was just wandering  about the difficulty level when it comes to polishing heads on your own.the kits arent that expensive but im not sure how difficult it would be to try this at home i mean i never tried this before but i think i'l rather try the polish before i try to port it just looks difficult to me or maybe its just me i dont know.please explain  

Are you polishing the exterior for decorative reasons or the inside of ports and combustion chambers?

I'm guessing you're referring to ports. In which case, this idea went away a long time ago with the Ford Flathead. It was found when people started using flow benches that flow through a port went down when they were polished. The reason is that air molecules stick to smooth surfaces and build up a slow moving layer against the surface. Additionally it's found that as shapes of the port change that it takes more energy to cause the air to transform from one contour to another if the surface is polished. So port surfaces are left with the surface roughness typically left by the porting tool be it a high speed stone or carbide/metal cutter or mill. The slightly rough surface causes little swirls to form that actually make it easier air molecules to move along the surface.

Polishing is used in combustion chambers to provide a surface that reflects heat back into the cylinder, producing more work instead of loosing that heat to the cooling system. A polish in the chamber also helps to prevent carbon from forming deposits on these surfaces.

The Harley guys persist in polishing ports, other than they tend to be 50 years behind the State of

Adding on to your statement, Bogie. How long does the polish really last? I've seen articles on Keith Black's website claim huge gains from polishing, like 6% increase in torque, and "they" stated that the polish even works after it is covered in a carbon film, could this be possible?

I've found that the effects of polishing chambers goes away within several hundered to a thousand miles on a street engine.

For a competition engine that sees frequent tear downs for cleaning and inspection, polishing makes more sense.

Bogie

This is the statement I'm referring to. I'm sorry for asking you about what others have said, but I just thought I'd ask if you believed there to be some truth to this.

Would cutting a sharp line just "outside" of the valve opening possibly add in turbulence that might be detrimental to high RPM engine?

I wonder why they "reccommend" not to "dimple" combustion chambers?

Experimental work to reduce piston heating of the incoming fuel mix has been very limited, but in theory a thin coating may prove to be beneficial. A thin, smooth coating over a polished piston should still reflect combustion heat while reducing caron buildup and protecting the piston polish. It is easier for a thin film to change temperature with each engine cycle than it is for the whole piston to do the same. A thin film can be cooled by the first small percentage of inlet fuel mix, allowing the main quantity of fuel mix to remain relatively cool. Tests have shown that polishing the combustion chamber, valves and piston top can increase Hp and fuel economy by 6%. So far it has proven difficult to keep a coating on a polished piston.

All this polishing is counter to the practice of dimpling the combustion chamber. Dimpling has been shown to put wet flow back into the air flow and improve combustion. We do not recommend dimpling, but do suggest cutting a small discontinuity close to the valve seat to turbulate wet flow. Some bench flowed cylinder heads encourage fuel separaton at the inlet port. If a small step is added at the valve seat to force the wet flow over the resulting sharp edge, fuel will re-enter the air stream and give you the same affect as dimpling, only without losing the benefit of a completely polished chamber.

In this case they're referring to wet flow where the fuel is separated from the air and needs to be mixed up again before the spark is applied. What happens here is that the fuel either didn't mix well to start with or became separated through the twists and turns of the port.

Without knowing that separation took place, to a tuner, the engine runs lean, but the exhaust smells and an O2 sensor reacts like its rich. This is the kind of a problem that can drive a sober man to drink.

The idea of dimpling the chamber roof was to create some local air turbulence to help it to draw the wet fuel back into suspension. Of course one of the contributing problems is that there is minimal airflow against this surface so dimpling usually did not provide sufficient mixing.

The idea of cutting a discontinuity is to do the same thing, which is to re-energize the boundary layer and to place the fuel in its path such that a remix would take place.

This is a place where the combination of the intake manifold and the port come together to cause the fuel to separate from the air. To some extent simply making turns causes this separation, especially as the throttle gets to WOT and the manifold vacuum drops. Vacuum helps evaporate the fuel so the problem of separation is somewhat mitigated at light throttle settings.

Cold intake manifolds on a carbureted engine also contribute to this problem. Exhaust heated intakes are much better at getting fuel into suspension with the air and keeping it there. Some solutions for a cold intake are smaller ports to speed the mixture up and cause greater mixing of the fuel and air. Purposeful mismatches of manifold to head port, where the manifold passage is slightly smaller than the head port such that liquid fuel traveling on passage walls is spun back into the mainstream by the turbulence of the mismatch. Always the mismatch is small and the manifold passage is smaller in any and all dimensions than the head port.

Vapor point of the fuel comes into play, where a more volatile fuel (low vapor point) is easier to mix with the air. One could think of this as running winter grade fuel in the summer time. Putting about 10 percent methanol into the fuel of an engine that tends to wash the chamber roof can help.

The port ahead of the valve on the long side can have a groove or a step just under the seat to catch any liquid fuel and turbulate the local boundary layer to cause mixing. Ditching or leaving a lip on the backside of the valve can also help at low lifts. Also, a sharp edge lip on the sparkplug side of the port merge into the chamber roof can be helpful in putting fuel traveling along this surface back into the air stream. Then of course there’s dimpling. Certainly leaving an as cast or as ground in the case of ported passages rather than polishing the ports is very helpful in keeping the boundary layer invigorated, thus mixing liquid fuel back into the air stream. This is especially important with divergent cross sections where a high adhesion surface causes a thick and viscous boundary layer to form. This is a place where fuel will condense and fall out of suspension as a liquid and dribble along the metal surface. The valve pocket is one of these places where the mixture is allowed to expand, loosing velocity and regaining pressure. The expansion reduces temperature, this combined with the loss of velocity is a recipe for fuel to condense out of the mixture and run as a liquid along the walls. Among other things this can tell you that the ports are too big for the engine’s displacement and rev range. This is more of a problem with hot street engines, short circle or road track engines where there are large changes in throttle position over fairly short periods of time.  

For a competition engine that sees frequent teardowns for inspection it’s pretty easy to see the effects of fuel wash. Street engines are harder to diagnose since we don’t tend to open them up very often. On the street this can get to be a big problem when long duration high lift cams are run at highway speeds and in stop and go traffic. Long duration, high lift cams kill port velocity at low speeds and fuel drop out can become a big problem. A simple solution is port injection where the fuel is put in at the valve under considerable pressure. The fuel stream is designed to hit the hot backside of the intake valve where there is a major air flow at all lifts. The physical impact of fuel on the valve breaks up the stream into a fine mist that is drawn in with the air flow. This gets around the manifold flow problems found with carburetors and throttle body fuel injection. This rather takes us back to intake design with a competition engine.

When performing head off inspection on an engine that isn’t accumulating hundreds or thousands of miles, one can plainly see fuel wash. On a cylinder by cylinder basis it’s possible to see individual cylinders that may be running rich due to fuel separation problems of the manifold. To some extent these mixture problems can be managed by changing passage shape, ditching and damming the passages to force fuel where it’s needed. But this typically entails a lot of “cut and try” effort.

There’s no way around the fact that you’ve pretty much have to go through a season with a set up, tuning and refining it to get it on the competitive edge for the next season.      

 Bogie