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Exhaust Ports

You need to actually calculate the effective area of the port - as measured in the cylinder. And compare it to the amount of area at the outlet. There should be a greater area at the outlet. Typically a 2 or 3 degree taper from exhaust port carried all the way through the exhaust port tunnel. It then should match the pipe. Be sure to re-read this page if you're having trouble remembering how it's done.

The ideal area of the exhaust port is always determined by its effective area and how that relates to the taper of the exhaust tunnel and its length. No matter what type of engine build we're talking about.

It is common to expand the exhaust tunnel to aid scavenging of the spent gasses. It's angle can be the same as the first section of the pipe but sometimes, actually, often times it won't be, because of the physical parameters of the tunnel itself. Things like running into water passages or seeing daylight prevent it. However, if correctly designed, the pipe designer will assume its length from the face of the piston and the angle being the same during the complete first section. So, making the exhaust port the same angle as the pipe is effectively making the pipe like the pipe designer intended. The exception to this is that mass production pipe builders know that their customers are not going to have their exhaust port tunnels matched to the pipe so they compensate the design to account for it. Typically this would be a small difference - maybe a half of a degree to a degree in that section of the pipe.

Having said that I should also point out that some pipes tend to work well on just about any cylinder they are attached to. This would tend to lend theory to the first section of the pipe not being as important as it once was. The second or third section of the pipe works double duty and makes everything right. I have no concrete explaination of this. I just know it seems to be true in some cases. I don't pretend to be a pipe builder and in fact I don't profess to knowing (really) anything about pipes. But, I have talked to a few pipe builders that make fine products and I know more about the pipe building theory than they do. This would lend theory to pipe theory being just that. Theory. Theory is great and it gives us something to talk about but real world testing is a hundred times more valuable. That is how and why some pipe builders sound like they have a sixth grade education but produce a killer pipe.

With that in mind, I can say as a general rule and without worry that if you smooth the transition from cylinder to pipe, and if necessary change the tunnel angle to make its entry as straight as possible - as in match its angle, you'll not undermine the work the designer put into its design.

There should probably be no large changes made to the tunnel walls. If there needs to be then be, then you're probably building a drag race engine and you'd better have a custom pipe made or know ahead of time its inlet diameter and angle of its first section. Or, I suppose hope to get lucky. It's hapened.

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