Overboosting on your 08+ WRX/STI…. who’s to blame?
Hello everyone, just wanted to take a minute to point something out to everyone that gets discussed from time to time but typically without data.
Most of you have heard of boost creep issues on 08+ WRX/STIs that comes from a wastegate efficiency issue on modded cars in colder weather. Some say it’s a “tune issue” some say you need an EBCS to solve the issue. The real answer is that it is a mechanical issue that cannot be addressed by the tune. We typically see this condition on VF48 (08+sti) and VF52 (09-14 wrx) turbos with aftermarket intake and aftermarket downpipes in colder weather. The colder and denser air combined with increased airflow to and from the turbo creates a situation where the wastegate cannot bypass enough exhaust gas past the turbine wheel and the turbo continues to make boost considerably higher than wastegate pressure regardless of wastegate duty cycle (WGDC). Not only is this dangerous due to uncontrollable boost, but on stock injector cars this can easily max out the stock injectors and create an extreme lean condition. The log below illustrates the exact issue. I suggest three things to remedy the situation. 1. Install an external wastegate. This will completely remedy the issue because it allows control over more exhaust gas to bypass the turbine wheel. 2. Install an aftermarket turbo (i know a guy!) 3. IF you cannot afford the EWG or new turbo and won’t leave the car stock in the meantime, DO NOT do 4th/5th gear pulls when it’s 18* F outside. Remember, 4th gear on a WRX 5 speed is about the same ratio as an STI’s 5th gear. Oddly enough, I couldn’t replicate the issue on my test drive, mainly because doing 4th gear pulls in a stock engine WRX tuned on pump gas is just asking for a ringland issue or a serious ticket! This overboost happened at over 110mph and the customer supplied the log.
This is an issue that can be tamed with driving style and thoughtfulness of the atmospheric conditions when you are driving.
As seen below, WGDC had plummeted to 3% (same as zero) because the turbo dynamics (PID control) has detected an overboost and is doing everything it can to reduce boost. The Wastegate is wide open, yet the car is still making over 19psi. Imagine this situation in 5th gear!
Hopefully this helps everyone understand the issue. If you are throwing overboost codes in cold weather in the cars with mods i noted, don’t be too quick to blame the tuner!