I am writing this tech article to address my thoughts on the practice of welding the Ford EFI lower #1 and #5 runners to “straighten” them for better flow. I have never been a proponent of doing this the wrong way, so I wanted to put my thoughts in writing for you to chew on.
The two front runners in Ford 5.0 lower intakes (as well as Ford carb intakes) angles forward in the car primarily due to the front mounted distributor in Ford engines. This causes the #1 runner to have a forward angle of about 30 degrees and the #5 runner to have a forward angle of about 50 degrees. They also have necessarily sharp turns into the head runners (which angle back the other direction by about 10 degrees), making the total turns 60 and 40 degrees. High velocity air does not negotiate tight turns well at all. The mass of the moving air column tends to over shoot the turn, “stack up” in the long side radius and not make efficient use of the runner cross section. This causes the problems of turbulence and inertia blocks to air flow.
Some feel the answer to this problem is to weld up the backs of the “knees” of these two runners on top the intake as well as welding the runners underneath so that the runner can be “straightened” by porting or machining. This is also done in some of the other runners with much smaller turns and they do not have the issue to the same degree as the front runners – they can be effectively straightened if need be. I drew up the following illustration to make my point. Notice that the physical angles of the runner and the head cannot change – the only area that can be worked is the transition at the head flange.
Notice the straight dotted line that represents a “straight” runner. What has been done is a radius that existed before has been turned into a “V” angle right at the head flange. The restriction has been moved from the intake runner to the head port entrance. High velocity air must have a large smooth radius to maximize its tendency to want to “stick” to the near side runner wall rather than being thrown against the far side runner wall. This tendency of air flow was first documented by a Romanian by the name of Henri-Marie Coanda in the 1930s.
Coanda noted that a stream of fluid or gas will tend to hug a convex contour when directed at a tangent to that surface.
You can check this out for yourself by turning on a tap, so that there's a steady but gentle continuous stream of water flowing. Now bring the back of a spoon into slight contact with the stream and you'll find that the water will no longer fall straight down but actually stick to the curve of the spoon.
This rather un-intuitive behavior is the Coanda effect in action.
It's pretty easy to understand why a flow would be deflected by a concave curve -- the curve will "push" the flow around the corner -- but isn't it odd that the flow seems to be pulled by a curved surface?
Both the concave (long side runner wall) and convex (short side runner wall) effects happen in a closed runner with high velocity air inside – the idea is to keep the velocity gradient in the runner as evenly spread as possible through the cross section of the runner. The higher the velocity and the tighter the turn (as in a "V" shape versus a large radius at the short side runner wall), the more the air is going to "stack up" on the long side of the runner and reduce the flow efficiency.
So, creating a smaller radius by “straightening” the end of the intake runner may not be a good move – what do you think? I think it is a much better idea to use the runner’s existing wall thickness and/or welding it more to make the radius larger and more gradual over a longer portion of the runner. This will help keep air distribution more even in the runner and produce more flow at higher velocity – and I’m ALL ABOUT keeping air column velocity as high as possible. You can get velocity too high, but that is another subject and another day. Below is an illustration of a runner wall that illustrates what I am saying. Keep track of the wall thickness at the thinnest part (back of the knee) and work around that point as a pivot point for runner design.
