You're offtopic, but my thought on this:
A spoiler does what it's name suggests, it disrupts the air flow. But that doesn't have to result in drag.
As you propably know moving cars always carry whirls of air behind them which cause a good part of their drag. A perfect 90° edge on the rear will support this a lot and cause a lot of drag (may be noticed when standing beside a road where trucks pass by). A spoiler will cause the air to not get dragged into these whirls as easily, more air passes without getting caught, less air is trapped behind the car/dragged through the static air, less drag is produced.
Any objections may be voiced and will be appreciated. I just made this up from my understanding of hydro-dynamics.

Vain

Quote from tristancliffe :

Not according to my Vehicle Aerodynamics lecturer...

That's what my 'Competition Car Downforce' book says, and I think I agree. How would keeping the flow attached longer cause less lift? Lift is generated by the attached flow curving over the convex surface at the rear of a car. Disrupting that flow generates less lift. What mechanism does your lecturer propose?

The separation will increase drag (as an element of turbulent flow must exist between the laminar and non-laminar flows), and the seperation will cause a local reduction in pressure relative to the underside = more lift.

But at the same time the spoiler will break up vorticies behind the car, and reduce the size of the wake, cancelling out the drag, and removing the lift = less drag than no spoiler, and less lift than no spoiler.

Remember lift is ultimately just a pressure difference, and if that arises though laminar flow or turbulent flow matters not on a road car.

Ah...I think I understand the problem...my definition of 'spoiler' seems to be a bit different to yours (and, so it would seem, the rest of the world!). I think of a spoiler as a device attached to the rear of a car along the whole width (like NASCAR) but the S1 cars (XRT, FXO, RB4 etc...) have devices which look more like wings attached to the tailgate (but are still called spoilers). In that case, your explanation is correct. Sorry for the misunderstanding, it's been a while since I drove a road car in LFS

no, those are spoilers too

wings go on planes, not cars

Yeah, they are wings Stewart (ignore the silly Yank between us, they don't even know what a bonnet is )

just kidding Nuse

its what we call a hood

Isn't that what keeps the rain off your head?

No, that's an umbrella

...mmm... Somebody should really think and revolutionize the shape of umbrellas. Their aerodynamics always causes them to lift and turn them inside out...