Boost Creep on RB26 Single Turbo

Good day all,

I am experiencing a boost creep on a built RB26 with single turbo. Here's a quick list of the mods :

DocRace Exhaust Manifold (Single Scroll) T4 Flange

Precision 46mm Wastegate with dump tube vented to atmosphere

Precision 6266 Gen 2 Ball Bearing 0.81 A/R

ECU Master Classic

So I can't control boost no matter what I do, it just creeps to around 25 PSI on the dyno, probably would go higher but I hit boost cut that I set there. I have lowered the spring pressure in the Precision Wastegate to 9 PSI, still creep like crazy. Tried another Wastegate (Tial 44 with 16psi spring), still creep. I thought my timing was a bit retarded so I gave it a couple of degrees which would get me close to MBT and nothing changed. I am using good quality parts, brand new from the box, the exhaust system and intake system is well designed. I have attached the dyno boost level of both wastegate I tried, and it looks like the creep is consistent with both wastegate. Also, I made a pull with the dump tube of the WG removed to make sure that it was not restrictive. Finally, I tried 2 different vacuum sources to the WG, same results. There is no boost solenoid installed for now.

I am pretty sure that a 46mm WG is able to bypass enough exhaust gases to control boost properly. But I am running out of ideas on what could cause that... too small of an A/R maybe causing too much backpressure?

Anyway, if you have any ideas, feel free to share!

Thank you, have a good day,

Frank

Update :

Did a test with no wastegate installed, the manifold WG port being vented directly to atmosphere, and I was able to boost 10psig at around 6500rpm... Both my dyno and the MAP sensor read 10psig! I'm starting to think it's a bad manifold design?

The wastegate is placed 90deg from the runner, and the runners come a bit higher than the middle of the wastegate port, which is probably not ideal to vent the exhaust flow to the atmosphere.

I also sent a email to DocRace about my issue a couple of days ago, never heard from them. I wrote to them again today, maybe I'll have more luck!

Frank

Yes that is a terrible design. You need the wastgate have the same or better flow than the turbo inlet. If you could make a new manifold, here is a diagram of the best way to do it if you have the room:

https://www.turbosmart.com/wp-content/uploads/2011/10/5529.jpeg

and this on what to avoid:

https://www.theturboforums.com/attachments/wastegateplacement-jpg.662720/

Thanks David,

Yeah I found those diagrams online as well. I knew the placement was not ideal, but I sold a couple of these before and never had any problem, although they were all in a T3 config.

My other option would be to add another wastegate at the back side of the manifold. I don’t really want to butcher this manifold, I hope that DocRace will answer me and we can find a solution together. But I will if I have to!

Well, you did the test to determine that it's the design that's the issue. Perhaps a larger wastegate pipe would help (even with the bad shape) -- it's certainly not the same cross section as the turbo port.

BTW -- try calling, it's a lot easier to ignore an email you don't want to deal with than a person on the phone.

They replied to our supplier this morning, asking for pictures and all. They had not listed their phone number on their website, but a quick google search did lead me to their Facebook page and they have their phone number listed there.

Thank you for your time David.

So we ended up adding a second wastegate, all is good now. Boost is now consistent.