Subaru Tuning – A Quick Look into Boost Creep

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Boost creep is something we tend to address quite a bit during the winter months.  Below we will go over the causes of boost creep, why you should be aware of it, and solutions to keep your boost levels in check.

Boost creep is defined as a condition of rising boost levels past what the predetermined level has been set at.   For example, if your target boost level is 15psi, you would see the boost levels increase beyond that as RPMs increase.  The short explanation for boost creep is that the wastegate is not able to flow enough air to bypass the turbocharger’s turbine housing.  When the volume of air flowing through the turbine housing continues to increase, boost will also continue to increase.  Boost creep is most commonly observed in areas that have very low ambient temperatures as well as high atmospheric pressure.  This often encompasses those in the Pacific Northwest as well as New England and East Coast.

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Boost creep is undesirable for a number of reasons.  When a car exhibits a large amount of boost creep it makes controlling the boost level next to impossible for a tuner.  A car that is on the edge can exhibit highly variable boost response under different environmental conditions.  For instance, if you have your car tuned during warmer months, the change in weather going into winter may be enough to cause a significant increase in boost.  Once the flow limit of the wastegate has been reached, exhaust gas back pressure will increase.  This is very inefficient and will cause temperatures to increase and introduce more detonation-prone conditions.

Boost Creep is not a condition that can be corrected solely by making tuning changes as it is a mechanical limitation.  The two easiest ways to address boost creep are to either port the wastegate housing of the turbocharger in order to allow it to flow more air or to switch to an external wastegate style uppipe.

If you’re looking for answers to other tuning related questions, head over to our Knowledge Base!


COMMENTS
Niko Balbin on March 29, 2020 says:

I am looking to purchase a cobb 20g turbo, I am in the ball park of 400whp and im looking to make power reliably,im doing a forged internal build. I am planning on running a catless downpipe and i was worried about boost creep. My question is im not sure if the stock internal wastegate will be sufficient in holding boost levels or will i need to go to an external wastegate in order to eliminate boost creep on this set up

Reply
    Marshall
    Marshall on March 30, 2020 says:

    Hey Niko,

    The power level you’re looking for is right at the top of the capabilities of our 20g turbocharger. While we have seen some vehicles produce much more than that (as much as 495hp), it’s generally on a much more “all-out” type of build. We cannot comment on using the turbocharger with a catless downpipe as that is not testing we have done. In general, if boost control becomes an issue based on wastegate capabilities you would need to seek out an alternate style wastegate solution (external, for example).

    Marshall@COBB

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