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Originally Posted by euromodder
You can defend them as whatever you wish, but they're just not controlled enough to accurately measure minor differences
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I agree it's true that the hardest mods to test are those with the smallest results.
I'm not at all saying my testing was/is perfect. I've done some crap tests in the past - but the Airtabs ones weren't among them.
So, I'll continue to defend...
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even less so when testing on an already fuel efficient vehicle.
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Depends on the units you're using on your fuel consumption display:
- Using L/100 km gives you the
poorest resolution in an efficient car.
- Change it to MPG (US) for testing purposes and you immediately have far better resolution.
- Change it to MPG Imperial and you have even more.
- KM/gallon Imperial, more still.
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I'm just saying we can't sufficiently control the test conditions to get really reliable data. There's far too many factors beyond our control out on the road.
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That depends on the effort you're willing to put in to doing a test. I believe I was able to control for the major factors that would affect results:
- waited for a windless day (by being patient);
- performed runs (in the middle of the afternoon, because I can) with no other traffic around (no confounding aero effects);
- averaged bidirectional runs on a very straight & level stretch of road;
- cruise control was set only once for all runs (driver's right foot removed from the equation);
- all comparisons were done immediately following one another (eliminating major potential changes in the vehicle/atmospheric factors).
The result of attempting to control these factors was I think I pretty respectable (low) standard deviation in the results: .31 mpg (US) for the A set and .20 mpg (US) for the B set in the Corolla tests. Using Airtab's claims, A 2-4% improvement in fuel economy improvement would have shown up as a ~1-2 mpg (US) difference on this 49 mpg vehicle (at the speed tested), well above the statistical "noise" in this case.
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From your testing, I recall they smoothed out the airflow.
So they are doing something.
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I agree they did something on the Corolla. I believe they directed flow through what would otherwise (in the absense of the VG's) be a separation bubble at the mid/base of the rear window. Yet there was no measurable effect on fuel economy, probably owing to the energy required to form the vortices which redirected the flow.
Of course on-road testing sucks in general compared to lab tests. But with patience & effort it's possible to control for more than you might think and get good data on the road.
[ /end-defending-my-honour ]
