Quote:
Originally Posted by aerohead
1) I'm in total agreement that we can.
2) Accessory load will increase at the new, higher velocity, after the aero modification, and we have no baseline data for that.
3) Same for rolling resistance
4) Same for powertrain mechanical efficiency ( it's at a new rpm and lower load )
5) Same for BSFC as the engine is 'blind' to some former signal input
6) From the EPA, we know that A-B-A testing doesn't cancel wind effects, as the vehicle demonstrates different reactions to, say, headwind / tailwind.
7) If we're in 'quartering' winds, there's no available data for that, one way, or another. A complete unknown.
8) Fuel Btu content won't vary.
9) Fuel density ( coefficient of thermal expansion ) WILL vary with temperature, however, probably not, in the timeframes we're working with.
10) As to accuracy, one thing not mentioned, but of some assistance to us, would simply record the fuel economy for a spectra of higher velocities which happen to include the new, higher, velocity-2 with no modifications, to compare to the modified car at velocity-2. Some things could be reverse-engineered from that kind of data.
11) The more 'book-keeping' we can perform, the higher the resolution when attempting to isolate the aerodynamic effects.
12) Qualitatively, the throttle-stop test will demonstrate the trend, without countless hours and liters ( gallons ) of fuel going up in smoke. It's worth the fuss.
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2-3) I think they will be a linear thing and can be all integrated into "rolling resistance"
4) Mechanical resistance is maybe 5% of the total drag, even if it changes by an extreme 10% it is only 5.5% of total drag and so while not irrelevant it is not a huge difference.
5) BSFC if it does change will be tiny and I believe will be negligible over the few percent increase/decrease in engine speed.
6) Wind, well nothing can change that, but on a calm day I see no problem with multiple bidirectional averaged runs a-b-a-b-a-b as a minimum. So, just don't do it on a particularly windy day, unless you are specifically testing crosswind drag.