[sperry]

As I've posted before, the issue with MBC's only comes into play in 2 situations:

1) if you have the boost turned way the hell up
2) if you are under a partial-throttle/full-boost (PTFB) condition

Think of it this way: you can run a ton of boost in a WRX motor as long as you can keep things cool enough to avoid detonation. The motor will not blow up simple because it's making a lot of power, it blows up when the a/f charge detonates early while the piston is still on the way up the cylinder. Preventing detonation is simply keeping things cool enough that they don't explode untill the spark plug goes off.

On a mostly stock motor, what keeps things cool is the fuel. If you have a rich enough mixture things won't detonate, especially at our high altitude. As the mixture leans out, things heat up. You can see this in the EGTs. On a WRX once you start getting to the 1550F EGTs, start thinking about not driving so hard, however I've heard the WRX motor should be good to 1600F.

When you throw a MBC in the mix, you have to remember the car's ECU is what controls fuel, which is the primary means to keeping things cool in a mostly stock car. If you have a MBC, you are putting more air into the motor than the ECU expects.

For example, you set the MBC at 18psi (not that the stock turbo can really put out that much boost, but for argument's sake). When you floor it, the car will open the fuel injectors all the way and try to dump enuf fuel for what it thinks is 14.5psi, since that's what the stock boost maxes at. Because you're forcing extra air in there, and because the stock fuel system can't handle all that boost, the mixture leans out. Do it enough and you blow up the car. Setting the MBC way high is obviously a bad thing.

Now let's look at setting the MBC at stock boost levels, say 15psi. At 15psi, you'd think it'd be impossible to over-boost. However, you need to remember that the ECU thinks it knows your boost curve. It thinks at 50% throttle at a certain MAF/MAP load, the car will have say 8psi of boost. But since there's a MBC on there, your car is actually producing 15psi! The ECU is dumping the proper amount of fuel for 8psi, not 15... so you can end up with a lean condition. This is where it's important to watch your EGT gauge... if for example, you're climing a long hill in 5th at 50% throttle, you will boost to max... the EGT's will rise. Once they start getting up there, you need to back off the boost, or floor it to trigger additional fuel. PTFB conditions don't occur too often, so it's usually okay to run a MBC set at stock levels, but you really need an EGT to watch things.

Now, regarding "overheating the cat" with a MBC. Let's remember what cats are for, and how they work. They're basically a metal mesh in the exhaust that heats up. The heated mesh is designed to burn off unburnt fuel in the exhaust so it doesn't blow out the back of the car.

What destroys cats is *not* hi temp exhaust, it's *rich* exhaust! If there's tons of unburnt fuel in the exhaust, the cat has to burn it... that's what overheats and destroys cats. Running lean due to a MBC will not destroy your up-pipe cat, since you're sending hardly any fuel out in the exhaust. Running lean due to a MBC break motors by causing detonation.