Polishing the intake- and exhaust port???

Hey guys,

Is the HP increased by polishing the intake- and exhaust port on a motorcycle or my KZ750 B1??

I read somewhere that the rough surface inside the intake port was helping to mix the fuel and air before entering the combustion chamber? Is that true?

Would you recommend to polish the intake and exhaust port?

Kind regards Andreas

It's worth polishing the exhaust to reduce carbon build up but leave the inlet un-polished to aid atomisation.
In truth you probably wont tell any difference as it is the port shape that aids power and removing material from the wrong area can create unwanted turbulence and actually reduce power.
If you don't know what you are doing I would minimise the work to just smoothing out any step between the intake rubber and port .

easiest way I can think to explain porting a head.its not size that's important,iits how it transisions from one end to the other that makes the biggest difference.

The debate over polishing is as old as the combustion engine. In theory a polished surface has more friction to an enclosed turbulent flow. Why? Because the laminar boundary layer acts as a buffer between the material surface and the free stream within the pipe (or intake runner). Reducing surface roughness reduces the thickness of the laminar boundary layer and thus increase the pressure drop though an enclosed flow duct due to friction. This is a well known principle of fluid mechanics. Strangely enough engine builders have shown in some cases that polishing can actually increase flow despite the fact that we would expect just the opposite.

What is my point with all of this? Well if you don't have any first hand knowledge with exactly what you are doing when polishing a intake port runner, then don't do it. In practice intake ports should be left with a some surface roughness like a bead blasted finish and exhaust ports are polished smooth to prevent carbon build up.

More important than the surface treatment, the transitions are what you need to spend time on, specifically the entrance to the intake manifold and the valve seat area ( bowl work). Again if you have no first hand knowledge of what to do, leave it alone.

On a purely stock engine, I'd be really surprised if polishing added any measurable power. Correctly reshaping the ports might gain a pony or two, but the ports on most of these engines already flow just about as much as a stock engine can take.

Again thank you guys!!

I really appreciate your time and respons. I will stay away from the inlet and polish up the exhaust outlet to reduce carbon blockage.