What is Boost Creep?

If your car is suffering from Boost Creep, here is the behavior you would see.

Assuming you are accelerating in one gear at full throttle, you will hit your target peak boost (the most boost that you want to make, say 15 psi) at a certain rpm, let’s say 3500 rpm. Then as you continue through the rpm range, you should see your boost pressure begin to taper off, say after 4500 rpm.

If your car is suffering from Boost Creep, what you will see is at some rpm after your boost begins to taper as normal, your boost pressure will begin to build again, and if it does, it most likely won’t stop until you let off the accelerator. It will also most likely build well beyond your target boost.

So what causes this?

What your car uses to control boost pressure is the turbo’s waste-gate. This is simply a door that opens up in order to direct exhaust around the turbine to relive pressure and slow the turbine down. Basically, the more the waste-gate stays shut, the more pressure builds, and the faster the turbine spins, and the more it stays open, the less pressure builds up, and the slower the turbine spins. Your car (or boost controller) will modulate the waste-gate frequently to maintain boost pressure…

In the case of Boost Creep, what happens is that in high rpm’s, your engine is pushing out so much exhaust that even with the waste-gate remaining open, it isn’t able to divert enough gas to keep the turbine from spinning faster, so your boost pressure continues to build. This is a problem because boost pressure will continue to build uncontrolled, and it may push past the point where you still have adequate fuel capacity, and cause a lean running condition.

Also, one of the reasons that boost normally tapers is because as the exhaust flow increases as rpm’s increase, it will actually push a turbo out of its peak efficiency range. This means that the turbo doesn’t make the same pressure as efficiently at a higher rpm as it does at a lower rpm. The less efficiently a turbo makes pressure the more heat it will put into the intake charge, which increases the chance of running into detonation, which usually requires more fuel to compensate.

So what causes this?

Well, basically, it is a simple problem of the waste-gate opening being insufficient to divert enough exhaust around the turbine. Sometimes this is because to small a turbo is paired with to large a displacement engine, or it can simply be that the waste-gate is just to small.

So what can be done to correct this?

Well, there are a lot of possible solutions, but we’ll stick to the simple ones.

First, sometimes Boost Creep can be helped by changing your car’s tune. Consult your local tuner to get their feedback as to whether this could be an option for you.

Another option would be to have your waste-gate ported. This simply means that you would go in and increase the size of the waste-gate opening so that when the door opens, it can divert more exhaust around the turbine. This is something that is probably best left to professionals, but most likely any company that builds turbo’s would be able to port your waste-gate for you. If you are ordering a new turbo, it wouldn’t be a bad idea to consider having this done when they are building the turbo for you.

Another option would be to go with an external waste-gate. One reason that the size of the waste-gate opening is limited is because it needs to fit inside the exhaust housing. If you go with an external waste-gate, you would actually have a second exhaust channel that skips over the turbo all together. This allows you to put in a much larger waste-gate, so it is much better able to divert exhaust around the turbo. The down side is that it requires some custom fabrication or buying new exhaust components, welding up your internal waste-gate, etc., but this can be a very effective solution.

If you feel that you are having this problem, it would be a good idea to take your car to a professional mechanic/tuner and get a second opinion before taking any of these steps. They should be able to help you decide on what solution would be best for your application.

Jon Cooley

Comments

[Varun Hurry]

so i was on cruise control going to work doing 70 mph at around 2700rpm with my 09 STi... it's bone stock, never raced and had it for only 2months now. the car jerked by it self and the check engine light went on, the sport light was flashing, the cruise light was also flashing and the traction was set on orange which means it was turned of! why did it happen and how can i help it not from happening again???

[Jon Cooley]

It is quite difficulty to speculate as to what the problem could be. My first recommendation would be to take your car to a Subaru dealer to see if the CEL code was stored in your ECU. That will give you a good idea what the problem was if a code was stored.

If there wasn't a code stored, I would just recommend that you keep an eye on it and see if it happens again. And if it does, see if you can find a specific set of circumstances that causes the issue. This will also help to diagnose the issue.

Best of luck and enjoy the new car.