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Old 07-22-2019, 01:13 PM   #1 (permalink)
Cyborg ECU
California98Civic's Avatar
Join Date: Mar 2011
Location: Coastal Southern California
Posts: 6,297

Black and Green - '98 Honda Civic DX Coupe
Team Honda
90 day: 66.42 mpg (US)

Black and Red - '00 Nashbar Custom built eBike
90 day: 3671.43 mpg (US)
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TESTED: flat decklid spoiler


I posted in my modding and maintenance thread how I built it. See:

It's just a cardboard prototype.

Testing method.

When I arrived at El Mirage at about 8:00am, I explored for good ground a bit. The place was fairly deserted. I chose a spot on the lakebed that I'm familiar with from previous years. I set up a brightly colored marker at one point and did a measurement run: I accelerated to 40 mph and when the car stopped I laid down a cone. Using mountain tops in the distance and these two markers I was able to guarantee my course crossed the same ground for each run in order to control for differences in the lay and firmness of the ground.

The procedure was A-B-C-A. I did three "A" runs with just the OEM configuration. Then I did three "B" runs with just the flat deck lid spoiler and three "C" runs with the spoiler + side panels. Finally I did a 4th set of three runs in the OEM configuration in order to control for the increasing temperature during the morning.

For each run I would accelerate to 40 mph & hold it. As I passed the brightly colored marker I would push in the clutch pedal & start tracking distance rolled using a GPS app on my tablet. At the moment that the car stopped I stopped recording data on the GPS. All runs were eastbound.


A (OEM shape)
0.55 miles
0.59 miles
0.59 miles

B (spoiler without side plates)
0.64 miles
0.61 miles
0.67 miles

C (spoiler with side plates)
0.66 miles
0.61 miles
0.67 miles

A (back to OEM shape)
0.62 miles
0.64 miles
0.61 miles

A = 0.5766
B = 0.64
C = 0.6466
A = 0.6233
Average of both A = 0.5999
C - A ave = 0.0466

C is a 7.23% increase over the average of the As (C is 3.61% over second A)
B is 6.27% over the average of the As (B is 2.61% over second A)

Obviously, The car rolled furthest with the spoiler "box cavity," though the distance with the spoiler alone is a close second. The box cavity produced a gain of about 7.23% and with just the spoiler I saw a gain of 6.27% rolling distance. A more conservative reading of the results would half those gains. So it seems it could produce between a 1.5% or 3.5% fuel economy increase.


I had problems with the heat and with battery voltage. At one point in the morning testing the voltage meter showed 10 volts. I knew that a problem was developing with the battery and so I had been running the alternator the entire way from home to El Mirage and on all of the testing runs. Something's wrong with the battery, and the battery is not that old. This was an especially difficult problem in the attempted afternoon testing.

During my afternoon testing, the sun on the black car in the heat began to melt some of the duct tape and neodymium magnet mounts.

During the afternoon testing, because I felt myself more in a hurry, I dispensed with using my markers & chose random markers on the ground that I then didn't stick to very well. The afternoon testing was a complete mess. I lost some afternoon data too.


Side view of the spoiler with support braces added before testing. The side plates for the box cavity configuration are not pictured.

Image of the dust pattern behind the rear bumper when the car is in its OEM configuration. Notice how the dust is distributed around the top of the bumper cover.

Image of the dust pattern after the runs with the spoiler, without the side plates. Notice the changes in the dust pattern on the bumper cover in particular.

Image of the dust pattern on the bumper cover after the runs with the spoiler in the Box cavity configuration. The dust pattern has changed again.

This picture is of the dust pattern it was revealed immediately upon removing the spoiler after the final run, the set with the spoiler on in the box cavity configuration. The dust has been gathering under the spoiler. The dust did not stick to the spoiler itself.

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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

Last edited by California98Civic; 07-23-2019 at 07:43 AM.. Reason: typos
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