Hey, somebody sent me an email! This person writes:
"The valve seat inserts overhang into my ports leaving a sharp edge. Should I try to blend these edges into the port?"
Let's see what happens when I try to blend the valve seat inserts into the ports on an aluminum head.
The illustration above is a cross section of a radiused steel valve seat insert showing typical issues where the insert transitions into the port wall, and where the top of the insert transitions to the chamber roof.
You can see at the transition from insert bottom to the port walls an overhang or underhang (or both) can occur leaving a sharp edge or uneven transition.
At the top of the insert, it is very typical to see a slight recession between the top of the insert and the combustion chamber roof (not to be confused with "valve seat recession"). I will not address this particular issue here, but only say that I have found only negative flow results in blending this area. Also, if this edge is very sharp, it may be wise to blend it, around the exhaust valve at least, for high compression or power adder applications to reduce the risk of predetonation.
This photo shows how a radiused steel valve seat insert is overhanging a valve pocket wall on this TFS aluminum head intake port on the outboard side of the pocket. You can even see a slight shadow under this overhang.
This photo shows the inboard side of the same pocket, where the seat insert is slightly underhanging the pocket wall. You can also see where the seat cutting tool has cut into the pocket material slightly.
An underhang and a sharp edge where the seat cutting tool cut into the aluminum pocket wall are shown on the short side radius of this exhaust port.
The head was flow tested in its stock form on a Flow Performance equipped micro bench. This system has shown that it has the sensitivity and repeatability to clearly show any changes in flow rates as each of the following steps were performed.
The underhang on the intake inboard pocket wall on the intake was blended into the pocket. This produced about a 1% reduction in flow at high valve lift!
Using an 1/8" ball burr on a Dremel, I carefully tried to blend the seat insert overhang into the pocket wall without getting too close to the throat cut on the outboard and back wall of the intake pocket. This produced no detectable change in flow.
The only change in flow for the exhaust port was the result of blending the underhange and tooling mark on the short side radius. This produced about a 3.5% increase in flow at .5" lift.
For a radiused valve seat insert, it would appear that minor transition "issues" produce negligable-to-none reductions in flow capacity except for the exhaust port short side radius. However, non-radiused valve seat inserts may be more sensitive to any transition variances.