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wmjinman · 11-09-2012, 04:16 PM

I figured that out back when I was doing my Bonneville Salt Flats landspeed racecar. I had another car of the same model & year (almost - 2 years newer, but with the same basic body style, at least) that I'd go out and test different air dams & stuff. It was too old to hook a ScanGauge up to, so I'd do coast-down testing.

But knowing the mods would have greater effect, and the results would be easier to see at higher speeds, rather than doing it on the freeway where the speed limit is 75 and lots of traffic and cops, I'd go way out - probably 80 miles from where I live - to a particular stretch of fairly flat road way off the beaten track with extremely light traffic & still a 70 mph speed limit. Of course, I was trying to exceed 200 mph in the "real car" too, so I wanted the tests to be a little closer to the speeds I'd actually be dealing with.

So I set up a 1-mile test section for my two-way tests & marked the ends with orange traffic cones. Then I'd go out more than a mile past the end, turn around & punch it up to 112-115 mph, then throw it into neutral, trying to guage it so I'd be down to going 110 at the "start" cone (or near it). When it hit 110, I'd start my stopwatch & then let it coast down to 60 & write down the time. Then I'd go a mile past that end, turn around, & repeat. And I'd try to do 3 sets of those.

To try to reduce the other variables, I had the tires pumped up to 70 psi, and would pour a couple gallons of gas in the tank after each set to try to keep the weight the same. Wind will slow down a lighter car faster than a heavier one, all things being equal, right? Of course, that big 455 Buick probably burned even more gas than that. I didn't do the easy to gauge "fill it all the way up" trick, because, trying to maximize the aerodynamic effect, I tried to run it as lightweight as possible, so with the tank less than half full. But with a couple of exceptions (which I think were probably due to intermittent wind gusts), I was pretty pleased with the consisancy of the data, and I was definitely able to tell which air dam (if any) cut drag the most.

Like I said earlier, I just love doing this type of testing. :-)

11-09-2012, 04:16 PM	#31 (permalink)
wmjinman Master EcoModder Join Date: Oct 2012 Location: Carson City, Nevada Posts: 612 Jimmy - '00 GMC Jimmy SLT 90 day: 21.18 mpg (US) The White Gnat - '99 Suzuki Swift Team Suzuki 90 day: 51.87 mpg (US) Thanks: 240 Thanked 114 Times in 90 Posts	I figured that out back when I was doing my Bonneville Salt Flats landspeed racecar. I had another car of the same model & year (almost - 2 years newer, but with the same basic body style, at least) that I'd go out and test different air dams & stuff. It was too old to hook a ScanGauge up to, so I'd do coast-down testing. But knowing the mods would have greater effect, and the results would be easier to see at higher speeds, rather than doing it on the freeway where the speed limit is 75 and lots of traffic and cops, I'd go way out - probably 80 miles from where I live - to a particular stretch of fairly flat road way off the beaten track with extremely light traffic & still a 70 mph speed limit. Of course, I was trying to exceed 200 mph in the "real car" too, so I wanted the tests to be a little closer to the speeds I'd actually be dealing with. So I set up a 1-mile test section for my two-way tests & marked the ends with orange traffic cones. Then I'd go out more than a mile past the end, turn around & punch it up to 112-115 mph, then throw it into neutral, trying to guage it so I'd be down to going 110 at the "start" cone (or near it). When it hit 110, I'd start my stopwatch & then let it coast down to 60 & write down the time. Then I'd go a mile past that end, turn around, & repeat. And I'd try to do 3 sets of those. To try to reduce the other variables, I had the tires pumped up to 70 psi, and would pour a couple gallons of gas in the tank after each set to try to keep the weight the same. Wind will slow down a lighter car faster than a heavier one, all things being equal, right? Of course, that big 455 Buick probably burned even more gas than that. I didn't do the easy to gauge "fill it all the way up" trick, because, trying to maximize the aerodynamic effect, I tried to run it as lightweight as possible, so with the tank less than half full. But with a couple of exceptions (which I think were probably due to intermittent wind gusts), I was pretty pleased with the consisancy of the data, and I was definitely able to tell which air dam (if any) cut drag the most. Like I said earlier, I just love doing this type of testing. :-) Last edited by wmjinman; 11-09-2012 at 04:23 PM..