buy a cheap rpm meter, voltage controller and the set of fan and motor.
here is the idea..
you have to wired the motor the voltage controller and the rpm meter together.
when the rpm needle goes up, it will send more voltage to the motor (by using the voltage controller) this whole idea just involve mechanical setup..
a bit more explanation..think of a volume knob/controller. we turn the knob to turn the volume high is it? think if the turn of rpm needle function like the volume knob. maybe the voltage controller need to install somewhere at the rpm needle itself (maybe open the meter and connect the rpm meter motor directly to the voltage controller?)
ive done this project before but just without the rpm meter and the voltage controller. just on off switch on my dashboard..the fan motor maybe can get from 12v cigarette lighter car vacuum cleaner selling RM30 at the market...
performance gain? dont try compare with turbo..hehehe..