The International Cessna^® 170 Association

Horsepower in an engine is determined by Displacement, RPM, and BMEP only. Port polishing will only increase power if it changes one of the three mentioned above.

The theory behind port/polish is the same as bigger/more valves; decrease the resistance of moving the fuel mixture to the cylinders allowing more volume thus increasing BMEP.

But, just like bigger/more valves or a bigger carb or anything else that increases intake efficiency, it requires very high reving engines to make use of. Even then, the difference is tiny. But in races where winners are defined by .01 seconds, it's worth it but even then it's the LAST thing you do to a motor for more power.

The O-300 (or any airplane motor) doen't turn fast enough to make use of this technique.

Thanks for the replies guys, I kind of figured it had something to do with racing. The guy asking me if I thought about polishing the ports is an ex race car driver. He sounded adamant doing this would increase power a bit and result in cooler temps as the engine doesn't have to work as hard to draw air/fuel. I thought I'd ask around anyways if there was a legal means to do it. Sounded like an easy way to gain a little power. I guess someone would have an STC out if it was worth doing. I think i'll just stick to my plan to buy a seaplane prop and reduce weight as much as possible.

About plus 3years ago ,we had to rebuild our o-300 and it needed new cylinders and pistons. The intake manifold was checked out and found the castings inside was looking at the Grand Canyon .This was polished out and the heads intake and exhaust wasn't much better and these were flowed and polished . TCM "gave "us some new pistons ( still waiting on the bill) and rings. They were domed with some small cut outs in the top of the pistons. The Carby had to be replaced as the float was made of brass and had been repaired crudely. A one piece Venturi was included. The cam shaft was replaced and a different grind was part of it. 99% of this was legal because many specifications are not included in the type approval. The whole rotating mass was balanced . The engine was then mounted on a test truck and run for over 12 hours to fix bugs and test its reliability . This engine was so smooth and produced 185 hp .at 2800rpm. Fuel consumption was about the same as the old one at the same rpm. However at higher rpm it really could eat the fuel. Oil temps were normal but a Air Wolf spin oil filter was fitted also . Chts were a little higher but 380deg was normal . Over all a good engine