OK folks. I have been thinking about this for a while. Remember the first NASCAR race this year? Someone was disqualified (I don't remember who, since I am not really a NASCAR fan) for having a leaking seal on the trunk at the back of the car. Apparently the air that is scooped into the cabin to help keep the drivers cool slightly pressurizes the cabin. If it is allowed to leak out the back it tends to reduce the partial vacuum behind the car, leading to less drag. I figure if it's bad enough to have a severe penalty, it's good enough for us to use.
Now, I realize there are two vents in the trunk area, one on each side. And they dump out into the area between the rear sheet metal and the bumper cover through the two louvered vents in the back panel. And then the air exits the car through the openings above the rear license plate.
Wouldn't it be better if an opening was made in the rear sheet metal just under the trunk lip? Of course the rear carpet would need to be cut and some louvers or screens put in place to prevent anything from falling into the bumper cover area.
I would think that you would get better air flow through the car with the windows rolled down a bit in the summer if the air had an easy way out. I wish the side windows would hinge out.
Anyway, just some thoughts I had. Any comments?
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2000 Silver MT Insight
WAI, HTR200, Kenwood MP3 head unit, 3 MTX stealth subs, very-high-mounted 4th brakelight, yellow-top, tinted, SG2, MIMA, center armrest.
The air is already escaping just above the license plate which is in the low pressure are behind the car. So why do you suspect it would make a difference if it escaped in the place you describe instead?
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2000 Insight 5spd with 290,000 Km
Daily driver (previously used for autoslalom & track lapping)
Collision avoidance and Track driving instructor
same way out, just a direct shot from inside the cabin, possibly ducted. basically a reduction in "cooling" drag, and a larger volume of air. i wouldnt be above putting visible outlets on the bumper cover if it helped. even if it gave me more airflow in the cabin without opening up the windows any more i would be happy. might keep me from turning the a/c on a bit longer into the summer.
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2000 Silver MT Insight
WAI, HTR200, Kenwood MP3 head unit, 3 MTX stealth subs, very-high-mounted 4th brakelight, yellow-top, tinted, SG2, MIMA, center armrest.
Increase speeds to around 200 MPH and you might be on to something
IIRC the windows open / closed "arguement" has never been attempted to be measured for the Insight and at highway speed in normal traffic. At the high end of Insight MPG abilities (90+ MPG) its probably measureable on the MPG gague.
But in "normal" traffic there will probably be other bigger forced (by traffic) losses.
It seems to me that it would take some fairly high-powered fluid dynamics or a lot of testing to know for sure how much drag reduction your idea would provide in an Insight at 60 vs Nascar at 160, but it has another merit too. If we could flow more air through the passenger compartment (perhaps especially if you could draw it from the high pressure area at the very front of the car) then there would be fewer days we would be likely to use the AC, and that could be a meaningful mileage bump, depending on where one lived and how spoiled one is
moses: yeah, i wish they would let me put my insight in the wind tunnel here. crank that thing up to 900 mph and see what the insight will do! most likely shred into a million pieces. take out the fan, and my job with it... nevermind!
kevin: thanks for the link to the exhaust pics. i saw those when i got my car, since i had to replace the bumper beam and cover. i dont know if giving the exhaust air a larger, straighter shot out the back would help much or not. i could always cut out the area in question and fold the carpet down to check it out. and if i dont like it i could weld or rivet in some thin aluminum panels. or just block the hatch up an inch and see what happens to the cabin air flow. hmmmmm......
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2000 Silver MT Insight
WAI, HTR200, Kenwood MP3 head unit, 3 MTX stealth subs, very-high-mounted 4th brakelight, yellow-top, tinted, SG2, MIMA, center armrest.
There was a great idea around a while ago, using a small onboard compressor to create high-pressure jets of air.
These jets of air were extruded through carefully placed and shaped nozzles at the rear of the truck to "lengthen" the area of less turbulent flow behind the vehicle. The reasoning is that an effectively longer vehicle has a lower Cd and less air resistance. I think they managed to cut the air resistance of a typically box-shaped truck by around 50% using only a 5 hp compressor.
Apparently the fuel used to run the compressor was more than made up for by the gain in fuel efficiency from lower drag.
All work on this went very quiet a couple of years ago and I heard a rumour that an F1 team had bought out the IP, though not sure if that's true.
The Insight follows Dr Kamm's principles from the 1930s http://www.design-classic-cars.de/jaray/kamm.html
As far as the wind is concerned, our cars already have a long tapered tail, it just isn't visible!
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NASCAR, drag reduction, and you.
