RS6 hybrid surging, can't figure out what's wrong

[rnagy86]

Hi

So this is mostly for the educated so please if you don't know what you are talking about
don't bother replying, no need for "pressure test the system" comments. I did not want to
sound rude but ...

Anyway, here are the specs of the car and then I will describe the problem.

- built bottom end rs4
- ported and polished 2.4 heads with built valvetrain
- tubular exhaust manifolds (in: 37mm, out: 41mm)
- RS6 hybrid turbos with 57mm pressure pipes and 60mm intake pipes, 45-50mm turbine, 46.5-60mm compressor
and also a bigger compressor cover (2.7TDI ones)
- all the other intake components are rs4
- dual 3" exhaust
- ER intercoolers



Car dynoed 665bhp, running it's best fats in 2.3s but the problem is the massive compressor
surge it is getting between 5000 and 5750 rpms, even at 1.5-1.6 bar of boost and I can't figure
out where the problem is. Below 1.5 bar it is not surging.
There is no boost pressure loss but the mass airflow and the sound clearly
shows its surge. I've measured exhaust back pressure pre-turbine and it is a constant 2.5 bar at 1.6 bar
of boost even during surge. That's a good ratio I think and should not cause issues.
Changing camshaft changeover has no effect on the surge what so ever.

Here are some graphs to illustrate the issue:



And a video from the airbox to illustrate:



I talked to several people and none has any idea what causes this so I am putting out here
to try my luck to see if anyone has any glory idea.

Thanks for the input!

[vavJETTAw36]

Just curious, have you tried running without the air box to check for any restriction? Just I thought, maybe the box is restrictive


Sent from my iPhone using Tapatalk

[rnagy86]

Just curious, have you tried running without the air box to check for any restriction? Just I thought, maybe the box is restrictive


Sent from my iPhone using Tapatalk

That was actually the first thing that got checked, by doing actual measurements of restrictions
An RS4 airbox is no restriction for such power levels but as you can see in the video I posted that
was taken with a big ass cone filter. Since I max out the RS4 MAF at redline I also have a 90mm
MAF housing which i tested as well. This is certainly not an intake restriction. That would also
manifest in an other way as you can see on the graph it can take more air just fine after passing
the surge.

Here is another graph which just got taken 15 minutes ago, where you can see the MAF maxing
out at redline, but that's fine, that's certainly not the issue.

[Monty23]

I'm thinking some sort of turbulent flow scenario (vortex shedding maybe?) before the turbos that are causing the compressor to stall in that limited rpm region. What do your turbo inlet pipes look like? Are there any significant neck downs right before the turbos?

[rnagy86]

I'm thinking some sort of turbulent flow scenario (vortex shedding maybe?) before the turbos that are causing the compressor to stall in that limited rpm region. What do your turbo inlet pipes look like? Are there any significant neck downs right before the turbos?

I have the OEM RS4 intake pipes modded to slide on to the bigger compressor cover with the same
type of O-ring seal. They were cut and a piece of pipe was widened and then tig welded. There is
also an air-flow straightener in the RS4 intake pipe which is also installed in my pipes.



The second image shows how the pressure pipes look like. No crazy bends.

[getslideways]

Do you have a means of limiting the boost pressure. I would be curious to see if you capped the boost pressure below 1.5 bar if it would still surge or if it would shift where it surged one direction or another in the RPM scale.

Also, do you have a graph of what the N75 duty cycle is doing during this?

[rnagy86]

Have you logged IATs?

I did yes, the car has been running like this for a couple of months now. My IATs are always +10C of ambient which means that
i usually go 30-40C IAT during a pull up to redline, only saw 70C on the dyno at redline.

[rnagy86]

Do you have a means of limiting the boost pressure. I would be curious to see if you capped the boost pressure below 1.5 bar if it would still surge or if it would shift where it surged one direction or another in the RPM scale.

Also, do you have a graph of what the N75 duty cycle is doing during this?

