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Boost Creep

Boost Control

Discussion and questions related to the course Boost Control


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I am about the begin the process of adjusting my boost control system on a 2004 Subaru STI.

I have been tuning fuel with my boost control system disabled. The turbo is a Cobb 20g, with a 14.5 spring. In addition, I have a catted aftermarket downpipe.

Looking at my full power pulls, even in 3rd gear, boost hits 13 by 3500rpm, but rises to about 17psi by redline.

Shouldn't my boost "curve" at wastegate pressure be flat?

I've attached a log.

Attached Files
  • WILDER-TUNED-V2-1.16.18-3rdgearWOTpull.csv
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This isn't uncommon actually and is a result of the wastegate being a little marginal in its flow capability. It's only a problem of course if you want less that 17 psi at high rpm. What you may also find is that if your ignition timing is currently quite conservative then you may see that boost drop down a little as you advance the timing up. This is because retarded timing puts more heat into the exhaust system and provides more energy to the turbo.

Specifically to answer your question though, I'd say about 1 in 10 of the turbocharged engines I tune provide a flat boost curve with no electronic control active and even if the boost is flat at a low boost level of perhaps 12-14 psi, you'll likely find that as you lean on the turbo harder and raise the boost, that the boost will then start to taper off at high rpm.

Awesome. This is great info.

Thanks for taking the time to look at my datalog!