Unknown sound similar to BOV on positive boost with partial throttle RB26

[nouveau_poor]

Hi everyone,

I've been wrestling with this for a while now and have been trying to find out the cause. For context, the turbos used are Garrett 2860 -5s, the BOV is a BNR32 HKS SSQV IV kit, the car is currently tuned to ~470 whp on 17.5 psi. The car drives normally, pulls well when it doesn't happen, and I can replicate it fairly easily. It does not sound like turbo shuffle or flutter. The engine has only a thousand or so miles and has had this behavior since it was completed.

After my engine was built for my R32 GTR, I noticed that the car now sometimes makes an air discharge sound on what appears to be positive boost pressure that sounds really similar to a BOV. I had thought that it was a BOV issue but even when replacing it with a brand new unit, the sound persisted. It seems like it's coming from the passenger side but I may be mistaken. The closest scenario I could find was this post here

https://www.gtr.co.uk/threads/maybe-a-boost-leak-opinions.147955/https://www.gtr.co.uk/threads/maybe-a-boost-leak-opinions.147955/

started by @yakshii and it sounds very similar. As in, at partial throttle once I reach positive boost I begin to hear the same psh psh psh psh psh sound like air is leaking somewhere when I keep the throttle in the same position. It most commonly happens in 4th gear at around 3-3.5k RPM and 5th gear around 2.5-3k RPM, which seems to coincide with normal positive boost thresholds. It might be similar to what @Austrian GTR mentioned about his own HKS SSQV. Notable difference would be that when he applies more throttle when it happens, it stops. In my scenario if I apply more throttle during this repeated psh psh psh sound, the cadence of the sound gets faster and louder rather than stopping. If I lift off slightly and apply throttle again, it will normally stop and pull without issue. I've checked all rubber couplers to ensure that they are tight, but have not gotten the opportunity to properly do a pressurized boost leak test.

If anyone has had similar experiences or thoughts on what might be the cause, I'd be very keen to hear them. I also have a video of it happening from inside the cabin, if that would make it easier to understand: https://youtu.be/2zqZXcx8jbA

[GTSBoy]

26 minutes ago, nouveau_poor said:

It does not sound like turbo shuffle

Yeah it does.

[nouveau_poor]

12 minutes ago, GTSBoy said:

Yeah it does.

Thanks for the quick reply. To be clear, when you say turbo shuffle do you mean turbo flutter "stustustu" or referring to something else? I had thought they were the same thing. When I wrote the post my intention was to say it wasn't a flutter/compression surge sound. My understanding was that a flutter sound would be occurring when throttle is released, whereas I can keep the throttle in the same position for this noise

Edited 14 hours ago by nouveau_poor

[GTSBoy]

No. Turbo shuffle and surge/flutter are not the same thing. Specifically, on a GTR, turbo shuffle has a definite meaning.

On a GTR, the twin turbos are assumed to be the same thing and to operate the same way, exactly. In reality, they do not. Their exhaust sides are fed and exhaust a little differently, to each other. Their inlet sides are fed and exhausted a little differently, to each other. Consequently, when they are "working" they are often at slightly different points on the compressor map compared to each other.

What this means, particularly when coming on boost, is that one of them will spool up and start producing extra flow compared to the other, which will put back pressure on that other compressor, which will push the operating point on that other compressor up (vertically). This will generally result in it bumping up against the surge line on the map, but even if it doesn't, it upsets the compressor and you get this surging shuffle back and forth between them

That is "turbo shuffle" on a GTR. It is related to other flutter effects heard on other turbo systems, but it is a particular feature of the somewhat crappy outlet piping arrangement on RB26s. There are plenty of mods that have been attempted with varying levels of success. People have ground out and/or welded more material into the twin turbo pipe to try to prevent it. Extending the divider inside it works, removing material doesn't. There are aftermarket replacement twin turbo pipes available, and these exist pretty mush purely because of this shuffle problem.

[nouveau_poor]

11 hours ago, GTSBoy said:

No. Turbo shuffle and surge/flutter are not the same thing. Specifically, on a GTR, turbo shuffle has a definite meaning.

On a GTR, the twin turbos are assumed to be the same thing and to operate the same way, exactly. In reality, they do not. Their exhaust sides are fed and exhaust a little differently, to each other. Their inlet sides are fed and exhausted a little differently, to each other. Consequently, when they are "working" they are often at slightly different points on the compressor map compared to each other.

What this means, particularly when coming on boost, is that one of them will spool up and start producing extra flow compared to the other, which will put back pressure on that other compressor, which will push the operating point on that other compressor up (vertically). This will generally result in it bumping up against the surge line on the map, but even if it doesn't, it upsets the compressor and you get this surging shuffle back and forth between them

That is "turbo shuffle" on a GTR. It is related to other flutter effects heard on other turbo systems, but it is a particular feature of the somewhat crappy outlet piping arrangement on RB26s. There are plenty of mods that have been attempted with varying levels of success. People have ground out and/or welded more material into the twin turbo pipe to try to prevent it. Extending the divider inside it works, removing material doesn't. There are aftermarket replacement twin turbo pipes available, and these exist pretty mush purely because of this shuffle problem.

I appreciate the detailed explanation, think I understand now. I spent the better part of last night reading what I could about shuffle and potential solutions. I had replaced the OEM twin turbo pipe with an alternate Y pipe that is separated further away from the turbo. The current one is from HKS and I had a previous pipe that was separated even further away, both have shuffle. I had heard that a divider can be welded in to the OEM pipe to remove turbulence, and figure that aftermarket pipes that are more separated would achieve the same thing. From what I read, most people with -10 turbos get shuffle due to their size, though it's a bit less common with -5s on a standard RB26. I think Nismoid mentioned somewhere it's because OEM recirculation piping is common in Australia with -5 cars.

It seems that the recommendation tends to vary between a few options, which I've ordered in what I think is most feasible for me: 

1. Retune the MAP or boost controller to try to eliminate shuffle

2. Install OEM recirculation piping

3. Something called a 'balance pipe' welded onto the exhaust manifolds. I don't know if kits for this are available, seems like pure fabrication work

4. simply go single turbo

My current layout is as follows:
Garrett 2860 -5s

HKS Racing Suction intake

MAF delete pipes

HKS racing chamber intake piping

hard intercooler piping, 

ARC intercooler

HKS SSQV BOV and pipe

Haltech 2500 elite ECU and boost solenoid/controller

HPI dump pipes

OEM exhaust manifolds

HKS VCAM step 1 and supporting head modifications

Built 2.6 bottom end

All OEM recirculation piping was removed, relevant areas sealed off

I'll keep an eye out for any alternative solutions but can get started with this. 

Only other question is, does shuffle harm the turbo (or anything else)? It seems like some people say your turbo shafts will explode because of the opposing forces after a while and others say they just live with it and adjust their pedal foot accordingly.