Of course I can do that for you. Here is a graph where N75 duty is fixed to 0%-10%-20%-30%-40%-50%.
No surge except for the 50% WDGC run which has one slight dip already.
I swear whoever tells me what's wrong, I am going to send that person a full set of RS4 carbon engine covers as a gift

[getslideways]

I swear whoever tells me what's wrong, I am going to send that person a full set of RS4 carbon engine covers as a gift

I better call in sick to work and put my thinking hat on!!!! lol

[NOTORIOUS VR]

Well here's my take on the situation... Surging in this case would the turbos trying to supply more air to the engine then it can ingest. I don't see that being the case w/ RS6 style turbos, even w/ hybrids. So I will move away from the turbo and direct my attention to the engine. Specifically the cams, cam timing and VVT activation as well as possible issues with the TB.

I would have a look at the cam timing and VVT change over because well, according to your graphs it looks like it might just be around where the cams are turning OFF. But I can't tell for sure since we don't have a full log available. But it would be interesting to see it with an overlay of the surging vs VVT switch over point.

Also it would be easy enough to do a test with the cams on both fully on and full off positions to see what happens w/ the compressor surge w/ different cam states.

[rnagy86]

Well here's my take on the situation... Surging in this case would the turbos trying to supply more air to the engine then it can ingest. I don't see that being the case w/ RS6 style turbos, even w/ hybrids. So I will move away from the turbo and direct my attention to the engine. Specifically the cams, cam timing and VVT activation as well as possible issues with the TB.

I would have a look at the cam timing and VVT change over because well, according to your graphs it looks like it might just be around where the cams are turning OFF. But I can't tell for sure since we don't have a full log available. But it would be interesting to see it with an overlay of the surging vs VVT switch over point.

Also it would be easy enough to do a test with the cams on both fully on and full off positions to see what happens w/ the compressor surge w/ different cam states.

I wrote in my initial post that VVT changeover has no effect on it, at least nothing noticable.
I can't find the log I did where I set changeover to 6500rpm from the OEM 5500rpm.
I could only find the one where i turned it off completely and the one that moved it to 6k.
There were less boost pressure requested on the 6k one though, I was trying everything I could. (That's why MAF is different a bit)



I was also thinking a lot about the hydraulic lifters not holding pressure and thus
not opening valves completely but I have zero valvetrain noise after hard pulls.

[NOTORIOUS VR]

Interesting... what about checking the gates on the turbos? Maybe they are out of sync and one turbo is making much more boost then the other?

[rnagy86]

Interesting... what about checking the gates on the turbos? Maybe they are out of sync and one turbo is making much more boost then the other?

I will check that, but they were completely in sync when the engine went in and it's surging since
the beginning, but it's never a bad idea to re-check them.

Also what I did today is that I tried to measure pressure difference between the compressor
housing at the N75 line and post IC to see what is what, but i forgot that the N75 is basically
a boost leak (compressor house pressure < post-IC pressure) so I had to electrically disconnect it to
have it closed and at wastegate pressure I am getting a constant +0.1bar pressure at the compressor housing,
that's a 10% difference so that might be a lot more at higher pressures, so what I was thinking is that tomorrow I should
fabricate pipes to bypass the ICs for a test run and see what happens. I am pretty sure other
high power cars run ER smics as well and don't have this problem, but I guess this has to be
checked as well.

[NOTORIOUS VR]

I will check that, but they were completely in sync when the engine went in and it's surging since
the beginning, but it's never a bad idea to re-check them.

Also what I did today is that I tried to measure pressure difference between the compressor
housing at the N75 line and post IC to see what is what, but i forgot that the N75 is basically
a boost leak so I had to electrically disconnect it to have it closed and at wastegate pressure
I am getting a constant +0.1bar pressure at the compressor housing, that's a 10% difference
so that might be a lot more at higher pressures, so what I was thinking is that tomorrow I should
fabricate pipes to bypass the ICs for a test run and see what happens. I am pretty sure other
high power cars run ER smics as well and don't have this problem, but I guess this has to be
checked as well.

There are plenty cars that have run same size and bigger turbos through ER's... My Tial 605 car included w/o and surging issues. So I don't think it will be the IC's either.

