[Diy_efi] BHP calculations

[Greg Hermann]

At 10:49 PM 7/29/02, Hugh Keir wrote:
>I am working on a spreadsheet to calculate BHP from datalogged information.
>
>The basic formula I am working to is:
>
>Engine BHP =3D acceleration BHP + wind drag BHP + transmission loss BHP.
>
>I need help with the formula to calculate acceleration BHP.
>
>I know the weight of the car and the change in engine RPM over time.
>
>I can therefore use F=3Dma  ( force =3D mass * acceleration ).
>Where I think F =3D lbs, mass =3D slugs ( 32.1 lbs each ) and acceleration =
=3D
>ft/second change in speed, to calculate the acceleration BHP
>
>Horsepower is 550ft lbs / second. I am not sure how you get BHP from F
>above, do I just divide by 550 to get BHP?

Pay attention to the units for your answer ! You have F right, you will
also need the car's speed (V?? in ft/sec) to plug into the calc to get HP.
As in: HP =3D F =B0 V/ 550
>
>I have used the wind drag calculations from
>http://members.home.net/rck/phor/06-Speed.html which seem OK and will be
>able to get some transmission loss figures from some rolling road runs.
>
>Your help will be greatly appreciated.
>
>I can copy the finished spreadsheet to anyone interested.
>
>I have used a variable air density input to the spredsheet and hope to add
>wind direction to normalise the readings.

Watch it here. You may be better off to plug the aero drag force into the
spread sheet at the "F" level rather than at the HP level--- The aero drag
=46ORCE will vary directly with the air density and with the SQUARE of the
vehicle speed, whereas, the aero HP will vary with the SQUARE of the air
density and with the CUBE of the vehicle speed. Humidity affects air
density at any given barometric pressure (density falls with increasing
humidity).

Greg
>
>Thanks
>
>Hugh



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