Boost creep. You have heard the term, but do you really know what it means and why it can be a dangerous thing to happen to your car? Below I will explain what boost creep is, what causes boost creep, and how to remedy it.
Boost creep can’t be tuned out, so how do you fix it?
Boost creep is when boost hits wastegate pressure, but then slowly rises or “creeps” as engine RPM increases. On an STI, the stock wastegate actuator has a spring pressure of ~8psi. That means when running wastegate pressure you should hit ~8psi and hold around 8psi until redline. When boost creep happens, boost hits wastegate pressure of 8psi and can rise to 18+ psi by redline. Boost creep can be dangerous because you can’t tune it out – i.e. you can’t lower the boost with a boost controller and a tune. It’s mechanical and has to be mechanically fixed. Running too much boost close to redline can easily max out the stock fuel system. When that happens, your AFRs can be dangerously lean, and knock becomes much more likely.
While that might not happen on a stock car, once you add some modifications to increase power or airflow, boost creep can sneak in. With a free flowing intake and aftermarket downpipe the problem is very prevalent. Some cars do fine and others have a lot of boost creep. The 2008-2014 STI seems to be the least affected by boost creep. The 2004-2007 and 2015-2021 STI can have a lot of boost creep once the free flowing downpipe and intake are installed.
Above is a graph illustrating boost creep. The blue line is what I typically see. Boost hits 8 psi and slowly creeps up to 12-13psi. That is fine since it’s low enough that we are not maxing out our fueling system. The red line is an example of extreme boost creep. Boost starts at the normal 8psi, from there it increases in pressure much faster than normal and hits 18psi around 5000rpm.
The next graph above shows the injector duty cycle as well. You can see at 5000rpm IDC is at 95% and increased to 100% by redline. While this was ok on the dyno and the AFRs were rich enough. Once some cold weather rolls around, and it always does here in Minnesota, boost may hit even higher. The higher boost along with the need for more fuel from the colder air can quickly max out the fuel system. Leaving your engine dangerously lean and the ECU unable to help the issue because it can’t richen up the AFR or drop boost. It can only drop ignition timing, which may not be enough.
How to stop boost creep once and for all
Worried? You shouldn’t be. We have several methods to keep boost creep at bay. The first and easiest way is to keep your stock intake when installing an aftermarket downpipe. We have found this very effective and is the reason why you can only find STG 1 tunes from Cobb for the 2015-2021 STI. The second method is to just let it happen. If you have large enough fuel injectors, boost creep is not an issue since you will always have plenty of fuel. An added bonus to installing larger fuel injectors is you are ready for E85. More info on why you should run E85 can be found here. The 3rd and most labor-intensive option is porting the stock wastegate. We do this on engine builds since the turbo is already removed. Porting the factory wastegate allows more exhaust gasses to escape. That reduces exhaust back pressure and can keep boost creep to a minimum. We used to install external wastegates to stop boost creep but since they are not emission compliant we no longer recommend them.