Originally Posted by Znod' post='274952

I have found a calculator that yields aerodynamic HP--the HP loss from aerodynamic factors at a given MPH. My maximum HP tends to be produced at about 6k RPM--which corresponds to 89.46 MPH. Given these values, plus a CD of .29 and a frontal area of about 22 sq. ft., etc., my aerodynamic HP is 32 HP. This value is .09846 or stated HP (32 / 325)--which is surprisingly close to the .10078 value caclulated above.

So, assuming that a 15% drive-train loss is reasonably accurate, here is a summary:

.15000 = Drive-train HP-loss percentage
.09646 = Aerodynamic-HP percentage
.01384 = HP-loss percentage from weather and altitude
.26030 = Calculated average percentage max HP loss

.26462 = Average percentage max HP loss as indicated by Pro RR--which is surprisingly close the above value)

From an overall perspective, my calculations provide independent confirmation of the RWHP values produced by my Pro RR assuming that 15% is a good drive-line loss value for our cars. They also suggest that my car-weight value of 4150 is reasonably accurate under this assumption.

Here is what the calculator for aerodynamic HP looks like.

Coefficient of drag: .29
Frontal Area (Square Feet): 22
Test Temperature in Degrees F: 55
Test Barometric Pressure in Inches Hg: 30.185
Vehicle Miles Per Hour (MPH): 89.46
Vehicle Weight in Lbs: 4150
Tire Inflation Pressure in psi:
Vehicle Coefficient of Frontal Lift: .075

Input Parameters Are the Following:
? Coefficient of drag = 0.3
? Frontal Area = 22.00 sq feet
? Test Temperature = 55.00 degrees F
? Test Barometer = 30.18 inches Hg
? Vehiche MPH = 89
Computation Results:
Air Density Computed is 0.00241
Aerodynamic "Drag Factor" is 0.01656
Rolling "Drag Factor" is 21.33690
? Computed Aerodynamic Horsepower Required is 32
? Computed Rolling Horsepower Required is 21
? Computed Frontal Lift Force is 34 Lbs.

Originally Posted by Znod' post='276205' date='Apr 30 2006, 04:34 PM

I have refined the above calculation of .01384 in several ways, although the refinements change my subsequent calculations very little. As I now understand things, the above corrrection factor is off small amounts because of (a) a minor inaccuracy in the calculator I was using and (b) an additional nuance in caculating the SAE dyno correction factor. First, calculating the correction factor using its actual formula gives 1.0166. Second, as I read several sources, this factor is not applied to the dyno-determined RWHP to obtain the final SAE dyno correction. Instead, it is applied to .85(FWHP)--which on my car is .85(325) = 276.25. So, the actual HP correction is .0166(276.25) = 4.5858--from both altitude and weather. Dividing this value by 239 gives .01919. So, based on RWHP, the correction factor actually is 1.01919. But, more importantly for my purposes, .01384 becomes .01411, a very small difference. Consistently, as implied above, the FWHP loss is 4.5858 = .01919(325).

Earlier, I generated confusion by relating a similar loss to my "documented 10 [FW]HP decrease" from weather alone, etc. I have deleted the confusing material both above and below. I am rethinking the whole issue.

And, actually, given the facts I am working with, the weather effect would have added FWHP minimally as evidenced by density altitude = 1,532.26' < actual altitude = 1600'. So, for all practical purposes the average FWHP loss I am dealing with is solely attributable to altitude. One lingering factor is that I have no clue about the altitude, etc., BMW uses in rating the 545i at 325 FWHP. But, pressing on below .....
Given the small change from .01384 to .01411 calculated above, I am closer to reconciling all the "facts." Repeating from above, here is where I stand.

So, assuming that a 15% drive-train loss is reasonably accurate, here is a [new] summary:

.15000 = Average Drive-Train HP-loss Percentage
.09646 = Average Aerodynamic-Drag HP-Loss Percentage
.01411 = Average HP-Loss Percentage from Altitude (almost exclusively) and Weather Effects
.26057 = Calculated Average percentage Max FWHP loss

.26462 = Average Percentage Max HP Loss as indicated by Pro RR--which remains surprisingly close the above value

Originally Posted by Znod' post='274952' date='Apr 27 2006, 03:23 PM

Another factor that could be a problem in my calculations is vehicle weight. I have made several attempts to estimate this value. My best estimate is 4150 lbs--which is one of the settings my Pro RR relies on in calculatng all-loss RWHP.

Originally Posted by E60VTA' post='279241' date='May 8 2006, 12:15 AM

Can you have your vehicle weighed on a truck scale rather than estimating its weight?

Originally Posted by Znod' post='279405' date='May 8 2006, 07:33 AM

Yes, but the nearest one is far far away and in a place I don't want to go.

Originally Posted by E60VTA' post='280219' date='May 10 2006, 12:13 AM

Here is a link that you may be aware of already to locate truck scales. Truck Scale Locator

Originally Posted by E60VTA' post='279241' date='May 7 2006, 11:15 PM

Can you have your vehicle weighed on a truck scale rather than estimating its weight?

Originally Posted by E60VTA' post='280219' date='May 9 2006, 11:13 PM

Here is a link that you may be aware of already to locate truck scales. Truck Scale Locator

Originally Posted by wolverine' post='280393' date='May 10 2006, 09:28 AM

You can go to any moving company, and they'll have a scale. I'll bet there are a few not too far away from you. You can just drive your car up and ask them for a weight slip. It shouldn't cost more than $10. Just make sure you note how much gas (7lbs per gallon) you have in the tank, and weigh or take out any other stuff you have in the car.