Front air dam vs underbody belly pan
 

On an old car, in my case a 1966 Chevelle, with a horrendously shaped underbody, is it better to use an air dam to try to keep the air out or try to fab a belly pan?
I already have an air dam I’ve made for the front so would it be worth it to remove it and smooth the underbody?
The Chevelle is a full framed car with dual exhaust the full length, two relatively huge mufflers, I live rear axle. It’s a mess. It would require a LOT of framing to support it and I’d still have to leave the exhaust area open.
This is more for mpg than downforce.
Opinions?

Most people seem to use the air dam because it's less fabricobbling.

If you do a belly pan start at the front instead of the back (or so I hear).

'Front air dam' cannot even be compared to 'underbody belly pan'.
It is a percentage of what 'underbody belly pan' can do.

So your vote is for a belly pan. Noted.

ecomodder.com/forum/showthread.php/aerocivic-how-drop-your-cd-0-31-0-a-290.html

The Aerocivic has dual side skirts between the wheels.

I think the general consensus, arrived at in other threads on this very topic, was that a good front air dam could deliver about 70-80% of the results of a comprehensive belly pan at 10% of the cost and effort. If you already have a dam built the obviously start there. Establish some baseline results without the dam for comparison, then run some tanks with to see how it fares.

Add the side skirts a la Aerocivic and it likely gets even better.

Very interesting. 70-80% of the results is a pretty good return considering how much easier it is.

That's why they're more popular. Bang for the buck.

Chevelle
 

GM's two lowest drag cars had full belly pans and no front airdam. Cd 0.137 and Cd 0.14 ( if we ignore the Cd 0.089 Sunraycer ).
A 'guess' for the '66 would be around Cd 0.51.
Today's 'Chevelle' Malibu is around Cd 0.28.
A look under a modern Malibu would say a great deal about GM's strategy for drag reduction over the decades.
HOT ROD Magazine used a full aluminum belly pan on their Cd 0.20, 200-mph Project Red Hat Camaro ( originally around Cd 0.49 ).
There are no low-drag cars without full belly pans.
You're the only one who can make the call on, how much you're willing to invest, time and moneywise.
In one example of a early- 1980s Cd 0.30 car, a full pan with 'slow' diffuser netted a delta- Cd 0.070 drag reduction.

I agree with everything you say. But a belly pan on a old full frame car would be a huge undertaking. So I’ll have to live with the less efficient airdam for now.