E46 roof spoiler

I don't know, why do you need one?

Funny! :) But seriously, why do they even exist? Do they actually help the air flow over the top? I would think that the smoother lines (without the spoiler) would flow the best.

A spoiler directs airflow, not improves it - it's chief use at the rear of a car is to increase downforce; to make the air push down on the car. A high speeds, the air moving under the car can create enough lift to make the car unstable and hard to control, since you're essentially creating hurricane-speed winds under and around the car. A front air-dam reduces the air moving under the car, while the rear spoiler interrupts the flow and causes it to push down on the car, counteracting the lift. So, do you need one? Definitely, if you're regularly driving at over 120 mph. At normal US traffic speeds, not so much. Lowering the car to Euro specs (are they different anymore?) would make the car stable up to Autobahn speeds, since that's what the car was designed to handle. You might notice a slight improvement in handling with a rear spoiler at such speeds, but you still wouldn't *need* a spoiler. Emanuel

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Except the OP is talking about a *ROOF* sunroof deflector, which is an entirely different subject than you are talking about.

FloydR

Yes. I'm referring to the roof spoiler. Maybe the two spoilers work together for optimum airflow? Could it be that they "engineered-in" an aerodynamic flaw so they could sell an accessory "fix" later ( to increase the bottom line)? Shrewd marketing perhaps? Does it extend very far over the rear window and act as a sun visor?