PORT MATCHING aka DO NO HARM

Now that we have that gasket matching issue out of the way let's finish the rest of it. It would be real nice if ports and their manifolds that pair up to them were the same size and shape, but that doesn't happen often enough. So we need to deal with what we have. Let's say the exhaust manifold has a smaller opening than the exit out of the head. In that case we need to open the manifold to match the head. But what if the manifold opening is larger, then what? Do we open the head up to match the exhaust manifold? No. If you do open the head to match you will create a "slow" spot in the air flow. Terrible! If the air flows through the runner and hits a wall, such as a protruding gasket or a manifold the air "trips" and "stumbles" causing turbulence, and impedes air travel. But if the flowing air falls over the edge and into the manifold there's little harm done. The point here is the "Doctors Oath" of "do no harm". It's not an ideal situation but it is better to leave it alone that to make things worse. The same logic applies to the intake airflow, it can fall into the intake runner of the head from the intake manifold, but you don't want it to stumble into the gasket or the lip of the intake runner. The idea here is to minimize the bad, maximize the good, and "Do no harm". So bigger is not always better. Grinding away where ever you get the notion can cause the air flow to slow and thus losing the pulsing, charging, ram air effect, and extraction benefits we need. Port matching is done after the head is ported. - Dr. Denny

Dr. Denny does custom head porting & polishing. For more information go to DR.DENNY.COM

GASKET MATCHING VS PORT MATCHING

This is a commonly misunderstood subject. Many people think and believe the proper method of port work is to use whatever gasket comes in the box as a template to determine what size the ports or runners should be. It's not that easy or simple. I have seen many sets of heads destroyed by using this method. They look impressive to the untrained eye. You might even show some improved performance over stock, but overall you've created some airflow issues that can't be fixed. To assume that the gasket is an intelligent being would also be to assume that the gasket maker knows what size your ports should be. That is to assume also that the gasket maker knows your cubic inches, cam size, carb cfm., gear ratio, valve size, and RPM operating range, and more. Here's the story on gaskets. If gaskets protrude into the runner then shave or cut the excess off so that it doesn't obstruct air flow. That's it! Don't use gaskets as templates. My next post will be discussing the rest of port matching that wasn't handled here. - The Head Doctor

Dr. Denny does custom head porting & polishing. For more information go to DR.DENNY.COM

BIGGER is BETTER.....RIGHT?

Here's the important question regarding cylinder heads and porting. Is bigger really better? The same could be asked about tires, carburetors, brakes, and bumpers, even sandwiches for that matter. This will be a short post now that I brought up sandwiches.

With heads here is the key: Everything in moderation. Bigger valves and ports breathe bigger air at bigger RPM. The question, however, is what does your engine need for the way it will be used? Make sense? The Ford 351 Cleveland 4-barrel engine had huge intake valves and runners. You could park a bus in those runners and still have a passing lane. And how did they perform? Terrible! They made very little power until 3500RPM. So they were sluggish at low RPM. The cure was to use the 2-barrel heads. The local dirt oval track racers that use 351C's never use the big heads. With valves, if you go too large, they will be shrouded up against the wall of the combustion chamber. Air will not flow in any volume in that area. With ports or runners some would-be porters just "hog" everything out as far as possible. Many times this will slow the air flow... not what we're after. We want velocity. Anything that slows the air flow is something we want to remove, not create. My next post I'll say some things about gasket matching and port matching. I can't wait. - Dr.Denny

Dr. Denny does custom head porting & polishing. For more information go to DR.DENNY.COM