And have you checked the TB for any issues/restrictions? And maybe you won't wanna hear this, but if anything was done timing belt/cam wise, double check all that as well.

[rnagy86]

There are plenty cars that have run same size and bigger turbos through ER's... My Tial 605 car included w/o and surging issues. So I don't think it will be the IC's either.

And have you checked the TB for any issues/restrictions? And maybe you won't wanna hear this, but if anything was done timing belt/cam wise, double check all that as well.

The TB opens completely at WOT (only checked while the car was stationary of course). The head was built, it got new lifters and 2.8 cams and timing is spot on. (-1 and 0 in block 93)
I really don't know what can go wrong there. :(

[Corradovolksb]

To rule out the throttle body look at requested throttle angle from the pedal and actual throttle angle and see if the throttle is opening and closing around that rpm. I know this is a basic question but have you done a throttle body adaption recently? I have had that cause some strange issues.

[rnagy86]

To rule out the throttle body look at requested throttle angle from the pedal and actual throttle angle and see if the throttle is opening and closing around that rpm. I know this is a basic question but have you done a throttle body adaption recently? I have had that cause some strange issues.

As I said previously I did check the TB and it was opening fully, the logs show it's completely open as well,
of course you can't make sure unless you have a borescope installed all the time and monitor it :D
I can do an alignment tomorrow but I fear that's not going to change a thing :(

[S4gasm aka LOTR]

Don't suppose you've tried any other intercoolers such as SRM? Maybe a long shot, but possible those ER intercoolers can't keep up.

[Corradovolksb]

Ok cool you said it opened fully while stationary so didn't know if its reading 100% during a pull. I would suggest Diverter valves are causing it but your boost curve looks constant. LOL a bore scope in the intake could be cool but yah not possible.
I'm not sure a TBA will fix it but I wanted to throw it out there.

[jballou]

I think I had a similar issue on a clients car. Had the turbos rebuilt and the problem went away. They changed the compressor wheels and rebuilt the wastegates. I thought all along it was the wastegates doing it. I think I even have a video of it.

[rnagy86]

I think I had a similar issue on a clients car. Had the turbos rebuilt and the problem went away. They changed the compressor wheels and rebuilt the wastegates. I thought all along it was the wastegates doing it. I think I even have a video of it.

This seems to be a completely different issue, I don't have any boost fluctuation.

[jballou]

Ahh, just saw your video... Yeah that is different.

Does this only happen at Full throttle? Or does partial throttle do this as well?

I am still thinking it is a wastegate issue. What are the Cracking pressures set to?

[rnagy86]

So I went crazy today and did two things.

#1 run the car without intercoolers, quickly bypassed them (no pictures sorry, i forgot to take 'em)

I saw a sudden overshoot of boost on ramp up, so I had to fix wgdc to 50% to see what's up and I noticed
that there is basically no surge at all at 1.6bar of boost, I stopped at 6k rpm because I knew IATs will be super high, well for sure
they were. 100C at 5.5k RPM and ECU pulling timing down to 2 degrees... thank you E85. ;)

Then I figured I have to try something else. I remembered from Flyboy's testing that the OEM RS4 intercoolers
flow better than the ERs, well the only problem is that I only have one handy, the driver side.
... but hey I have a set of C5 RS6 intercoolers which are GIGANTISCH (ref.: https://www.youtube.com/watch?v=5WSkKUjizxU).
Since I did not have enough silicone couplers for doing both sides with the RS6 coolers,
I did the driver side with the RS4 and the passenger side with the RS6 cooler.
I think I could not fit both RS6 anyway, they are really huge.... I mean it.
Don't laugh at me, this is eastern european style.





I also did a reference run with the ER coolers an hour before I installed these here are the results.
First is the ER then the RS6-RS4 combo.





And an overlay of the two:



What do you guys think?

[NOTORIOUS VR]

And an overlay of the two:



What do you guys think?

My opinion? It's the turbos themselves. Maybe something didn't go right with the machining, or the compressor vs turbine wheel selection is bad.

Did you get around to checking cracking pressure on the turbos, making sure they're sync'd?

