Port and Polish

First the intake: there is a phenomenon known as laminar flow -- when fluid moves inside a conduit, it tends to move in "layers" that may travel at different speeds, but they tend not to inter-mix. Something like this happens in the intake. We would prefer turbulent flow between the venturi and mid-port and again once inside the cylinder; in this case there is much homogenation of the fuel into the air stream. There is also something called the "Tau Factor" -- usually only mentioned in relation to fuel injected motors, but it happens with carbs, too.

When the conduit wall is very smooth, particles of mixture tend to fall out of suspension and wet the walls. Sheets of fuel run down onto the hot back of the intake valve and vaporize. This makes for a very rich mixture around the plug which is (for the wrong reasons) adjusted for by leaning out the mixture over-all to even things out.

Rough port walls maintain a turbulence that keeps the fuel in suspension. There is much discussion on what parts of the port walls should be "rough" and just how rough they should be. Personally, I like about a 240-280 (sandpaper grit) through most of the port with 320 or a little finer downstream of the guide.

Exhaust: Conventional wisdom dictates that the most power is realized when the exhaust gases stay as close as possible to combustion temperature all the way to the atmosphere. So polish those suckers until you can shave in them. Also consider having the inside of your pipes enameled at least as far down as just past the first bend. Rough port walls encourage carbon to nucleate onto and coat the walls. This also has an insulating effect, but the carbon continues to build (especially in the areas that are hard to polish -- which is where rapid flow is generally needed more) choking flow down.

Combustion chambers: When fuel ignites, it generates heat while it expands. We'd like as much of the energy released to be harnessed from the expansion and as little wasted in the shedding of heat. The smoother the head (and valve face) surfaces, the more heat is "reflected" (it's really a lot more complicated than that) back into the gas; forcing/allowing more expansion to push down the piston. You sometimes see racers having Teflon or ceramic applied to their piston skirts to reduce friction, but also to the inside of the combustion chambers and the crowns of the pistons to reflect heat better. Also to keep carbon from depositing.

More to come . . .

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