I have the head off my engine at present (ST170) and stripped down. Is it worthwhile polishing the ports while its off. The ally casting is quite rough but I don't think I want to spend hours polishing to gain a few HP.
Thanks
Ian

If you don't want to spend time doing it then you've answered your own question

Do you want the best horsepower from the engine or a cheap engine?

Perhaps a better question then would be What sort of gains can be expected from polishing the ports

Taking out any casting irregularities, and producing a port of as near common ‘diameter’ will provide better results.

A 300 - 400 grit finish is said to be preferable to a mirror finish.

Thanks Pete....advice always appreciated

It's more about what other parts you mix it with. With just standard cams and inlet etc then very little gains but if you get throttle bodies and bigger exhaust, cams and Pistons then it's more that it needs to be done as it will maximise the results.

That's helpful thanks, I'll be putting bike carbs on but don't intend to swap the cams or pistons so im thinking it may be unnecessary...

Good luck with the build.

Port polishing does not produce any more power, the rough cast finish actually helps prevent fuel drop out and pooling.
In the bad old days polishing ports was used to convince people a good porting job had been done when in reality it was merely a cosmetic con job.
I would look for 3 angle valve seats bigger valves, waisted stem valves with penny on a stick valve heads rather than shiny shiny ports.
Matching the inlet port to the head is a worthwhile job though if you want to do something to help flow and will probably take less time than polishing your ports.

As snapper says you don't actually want polished ports, just remove sharp edges. Most 4 valve heads are pretty good these days straight off the line. Race engines often have polished ports but they are looking for max port velocity at very high engine speeds and even then the % gains are small. The Zetec head design is also aimed at exhaust emissions performance rather than max power and the ST170 was a cosworth tune up on the original engine so I doubt there would be anything gained in polishing the ports. As for bigger valves etc if memory serves the valve size increases are small as the valves are quite close together. I would work on cam design and intake and exhaust design for maximising performance.

The latest vogue is to machine dimples into the inlet tract, like the ones on a golf ball. Some dimple fanboys go even further and dimple the combustion chamber and the piston top!
There is something to be said for polishing the exhaust port to slow down the build up of carbon but even that would seem to be a waste of time given your average kit car mileage.

Don't think I'll bother with the polishing , thanks for the advice everyone, much appreciated
Ian