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Boost dips after wastegate opens

Boost Control

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Hey Guys,

Have something I haven not seen before. Boost builds normally and when the waste gate opens drops off sharply. Builds back up again until creep takes over.

The engine is a non VCT SR20DET and the boost graph is on wastegate spring pressure only. Boost control is possible in open loop set to 18psi and touches 19-20 psi at the very top end.

Clearly it appears the waste gate needs to be ported. For the boost dropping off after the gate open I am guessing that the spring crack pressure is good but one cracked open it is weak and goes full open (boost drops) and stays open and the boost creep then takes over.

Happy to listen to anyone thoughts


Attached Files

If the boost continues to build after the wastegate opens, doesn't that indicate the wastegate is too small, or is not located to flow sufficiently? Or is your boost control then closing the waste gate?

Hey David,

My main point of interest is the large dip after the gate opens and the potential cause for that.

Yes I do believe boost creep is a problem also. At the time the log was taken the turbo was operating on gate pressure only. No boost controller was plumbed in circuit. No electronic control was occurring. The turbo is internally gated.


Maybe since the engine doesn't have VCT, it just can't take the air the turbo produces at 4000 RPM, but then it's efficiency increases as the cam comes on, reducing the boost pressure. Have you tried changing cam timing to see how that changes that curve?

So, are you sure the wastegate is actually opening? Do you have a a wastegate position sensor you could monitor?

Hey David,

We can physically watch the gate open. This is a street car so nothing fancy like a waste gate position sensor. But it is definitely opening. We have used this size turbo GT2860RS with very similar combination of parts on a number of cars. The only real difference in this case is the turbo is not a genuine Garrett.

We will be removing the actuator and striping it to confirm spring size and internal condition.

I will let you know when we find a solution.


There is possibility that the wastegate is stuck?

Hey Guys,

Interestingly the BOV was removed and the dip is no longer occurring.

Once the waste gate is ported the boost creep will be under control .

It was the BOV causing the issue of the dip.


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