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Underbody panels - how important?

43K views 80 replies 37 participants last post by  MparkH 
#1 ·
It looks like I am missing the underbody panels on my 2002 CVT, from the radiator to the rear of the engine bay. Aerodynamically speaking, how important are these?
 
#2 ·
Like everything else, maybe 1%. It's when you add them all up that it makes a big difference. Probably the tires are the most important thing. The panels may affect engine cooling a bit, but I doubt if it's very much.
 
#3 ·
I can't talk about the underbody panel itself, but like the earlier poster said, everything counts.

I had lost the driver side "air dam" that sits in front of the tire, and also the inner fender. (I hit some road debris...)

I've been doing some long distance commuting (700 plus miles each way) on Interstate 80. With the damage, my mpg was hovering around the 50 mpg number. Once I replaced them I slid up to the 60 range. This all at "highway speeds"....

(I also pulled off a 70 mpg trip, but in that one I kept my eye on the display the whole time, drove in the Carter speed range, and despite teh weather, kept my a/c off. I ain't doing that again).

Oh, a picture of my mileage display on that last trip:
http://www.panix.com/~dannyb/images/mich-to-nyc.jpg

(2001 CVT)
 
#26 ·
I do not have any air dams, and no panels under the engine. Is this really possible to jump from 60 to 70MPG? This may be the reason why I can only max out my MPG on the highway in the mid 60's.

When I push the car hard at 65MPH and higher, I only get around 55MPG average.

Some people say here 1%, while he says over 10% difference. What should I expect on full highway miles. I like to drive fast in the insight, like 70MPH, so maybe it is a must for me to get those under body panels at those speeds.
 
#6 ·
I tore mine off by driving through a deep puddle at too high a speed (right after a night of heavy rains). The front screws tore through the dam material itself, and then the leading edge came into contact with the road, and it basically dragged itself down back and under itself until it was more than halfway off. I pulled over and got wet and muddy pulling it the rest of the way off.

Amazingly, the Honda dealer near me replaced the thing for $102.25 -- basically the cost of the part alone. He didn't charge me for the labor (although it wouldn't have been much).

When it was off the car, I could see exposed wires running along the underbody; I was worried they might get snagged on something. I was glad to get it replaced so cheaply.

Good luck

MF
 
#8 ·
jack9994 said:
i was doing 112mph yesterday (overtaking some guy in his new Merc)
Whoa! Got any points on your licence yet?!
 
#10 ·
Christian

Yes, i got 3 points in the Insight last month - doing 84mph on the motorway by one of the Cumbria Road Safety vans parked on a bridge - wasnt paying attention - i'm sure the police would have ignored me at that speed. First points for 23 years!!!

Cheers
John
 
#11 ·
By all means, replace the panels. It's easy to underestimate the importance of under-body airflow management. It's no killer to MPG at very low, urban speeds. But on the highway, it's major. At high speeds with NO panels, substantial turbulence is created. The panels will help to keep the airflow laminar.

My experience with this, comes from being a test-pilot back in the 80's and 90's. You see, airplanes with retractable gear have doors that fit up tightly to hide the wheels and smooth the airflow. For example, one of our single-engine planes (Glasair) had a top speed of 244 mph at sea level. Testing without the gear doors showed a top speed of only 230 mph.

Anytime you can smooth, hide, streamline, etc, both parasite and induced drag is reduced.
 
#13 ·
Hi Bagwell; From my flying notes back in 1993, as quoted in "Air & Space Progress" magazine. "None of the 737 series have ever had full main gear doors. Instead, partial doors cover the legs, while the outer wall of the tires meet with aerodynamic seals to make a smooth surface on the underside of the aircraft."

afaik, both the Embraer 145 and 170 employ main and nose gear doors supplied by Gamesa, inc. or the Sonaca Group......
 
#14 ·
Thanks to all.

Thanks everyone, I got underneath the car tonight and realized I was missing the Front Lower cover and both lower engine covers. I have new ones on order.

I think I am going to make templates of them before I install them. With the Chicago snow, I may make a set out of 1/4" aluminum to prevent them from ripping off. I'll have to see how sturdy the attachment points are first.
 
#15 ·
Re: Thanks to all.

WindyCityInsight said:
I may make a set out of 1/4" aluminum to prevent them from ripping off.
Planning to bobsled with your Insight :?: :p

IMO you'd end up ripping up the floor pan before 1/4" AL panels would give way. And if one did come off you'd better pray that it dosen't slice open the fuel tank on its way :!: :shock:

Ummm. Looks good "on paper" 'til you consider all the other consequences. I'd strongly recommend don't do it.

