of friction changes with temperature (which affects the viscosity of the lubricant, the size of the oil wedge between the part and the bearing). Temperature can be changed by horsepower introduced into the system, as well as speed. As LT4BUD also mentioned, there are windage losses as the U-joints on the driveshaft and half shafts turn through the air, as the gears in the transmission and differential slosh through the oil, etc, etc. The density of the air (which is a function of altitude, humidity, barometric pressure, temperature, etc, etc) affects the windage on the external drivetrain components, pinion depth can affect the losses in the differential, as can the weight of the oil, etc, etc. For inertial dynos, the weights of the flywheel, clutch assembly, gears, driveshaft and half shafts, wheels, tires, brake rotors, wheel studs, lug nuts, etc, etc ALL affect the parasitic losses in the drivetrain. Obviously things like the lug nuts have a negligeable effect on the loss, but it's just a hyperbole to illustrate the number of factors we're dealing with here.

GusBustamanteJr

^^^^^^^^^^^^

I was gonna say..... what he said^^^^^^^^!!!!