Nice to see this has turned into a global discussion!
This is what I usually post on the UK FTOOC when this topic gets discussed on a fortnightly basis!
Not my words but have a read of them anyway:
Electric 'superchargers' will not provide boost on an automobile engine and will most likely rob a good running engine of high-RPM power by RESTRICTING airflow.
Consider for a moment what a supercharger must do to increase power. It must actually PUSH more air into the motor than it draws in naturally. Creating boost means moving a large volume of air (CFM) at increased pressure (PSI.) Most of these on eBay are nothing more than bilge exhaust fans designed for use on inboard boats. They might *seem* to move a lot of air but you honestly have no point of refrence, you don't know how much the motor sucks naturally. I assure you, it's substancially more than a plastic fan can provide.
Small true superchargers like the Eaton M-90 push only a small volume of air for a boost application. However, this small supercharger will push MORE boost than a commercial quality, gas-powered, backpack leaf blower and I'm sure we've all seen what they can do. Toss an electric vent fan like these into the comparison and you have effectively brought a rubberband to a gunfight.
How can an electric blower actually rob power? Simply put, the electric fan cannot push as much air as the motor demands at high RPM so the engine has to not only suck air but it also has to pull the fan faster than it's already moving.
Notice that few of the electric 'supercharger' auctions will provide the Boost @ CFM rating of the fan and that's for good reason. Simple math can determine the point at which they ROB power from YOUR engine if you simply know the size of your engine.
If electric fans could boost a car engine, manufacturers would be doing it.
Simply cleaning the junk out of your car and reducing weight will have more impact on your performance than a bilge fan.
But hey, if you have money to burn, buy one, test it on a dyno and prove everyone wrong.