The definitive answer to why our turbos leak oil once we fit dump pipes.

[BIG-D]

Well?

Does anyone actually know why??

The guy that rebuild my turbos just laughed at the back pressure myth .

I have heard tuning can fix it, I have seen cars blow lots of smoke after being tuned especially the wastegate not opening correctly or too soon.

I refitted secondary cats and the smoke eventually stopped, in fact the exh has never been cleaner, so why the hell is this particular make of car bamboozling the experts and has no clear concise explanation as to why a free flowing exhaust upsets the oil feed and drain on our turbos so much?

No other car seems to suffer from this problem and good luck finding a rebuild kit for our turbos.

Everything is secret squirrel, or buy OEM replacements... yeh right phuck off.

Not good enough !

I will show and exlain what I did one day , but only because I phuckin' had too...

Feel free to add insight into my conundrum please.

It may just benefit some other Stinger owners that keep getting bullshitted too.

Peace

 

[Ohiocruiser]

It's not only stinger problem. On my previous car (hhr ss), once I installed free flowing downpipe, oil started leaking from exhaust pipe. So, your rebuild guy can laugh all he wants, but stock turbos are designed to work and seal properly with some backpressure.

 

[BIG-D]

So you don't know?

Ok thanks

 

[Thomby]

I was also skeptical of the claim that it's just "less downstream pressure = oil blowby". In reading about turbo backpressure and interrogating chatgpt, one explanation I saw was this:

- Normally your pressure hierarchy under boost is: input oil feed (say 40-70psi), then exhaust pressure upstream of turbine (say 20-40psi), then exhaust pressure downstream of turbine (a few psi), then oil return ie crankcase (ideally no pressure but could be couple psi under boost).

- Reducing restriction downstream of the turbine mostly drops the exhaust pressure upstream of the turbine (via increased flow), and a smaller amount downstream (since it's not as high there to start). The latter may bring it closer to or even lower than crankcase pressure, especially if the PCV valve is closed off (since manifold is pressurized) and the high boost creates enough blowby to take crankcase pressure slightly positive. Oil sits in the CHRA instead of draining freely, and the reduced exhaust pressure pulls it past the seals.

There are a couple unknowns with this theory, which maybe someone can clarify:

- What our actual crankcase pressure is under heavy boost. The driver's side line from just in front of the turbo inlet to the crankcase would normally pull fresh air into the crankcase, and under boost probably sees a couple psi of vacuum from the turbo...don't know if it has any kind of valving but that may relieve crankcase pressure under boost.

- There's also the "vacuum intensifier" 4-way valve at the top rear driver's side, which connects to the manifold, the intake 6-8" upstream of the turbo, the brake booster, and the vacuum pump (I think). I assume under boost it seals off the pressurized manifold, that the intake piping connection stays at roughly atmospheric pressure and is therefore just a source for fresh air, and that the vacuum pump only serves to provide brake assist, but I guess there are some systems that also connect the pump to the crankcase...

 

[BIG-D]

Awesome response mate thank you very much... I wonder how that would affect people that fit a breather on your drivers side rather than rerouting it back through the intake then ?

 

[Thomby]

Awesome response mate thank you very much... I wonder how that would affect people that fit a breather on your drivers side rather than rerouting it back through the intake then ?

In stock form, I believe the system operates in two modes:
- At idle and low load, manifold vacuum pulls air/vapor through the passenger side hose+PCV valve, putting the crankcase under slight vacuum, with the driver's side hose pulling fresh, filtered air from the intake piping. Moving the driver's side hose to a breather would let it pull fresh air, and assuming a small filter, not change anything.

- Under boost, manifold pressure closes the PCV valve, and the crankcase is either neutral or slightly positive pressure due to blowby. The driver's side hose connects right at the turbo inlet and sees mild vacuum, so flow in that hose reverses as crankcase vapor is pulled into the turbo (chatGPT suggests a couple psi for each, but I don't have any actual data). Moving the hose to a breather would eliminate whatever vacuum the turbo inlet is pulling, but still let positive crankcase pressure vent.

If you instead vented the passenger side hose (after the PCV valve), you would give up the vacuum at idle & low load that pulls fresh air through. Crankcase pressure could of course exit through the vent, and I'm unsure if it would also exit into the driver's side intake (since that hose is open/valveless, I think, but the turbo inlet isn't usually generating vacuum).

 

[oddball]

I'm not sure why they call it an "intensifier" - maybe a humorous translation subtlety? I'm certain there's not a venturi in there or anything and i's just a stack of check valves. I recreated the same basic concept for my Cutlass because idle vacuum is too low for power brakes. The idea is to use manifold vacuum when it's useful - the engine moves a ton of air, so it'll quickly pull the booster to the same vacuum. That way the vacuum pump only has to pull a deeper vacuum, usually up to ~18" HG. Going from atmosphere to 18" HG with a small vacuum pump takes close to a minute, but raising from 14 to 18 only takes a few seconds. The check valves keep the air flowing in the right direction depending on conditions.

Sending the driver side crankcase vent to a little filter instead of the intake will do absolutely nothing except get some oil in your engine bay. That's why guys with classic engines that just run breathers tend to wrap rags around the breathers to catch the oil.
There will be oil coming out of there, but it should be extremely little. Tork sells a catch can for that side, but it'd take tens of thousands of miles to fill up - unless you're doing really exciting things with the car or completely destroyed the rings.

 

[Thomby]

Sending the driver side crankcase vent to a little filter instead of the intake will do absolutely nothing except get some oil in your engine bay. That's why guys with classic engines that just run breathers tend to wrap rags around the breathers to catch the oil. There will be oil coming out of there, but it should be extremely little.