Be careful out there guys :!:

HTH! :)
 
#17 ·
Over the past 1200 miles I have averaged 70.6 mpg with no panels. I go from one small town to another (15 miles each way) daily at an average of 55mph.

If somebody has the bottom panels off of their Insight I sure would appreciate it if templates could be made. I will pay money for your trouble!

I believe some sort of rigid plastic sheeting will be a decent substitute.

My 2000 Insight was missing them when I purchased it. It has over 140,000 on it and I don't want to spend the money for new ones so I drive carefully and keep the car on the highway.
 
#19 ·
I noticed about a month ago that the black plastic "dam" in the front (on the drivers side) was hanging loose from the bottom of the bumper. I took a quick look and saw that two screws were missing and just assumed that they needed to be replaced and it could wait.

Last weekend I finally got around to looking closer and found out that the reason the screws are missing is that the plastic they screw into (the bottom of the front bumper) is broken in one place and stripped in another.

There are scuffs on the bumper in this area so I'm guessing that the previous owner may have bumped a curb or something along those lines.

Is there any recommended way to repair this other than replacing the entire bumper? I have a few ideas but I wanted to see if anyone has come up with something that works.
 
#20 ·
I don't know about those parts specifically, but I've repaired minor cracks on some panels (the rocker panel under the door, and trim piece behind the front wheel) with ABS pipe cement from the hardware store. (ABS is the black pipe, the white is PVC which uses a different cement.) I don't know how well the repairs would stand up to stress, though.
 
#24 ·
Body shops have a system for fixing minor damage, including holes, in plastic bumpers. According to the shop manual, the bumpers are polypropylene, marked with a PP stamped on the part. Other possible symbols are GTX, PAMXD6~G+T, ABS+PA6, E/VAC, ABS, AES, and PC. It doesn't show the underbody panel on the picture, but if you can find out the plastic type a body shop might know what sort of kit is needed to be compatible with the plastic...
 
#27 · (Edited)
Having just installed the right side under body panel for the engine [ $40] the only difference I noticed was a bit higher intake air temperatures for my very simple hot air intake modification ducting added earlier once the panel was installed . The intake draws off the catalytic converter heat shield and the panel must have quietened down some turbulence and outside air circulation i guess.
Didn't notice any change to coolamt temp via the Scangauge from the addition of the previously missing panel. My mileage following installation actually went down on the first long highway trip at speed after the panel was installed, but not because of the panel.

If you're going fast, no doubt every little bit will help somewhat; but to pin down how much benefit accrued to economy as a percentage will be washed out by variables such as tires, temperature, traffic on any particular day, and wind speed, especially in winter with denser air. That's stuff it would take a wind tunnel test to figure at a guess.
My rough estimate/experience is from 100kph to 110 kph there's a real hit to economy; ten percent faster, maybe 20 percent more fuel, to use round numbers. It's a square of the velocity type of thing.
________________
A major reason I installed the missing underbody panel, was not only for the aerodynamics and completeness, but also because of the deep snow anticipated in coming months that can pack up under the car; melt; freeze; drop down in nasty ice chunks; throw belts; wreck seals!; and worse.
[It may be a moot point for many without deep snow, but I had real bad luck with another small low ground clearance vehicle where similar under engine panels had been removed. [Done at the urging of a good mechanic 'to keep the car running cooler' in summer and save me 'a bunch of problems']. Ya,..True, :) but: ...
Result in Winter: two new serpentine belts; replace bent crank pulley; replace crank seal damaged by a thrown fouled belt; a rebuilt alternator, due to oil-up and short-out of original; hrs of $hop Time; Loss of Use x2; Aggravation 2x+ .. all from snow packing up where it shouldn't go, forming ice and then melting and falling into pulley belt mechanism because there was no plastic panel to prevent debris from getting up there in the first place! 'nuf said. :( rant/]

There's a pulley very low down that could present similar grief in the Insight.
__________________
[complete parts list uk/us pricing see: front fender]
If you do decide to order a set of panels #12 #17, the one clip [#36] that attaches to the front of each side of the panels to the horizontal aluminum part back of the bumper must be ordered as well; there _is_ a clip [#26] attached with the panel that a 6x16mm 1.25???pitch [best check pitch!] 10mm headed bolt passes though.
fwiw??!: The other four 10mm headed one piece washer-bolts are 6x20mm long with the same pitch cost $2.40/bolt from Honda.
The four 6x20mm 10mm washer-bolts were Insight specific and special order out of Japan = ~4 wks.
[..a metric nut and bolt supplier will have bolts and separate washers to accomplish the same task for much less with no waiting.]
 
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