Boost Control Tech

On a turbocharged car, boost is your friend - packing extra air into the combustion chamber results in dramatic increases in power. But, there can be too much of a good thing - too much boost, and your motor can let go. Not enough boost, and you're not making enough power.

Controlling boost is actually pretty easy in principle, but there's so many ways to do it, people get confused. So, let's go over the basics.

A turbo makes power by compressing air into the motor. There are 2 wheels on a turbo - the turbine in the exhaust, and the compressor in the intake. Exhaust gasses spin the turbine, which is connected on a shaft to the compressor wheel. The exhaust spinning the turbine wheel makes the compressor spin, which sucks and forces air into the motor - this is boost.

Now, if ALL the exhaust gasses went through the turbine, the turbo would keep making more and more boost and spin until the velocity of the exhaust gas overcame the turbine. This situation would have uncontrolled boost - it would skyrocket up, making TONS of boost. Pop goes the motor, and no more fun.

So, we have a wastegate. Simply put, it's a pathway for the exhaust to travel around the turbine blade, not hitting it, and go into the exhaust. So, with this pathway, you're bleeding pressure off, keeping all the energy from spinning the turbine.

Now, if we just had this pathway around the turbine, we would have controlled boost, but it would be limited by the size of the pathway. A large pathway would bleed off lots of exhaust pressure, resulting in low boost. Too small, and you have too much energy and boost. So, we put a doorway of some kind in the path. Using this doorway, we can keep the path shut to use all the exhaust energy to spin up the turbo, then open it when we have enough boost to limit the energy hitting the turbine. This doorway is the wastegate.

Now, how are we going to open the wastegate at the right time? Using a diaphragm. The diaphragm is connected to a rod leading to the flapper door - the wastegate. There's a spring behind the diaphragm, so the wastegate door normally stays tightly shut. But, if we put air pressure in front of the diaphragm, we can push against the spring. The diaphragm moves back against the force of the spring, and as it moves, the rod connected to the wastegate door moves, opening the wastegate.

So where are we going to get this pressure? Well, the turbo is making boost pressure. On most turbos, there's a hose nipple on the outlet of the compressor housing (this is where the compressor wheel is located in) - the outlet is where the compressed air comes out of the turbo. Hook a hose up to that nipple, connect to the diaphragm, and voila! Boost control! When there's enough boost to press the diaphragm against the spring, the door opens, and boost stabilizes. Neat, eh?

The manufacturer of the turbo does an extensive testing process to find the best size of diaphragm, the size of the flapper door, the tension of the spring, etc. to make the turbo produce however much boost it's supposed to. On a stock TurboII, the boost is set to 5 or 7 psi, depending on model year. It will give exactly that much boost, time and time again.

Now, here's where it gets interesting. Let's say you start modifying the car. You remove the catalytic converters, replace the small stock exhaust with a nice large 3" exhaust, and install some high-flow mufflers. Well, now we have a problem. The turbo was designed around parameters of a stock car, including the amount of restriction of the stock exhaust. With the exhaust breathing easier, exhaust can get out of the engine MUCH easier and faster. Since it's getting out easier and faster, the stock wastegate just can't keep up - it's bleeding pressure off as fast as it can, but the exhaust energy is so much more, and moving so much faster, it overwhelms it. So, your boost goes up. This can be good to a point. But, with the small stock wastegate pathway, you lose control over how much boost you're running.

Well, what about boost controllers? Let's talk about that. Remember how air from the compressor housing nipple goes to the diaphragm, and pushes against the spring? Well, let's say we use a device to take some of that air and bleed it off. The turbo is creating boost, but the wastegate doesn't see it all, because we're taking some away. We can use a boost controller to raise our boost, then, over what it is normally. A boost controller, however, can never LOWER boost, just RAISE it. We're still limited by the tension of the wastegate spring, the size of the path going around the turbine blade, and the size of the wastegate flapper itself.

Now, in '89, Mazda changed the turbo to help address this issue. Instead of one pathway, there's now two. This gives much better boost control, since we have a LOT bigger path to bleed boost off with.

With the stock setup, you really can't do a whole bunch to lower boost. One technique is to bore out the wastegate. This involves usning a grinding tool to enlarge the size of the path under the wastegate flapper. This actually works incredibly well - enlarging the path gives more potential to bleed off boost, helping to bring our boost back in control.

So, let's say we want to make a LOT of power, and we absolutely, positively have to have rock-solid boost control. This is where an external wastegate comes in. The stock wastegate is considered internal - it's built into the turbine housing of the turbo. A "full" turbo setup, however, doesn't have a wastegate built in - it's an extra piece that bolts on. With a full turbo, you have a new exhaust manifold on the motor. This manifold has a flange to bolt the turbo on, and another flange to bolt on the external wastegate. The external wastegate itself has a diaphragm inside it, like a stock wastegate, and a valve to bleed off boost pressure. The valve is designed kind of like the valve from a piston motor. When the air pressure against the diaphragm in the external wastegate is great enough to push agains the spring, the valve comes off its seat, allowing exhaust pressure to bleed off. The external wastegate has two ports - an "in" where it bolts to the manifold, and an "out" where it bolts to the exhaust downpipe. This is the pathway to bleed off exhaust pressure.

External wastegates work so much better because the bleed path is REMARKABLY larger than stock, and so is the size of the valve itself. Typically, an external wastegate will NEVER have boost creep problems. You can even change the springs out in an external wastegate to raise your "base" level of boost - the springs are rated by how many PSI of boost it takes to open the wastegate. Most common is a 5 psi spring - put it in the external wastegate, and you'll always run 5 psi of boost.

Another feature of the external wastegate is it has 2 air inlets. Remember the wastegate diaphragm? Well, the air inlets let air go in both above and below the diaphragm. So, you can use air pressure not just to open the wastegate (pushing against the spring), but you can use air pressure to hold the wastegate shut (pushing with the spring). A good boost controller will use this capabilty, forcing the wastegate to stay shut untiil the desired boost is reached, then opening it up.

Hope this makes sense!

Dale