Boost creep, is my wastegate flapper opening enough?
 

2560, begi seperated gases DP.
Pictures attached, one is closed all the way, the other is open as far as I can get it. Does the wastegate actuator even try to open it much more than this?

Opening the flapper by hand, it definitely feels like its getting stuck on the DP. I remember when I had the DP off that the flapper opened a lot more. My main concern is that I'm trying to run 7-8 psi, and I'm ending up at 10 psi before hitting overboost protection. It seems to want to stay at 11 psi, so its not much overboost, but its annoying. This flapper issue was my first suspicion.

I was thinking of having a bell mouthed pipe welded on to make the diameter at the turbo side bigger and leaving the rest alone, but don't know if that hurts flow, make overboost worse or something weird that I wouldn't understand because physics are over my head currently. Also the bolts I'm using to hold the turbo and DP together are already in a tight area. If I were to switch to studs eventually it would make putting this together impossible if I were to make the pipe bigger on the turbo side.

tl;dr WG flapper hitting DP, wat do?

i was getting boost creep from a separated gas DP... I am 90 percent sure it was because the 'wastegate' tube was too long, and was creating a flow striction for the wastegate gasses...thus the creep. My flapper would hit the downpipe as well, but the wastegate actuator would not even extend it to that point...so flapper not opening enough was not the issue in my case.

You can hook up an air compressor or something to the wastegate actuator, and see how far it moves the flapper to see if that's a possible cause. If this is one of the SGDPs that doesn't recirc the pipe back into the main one immediately, I would guess that is the real issue at hand here.

It does recirculate it, probably about 20 inches down. I'm gonna give the actuator some pressure and see how far the flapper actually opens.

Honestly I had the same problem with my bell downpipe. The way I think of it is that the flapper opens right into the hole in the flange and pretty much restricts the flow into the tube whether or not it opens all the way, and i'm not convinced that it does.

My problems were all solved by switching to a bell mouthed downpipe.
https://www.miataturbo.net/attachmen...ine=1312486009

Did you have boost creep as well? I'm considering something like that but I'm unsure of the downsides/effects to not having the gases separated, if there are any.

inb4 EWG, commiefornia laws suck ass and I'd at least like to resemble the CARB legal kit.

Yes I had boost creep like a mother before going to a bell mouth. Everything seemed to improve for me, spool, power, though I have no dyno sheets to prove it. Only creep I get now is like ~1psi at redline.

As for Carb legality, I have no clue. I dont know how closely they inspect this sort of thing. My guess is that if anything it looks less fancy than the divorced gasses downpipe so it might just slide by. That and Bell offers downpipes that are not divorced so it shouldn't make a difference.

Interesting. Begi never answered my email so I guess I may hack up my downpipe.

Found this.
"As for the geometry of the exhaust at the turbine discharge, the most optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter-- via a straight conical diffuser of 7-12° included angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge. Many turbochargers found in diesels have this diffuser section cast right into the turbine housing. A hyperbolic increase in diameter (like a trumpet snorkus) is theoretically ideal but I've never seen one in use (and doubt it would be measurably superior to a straight diffuser). The wastegate flow would be via a completely divorced (separated from the main turbine discharge flow) dumptube. Due the realities of packaging, cost, and emissions compliance this config is rarely possible on street cars. You will, however, see this type of layout on dedicated race vehicles.


A large "bellmouth" config which combines the turbine discharge and wastegate flow (without a divider between the two) is certainly better than the compromised stock routing, but not as effective as the above.

If an integrated exhaust (non-divorced wastegate flow) is required, keep the wastegate flow separate from the main turbine discharge flow for ~12-18" before reintroducing it. This will minimize the impact on turbine efficiency-- the introduction of the wastegate flow disrupts the flow field of the main turbine discharge flow."

If only you had hard data for improved spool and power. My 2560 has the little divider plate between the turbine and wastegate, not sure if that would make a bellmouthed DP better at all.

Uhh...
so my wastegate actuator barely starts to do anything until 7 psi. It isn't what I would call "fully" open until 25 psi. My gauge is somewhat accurate.
Is this normal? I know theres considerable pressure behind the wastegate flapper door when the car is actually running but wow. This is supposedly a 7 pound spring.

EDIT: Derp, this is apparently normal.

I read through that thread, I'm not overboosting that much and this is a brand new Garrett 2560.

FWIW: I hit my boost target when its warm. Cool nights = 11 psi.

Try loosening your wastegate rod a few threads?

Also, i assume you're not using any kind of boost controller, correct?

I'll try that today. Yep, the actuator is running off a pre-TB line.

Drove it last night in cool weather, it was in the 50's. I kept hitting my 10 psi overboost protection hard. Had to keep it around 1/2 throttle just to stay under.

:vash:

wastegate dump tube being that long is going to really be inclinded to creep is still my guess.

I've talked with ARTech about my old downpipe (same design as yours pretty much) and it's the only one he's made with a recirc tube that long and the only one that's creeped. He's made many many others that recirculate back in almost right away, and never had a creep issue...said on his own car he's running a steady 5 or 6 psi with a downpipe like that.

Interesting. I plan to either modify my Begi one, or have another one made with a bellmouth on it. Tim from Absurdflow says he only made bell mouthed ones for begi setups.

After some testing I realize that my wastegate actuator does not run out of travel before the flapper hits the DP.

I already knew this going into it but EWG or go home. If I wasn't trying to at least resemble the legal kit I would've gone with EWG.

Thats a great point, loosen the rod maybe 2 turns, i put a good amount of preload on it.

Already did that just for the sake of experimenting. I need to cut that damn wastegate pipe off.

Did you ever get this fixed? I seem to have the same issue with my used BEGI downpipe.

Sir, I give you props for your level of commitment to searching rather than just starting a new thread. Hopefully one of those gents that was posting 6 years ago has a solid answer for you. If not, disconnect the actuator arm so that the flapper swings freely and see if you still get creep. If so, the hole will need to be opened larger or the flapper allowed to swing further, depending upon the root cause of the restriction.

Thanks sixshooter, I've been trolling the site for a year and a half and know better. :) I'm going to give your advice a try but we just got a foot of snow today.

I did also see this thread: Boost creep, is my wastegate flapper opening enough? - Miata Turbo Forum - Boost cars, acquire cats.

Take a look at some of the MK TURBO user build threads and maybe even the sale thread for the systems themselves. Those guys were opening up the wastegate hole for the same basic reason. There were pictures of before and after.