The Flow Fallacy of Straightening the #1 and #5 Ford Intake Runners

The Fallacy of “Straightening” the #1 & #5 Ford Intake Runners

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 must be angled 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. Physically, the cast runner intake exit is located behind the head port entry so they 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. 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 runner 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 in negotiating the turn. This tendency of air flow was first documented by a Romanian by the name of Henri-Marie Coanda in the 1930s. The very best radius would be infinite, but since we can’t get that, we want to use the largest radius we can in the short side wall transition to the port entry.

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 unintuitive 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 happen in a closed runner with high air speed inside – the idea is to keep the velocity gradient in the runner as evenly spread through the cross section to the runner as possible. The higher the velocity and the tighter the turn, 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 – and I’m ALL ABOUT keeping the air column cross section air speed profile high.