On carberated motors, it allows more space for the fuel to atomize and seperate more evenly before getting sucked down to the pistons, has been proven time over again to give gains of about 10 hp for 300hp and above applications. On direct port injection, which we have, it cannot provide you with anything. They have ones that spiral to increase the airflow into the motor, but 1" of spiral groves in the intake is not going to swirl all your air into the motors intake manifold. They show you how spinning water in a sink will drain faster, which it will, but you have about 5 bends in a motor before the air gets into the piston, do you think the air will still be spiraling.
We have the hemispherical head design with opposed intake and exhaust valves with centralized sparkplugs. This design is what gives us such huge horsepower with just a 350 motor. Helping the air fall into and out of the motor. The head design, and the head design alone is what gives us the power and effeciency, not the spiraling of a 1" spacer. maybe if they swirl ported every bit of the intake track to the piston, like they do on some Harley evo motors, then maybe we will see a nice gain in performance like the evo motors do.
There are several magazine articles and dynoes done all over the place showing no performance gains, and actual losses in power in some circumstances using tornadoes, or throttle spacers on fuel injected motors. Only on a throttle body injection system like the old vortec motors have there been gains, but again, the fuel is coming into the motor from the carb or throttle bodies, and being shot right at the bottom of the intake manifold, thus a spacer to allow the fuel to stay attomized better before getting into the piston can help, keeping the fuel from slapping the bottom of the intake manifold, but on multiport injection it makes no difference since the fuel is shot right into the piston area, sorry I messed up earlier when I said direct port injection, that is a desiel application, except in one or two experimental motors that volkswagon is working on.