smaller ports
 

mototuneusa has several articles on head porting in which they claim smaller intake and exhaust ports actually increase horsepower, and larger ports decrease horsepower. they only have experience with 4 valve heads, but feel this would also work with 2 valve heads. any thoughts on this?

Velocity = momentum = torque.

A lot of the reason for largish ports is due to trying to avoid laminar to turbulent flow transition in the Re=3000 range. It's easier for a manufacturer to make 'em bigger and not worry about how messy or suboptimal the casting is.

However, do two things, provoke a graceful laminar to turbulent transition and emphasise shape over size and you get a lot more torque and low end power. High end breathing may be restricted a little but that's just for big shiny numbers, the fastest vehicle and more efficient vehicle has the most area under the power curve, not the biggest spike in it.

Graceful flow transition may be achieved by such methods as mpgmike's "Powre Lynz" or coarse sanding or burring the intake. This provokes microturbulence before the typical flow transition velocity, preventing flow discontinuities and sub-optimal port shapes from forming large scale turbulence that restricts flow. Think of the small scale turbulence as ball bearings and think of the large scale turbulence as golf balls, if you try to put both of them through a twisty tube with sharp corners, the ball bearings will pour through and the golf balls will probably jam up somewhere.

Additionally mpgmike has discovered that "lynz" have a variable intake effect, by apparently increasing the effective size of the boundary layer at lower intake speeds, making a higher velocity core airflow stream in the middle, then as the flow gets higher, the boundary layer compresses and the area of the high velocity stream is greater.

A further aspect of their use on FI motors is at low demand when injectors are spraying into the closed ports, a sharp bottomed surface feature, with a dimension of under .2mm should allow capilliary action and "wicking" to take place, spreading droplets of fuel round the whole area of the port, allowing quick evaporation before the port opens, instead of just puddling as they would in a smooth port.

Another addtional effect for pits and grooves with dimensions of about .5mm to 2mm deep is that they should function as helmholtz resonators (think blowing over a beer bottle) in the ultrasonic range. The ultrasonic frequencies generated may work to break up large fuel droplets.

With regards to port shape a wider inside radius and smoother entrance to the valve is always better, if it takes volume from the port, no biggie, the flow velocity should be better.