[Jeffy]

I removed the throttle body on my Cooper and found that the throttle plate is pretty thick. The thickness is not an issue, but the edges are. When the throttle is wide open, the sharp edges of the plate cause an aerodynamic disturbance, both at the leading edge, and trailing edge. So, I filed the edges off, to form more rounded, blunt edges. Obviously, you shouldn't remove any of the diameter, or the idle speed will be affected. After re-installing the throttle body, I replaced the stock air intake and filter box with a cone-shaped filter like they sell at Moss Mini. Passing was noticeably improved. So, the performance gains come at high RPM's, but you need it in the U.S., because high performance engines are common in just about any kind of vehicle on the road, and sometimes you have to out-run them just to pass (people don't like being passed for some reason.). Plus, you can hear an angry growl as you wind it up, instead of almost no noise. Personally, I like a performance sound to go with my performance car. It's cheap and it works. -love, Jeffy.

[Paul]

Did you try it just with the filter change first, as that makes the engine purr/growl nicely on it's own, quite dramatic for such a small mod really!

[Jeffy]

That's where the growl came from. I don't know how much the rounded edged throttle plate is worth in horse power, but its easy to do, and it corrects a flaw that shouldn't have been, anyway. -Jeffy.

[Paul]

Indeed, and if you can DIY it easy enough, then why not.

[twouptons]

Smoothing the plate out probably doesn't help all that much (though I'm sure it looks better).

If you've ever had one of those balsa gliders when you were a kid, the leading edges are always squared-off and never rounded or really aerodynamically shaped at all. The difference in performance between the flat plate and something with a rounded edge is so small that it's not worth doing.

I'd guess that pretty much all of your performance/sound improvement came from the filter.

-Eric