Build a Porting Machine
(thanks to Tony (maytag) D of zcar.com)

Electric vs. Air
Electric Die Grinders are extremely efficient, and as a matter of fact are far more common and 
powerful than the air-powered equivalent.

Mr Clean
Even more cost-effective is to use what I used: a converted washing machine motor with a flex-shaft. 
The torque provided by a slow-speed (1750 rpm) 1/4 hp motor really gets the work done, without the 
high-speed chattering or control problems. This is a very common adaptation in Japan. 

There are couplers available using setscrews that have a 5/8" hole on one side, and the 1/4" hole on 
the other. Simply slide it on the motor, and slip a universal flex-shaft with collet end into the other,
 tighten the set screws, and you are on your way.

Comes in handy when you have tapered buffs and decide to polish aluminum wheels!
But the TORQUE! Man! You can hog the hell out of a coarse double-cut carbide, and the chips just FLY! 
And when running a 2" 80grit Merit Abrasives flapper wheel, you can actually BEAR DOWN and make dust 
fly out of the intake! 

Works well with those cartridge rolls on 6" extension arbores to port the intake runners, and with flapper 
wheels to smooth is all out in the end.

It's not as fast as the die-grinder, but it doesn't over cut, doesn't chatter, and doesn't fly out of your hand 
if you hit the wrong quadrant of the grinding bit.

I love my washing machine motor grinder. I don't know what I'd do without it. And if you really need one 
and don't have a trashed washing machine, go to harbour freight and pick up a 1 HP compressor motor 
for $59 on sale, and you're in big business compared to my anaemic 1/4 hp model!
And those you can get in 3500rpm, so you can actually cut even FASTER! muahahahaha!