Boost Creep


Catted vs Catless Downpipes in Cold Weather (Boost Creep)

Does boost creep happen on 15+ WRX's?  

     -  YES, it can.

What is boost creep?  And how do I know if I have it?

     -  Boost creep is when the boost uncontrollably rises at high rpm.  Boost creep can NOT be tuned out, it is a mechanical limitation of the wastegate that occurs from running a catless exhausts and intakes combined with cold temperatures (typically below 40* F). You can see in the pictures at the bottom of the page that the boost pressure continues increasing uncontrollably after 6000rpm with the wastegate duty cycle at 0%.  Boost creep is exaggerated with every restriction that is removed from the exhaust.  Boost creep is caused from the lack of back pressure and the physical wastegate hole being too small to bypass all the exhaust gas.  Since the exhaust can’t fit through the wastegate hole, it is forced through the turbine wheel which causes the boost to uncontrollably rise.  

But I've been running a catless exhaust for years and I don't boost creeep!

     -  Are you sure?  Because ALL subarus with even a catted TBE experience boost creep.  So if you have an aftermarket exhaust on your car, you are already experiencing boost creep, it simply isn't bad enough to cause an issue.  Have you ever actually plotted out your boost curve?  Boost creep does NOT mean you are exceeding your PEAK BOOST value.  Boost creep only occurs at high rpm where the boost normally tapers off.  Not all cars will experience detrimental boost creep with a catless exhaust.  It’s about the backpressure of the complete exhaust system, plus the intake system, plus the outside temperatures. 

But my friends brothers cousins sisters dad says boost creep doesn't happen on the FA!

     -  Just because one person doesn’t experience boost creep does not mean boost creep can’t happen.  At the bottom of the page you can see a boost plot FROM THE SAME CAR with a catless jpipe vs catted jpipe.  You can clearly see the boost uncontrollably rises to redline with the catless setup. The catted setup is also boost creeping, just not as severely.  The major issue with boost creep is the boost uncontrollably rising and maxing out the fuel system.  Maxing out the fuel system will cause the car to lean out and misfire at high rpm and potentially reach failure.  Porting the internal wg hole may reduce boost creep, but the only real cure for it is an external wastegate.  An electronic boost controller will not prevent boost creep. A manual boost controller will not prevent boost creep.  Boost creep can not be tuned out.

 For these reasons, I highly recommend staying with a catted exhaust.  

Q) What is boost creep?
A) Boost creep occurs when the wastegate is physically unable to bypass enough exhaust around the turbine on the turbo.

Q) Is boost creep harmful?
A) Yes, boost creep is one of the worst problems you can have with a turbo engine. Boost creep means that there is NO way for the boost pressure to be controlled. Boost pressures in excess of 20psi!!! have been recorded on STIs with TBE before the ECU cuts fuel.

Q) What causes boost creep?
A) Installing a turbo back exhaust whether aftermarket catted or not will in almost all cases cause boost creep to a certain extent although Catless exhaust with an aftermarket intake is the worse case.

Q) Are there any other options to prevent boost creep besides porting the wastegate?
A) Yes, Installation of an exhaust that either keeps the stock OEM muffler or one of the stock cats should not cause boost creep as severely as a fully open exhaust.

Q) How do I know if I have boost creep?
A) The best way is to have data and graph out the boost like the plot below. It’s much easier to visually see creep on a graph that by trying to watch an AP while you’re driving.

Q) What is the overboost CEL?
A) P0244 -  Boost Limit Exceeded

Q) Is there anything else I can do besides porting the wastegate to prevent the creeping....MBC, EBCS, engine management etc?
A) Yes, get an external wastegate.


If you think of it as a hierarchy, then it should make sense.

level 1) wastegate controls boost pressure
level 2) boost solenoid (EBCS or MBC) controls wastegate
level 3) ECU controls boost solenoid

Using this logic, if the problem resides on the 1st level, no matter what you do on top of it, you will still have the problem. Whatever (MBC, EBCS, UTEC, ECUTEK, COBB) is controlling the wastegate CANNOT prevent boost creep if the WASTEGATE is UNABLE TO FLOW ENOUGH EXHAUST to prevent boost creep. A fuel cut defender will not eliminate the boost creep problem, but it will stop the fuel cut. Your boost will still creep up to 20psi, but the ECU won't be able to tell due to the altered signal from the FCD.

Q) My car seems to boost creep when it is cold out. Why is that?
A) Ambient temperature plays a huge role in how well your engine runs. In general, the cooler ambient temperature, the better your engine runs due to the laws of thermodynamics. The colder air causes your engine to make more HP (more work under the curve with lower temps), therefore more dense exhaust gasses; therefore your car is more prone to boost creep when it is cold out. If you live in an area that does not see colder temperatures (AZ for example) too often, then boost creep may never become a problem.

Q) I want to port my wastegate. What do I do?
A) Can you remove and install a turbo? If yes, there are a couple of options. You can send it to a local machine shop port it, or do it yourself with a "Dremmel like" tool.