Fuel Pumps

[Shoppinit]

- Those wastegates you are referring to are part 36 below, right? Maybe it's just lazyness, but wouldn't it be enough just to swap left and right and see how it goes? And BTW, where are the dump valves on our cars? Directly attached to the turbos?

Some confusion here. Number 36 are the DVs. They're not dump valves on the RS6, they are diverter valves because the charge pressure is not lost to atmosphere but recycled (since the air has been metered).

The wastegates are actually part of the turbo and are valves to let the exhaust gas bypass the turbine to prevent the compressor wheel spinning up (and causing boost).

The waste gates operate in one of 2 ways:

1. The pressure builds up to a point where the spring holding the wastegate closed is overcome and (some of) the exhaust bypasses the turbine wheel. This way the turbo can't spin too fast (bad).
2. The wastegate has a external actuator which is operated by pressure from the compressor side of the turbo (via the N75). When the pressure is sufficient, this causes the wastegate to open, causing exhaust gas to bypass the turbine. The N75 controls this compressed air flow to regulate the turbo.

The wastegate's function is to allow limiting rotational speed of the turbine to safe levels (for longevity) and to allow regulation of the boost.

The diverter valve has no regulating effect on the turbo. When you lift of during boost, the compressed air needs to go somewhere. It can't go into the engine because the throttle plate has closed, so it needs to be diverted back to the intake pre-compressor so that is can continue to circulate. If you didn't have a DV then the back pressure from the air being able to go nowhere would slow the turbine right down. This means that when you require boost again, you need to do all that work to get it up to speed to compress the inlet air again. [A blow-off valve is a crude device that opens and lets this excess pressure vent to atmosphere. RS6s don't have em. No production cars do, I suspect. Probably lots of Subarus have them fitted, judging by the silly noises they make.]

So... when you said you changed the wastegates previously, did you mean you changed the DVs?

[Shoppinit]

Yeah, time to find that guy/girl from St Denis..

What could possibly go wrong...

- coming back to a previous question you've asked : yes I did check all pipes surrounding the N75, but not up to the turbos. I hope this can be done without having the engine out..

If you do a pressure test to see what pressure the wastegates (not DVs ) are opening then this should show you if there are any leaks. The output pipe goes from the N75 to the turbo wastegates and no further, so they should hold pressure (careful, not too much).

The input to the N75 only comes from one turbo. The assumption being that the turbo pressures are balanced. Might be tricker to test this pipe. Hopefully you'll be able to see it, maybe with your borescope.

- As far as the blocked exhaust scenario is concerned, could that explain the discrepancies knowing that the air coming from both turbos is mutualised at the entrance of the manifold?

The thing is, the turbines are driven by exhaust gas movement. If the gas is moving less quickly in one turbo, then the turbing wil be driven less and the compressor wheel will compress less. You get uneven boost and the pressure finds a way to go from one side to the other. This definitely happens in the manifold, but also, as you've discovered, it tries to find other ways. Hence all the non-return valves etc. in the PCV and vac system.

In extreme cases, if you completely stop one turbo (e.g. after an exhaust manifold hari-kariing - ask a B5 S4 owner about this pain) then the working turbo will push air out wherever it can. I saw this on an RS6 where the air was being pushed back out of the MAF of the broken turbo by the working turbo. The MAF doesn't know which way the air is travelling, only that it is moving through.

[Classik]

- Those wastegates you are referring to are part 36 below, right? Maybe it's just lazyness, but wouldn't it be enough just to swap left and right and see how it goes? And BTW, where are the dump valves on our cars? Directly attached to the turbos?

Some confusion here. Number 36 are the DVs. They're not dump valves on the RS6, they are diverter valves because the charge pressure is not lost to atmosphere but recycled (since the air has been metered).

The wastegates are actually part of the turbo and are valves to let the exhaust gas bypass the turbine to prevent the compressor wheel spinning up (and causing boost).
(...)

So... when you said you changed the wastegates previously, did you mean you changed the DVs?

Thanks for the clarification and the detailed explanation, obviously it was well worth it! So in short WGs regulate turbos speed and DVs recycle excess pressure pre-turbos/post-MAFs when the throttle pedal is released. Now it makes perfect sense
And yes, when I said I changed the wastegates, I meant the DVs (part 36 above^^)! Ooops

Having said that, I now wonder how could I change those wastegates without changing the turbos.. Have I missed something again?

