If the flow off the roof doesn't reattach at the trunklid you have negative pressure above the rear creating rear lift
![](http://i1271.photobucket.com/albums/jj622/aerohead2/12-19.jpg)
Adding around 35mm of rear spoiler gets reattachment and some lift reduction
![](http://i1271.photobucket.com/albums/jj622/aerohead2/12-195.jpg)
Going over 100 mm you get more down-force,but greater overall drag,defeating the purpose of the spoiler
![](http://i1271.photobucket.com/albums/jj622/aerohead2/12-194.jpg)
*The spoiler 'sees' some of the more energetic air and captures it's momentum
*An early Firebird spoiler goes all the way to the 'Template'
![](http://i1271.photobucket.com/albums/jj622/aerohead2/06-2829.jpg)
The Audi TT goes near it,but not quite to it.
![](http://i1271.photobucket.com/albums/jj622/aerohead2/06-2827.jpg)
*Without a wind tunnel it's hard to know what actually happened with these cars.
*I'd recommend staying with the spoiler if for no other reason,to reduce rear lift,and chance of hydroplaning back there.