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Old 09-29-2008, 10:48 PM   #1 (permalink)
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Airdam vs partial bellypan?

About a week ago I decided to remove the airdam and attach a partial bellypan using the same screw holes. Sorry my camera is on the fritz, but it runs from the lower edge of the valence to the engine cradle (lowest part of the undercarriage), about 16". I also took some of what was left over and blocked off about half of the breather openings in the valence.

The results - hard to say. It immediately feels like less resistance, just like when I put the airdam on it immediately felt like more resistance. That makes sense because the airdam hung a tad below the engine cradle and therefore created a bit more profile. However even despite two pretty warm and calm highway drives, it seemed to lose some mileage. Last week we had some warm days and I lost some mileage to work and back too, plus I seem to be doing worse than similar days that were at even lower temperatures. I know the fan isnt running, because the car runs at pretty much the same temperature as usual, and I know where the temp gauge has to be to get the fan to come on and its nowhere close.

Does this make any sense? I wonder if the airdam was pushing up a roll of air to help get the rest of the air over top of the car? Sort of like a pickup with gate up gets better mileage from the roll of air that the tailgate pushes up? It seems counterintuitive, foolishness like saying a parachuter will go faster from the wad of air created by the parachute, but in the pickup case its supposedly been proven.

And if that is the case, it seems that I would really benefit from a good sized false nose that would mimic this roll of air, and reduce any parachute effect.

Has anybody else had any other similar or not so similar experiences?

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Old 09-30-2008, 09:31 AM   #2 (permalink)
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it's difficult to juge a feeling but i remember my partial bellypan really felt different the first tim i had it on, the car seemed much more quite and felt much smoother. the car does about 2 mpg's better than before although it's difficult to pinpoint the exact improvement.
another benefit is that the engine stays much cleaner.
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Old 09-30-2008, 07:15 PM   #3 (permalink)
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I made something similar for my xB, and combined with the 1/2 grille block at the same time, my butt-dyno tells me it helps at least a little.
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Old 10-08-2008, 08:11 AM   #4 (permalink)
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Well I think I gave it a fair shot. Overall it seemed to lose mpg. Last week was bad weather, but the last couple days have been nice. And both days I still struggled to get 30 mpg, even yesterday on the way home, 60 degrees and no rain, I should have gotten more like 33 without too much trouble. Even after airing up the tires some more.

So last night I took off the partial bellypan and put the airdam back on, and touched up the chipped paint. So far so good, I got 32 this morning on the way to work, cold engine, 40 degrees, and rain. Lets see what a nice day can do.
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Old 10-08-2008, 09:27 AM   #5 (permalink)
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there's a couple of things you have to considder with a bellypan.

one thing is that any air that enters the engine bay trough the grill will also have to exit somewhere. the opening at the bottom of the engine may serve as a very crude exit duct wich, when blocked could cause the airflow to exit elsewhere where it might cause a disruption in the airflow underneath and around the car.

it's also possible that it restricts the airflow to your engine, creating pumping losses, or cause it to ingest hotter air wich depending on how your engine management reacts could also cause worse FE.

on the other hand if the underside of your car beyond the bellypan causes enough flow restriction this may undo any benefits from the bellypan.

were a dam when proppery sized could reduce underbody airflow slightly.

if the dam works, keep it on by all means and take whatever gains it has, but the bellypan doesn't work it might be trying to find out why that is.
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Old 10-08-2008, 10:25 AM   #6 (permalink)
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I wonder if it would make more of a difference if the belly pan was put on the rear of the vehicle? This would maybe straighten the flow for better rear end aero effects. I'd imagine that the effects would be more noticeable without any front end mods, but it could increase the efficiency when combined with the air dam also- straightening the air flow that gets by the dam.
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Old 10-08-2008, 12:18 PM   #7 (permalink)
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Quote:
Originally Posted by woohoo207 View Post
I wonder if it would make more of a difference if the belly pan was put on the rear of the vehicle? This would maybe straighten the flow for better rear end aero effects. I'd imagine that the effects would be more noticeable without any front end mods, but it could increase the efficiency when combined with the air dam also- straightening the air flow that gets by the dam.
This was the idea I had in mind for doing on one of my cars. The front air dam prevents the air from going straight under... but the air gets under the car from the sides, too. Massive side skirts are not always practical, but smoothing out the rear of the car would lessen the drag where the air enters.
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Old 10-08-2008, 12:26 PM   #8 (permalink)
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Quote:
one thing is that any air that enters the engine bay trough the grill will also have to exit somewhere. the opening at the bottom of the engine may serve as a very crude exit duct wich, when blocked could cause the airflow to exit elsewhere where it might cause a disruption in the airflow underneath and around the car.
This could be true. Behind the grille openings in the valence is wide open underneath for like 16 inches. The engine and transaxle combo take up most of the rest of the space back to the firewall. So the air coming through the bottom part of the radiator (the top is blocked with my homemade solid grille) would most likely go down under the car. The transverse V6 would help too, since it is much wider at the top than the bottom, guiding the air down and under the car. With the bellypan, who knows where it goes.

Quote:
it's also possible that it restricts the airflow to your engine, creating pumping losses, or cause it to ingest hotter air wich depending on how your engine management reacts could also cause worse FE.
Definitely possible. The air intake is behind the drivers side headlight. The air normally would normally be drawn straight up from the bottom, but with the bellypan, it would have to take a more roundabout way and would probably be warmer. I didnt notice much of a difference in my temperature gauge, could have been slightly higher but if so then just a sliver.

Quote:
on the other hand if the underside of your car beyond the bellypan causes enough flow restriction this may undo any benefits from the bellypan.
The engine and transaxle are close to the firewall, and the floorpan is pretty smooth, but the solid beam rear axle probably screws up the airflow pretty well. I would probably need to take a big sheet of rubber and fasten it to the floorpan and to the rear bumper, draping it under the rear axle with enough room to stretch as needed. But then theres the exhaust too.

Quote:
were a dam when proppery sized could reduce underbody airflow slightly.
The level of the airdam is a bit below the engine cradle, which is the lowest part, so it probably keeps more air from under the car, where the bellypan was stretched from the valence to the cradle and helped direct air under the car.
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Old 10-08-2008, 01:17 PM   #9 (permalink)
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If your belly pan was angled down, rather than horizontal, it was creating a pressure bubble underneath the front of your car, creating lift as well as drag. Not so good for stability.

Belly pans should either be horizontal, or angled slightly up toward the back to increase stability, as well as smoothing the air flow.
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Old 10-08-2008, 05:11 PM   #10 (permalink)
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Hmm thats good to know. I just threw it in the trunk for now. Maybe sometime here Ill try reattaching it to the bottom of the airdam and spanning to the cradle, so itll be sloped up a bit as it goes back.

Edit: 33.6 mpg on the way home today, 50 degrees, some roads were still wet. Im convinced the airdam is working. I go downstate Friday and Sunday, I should do well then too.

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Last edited by wagonman76; 10-08-2008 at 09:30 PM..
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