belly pan vs. air dam ?
 

so how much more aero is a belly pan compared to an air dam?

1 mpg
2 mpg
3 mpg
3% ???

Depends on the vehicle.

I have no data to quote, but from what I've read here by those who know more than I (basjoos, aerohead), I have learned that the belly pan is the better of the two. If you don't/can't/won't install a belly pan, then the air dam will at least direct air around the car, instead of allowing it to go underneath where it becomes turbulent. The air dam increases the frontal area of the car, so it is usually recommended to have it extend only an inch or so below the lowest part of the car.
The difference between the two would depend on air dam size, shape, angle, and fairness to the rest of the car; and bellypanel extent of coverage and 'smoothness'. And weight difference between the two.

corsa underside
 

Modmonster,as Big Dave has said,it really does depend on the particular vehicle.And as Tango Charlie has mentioned,as the full bellypan is always favored by die- hard aero guys (and gals!),if you can't go that deep,the the airdam can serve as a palliative.------------------------ Here's the rub for you.Adam Opel AG,who developed the Corsa,does a lot of wind tunnel development with their cars,and I suspect that they "looked" very hard at both it's nose and underside.The car should already have a form of front airdam integrated into it's fascia.Also,being a uni-body type vehicle,lacking the deep recesses of a body-on-frame chassis ( as in a pickup truck,or truck based SUV ),the platform no doubt,has already received many small refinements to shield undercarriage components from the air. Auto makers have done a lot of this sort of "unseen" development since the 1970s.-------------------- So,this area of the car may have already seen much "optimization" and might not "show" remarkable improvements from more "radical" modifications.------------------------------------- For some,it comes down to,"I have to know!" And they just jump in, with a leap of faith.I'm willing to lose money on my own experiments,although am reluctant to waste other's hard earned money.--------------------------------- So where does this leave you? If you can find my installment on aero mods,% mpg change (under vehicle modifications)in the archives or earlier pages,I've listed all the quantitative values I have for these kinds of mods.If you'll study those numbers,it may give you some insight as to what you might expect with the Corsa.-------------------------- If you run into a snag,let us know,and we'll jump back in.Good luck with the project!

Individual results are likely variable in large amounts depending up many factors, including execution.

Found this link over at Gassavers, not the absolute answer but you will find a table about midway into the article and it might be useful.
Browser Warning

Some pretty good info from the folks "Down-under."

I'm still doing a lot of reading before I try to smooth out the back 2/3rds of underside of the of my Solectria Force. Solectria did a pretty good pan under the front. But it needs some filling also.

EVDRVR

<http://www.autospeed.com/cms/A_2456/article.html>

Above is the link to the second part of the article I posted above.

The url for the first is:
<http://www.autospeed.com/cms/A_2455/article.html>

Not sure what the "Browser Warning" is about. Autospeed site seems pretty passive to me.


EVDRVR

13,433 Km 8,347 Mile Trip
 

Check my belly pan thread here

Road Trip! ROAD TRIP!! 13,433 Km 8,347 Mile Road Trip!!!

The EPA highway rating for my car is 28 MPG (US Gallon) the old EPA rating was 30 MPG (US Gallon) and we all know that he old EPA ratings were almost impossible to achieve.

My 32.8 MPG (US Gallon) is 9.33% better than the old EPA and
17.14% Better than the "NEW" PA ratings.

I would say that the full belly pan works well! Truse me the EPA did not test at my speeds and the did not test at 105+ degree temepratures of Palm Springs and Phoenix or LA, San Francisco and San Diego grid lock traffic, nor did they cross the continental divide.

Check out the photos.
First 50 photos

Second 50 photos


3rd set I did mentioin it was 13,433 Km 8,347 Miles!!!