[Shoppinit]

You can't change the wastegates because they are internal to the turbo, but you might be able to change the actuators (next to 24, the bit that the air pipe from the N75 attaches to). I assumed this is what you were talking about.

Also, the pre-load on the wastegate can be adjusted, but I wouldn't mess with this unless I was sure there was a problem.

[Ajc]

You can't change the wastegates because they are internal to the turbo, but you might be able to change the actuators (next to 24, the bit that the air pipe from the N75 attaches to). I assumed this is what you were talking about.

Also, the pre-load on the wastegate can be adjusted, but I wouldn't mess with this unless I was sure there was a problem.

Just too add , the amount of preloaded on the actuators will have an effect on when the waste gates open and at what pressure . So if they are in sync you could be getting a gate opening slightly and this could possibly be giving you your odd readings . It's a shame it's not possible too log the boost pressures on each turbo . That said it wouldn't be very difficult too do if needed .

[Classik]

You can't change the wastegates because they are internal to the turbo, but you might be able to change the actuators (next to 24, the bit that the air pipe from the N75 attaches to).

I might be wrong but it doesn't sound like an easy job without taking the engine out... What should be done though is to check again those hoses (that was supposedly done when the gearbox was changed) and lub the rotating part of the actuators, just below number 19. Of course I'll do the other inspections with the borescope to leave no stones unturned; this will be done as soon as I'll be back from vacations (2nd week of August)

[Shoppinit]

Yes, I don't think changing the actuators would be much fun with the engine in situ. If it's even possible.

Should be easy enough to pressurise the actuators to see if they are balanced, though. Just disconnect the hose from the N75 and put your compressed air on there instead. Slowly increase the pressure from zero, observing both actuators (with the borescope if necessary) to see when the wastegates begin to move.

[Classik]

Just too add , the amount of preloaded on the actuators will have an effect on when the waste gates open and at what pressure . So if they are in sync you could be getting a gate opening slightly and this could possibly be giving you your odd readings . It's a shame it's not possible too log the boost pressures on each turbo . That said it wouldn't be very difficult too do if needed .

Thanks for chiming in ; I suppose you meant 'not in sync'.

As far as measuring the respective turbo pressures, I think we have only access to the sum through the pressure sensor at the entrance of the manifold, right? Is there any info provided by the N75 ? And let's suppose I find a way to measure both pressures, how could I change the actuators settings? Many questions, sorry...

[Shoppinit]

You can put a T in the pipe *to* the N75 and use a pressure gauge to check the pressure from the left hand turbo. More tricky to test the right hand turbo pressure.

Dalton's law suggests to me that pressures are additive, so if you get say 0.6bar from that turbo and the manifold pressure is 1.5 bar then the pressure from the other turbo can be deduced to be 0.9 bar. This seems too easy to me, but I'd definitely give it a try. I guess that assuming there's no other point that air can mix other than in the manifold.

Here's a clear video showing preload adjustment:

https://www.youtube.com/watch?v=p12g1PpIcjE

another one showing actuator testing:

https://youtu.be/NN1Hs3L0YYc?t=112

[Classik]

Great stuff Shoppinit, I'll see what can be done on my return. But surely enough we're getting close! (well I hope )

[Ajc]

Just too add , the amount of preloaded on the actuators will have an effect on when the waste gates open and at what pressure . So if they are in sync you could be getting a gate opening slightly and this could possibly be giving you your odd readings . It's a shame it's not possible too log the boost pressures on each turbo . That said it wouldn't be very difficult too do if needed .

Thanks for chiming in ; I suppose you meant 'not in sync'.

As far as measuring the respective turbo pressures, I think we have only access to the sum through the pressure sensor at the entrance of the manifold, right? Is there any info provided by the N75 ? And let's suppose I find a way to measure both pressures, how could I change the actuators settings? Many questions, sorry...

Yeah sorry , not in sync . I assume the boost is logged via the map sensor which would only give you an overall boost . But as shopp suggests, teeing into each turbo and using an external gauge is doable.

You can put a T in the pipe *to* the N75 and use a pressure gauge to check the pressure from the left hand turbo. More tricky to test the right hand turbo pressure.

Dalton's law suggests to me that pressures are additive, so if you get say 0.6bar from that turbo and the manifold pressure is 1.5 bar then the pressure from the other turbo can be deduced to be 0.9 bar. This seems too easy to me, but I'd definitely give it a try. I guess that assuming there's no other point that air can mix other than in the manifold.

