Boost creep 911 Turbo

[proffighter]

Hi Fellows

A friend of mine asked me for help with his boost creep issue. We discussed already bit and checked some things already. I try to describe:

- 3.3l Turbo 82
- Precision Turbo 5862 CHRA Ceramic bearings / Trim 0.82
- 964Turbo IC
- K-Jetronic, enrichement by cold start injector (boost steered)
- GHL headers
- Powerhouse dual outlet muffler
- Tial F46P Wastegate (0.7bar spring -> 1.0bar in reality)
- 964 BOV (plastic one)
- electronic boost control (via wastegate ventilation)

Some small other upgrades, but not relevant here I guess, because motor was running fine.

After winter time he went out for a ride, but boost was increasing, at 1.1bar he stopped. For some unknown reason, max boost increased.

Different measurements:

0.4bar spring with wastegate muffler:









He checked and tried a couple of things:

- Tial F46P Wastegate with 0.4bar spring: No change, stock seems even bader
- Boost hose from IC to Tial checked and replaced
- Boost control removed, wastegate ventilation into the open
- Wastegate muffler replaced
- BOV removed and IC port closed
- checked for general leaks (only hearing and watching, no pressure tests)
- Wastegate spring removed and valve block full open: Still about 0.4bar
- SDS ignition

I was not on site yet, but maybe someone has an idea or even better has similar issue. For me it's sound quite strange. What else could have such an effect without changing some components?

All inputs are welcome

BTW Boost pressure reading by stock clock and Zeitronix ZT-2

Seems to lean from 0.7bar up
Seemst to be fine (plateau at 0.4bar) and then loose control and increase, visible on all 3 pics

[David]

Hopefully Chris chimes in soon since he's the guru here. Seems really strange to have boost creep from just sitting with no changes. I can't think of anything you haven't looked at.

I assume when the wastegate spring was removed, the valve moved freely?

[greglepore]

Is it possible something is blocking the inlet to the wastegate? Has the wastegate been off to check?

[TurboKraft]

If I'm understanding correctly from your datalogs:
- 0,4 Feder => 0,36bar @ 2847UPM; 0,49bar @ 4227, 0,98 @ 6018
- 0,8 Feder => 1,1bar and then shut off for safety

Since changing the wastegate spring isn't resulting in lower peak manifold pressure, I recommend:
1. check that the wastegate's control lines are connected correctly -- always review the basics first...
2. get rid of the "wastegate muffler" and repeating the test. Those pipes (resonator / spark arrestor) severely limit exhaust flow out of the wastegate, and we have had them produce significant boost creep. This is also an easy test.

After that, I would normally suggest inspecting the wastegate to see if the valve travel is being limited in some way (obstruction?), or if the diaphragm has a leak (pressure check), but since you just had it apart, I will assume you checked both of these items.


Headers:

The GHL headers have some inherent boost creep because of the unfavorable exhaust flow from the turbo collector into the wastegate pipe. It's a design that is simple to construct and thus low cost.

B&B headers do, too, but inconsistently -- some better and some worse -- because the angle of the wastegate inlet pipe off of the turbo collector is inconsistent. I remember on (EFI) engine that went right past the 11.6psi (0,8bar) spring and made 16psi manifold pressure. It wasn't a problem as that engine was built to run 18psi daily on pump gas, but it was incapable of running *less* maximum boost.

Fabspeed headers also have some boost creep because of the same basic design.

[Mike930]

Hello everyone,

Thank you for your feedback

My name is mike and this concerns my car.

I once had an original wastegate that was fully open mechanically blockd, and drove with it.
There is also a gap of 4 mm between the wastegate and the exhaust manifold where the pressure can be released.
With the wastegate fully open, the engine produces 0.85 bar.




So i think my Tial Wastegate and the lines are connectet correctly.

Even if the GHL header is not ideal, i think that is still far too much.
I am sure that it was not always that high.
Next, I will probably change the exhaust to see if it is still the same.


Does anyone else have any other ideas?

[Mike930]

I only installed the wastegate in the picture for a test.
The valve is fully open and mechanically blocked in this position.
Nevertheless, the boost pressure is almost 1 bar

[TurboKraft]

Originally Posted by Mike930

Wow -- that is really strange.
...Wastegate locked fully open, bypassing as much exhaust gas as possible...
...plus, wastegate spaced an additional 4mm off of the headers to bleed off more exhaust flow...
...and still making 0.85bar.

You effectively have no wastegate in the system, just an open pipe bleeding off exhaust gas (energy), and there is still enough going through the turbocharger to make 0.85bar boost.

The turbocharger's turbine wheel is comparable to a GT35R's, and a 35R's exhaust flow is adequate, as well as the 0.82A/R turbine housing.

If there was too much backpressure, ex: restrictive cat/muffler, or turbine, then you should not be making boost.
Conversely, if the turbine is the path of least resistance (minimal backpressure) and all the exhaust wants to flow through it and not the wastegate circuit, then you'll be making too much boost.

Question: is there anything new about this turbo system, something you have recently changed?
Or is it all parts you have used with success for a while, and this is suddenly a new problem?

[Mike930]

Hi Chris

No, i've been running the hardware (headers, 964 intercooler, turbocharger, exhaust) like this for a long time.
The only thing i have changed is the setting of the warm-up controller of the k-jetronic and tried an additional injection.

A colleague has the same supercharger in his 930 and run it at a minimum pressure of 0.5 bar

iIthink i will convert to efi, but this will not solve my current problem with the high boost pressure.

As described i will change the muffler next and i still have an exhaust housing a/r 0.63 for the turbocharger.

do you have any other ideas what i could change?

Mike

[Mike930]

Hi Chris

No, i've been running the hardware (headers, 964 intercooler, turbocharger, exhaust) like this for a long time.
The only thing i have changed is the setting of the warm-up controller of the k-jetronic and tried an additional injection.

A colleague has the same supercharger in his 930 and run it at a minimum pressure of 0.5 bar

iIthink i will convert to efi, but this will not solve my current problem with the high boost pressure.

As described i will change the muffler next and i still have an exhaust housing a/r 0.63 for the turbocharger.

do you have any other ideas what i could change?

Mike

[freddy67]

A proper pressure test on the intake tract is worth doing — “looking and listening” can miss small leaks that only open up under boost. Also double-check the wastegate seating surface and make sure it isn’t sticking or leaking around the valve. I’ve seen similar creep when the wastegate flow path was restricted by carbon build-up or a mismatch with the headers. Since he already tried different springs and even removed the boost control, I’d lean toward a mechanical flow issue with the gate itself or the way it’s plumbed.