Cylinder head port designs

[dan filipi]

I'm trying to learn myself about head porting.

I'm an electrician so for me I think of flows like water in a pipe, larger wire and water pipe allows more current and water volume and flow.

With that sense in mind when I look at the 1200 ports, both intake and exhaust, they are smaller than the earlier heads and so are the valves.
It seems to me (since air is a fluid like water) that a smaller port will let less volume flow through it which means a smaller fuel/air charge can enter and is more restrictive in it's discharge.

Is this the case or am I missing something?

 

[AApple]

Bigger is NOT always better(says the guy in Texas!).
It's not simply a matter of volume...it has to do with velocity. Smaller ports can raise the velocity of the air/fuel going into the cylinder, creating a ram effect of sorts. This will also help with fill volume, since the air/fuel mixture going thu the ports of the head accelerate quicker, bringing more volume in, without an increase in stroke, or depending fully on piston speed, valve lift or size, to fill the chamber. Works really well for low end torque, and fuel efficiency, but it does tend to hamper high rpm performance to some degree.
That's about as tech as I can get. It's a lot more involved, tho. I'm sure someone here has more knowledge of fluid dynamics than I do :read:

 

[slabghost]

There is a good deal of difference between pneumatics and hydraulics. Fluids are not compressable. Gasses compress smaller passages properly designed allow for higher flow with closer input pulses evening the pressures. I'm obviously not an expert but restrictions in passages create a higher pressure just before the restriction and a low pressure just after. Strategicly these can even the pulses and provide a much faster flow of gasses through smaller passages than is possible in larger passages.

 

[user 92]

I haven't been bitten by the racing bug either, but to the best of my knowledge I agree with the replies so far. The pressure differences caused by the venturi effect increases velocity.
Keep in mind also that with the elasticty of gas, it doesn't start and stop moving instantly. When the intake valve opens, the gas takes time to get moving. And when that valve closes, as the gas that is moving has nowhere to go it will start compressing behind the closed valve untill it reaches enough pressure to come to a stop. As the piston is creating a low pressure area, when the valve opens again that compressed gas now has a little expansion help to get moving again. Everything affects everything. Bore, stroke, port size, valve lift, timing-duration, etc. It's all calculated into the formula. (Which I'll admit is out of my league. Different school.)

I don't need to race my GoldWing. My only interest in following all this is that it would be kinda cool to have a little stronger pull from a stop with smooth low end, without loseing the nice cruise at the top end. I generally ride pretty easy, but I'd like to know the bike can haul a couple fat people and pull a trailer without coughing and wheezing. If I can learn from others here, I might consider as much as a cam swap, but I don't think I'd go as far as swapping heads and carb racks, etc. I know, I'm no fun. :roll:

 

[dan filipi]

Thanks for the input so far, I'm learning.

So when a head is "ported" and "polished" what does that mean?

 

[MSGT-R]

Yep, everyone is on the right track and adding good points along the way.

Hight velocity is needed at low RPM to keep the fuel droplets in suspension from the carb to the combustion chamber. Choke is needed while the engine is cold to compensate for the wetting down of the passage walls like fog on a cold mirror; heat aids vaporization (that can be good and bad depending upon how much and where). Intake passages must never be polished to a shiny finish because of this. Go crazy on the exhaust, but this will eventually carbon up again.

Most airflow from carb to chamber will travel the fastest around the outsides of each bend, the most critical being in the "pocket" just above the valve seat. The tightest turns and the worst obstructions (valve stem) are here, and grinding can either improve or destroy the performance of a cylinder head. Shaping the valve seat angles further aids in air flow past the valve.

Can I complicate things further? The airflow past the valve depends upon the amount of lift, the shape of the back side of the valve, and its closeness to the chamber wall. Hemispherical chambers are the best shape.

Most head castings on motorcycles are quite good and often only a port-match to the mating surfaces and/or gaskets is needed. Occasionally you will find a really crappy one and you have to be careful on how much and where you can grind without busting through to the cooling jacket. Bad juju.

Here's a qucky in-bike porting (exhaust) that I did last weekend on my KLR250. The port was way smaller than the gasket and tail pipe diameter. It was like a child with tonsilitis it was so small. I took almost a handful of aluminum out of it. I'll refine it later when I have the head out of it.