[rnagy86]

My opinion? It's the turbos themselves. Maybe something didn't go right with the machining, or the compressor vs turbine wheel selection is bad.

Did you get around to checking cracking pressure on the turbos, making sure they're sync'd?

Yup I also did that and they are completely in sync 0.9 bar cracking pressure each and both have 1bar
at the compressor housing when measured separately.

[NOTORIOUS VR]

Yup I also did that and they are completely in sync 0.9 bar cracking pressure each and both have 1bar
at the compressor housing when measured separately.

Sounds good...

I'm going to stay with the turbos being the culprit at this point. Who built them and decided on using the TDi covers etc? Was this a previously proven combination (exact wheels + covers + housings, etc) or a one off?

[rnagy86]

Sounds good...

I'm going to stay with the turbos being the culprit at this point. Who built them and decided on using the TDi covers etc? Was this a previously proven combination (exact wheels + covers + housings, etc) or a one off?

It was built buy a local turbo shop whom I am friends with and we tried to pick the best solution in order to have
the best possible spool and top-end power as well. The TDI covers were picked because that's the biggest compressor
cover that can fit the K04 with the widest exit, all the other hybrids are using either OEM K04 or chinese covers
which are even smaller than the OEM ones. This is not a previously proven combination it's a one off. I don't like
all the chinese crap out there and the German hybrids have a ridiculous price tag, for what they are.

I've been talking to the shop as well for hours and we always get to the conclusion that the engine either
can't take the air or we have to go with a different turbine wheel, because what I have now might not be
perfect on the this engine. 12 blade wheel, unclipped. Clipping is an ugly solution (even though everyone else does it)
and going with a 9 blade wheel will sacrifice lots of spool.

Before going with another turbo I just want to rule everything else out. But if I have to go with another set
I might just go with gt28s.

[S4gasm aka LOTR]

Regardless if the turbos are fubar'd or not, you should seriously invest in a pair of SRM intercoolers. Those ER intercoolers flow barely better then stock IC's.
The test you're doing with mismatching IC's isn't going to yield very good results, obviously.

[rnagy86]

Oh i also forgot these. This is the same test ER vs mixed cooler with less boost.
This looks more interesting... Yes I would get SRMs if it would not take a month
and insane shipping prices +27% VAT + 8% duties.





And here is the log with NO intercoolers (fixed 50% wgdc).

[NOTORIOUS VR]

It was built buy a local turbo shop whom I am friends with and we tried to pick the best solution in order to have
the best possible spool and top-end power as well. The TDI covers were picked because that's the biggest compressor
cover that can fit the K04 with the widest exit, all the other hybrids are using either OEM K04 or chinese covers
which are even smaller than the OEM ones. This is not a previously proven combination it's a one off. I don't like
all the chinese crap out there and the German hybrids have a ridiculous price tag, for what they are.

Understandable, but one off solutions like this you can't be surprised if you run into issues like this. :)

I've been talking to the shop as well for hours and we always get to the conclusion that the engine either
can't take the air or we have to go with a different turbine wheel, because what I have now might not be
perfect on the this engine. 12 blade wheel, unclipped. Clipping is an ugly solution (even though everyone else does it)
and going with a 9 blade wheel will sacrifice lots of spool.

I don't see the need to do any clipping of the turbine, in your OP you reported that your EBP is under 2:1 which is very good. Clipping at this point will probably only induce later spool (with a possible gain up top). Maybe bandaiding the surge issue by slowing down the compressor side as well. But I personally don't think that is the right way to "fix" this issue. Then again, I'm not a turbo engineer :P

Before going with another turbo I just want to rule everything else out. But if I have to go with another set
I might just go with gt28s.

