Quote:
Originally Posted by redpoint5
It never was explained why a change in speed and therefore a change in BSFC was not relevant to quantifying aerodynamic treatments at a given engine load (throttle position). Maybe the method gets close enough that the BSFC can be ignored?
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It
was explained several times: BSFC has
nothing to do with throttle-stop testing since throttle-stop testing
does not use fuel consumption as a measurement parameter.
Throttle-stop testing works on the principle that for a given throttle opening and over small changes in engine RPM, torque output is nearly constant. On a vehicle with fixed transmission gear ratios, constant torque = constant force at the wheels, so a change in drag force will show up as a change in speed, and the ratio of the squares of each speed will be equal to the ratio of each drag area. That's it. No BSFC. Has nothing to do with this type of test.
The caveats for throttle-stop testing follow, and also have nothing to do with BSFC:
-may not be accurate over large changes in speed (engine torque may not be constant in this case)
-may not work on vehicles with electronically-controlled throttles (actual throttle position and engine torque may vary in this case despite constant throttle pedal position)
-may not work on vehicles with constantly-variable transmission gear ratios (engine torque and wheel torque not necessarily the same ratio in this case)
As always, with any type of testing, you have to try it yourself with a known change in drag such as windows up/windows down to see if it works on your car.