Don's Head Porting Extravaganza
Special Thanks to Don (Drothgeb) For the Time, Information and Pictures present in this article. Thanks Don!
There are different stages of porting, as Don describes below.
Stage 1, is removing the collar in the bowls just below the valve seat. It's pretty easy to see the area I'm talking about in the pictures below. Just removing that without removing anything around the valve guide or runners is a huge improvement. It will actually increase low end performance. Just doing that minimal amount of work, helped spooling a touch, dropped my maximum boost more than 10lb, and knocked .3-.4 sec off my quarter mile times. I ran this level of porting for more than a year, and was finally able to get the truck into the 10s with no other head work. As a comment toward cams... Adding a popular cam to this configuration actually cost me more than .2 sec. Like my earlier comment, the cam was costing me more on one end than it could make up for on the other end. Basically, the head was still holding me back. You really need at least this amount of work to take advantage of compound turbos
Here's a stock head. As you can see there's a lot of excess material just under the valve seat.
Everything inside the red circle is protruding into the air stream, simply removing this on each cylinder achieves what Don refers to as a “Stage 1 Port Job”
Stage 2, is a Stage 1, as well as removing the material around the valve guides, smoothing out and enlarging the bowls and swirl ramps, plus some work to the intake runners. This level really increases flow, and as long as you're careful with the bowls and swirl ramps, has very little if any negative effects. Any performance configuration is going to spool better and produce more power. This is also the minimum level that you are going to see much benefit from a cam. Although, that's still a tough call for a moderate hp truck that doesn't see much higher than the mid 3000 rpm range.
Here you can see a Stage 2 Port Job. You can compare this to the stock picture above.
And the material required to take off.
Stage 3, is when you modify the head to gain access to the intake runners through milling off the intake plenum or the npt plug trick (shown below). This level still spools and drives well and works with stock or moderate cams. I have this configuration on my street truck and see 10psi of boost at 1700-1800rpm. To me that's not bad for a 66/91 combo.
Here's a couple of slices through the intake. They are not in exactly the same location, but it gives you a good idea of the metal that needs to be removed.
Here's a good view of the intake after porting. Again, this still needs to be hit with a sanding wheel to smooth it out.
Now the stock exhaust runner is pretty open, so it just needs some cleaning up. But the stock intake runner is pretty tight. Looking through the plenum here, you can see what I mean.
Here's after a little work (still needs some more though)
Here's a side by side before and after exhaust port. This head actually had a "stage 1 porting" from a couple of years ago, so a stock head is even tighter than the before one here.
Above this is competition level. More extensive runner work, larger valves, swirl ramps are removed. The Hamilton Head starts at this level. At this level low end performance definitely takes a hit. Not going to spool well below 2000rpm, but it will flow on the top end. At this level you definitely need a performance cam to run in the rpm range that suits the head. Not suited to small turbo(s) either, needs air to make it work.
NPT Plug Trick
Now some of you are thinking that's great on the 4 runners you can reach. But what about the 2 on the ends. Well, I drill 2 holes in the top of the plenum that let you reach where you need to get to. I then tap the head for a 3/4"npt plug to seal the openings.
You just need to get one of these carbide bits with a 6" shank.
With one of those I can reach every surface in the intake runner without milling it off. And once a couple of the humps and bumps are cleaned out of the stock plenum, I think it works as good or better than the runner type intakes.
Well, that's the basics. Just remember, a lot of the runner walls are only a little over a 1/4" thick, so don't get carried away without knowing what's behind it. I've got this old head that I sliced up and, I drilled a bunch of holes in the slices too. It's pretty handy to have around for reference.