SM Head Modifications on a budget
Here's what the port mold revealed. Mold was cut in 3 locations, with location number 1 being in the center of the head bolt bulge which is also the apex of the short turn. Area at this port section is 2.04 in². Area at cut 2 is 2.42 in² and area at cut 3 is 2.80 in². Port length distance from the apex of the short turn to cut section 3 is less than 2 in. So in a distance of less than 2 inches, the port area increases by 37%. No wonder we have flow separation at high speeds. It's impossible not to. The paper labeled 'Port Mold Section 3' shows a circle of 1.88 diameter which is 2.80 in², the area of that section. If average port velocity is constant from section 1 to section 3, only the upper 2.04 in² of the area is needed. The bottom 0.76 in² marked in pink are not needed for flow and is the area where flow separation occurs. Obviously this is an oversimplification, but the point is evident.
We have established that there are SSR issues with this head. It also appears that area at the apex is a huge problem. I have included a flow graph of the port being discussed in this post (port 3/6) and a flow graph of another port in this same head that I've been working on for quite some time (port 4/5). Port 4/5 has had significant porting and widening of the apex along with chamber and PRP mods. It has had some minor SSR work. Port 3/6 has had no SSR work and no apex area work. Port 4/5 is peaking close to 300 cfm on a SSR that has been widened and laid back but not lowered.
The port being worked in this post (3/6) is stuck around 260 cfm. I believe the biggest issue is lack of area at the apex and too much expansion immediately thereafter. The SSR controls the instability at 0.500" lift and up and probably reduces peak flow a little.
This leads into another topic about the purpose for large bowls. One theory says that a large bowl slows the air down to help it get around a turn. In this case that seems like hogwash. If the air is in the turn already, it's too late to slow it down. At high speeds the large bowl just promotes separation.
Here's what the port mold revealed. Mold was cut in 3 locations, with location number 1 being in the center of the head bolt bulge which is also the apex of the short turn. Area at this port section is 2.04 in². Area at cut 2 is 2.42 in² and area at cut 3 is 2.80 in². Port length distance from the apex of the short turn to cut section 3 is less than 2 in. So in a distance of less than 2 inches, the port area increases by 37%. No wonder we have flow separation at high speeds. It's impossible not to. The paper labeled 'Port Mold Section 3' shows a circle of 1.88 diameter which is 2.80 in², the area of that section. If average port velocity is constant from section 1 to section 3, only the upper 2.04 in² of the area is needed. The bottom 0.76 in² marked in pink are not needed for flow and is the area where flow separation occurs. Obviously this is an oversimplification, but the point is evident.
We have established that there are SSR issues with this head. It also appears that area at the apex is a huge problem. I have included a flow graph of the port being discussed in this post (port 3/6) and a flow graph of another port in this same head that I've been working on for quite some time (port 4/5). Port 4/5 has had significant porting and widening of the apex along with chamber and PRP mods. It has had some minor SSR work. Port 3/6 has had no SSR work and no apex area work. Port 4/5 is peaking close to 300 cfm on a SSR that has been widened and laid back but not lowered.
The port being worked in this post (3/6) is stuck around 260 cfm. I believe the biggest issue is lack of area at the apex and too much expansion immediately thereafter. The SSR controls the instability at 0.500" lift and up and probably reduces peak flow a little.
This leads into another topic about the purpose for large bowls. One theory says that a large bowl slows the air down to help it get around a turn. In this case that seems like hogwash. If the air is in the turn already, it's too late to slow it down. At high speeds the large bowl just promotes separation.