Fair enough, although a set of EFR's would be a better choice IMO :)

[rnagy86]

Understandable, but one off solutions like this you can't be surprised if you run into issues like this. :)



I don't see the need to do any clipping of the turbine, in your OP you reported that your EBP is under 2:1 which is very good. Clipping at this point will probably only induce later spool (with a possible gain up top). Maybe bandaiding the surge issue by slowing down the compressor side as well. But I personally don't think that is the right way to "fix" this issue. Then again, I'm not a turbo engineer :P



Fair enough, although a set of EFR's would be a better choice IMO :)

The shop said the same, clipping or going with 9 blade wheel might just slow things down to
avoid surge, but it's not the perfect solution.
Or maybe I need bigger cams to let the engine breathe more, and better ICs.
I really can't make my mind up... either solution costs a LOT :)

[NOTORIOUS VR]

The shop said the same, clipping or going with 9 blade wheel might just slow things down to
avoid surge, but it's not the perfect solution.
Or maybe I need bigger cams to let the engine breathe more, and better ICs.
I really can't make my mind up... either solution costs a LOT :)

I honestly don't think IC's are the answer here. ER's cool very well, for that you give up some flow yes. But even with your other IC's and even without IC's it still seems like they are surging a little unfortunately.

Cams are always a good thing IMO and always underrated/overlooked on these platforms, but again I have doubts about your RS6 hybrids out flowing the likes of Tial 605's or 770's or K24 RS6's, all of which don't have issues with surging on stock 2.7 hardware or 2.8 top end.

Wondering if you're able to try a different compressor housing to see if that helps?

[rnagy86]

I honestly don't think IC's are the answer here. ER's cool very well, for that you give up some flow yes. But even with your other IC's and even without IC's it still seems like they are surging a little unfortunately.

Cams are always a good thing IMO and always underrated/overlooked on these platforms, but again I have doubts about your RS6 hybrids out flowing the likes of Tial 605's or 770's or K24 RS6's, all of which don't have issues with surging on stock 2.7 hardware or 2.8 top end.

Wondering if you're able to try a different compressor housing to see if that helps?

I don't know about Tials and K24 RS6s. I can't comment on them. I could try a different compressor housing,
but that needs machining and lots of work :) Here is my dyno btw (4th gear, 1.8bar @ 20-25 deg timing 70C IATS and surging):

- engine (corrected)
- engine (measured)
- wheel (measured)

[rnagy86]

Just a quick update on this... I've received my SRM intercoolers and of course it made no difference at all.
Next up, I am installing Loba camshafts to see if that makes a difference, then I guess it's new turbo time.

[JPM-Engineering]

I would had try external wastegates... what about the DP ? do they have divided flow path for the turbine and wastegate port?
https://www.google.cl/search?q=divor...CMy3wATI_5ywDA
hope you got the solution to your prob.
Regards man.

[Audi9]

Very curious if a solution was ever found? My money would be on the turbos. It's consistently happening at the 400 g/s mark despite all the changes you've made (the biggest being the cam changeover)

Can you swap the compressor housing? To me, it sticks out like a sore thumb

[rnagy86]

I did not get to installing the cams just yet, next week maybe, then I will update.

[rnagy86]

Okay so I've finished switching the cams to the Loba ones, without changing anything in the tune the car feels
more alive down low in the rpm range, and the surge moved down a good 200 rpm.

[Monty23]

Sounds like you have poorly matched compressor wheels that are being run into the surge region. Your options are to increase flow through your motor (cams didn't do enough it seems), decrease pressure ratio (lame), or change the turbo specs with your target pressure ratio in mind. Maybe chat with TTE about your current hardware and get their opinions?

[rnagy86]

Sounds like you have poorly matched compressor wheels that are being run into the surge region. Your options are to increase flow through your motor (cams didn't do enough it seems), decrease pressure ratio (lame), or change the turbo specs with your target pressure ratio in mind. Maybe chat with TTE about your current hardware and get their opinions?

Well what I can do is to get the turbos off and get the turbine wheels clipped because that's all for free and I only need to wrench on the car. The other thing I can do is to
not give a damn about it and just enjoy the car because I've talked to some people and they all told me that these compressor wheels surge on other engine as well
and I got a log from an S3 2l TFSI which surges like crazy (real surge as there is boost fluctuation as well, unlike my surge which is really on the margin as it does not affect
boost) and they all said that it never affected performance or reliability. Some new TTs are even worse and the OEM turbo can surge as well with the OEM tune.