Only as much as a fuckoff whale tail spoiler and In theory this setup works like a spoiler as if your going fast enough it'd produce downforce.

Just make sure the opening in the floor isnt too large...of you will probably (at speed) blow the thing up due to the pressure inside the encloseure...even the rad backing it up will produce some serious stress at speed.

I reckon you need 1/2-2/3 the size intake on the floor as the exit out back of the car....as like said before you'd end up inneficient the second the air has to figt its way in....

I'm going to have to go for a read mounted rad in the mk1 project so very very interested in your setup Phil keep the info flowing.

Dan