I recently remembered that the first time I pulled the driver's side intake hose off, I saw a tiny amount of oil on the compressor. I assume anything coming from the turbo's oil supply at the CHRA would go straight out the back to the turbine side, so this must be from the PCV flow that reverses under boost.

Another thing I've been wondering about is how an in-line gravity drain catch can on the passenger side actually drains. At idle/low load, intake manifold vacuum pulls the PCV valve open and air from the crankcase comes out. Under boost, intake manifold closes the PCV valve. I thought maybe at engine shutoff the valve would be open, but blowing into that hose confirmed it was closed & airtight.

I only had very minimal oil after ~10-15k miles, basically a light coating inside the catch can and hose, but I assume if it wasn't able to drain it would eventually back up in the line. Maybe when the PCV valve is pulled open by intake vacuum the oil can seep down through it?

 

[oddball]

The oil can drain while air is flowing through it. It'll just be a drop at a time. I'm running Red Line oil which is particularly low NOACK, and my PCV super-special catch can only catches a tablespoon of oil every few thousand miles. It's not a ton.

 

[BIG-D]

Anyway I fixed the issue by removing the down pipes... back to factory including secondary cats and no more oil smoke and it feels just as fast but a lot quieter...

 

[Thomby]

Anyway I fixed the issue by removing the down pipes

So what ended up being the reason for the leak?

 

[BIG-D]

Ask anyone on the internet because I'm not a scientist...

Even the turbo rebuild guys think the back pressure myth is a load of shit!

 

[BIG-D]

So what ended up being the reason for the leak?

Sorry... I'm not qualified to answer that question.

 

[Thomby]

Sorry... I'm not qualified to answer that question.

The TurboKits guys think they've "all but solved" it with their TK700s (ball-bearing hybrids, with upgraded turbine shafts):

The current version runs a dual ball bearing center section with twin turbine end piston rings, which has all but solved the smoking / oil issue when people go high flow CAT with the stock-based turbos.

*****Light smoking is common on 100% flow through set ups, same as the stock turbos, although reduced greatly, can still occur. This is NOT a turbo issue, but a pressure differential issue with the motor and oiling system.

 

[BIG-D]

Yeah I don't buy it... and it was a deal breaker for me , hence I never fitted BIGGER turbos to this and instead returned it back to stock!

A car that blows smoke is just an embarrassment no matter how quick it is!!

 

[Tigershark]

*Please take my input with a grain of salt here but I'm trying to pull from previous experience for a possible explanation/solution*

In the past with my old 335Xi when I had converted to 6266 top mount (N54), the first install went great, right up until about 1500 miles in. The oil feed we had for the turbo was to Precision's spec to ensure we wouldn't have any bearing issues. Unfortunately for whatever reason, we were still seeing oil leaks from the top side input port. We figured this was a warped housing and so did Precision so they sent a new turbo out and took the old one in for inspection. What they found however wasn't a case of bad casting but oil overfeed which had started to cause the seals to fail. Precision still warrantied the turbo and advised we install a restrictor on the feed line to resolve the problem. Sure enough, this was the silver bullet and the car ran like a champ with zero issues from then on.

Again just my .02, but based on whats been discussed so far, the only solution I can think of for this problem would be a restrictor on the oil feed if the drain cannot flow enough positive return to eliminate the blowby. Then again, it's possible this solution could also cause bearing failure from insufficient feed, but to my knowledge no one has really done any math to figure out if these turbos are being delivered an over abundance of oil whether the downpipes go buh bye or not.

 

[BIG-D]

Oil Burning After Catless Downpipe? | SleepyTuned

Understand oil consumption issues after going catless on your 3.3T Stinger or G70.

www.sleepytuned.com

 

[Thomby]

Oil Burning After Catless Downpipe? | SleepyTuned

Understand oil consumption issues after going catless on your 3.3T Stinger or G70.

www.sleepytuned.com

So the theory is that in stock form you have oil inlet pressure > turbine/exhaust backpressure > oil outlet pressure, and that freer-flowing exhausts reduce the turbine/exhaust backpressure below that of (or at least competitive with) the oil outlet (which is gravity drain, but sees crankcase pressure)?

If it's the crankcase, I'd expect turbo smoking through open exhausts to only happen under high boost, right? Wonder if anyone has put a gauge on the crankcase to see how its pressure varies.

Guess we could go dry-sump to keep the crankcase under vacuum...

 

[oddball]

The most common is smoking under load which can have lots of causes including blow-by. The crankcase ventilation isn't terrible. It's not amazing, but it isn't terrible. I would be astonished if there was more than 1/2 PSI pressure in the crankcase even in the worst case, so I don't think that's a big cause.

The key thing with turbos is the shafts spin so crazy fast that they can't use an actual seal. There's no rubber lip seal - it would burn away the first time you jabbed the throttle. It's just a tight clearance between the shaft and its bushing, which means it's *designed* to leak a certain amount. Changing up the exhaust system, especially the cat and the manifold coming off the turbo, changes the environment.

I'm not 100% certain, but I expect the oil pump in the 3.3 is also variable pressure, and the pressure is raised whenever power output is raised. So there's more oil flow going into the turbo. The turbo shaft is a restriction to oil flow - to keep oil around the bushings - so it'll try to squeeze out wherever it can.

 

[BIG-D]

The weird thing is mine only smoked under deceleration and only after hitting redline a few times beforehand...

Never under power .

I did notice my dipstick had popped out about 1/8" last time I checked the oil.

But that was after a whole bunch of dragy runs...