1zzfe boost creep troubles

Hi

I am trying to fight with boost creep in 1zzfe.

The specs of engine:

- unopened stock and healthy 1zzfe

- gtx2863 a/r .64 with rebored wg hole - in abouts of 26mm of diameter - 0.4 bar actuator, waste gate lever has been shortened to match expansion of the actuator to the fully opened waste gate position

- 4-1 equal length exhaust manifold with reasonably sized runners about 35-40cm

- 3” to 2.25” dp about 40cm of length, it narrows in the middle of it to 2.25”

- 2.25” dual exhaust

- short intake with W2A cooler approximately whole intake with the intercooler should be about 60cm, with 2,75" diameter.

- 510cc injectors flow checked/matched

I am wondering if it can be tune related, or the trouble lays in the “hardware” setup.

In previous setup there was china gt2860, but it threw up its seals, but it was also boost creeping.

The electronic boost control is turned off, because first I want to stabilise wastegate’s pressure boost.

At this point during WOT there is gradual spool from 2000 rpms up to 4000/4200. Above that the turbo gradually creeps to 0.8 bar on long high gear pulls.

I’ve tried (I think) everything now to stabilise boost. The engine is running a bit overfueled according to the boost level. It’s running .75-78 lambda with about 19 degrees of ignition advance at 0.5-0.8bar of positive pressure. There is no more room for advancing it more due to knock. EGT is not exceeding 800 Celsius degree so it should be fine…

I suppose the exhaust is too restrictive now due to do narrowing too fast. But maybe there is something I am missing ?

EDIT:

I've forgotten to mention that there was a situation when downpipe got loosened, and exhaust was leaking - then the boost was stable - which makes me think it's still exhaust related problem. Am I right?

Hello Easy G,

You’re spot on about the exhaust leak—that confirms it's a backpressure issue, not a tuning fault.

I'm still pretty new to EFI and forced induction, but here are a few theoretical points that might be worth checking:

Downpipe: Narrowing from 3" to 2.25" so close to the turbo creates a huge bottleneck. Running a full 3" downpipe further down will drop backpressure and fix the creep. Depending on your noise preference, you can add a good 3" resonator/muffler if you want a deeper, quieter tone, or keep it more open.

Wastegate Entry: Besides enlarging the hole to 26mm, try chamfering/smoothing the inlet edge like a funnel. Sharp 90° edges restrict actual air flow (Vena Contracta effect).

Fixing the downpipe flow and smoothing the WG entry should give the exhaust gas a much easier path. Let us know if this solves it for you!

Yeah, I did also tear drop shaped entry to the wastegate hole, it’s chamfered as you mentioned. I am highly convinced it’s downpipe issue. Just wanted to consult it before doing another exhaust job. The friend of mine who is the owner of this car brought it to me just to complete someone’s else job. I was about to put there stand alone ECU, assembly turbo kit on car and just tune it, but then issues started to multiple - like Chinese turbo was leaking oil to exhaust housing, boost creep etc. I asked another friend of mine to weld new header according to my instructions and it helped a lot, but it wouldn’t fully eliminate boost creeping issues. It’s just creeping now softer, more gradually.

In previous setup there was also the boost creep issue with kinda short 4-2-1 header. But then it was creeping even faster and higher because it Was over boosting to 0.9 bar at abouts of 4200 rpm.

Yep, teardrop porting is definitely a great touch for flow entry! Given that the wastegate geometry is already optimized, that downpipe transition seems like the most likely restriction left causing the creep—especially with how the boost stabilized during that exhaust leak.

Hope freeing up the downpipe flow turns out to be the final piece of the puzzle. Definitely keep us updated on how it goes!

Will do ;)

In two weeks i'll be able to modify downpipe.

Sure, hope to hear good news from you soon!👌👌👌👌