[bondvagabond]

There is a good book that explains called "performance with economy". Came out in the early 80's for hotrodders that wanted to be able to afford to drive their car.

The short version is, on carbed or throttle body injected gas engines you don't want to polish your intake at all, because the gas is actually small drops when it gets to the cylinder instead of a mist, so the turbulence from the rough intake helps to decrease drop size, increasing torque and fuel economy.

Super simplified explanation of porting is taking a cylinder-ish shape and changing the ends to be more cone like. This allows more flow of air. Sounds good but with a gas engine or a non turbo diesel there is an effect called scavenging, where the exhaust in the exhaust system is traveling fast enough it helps suck the burnt gasses out of the cylinder on the exhaust stroke. This can help by putting less back pressure on the piston and/or ridding the cylinder more completely of burnt gasses so fresh fuel and air can enter more easily. Porting can increase air flow to the point that it reduces this scavenging effect.

I welded up a set of headers for a geo metro optimizing for this scavenging effect, one of the real engineers one here helped me calculate size and length. The runners were TINY! As in, they don't sell exhaust tubing that small. Torque and fe both increased. This custom header was essentially the opposite of porting.

The book I mentioned has nice dyno charts showing these effects.