For simplicity's sake, we'll say the wastegate opens at 20 psi and regulates the boost pressure to 20 psi.

The way any mechanical boost controller works is fool the wastegate into thinking you are are making less boost then you actually are. So if you want to build 40psi, you need to make the wastegate think that you are only building 20psi at your set pressure, so in this case, you want it to think the boost is half of what it actually is. There are a couple designs.

The Needle Valve

This is simple and cheap, and works OK for people that use there trucks for fun. How it works is it delays the pressure going to the wastegate, so when you floor it, the boost has time to build up to your set pressure before the wastegate realizes it. The problem is when you're towing, or at/above 20psi for extended periods, the wastegate will get the full boost pressure you are making and will open way up to try and get boost back down. And because of the restriction, when you let of the skinny pedal to shift (with a manual) and you boost goes down, the wastegate will take a while to close again, making it harder to re-spool the turbo once you're done shifting.

Boost Elbow

The boost elbow is basically like a needle valve, but it has a small orifice after the needle valve that bleeds pressure to the atmosphere, if anyone here is familiar with a "resistor devider" (electronic circuit) it works in exactly the same way. By bleeding pressure after the valve you lower the pressure that the wastegate sees, making it so it will always see lower pressure then what you're actually making. Using the valve, you can change the amount of pressure drop. This is a step above the plain needle valve, and works very well for many, however there is still one issue. In order to wastegate the boost to 20 psi, the wastegate has to start to open around 10 psi. So if you set your pressure to 40 psi with the boost elbow, then it will start to open around 20 psi. And because of the orifice, you will always have a small boost leak, nothing huge, but it is there.

Ball & Spring Boost Controller

Learn how to make one HERE.
This is another step above the boost elbow. The Spring pushes the ball into a seat, sealing off the passage to the wastegate. The spring is on one side of the ball, and on the other side, there is the boost from the engine pushing the ball open. Once the boost pressure is enough to overcome the pressure given from the spring, the ball unseats and lets air into the waste gate opening it. On paper it looks like it will keep the wastegate closed until you your set pressure, however that's not quite the case. Because once some air pressure goes around the ball, it is now applying pressure trying to close the ball again. So in reality this design makes the wastegate think your at X less pressure then you actually are. So if you have it open at 40 psi, it will think you are at 20, and at 30 psi, it will think you are at 10. This design works very well, and should be good for all but the biggest perfectionists out there. The only real downside to this design is it still has a pressure spread between starting to open, and open enough to regulate the boost to that pressure.

Boost Governor

This design's only advantage over the "Ball & Spring" design is that it fixes the "problem" of the pressure difference between starting to open, and open enough to regulate the boost. This controller works by doing it's best to keep the pressure exactly where it's set to, so fully closed @ 38 psi, and fully open by 42 psi. There are some "over shooting" issues with ones I've played with though, due to the issue that the wastegate has only the time it takes to build ~2 psi to go from fully closed to open enough to regulate the pressure.
This is a much rarer design that only a few I know of have, this works extremely well for those who use there truck for towing, as they never really "punch" the throttle, giving the wastegate plenty of time to open.

Note: "Boost Governor" is a name I came up with, as I'm not sure what this type is actually called.

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.

Before:

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.

When we're trying to get as much out of our turbo as possible, it is very common to port the the wastegate out to help keep drive pressure in check.  However, we are limited to how much we can port the wastegate out by the size of the wastegate flapper.  The size of the little HX35's flapper could use some serious help, and it wouldn't hurt to give a HE351 a little love as well.  So what we're going to do is install a larger flapper, so we can make the hole even bigger, without having to worry about how much sealing surface is left.  The stock wastegate port diameter on both HX35 and HE351 are right around 21.75mm (just shy of 7/8).  While the stock wastegate flapper is considerably larger on the HE351, where you can take it out to 1” without much issue. The HX35’s flapper is considerably smaller, so 15/16” is the largest I would consider without upgrading the flapper to a larger diameter.  Lucky for us, making a new flapper is a simple job, and takes all of 15 minutes.

Here is a Stock HX35 wastegate flapper.

And here is a Stock HE351CW wastegate flapper.

To make a new flapper, I used an old exhaust valve, but this is as great of place as any to get creative.

First we’ll need to remove the old wastegate flapper.  Just drill out the center of the “rivet” holding the flapper to the wastegate arm.

To grind the exhaust valve to the correct size, I installed the valve into a drill press, did the rough grinding with a hand grinder, and then did the finishing touches with a hand file.

Here’s our new HX35 wastegate flapper, after I got it to the dimensions I wanted.

I then cut the stem off the valve, and installed it to make sure there was a nice fit.  On the HX35 I used a ¼” nut and hammered it onto the shaft, to hold the wastegate flapper on.  We’ll weld it on in a second, first we need make sure the wastegate has enough “slop” in it to make full contact with the housing, for a positive seal.  A little extra slop is a good thing.

Here is our wastegate at this point.

After making sure everything will work, weld the nut onto the shaft.  Make sure no weld gets in the way of being able to put a socket on the nut, this will help for later.

Now lets go to the HE351CW, we already made a new flapper for it, and welded it on (for the HE351CW, a 5/16” nut worked best)

Now we’re going to “lap” the wastegate, to insure that everything sits together well and there are no leaks.  Remember!  Leaks mean slower spooling!  Even if you don’t upgrade the flapper, I would recommend doing this if the wastegate has ever been ported, as porting can create a lip, and when porting.  Even if it was sealing before, you may have removed the sealing surface for the wastegate.

For lapping the wastegate, we’re going to use Valve Grinding Compound, last time I got some it was about $6 from Napa, you should be able to get it from most any auto parts store.

Then we need to apply it on the flapper, make sure to put more on the flapper then you think you’ll need, as we use the compound, the grit becomes finer and finer, so by reusing the compound that goes to the edges, we can get a progressively finer finish.  

Now we need to be able to spin the flapper, to grind away at the high spots in the housing.  When doing this, I would recommend going in circles instead of back and forth, that way the whole valve gets ground evenly.  Also make sure to apply the pressure to the flapper by the wastegate arm, same as it would in the truck, otherwise the pressure won’t be the same when you hook the wastegate actuator up, and can grind away one side of the hole more than the other.

Now here is where the nut welded to the top comes in handy!  Just take a screw driver, or a drill and spin the flapper.  Remember not to get carried away if using a drill, and don’t spin it to fast.  Make sure to check the progress repeatedly along the way, make sure that the compound never gets dry, as it will start to scor the surface.

Before:

After:

So there you have it, a wastegate that will seal wonderfully, but can flow enough for you performance needs!