Exhausts ports are polished, intake ports are left a little rougher deliberately so as to allow the evaporation of fuel.  Porting of a combustion engine has a lot to do with acoustics an inertia.  There is no steady flow.  Its all pulses, so surface finish largely of secondary importance compared to that. 

Am I supposed to see pictures in these threads?   I never see any. What so I need to do to see the pics?   I want to see how BL is progressing. :)

On Wed, Sep 7, 2016 at 10:02 PM, Robin Warde robin.warde@gmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:

 

The more turbulent the flow of gas into the engine chamber the better, because it creates a more even mix with air and therefore a bigger explosion. By contrast, the inlet manifold walls are often polished to a very fine finish to reduce drag and allow as much air as possible to enter the chamber during each cycle.

On Wed, Sep 7, 2016 at 10:57 AM, dennisjzr31@gmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:

 

The gas flow in an engine is always turbulent.  Its just too fast to be laminar.   A smooth golf ball drags a bigger vortex behind it.  The turbulent boundary layer of a dimpled ball projects a bigger object to the oncoming air, however this is more than made up for in drag reduction as the air sucks around the back of the ball thanks to the boundary layer.   A foil or boat hull is designed to bring the flow gently back together behind it unlike a golf ball, so turbulence is usually a hindrance.  However in some cases it works in a similar way.  For example, foils at a high angle of attack that are stalled.  If vortexes are promoted in this case the flow will stay attached a little longer just like a gold ball.   Google foil tubercles.