Here's a clear video showing preload adjustment:

https://www.youtube.com/watch?v=p12g1PpIcjE

another one showing actuator testing:

https://youtu.be/NN1Hs3L0YYc?t=112

Is it possible that if one gate isnt seating correctly then the other turbo is going too tey too compensate? , after all the boost is read after the two meet and the ECU doesn't know where the boost is coming from . All it wants too know is if it hits its desired level regardless weather that's from 1 turbo or two .

[Classik]

Dalton's law suggests to me that pressures are additive, so if you get say 0.6bar from that turbo and the manifold pressure is 1.5 bar then the pressure from the other turbo can be deduced to be 0.9 bar. This seems too easy to me, but I'd definitely give it a try. I guess that assuming there's no other point that air can mix other than in the manifold.

Not trying to contradict you, but just thinking out loud trying to understand how it works : supposing air is a perfect gas (which it's not ) and both turbos deliver the same flow of air at the same pressure, the sum of both flows should not increase the pressure at the manifold but just double the quantity of matter reaching the cylinders. Surely enough, air is not a perfect gas so some pressure increase will occur due to the gas temperature increase (van der Waals extension of pv=nrt), but I see no reason why pressures would simply add up. Again what's important is the quantity of matter (air) and that's what those MAFs are for.
Even worse : except during short periods of time (time for pressure to balance out through the plumber), and knowing that both turbos outputs are connected to the same point (throttle body), I even expect the same pressure would be measured on each turbo even if they are not matched! Maybe that would explain why there's only a single pressure sensor and not one on each side..

Pardon my naive and lengthy tentative of explanation above, but except if my reasoning is wrong, we are back either to chasing air leaks, or to find another way to measure mismatch between turbos. Spinning speed?
Or maybe I'm just talking nonsense and it's high time to get some sleep

[Ajc]

I dont think the volume of air being produced is the issue , I think possibly one turbo is dumping its boost through the exhaust due too a wastegate not being shut at the correct time .

[Shoppinit]

Yeah, it makes a lot of sense. There will be other influencing factors like air flow inertia and ducting geometry, but essentially, thinking about it logically, the pressure should be the same more or less everywhere.

By measuring the pressure very close to the compressor, you might be able to detect differences in performance, but it's probably not likely to bear much diagnostic fruit. Probably not worth it, especially given the difficulty of measuring the other turbo.

Turbo imbalance could be coming from wastegate actuator, preload or incorrect gate operation (sticking, not sealing, etc.); or exhaust blockage.

I tend to agree with ajc. More likely to be a wastegate issue than exhaust.

Thinking aloud here... if there was a partial blockage, that would affect turbo performance, but also would suffocate the engine at higher revs. Aren't you seeing fuelling problems at higher revs?

One thing you can check is the EGT. See if you are getting different temps, especially after a good run. The thinking is that is gas isn't flowing as well, then the temps will be higher. Or lower. Either way, could be useful to compare to the other bank.

Do you know if the car has ever had a new turbo? You're supposed to change them as a pair but since they are expensive some people only change one.

[Ajc]

Yeah, it makes a lot of sense. There will be other influencing factors like air flow inertia and ducting geometry, but essentially, thinking about it logically, the pressure should be the same more or less everywhere.

By measuring the pressure very close to the compressor, you might be able to detect differences in performance, but it's probably not likely to bear much diagnostic fruit. Probably not worth it, especially given the difficulty of measuring the other turbo.

Turbo imbalance could be coming from wastegate actuator, preload or incorrect gate operation (sticking, not sealing, etc.); or exhaust blockage.

I tend to agree with ajc. More likely to be a wastegate issue than exhaust.

Thinking aloud here... if there was a partial blockage, that would affect turbo performance, but also would suffocate the engine at higher revs. Aren't you seeing fuelling problems at higher revs?

One thing you can check is the EGT. See if you are getting different temps, especially after a good run. The thinking is that is gas isn't flowing as well, then the temps will be higher. Or lower. Either way, could be useful to compare to the other bank.

Do you know if the car has ever had a new turbo? You're supposed to change them as a pair but since they are expensive some people only change one.

Egt would be a really good shout here , if your seeing a difference in temps then that may point in the right direction. A blocked cat or precat would dring your temps up most certainly.