[lunarhighway]

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.