Kelly,

Ah so! I wasn't aware of that one and was hoping Lynn would make some comment.

 

 

Everything affects everything. Unless there is a need to operate the engine well above 9,000 RPM, there is no need to modify the exhaust port to any great extent.

 

Later closing the exhaust port (Port cut taller) extends the open time and increases overlap with the intake ports. This is quite helpfull at very high RPM. The same for early opening of the port. (Cuting it lower) there is little to be lost here at high RPM. The available work left in the charge has been reduced to near zero, and the late opening is to help sound by allowing more complete expansion before the charge is released. Radiusing the opening lip of the port gets flow started along a smooth curve where much of the flow stays along the bottom of the runner. There is some interest in removing the exhaust flow completely before uncovering the intakes. Adding the radius along the opening line and up the sides of the port does that.

 

Other than removing tooling marks and smoothing the top of the port there is not much to do. You don't want flow from the top of the port in either direction. So leave the top as per factory. I would reduce the chrome and iron  around the port in all directions just s few thousandths to avoid any possibility of snagging an apex seal. Just a 16th of an inch or so is fine.

 

If a chunk of anything goes through the port you can stand a burr being stood up or a scratch along the edge of the port. In the stock situation this would be seen by the next apex seal. If the surface next to the "hole" has been reduced slightly you may survive that gremlin without any drama at all.

 

If you want more exhaust flow, you can go wider and not change port timing at all.

 

The engine suffers from exhaust dilution of the intake charge well into the RPM range. Adding a big radius to the top of the port opening would raise the RPM where dilution more or less stops. So I see no point in changing it much.

 

Lynn E. Hanover

 

 

 

Yes your right the top edge of the exhaust port is correct.

I had spoken to Lynn in the past on the bottom edge ( leading edge) but not the top edge - I didn't know he suggested to slightly radius that edge - if he did I can't remember.

 

If we are singing from the same sheet of music - radiusing the top edge, does increase exhaust flow characteristics. See attached drawing.

Jump inhere if you wish Lynn!

George ( down under)

George and All,

          I do not follow !! I am familar with Lynn's suggestion to add slightly more

radius to what I call the top edge of the exhaust port.........Are you suggesting

additional work to this edge ?? Perhaps a simple drawing would help my simple

mind visualize what you are suggesting !!

--
Kelly Troyer
Dyke Delta/13B/RD1C/EC2