Q&A / Tech

Ask Away! with Jeff Smith: Port Matching vs. Pocket Porting — Which is a Better Use of Time?

I’m still buying parts to assemble a 355-cubic-inch small block Chevy for my ’55 Chevy. My buddy was over this past weekend and told me that I should make sure to port-match the intake manifold to the heads. Is that something I should do? He says he read where that could be worth 10 horsepower or more. If that’s true, then I will certainly make the effort, but I thought I’d check that out before I do the work. Thanks


Jeff Smith: Port matching the interface between the intake manifold and the intake port opening on the cylinder head is one of those classic mod stories that have appeared probably dozens of times in various magazines.

On the surface, this would appear to be an important part of any high performance engine build. This would seem especially important when the intake port opening is smaller than the manifold. This is even supported by flow bench testing where any obstruction or sharp edge at the port inlet results in major flow losses.

But this is one situation where the flow bench can lie to you.

I can give you a real-world example. We were testing intake manifolds at Westech for a story not long ago and one prototype manifold had a larger port exit than the intake port opening in the heads. This created a protruding ledge that was roughly 1/8-inch (0.125-inch) around the entire circumference of the port. The manifold designer was hesitant to test the manifold because he had experienced a large flow loss whenever there was a port obstruction like this. Both dyno operator Steve Brule and I offered that this was something that he didn’t have to worry about. The designer disagreed and that discussion meant we had to run the test. In this particular situation, we were using a 450-horsepower small block Chevy.

Our manifold designer buddy (whose name we will withhold to protect the innocent!), was convinced we’d see at least a 5-horsepower loss in power. Conversely, our position was that the engine wouldn’t know the difference and that the power would be the same. Or, perhaps the power might improve if this latest manifold was indeed better relative to the previous manifold with a smaller port opening. We quickly ran the test and the power curves for the two manifolds overlapped perfectly. The power was exactly the same for both manifolds with no difference in either torque or horsepower.

Here’s the explanation. If we were to plot the speed of the air travelling down an intake port, the highest speed would be in the center of the port. As the air moves closer to the port wall, the velocity begins to slow until it reaches the port wall where the speed is zero. The friction from the interface between the air and the port walls is what contributes to this effect. This is sometimes called the boundary layer effect. But you don’t have to take our word for this. We’ve included an image straight from NASA on how this works.


This NASA drawing illustrates how air velocity begins to taper off as the air gets closer to the surface of the port.

If the air is travelling very slowly at the walls of the manifold/intake port, it should be easy to see that even a 1/8-inch of ledge protruding into the port will have a minimal effect on port flow. The opposite is also true where the intake port is larger. The effect is essentially inconsequential. Would an NHRA Pro Stock or NASCAR engine builder allow this to happen? The answer is no, but for your average street engine builder, it’s an area of minimal concern.

This sounds crazy but we’ve seen big block Chevys where an oval port manifold was used on an engine with rectangle port heads and it seemed to run just fine. We’re not recommending that you do this, but in the case of an emergency, it certainly can be done.

One reason this port-matching modification continues to be popular is because the openings are easy to access. But if you’d rather maximize your time and effort, the smarter move is to spend all of your porting concern on the area within roughly ¾-inch on either side of the valve seat. That’s where a little bit of pocket porting work and radius work can have a measurable effect on power – if performed correctly. To illustrate this, we ran a test many years ago on a set of iron 1.94-/1.50-inch valve small block Chevy heads. We spent 8 hours doing pocket work on both the intake and exhaust ports just around the valve seat area. This was worth a solid 25 horsepower on a 350 horsepower engine. Not bad considering we did not increase the valve size.

So if you are going to spend some time on your cylinder heads, learn the techniques for pocket porting and spend your time there.

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  1. I would say that this is something that will worry you to know end (port matching), Say you go in and do the pocket work and not the ports. You know the feeling, why didn’t I do it? I was right there. WHY? WHY? WHY? Oh yeah I read an article that said it wasn’t really needed. Just do the pockets.

  2. So you didnt match port the intake to the head. And didnt see a difference. (A decrease would have been expected)
    But what if you had? (Could have resulted in an increase?)

  3. when ya do a comparison like this you MUST cover all aspects. I agree that a 3rd run should have been made with matched ports. Now that run would have been the golden one info wise

  4. The NASA reference here is a little misleading in the context of the subject. What the illustration fails to mention, intentionally or not, is the fact that a smooth surface, like that of a smooth polished intake, is where a boundary layer forms. Air gets lazy on a smooth surface (think of the water “cruzing” across your car windows while driving in the rain). This “boundary layer effect” is in effect at approximately 50-300 mph [I have witnessed this in jet planes during take off in the rain and fog]) after which the layer of low pressure is squeezed shut from the surrounding pressure. This is why it’s never good to port AND polish your heads (reduces midrange power [air in the ports can travel upwards of 400 miles per hour]). So the boundary layer you reference (especially if the heads have been done right) isn’t necessarily there. And turbulence, created by sharp edges (via a lack of port matching) and other uneven surfaces in the ports, can create unnecessary turbulence and impede the flow of air. (P.S. A dimpled surface will disrupt the boundary layer reducing turbulence and maximizing airflow.)

  5. I have been doing “alot” of reading on port matching, throttle body and diameter in reference to a my harley 2003 softail tc88 with all the bells and whistles on the rebuild. But I did not do port matching.
    Your article is informative and will help me in future decisions.
    Big thanks

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