Boost creep: is the turbo outlet nipple the best place for boost controller?

[Turbo6inKY]

Long title, and I'm going to fill this post with keywords, too. So please excuse the obvious stuff in parentheses, I'm just trying to make sure this thing is easy to find for people searching in the future.

I've searched, and I'm not convinced, so I'm making a new thread.

I've got a third gear boost creep problem. Widely reported... with internal wastegate people. I have an external. I'm using a GrimmSpeed boost controller, which is essentially a bleed valve like the RJC or whatever you might fab up from the McMaster catalog. I hit my set points in 1st and 2nd (first and second) reliably, but in third (3rd) and up, I get creep from my 17psi target to 20 before I let out of it. I haven't stayed in it long enough to see if it stabilizes because I don't want to be arrested or blow the motor.

The combo is far from stock, but for completeness, I'll enumerate the high points here:

• Comp 260H cam
• Ported heads
• .030 overbore
• PTE 5658 turbo with billet impeller wheel
• KB-style headers with external gate from buickgn.com (46mm gate, 10psi spring)
• Stock IC with MPE big neck
• Stock plenum and intake manifold
  
• aforementioned Grimmspeed bleed-type boost controller

Now, the crux of the question:

The MAP sensor, which is plumbed into the plenum via the vacuum block, is reading a boost creep in third gear. The boost controller is getting its reference signal from the turbocharger outlet. First and second are solid at 17psi (or whatever I set it to).

Would I rather want to tee the boost controller in with the MAP sensor, so they both see the same signal? Is it possible the turbo outlet is seeing steady pressure while pressure builds in the plenum?

 

[Chuck Leeper]

From what I've seen/read, the compressor tap is where it works best.

 

[bison]

You need to increase the wastegate cross section (flow area) if its creeping. Youre trying to control manifold pressure by regulating exhaust pressure.

 

[Turbo6inKY]

You need to increase the wastegate cross section (flow area) if its creeping. Youre trying to control manifold pressure by regulating exhaust pressure.

That's what I'm finding. It just seems a shame to be grinding on a turbocharger to increase the size of the wastegate hole. This is an aftermarket turbo, shouldn't the hole be big enough from the get-go?

 

[Turbo6inKY]

So, I pulled the turbocharger off last night to port out the wastegate hole. Lo and behold, the wastegate hole in the gasket isn't perfectly lined up, and it's been blocking a portion of the wastegate hole. Doh!

I went ahead and radiused the inside of the hole and took the sharp edge off the outer part. Carbon build up has left me a perfect outline of where I need to trim the gasket, so I'll get that done tonight and button it all up. It's supposed to rain for a few days, so it might be the weekend before I have any results to report, but report I will.

 

[bison]

That's what I'm finding. It just seems a shame to be grinding on a turbocharger to increase the size of the wastegate hole. This is an aftermarket turbo, shouldn't the hole be big enough from the get-go?

No. The wheel combo and what is trying to be achieved should be done with a t4 hot side and an external wastegate. If the hole was opened and didn't line up with your puck you'd have another problem. The choices you have are run the turbo at 80% or higher of it potential or waste more exhaust energy. We can't change the physics involved with the turbo so we work with them.


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