I came across something interesting that turned up in a search. I had not heard of this before, so I thought I'd post it for discussion:


In the December 2001 issue of Muscle Mustangs & Fast Fords toward the back of the magazine is an article by David Vizard about porting a pair of Windsor Jr. Lite cylinder heads. This article discusses porting for velocity and anti-reversion characteristics. For me this article opened up a whole new area of thought with regard to tuning an engine for power.

I'll try to characterize what the theory is.

Air passing through both intake and exhaust ports does not travel at the same speeds throughout the cross section of the port. For example, in the intake port the port wall closest to the centerline of the cylinder has more flow than the opposite side. The variation in the Windsor Jr. head was 300 ft/sec on the fast wall and 200 ft/sec on the slow wall. The slower velocity side of the port is where reversion occurs.

The article goes on to describe some simple steps to reduce the difference in the cross port velocity as well as move the center of the high velociy area closer to the middle of the port in the case of the exhaust port. On the intake side the before variation was 300 vs 200 ft/sec (fast side vs slow side), the after was 350 vs 300. On the exhaust side the before and after was 260/60 and 300/150.

I wish there was some dyno charts to show the effect on torque but David theorized that the engine would produce more torque throughout the power band, have a wider torque band, and improve low-end torque for a given camshaft. To me this is a win,win,win situation.

Strangely, I haven't heard much else about the subject since. I realize that most magazines tend to port and tune engines for maximum horsepower (at the expense of torque, driveability and efficiency). Big hp numbers are what sells. But some of us want more versatility from all our vehicles.

So far my porting skills have been confined to my squarebore to spreadbore adapter, and I'm certain it flows major impressive cfm's!!

 

Here is a more recent article written by David Vizard:

Cylinder Head Porting Mistakes - Engine Masters Magazine

 

Intentional mis-matching has been around for quite a long time but not discussed much in the magazines. I think they first mentioned it as a simple way to get a little of the effect of anti-reversion headers.

You've looked at how factory ports and manifolds never line up at the edges. The ancient notion of porting was that you would even up the edges all around. But what that leaves you (with the manifold bolted on, let's say, an intake manifold) is an intake runner that has a wide spot in it (unless you opened up the entire runner to match, which you probably don't want to do unless you're seriously opening up the ports for racing).

But if you think again about those mismatched ports you've seen, they are nearly always somewhat misaligned. Now you expect and want the airflow to be going only one way, from the carb to the cylinder. Think about that moving slug of air/fuel charge as it gets to the misaligned holes (boy, I wish I knew how to do a drawing!). Part of the hole will have a square edge sticking up in the way of the airflow. Another part of the hole will have a square edge that the airflow flows over, drops over. Well, the only part you really need to grind away is the edge that protrudes into the airflow. The other edge might have a little turbulence behind it, but it's not an impediment to the flow.

Annnnd, as somebody realized, this second edge has a bit of anti-reversion effect, that is, it tends to block the flow of any pressure waves that might be trying to come "UP-stream". These wrong-way waves can produce, usually in competition engines, a phenomenon called, "fuel stand-off" (undesirable; you can google it). So in the interest of damping down these wrong-way effects, some engine builders started to purposely grind a bell-mouth shape at the manifold-to-port junction, so that the wrong-way waves met a square edge that tended to weaken them. This can and is done with both the intake and exhaust side; you just have to keep track of which way the flow is supposed to go. I've done it for twenty years or so, though on mild street motors, therefore it made no difference detectable by seat-of-the-pants. Well, as an old 2-stroke racer, I just like to "improve" stuff with my port-grinder

Some more recent observations have been made on this port matching/mismatching deal. First, David Vizard, who has done more dyno pulls than almost anyone, has found that anti-reversion exhaust headers (google it if you haven't seen how they're built) are surprisingly helpful on street motors of all kinds. That would indicate that the intentional mismatches would be too (though not nearly as dramatically as A.R. headers); hey, maybe my grinding really did benefit my streetmotors! Second, from the same source, if you're purposely grinding a mismatch, it is much more effective if it is mainly or entirely done on the floor of the port that goes to (intake) or comes from (exhaust) the short turn side of the port. Third, I.R. (individual runner) intake systems, for instance three 2bbl Webers on an inline six, are somewhat prone to metering glitches from those pesky wrong-way waves; the top tuner who has been talking about I.R. systems says that creating a bell-mouthed mismatch in entry to each runner at the carburetor mounting flange will damp a lot of this out.

 

I liked this statement form Vizard's later article
"Shape It, Don't Shine It
Pro-ported heads often have a high shine finish, but understand that this is done to add perceived value for those with limited porting knowledge. It is shape not shine that makes horsepower."

 

2-3 years ago there was a thread on the Speed Talk Forum about porting. That Forum is populated by Engine Masters types. The consensus of opinion is that gasket matching was productive in only a very few applications on engines built for a specific performance purpose. It was also agreed that port matching was unnecessary except for the removal/blending of ledges sticking out in the downstream flow. Some intakes and/or heads have areas that are even filled to achieve a particular desired flow characteristic.

If one had a generous supply of heads to experiment with and a flow bench and dyno, a head could be ported to match a particular application to achieve a particular purpose. For most 300 applications the removal of casting flash, bumps, etc., polishing of the combustion chamber and exhaust ports, and the rounding of the short radius in the head will probably be the most one needs to be concerned with.