twin turbo surge issue. advice?

[markrolston@mac.com]

I'm looking for some advice on a turbo issue I'm experiencing:

 

At part throttle (almost exactly 3k) the turbos begin to create a whoosh-whoosh cycle like a slow choo choo. As I accelerate this cycling sound also accelerates until it disappears. Also it feels like a studdered acceleration force rather than a smooth engagement. But that could be my imagination given the focus on the sound. It sounds to me like boost surge in one or both of the turbos. The turbos are essentially spinning up more boost than the throttles will allow the engine at that point.

 

The setup is two 2871s on Tomei Expreme manifolds controlled by an eBoost2 controlling two separate boost controllers. The internal wastegate actuators are 14lb springs and the boost is set to 16 or 19lbs. Both have this effect. (there is a 25lb setting as well but that's off the table with this issue)

 

So far I've received lots of competing advice:

 

1. It's surge and I should put seperate BOVs on each hot pipe and recirc those into the intakes of each turbo. I'm not sure this would have any effect. you tell me. I already have one BOV venting to atmosphere.

 

2. It's an imbalance caused by cylinder firing order causing the turbos to get out of sync and I need a balance tube.

 

3. The turbos are the wrong size for the engine. I need a different size.

 

4. all or a combination of the above.

 

advice? The problem hasn't been solved by tuning.

[mattyice]

ive never heard of a turbo being the wrong size for an engine, you can put any turbo on a car you want...twin 2871R's should be perfect as a twin turbo setup

 

It shouldnt be your BOV since the problem is on spool up from what you said

 

I could see a balance tube helping somewhat, but the stock manifolds dont have one why would your aftermarket ones need one?

 

maybe since the 2871R is a slower spooling turbo for a twin turbo setup and when it hits it hits hard

 

from the sound of it though its more like there is some restriction in the charge pipes that is forcing the air back out casuing the surge or the turbo is spinning too fast and its falling back on itself

[stony]

Marc are you on the rb26 yahoo forum?

 

if you are shoot that question to Matt he should be able to help you out.

[BirdmanZ]

The short answer is, that's what those turbos do on that engine. It's the turbos surging or shuffling as some people call it. If you spend some time on forums.freshalloy.com in the Skyline section you'll see people complaining of the same problem. The only way to completely rid yourself of them problem to my knowledge is to change the turbos, or increase the displacement of the engine. i.e. RB30.

[RB26powered74zcar]

I'm running twin 2876r's and I had that same senerio happen to me when I first got my new motor up and running.

 

It would idle and make strange swooshing sounds out of the compressor housings. I put my hand up to the inlets on the turbo's and felt air actually pushing out from the comp wheels as they were making those sounds.

 

Come to find out, it was caused by me being stupid by hooking both breather outlets on the cam covers up to a catch can, but with no vent filter in it...

The crankcase pressure was venting thru the cam cover vents, into the catch can with no vent, so it was caused by back pressure.

 

I removed both lines from the cam covers and the problem instantly went away. I ended up adding a vent/ air filter onto the catch can.

 

I'm not saying this is your problem, but its something to look into.

[markrolston@mac.com]

Thanks for the advice. I have small air filters attached to the cam covers so I don't think that's the issue.

 

Precision Turbo is going to put ported shrouds on the turbos to see if that fixes it. I'll post results next week.

[amorfin]

So if the turbos are the problem... this should not occur with the N1s right?

[Bob_H]

I'll second sending that question to Matt on the RB26 yahoo group - he's built more than any of us in very high power configurations.

I doubt its the BOV's as long as it is working properly since you are having the issue under boost, not after letting off the throttle.

Again, Matt has a lot of experience with this and likely can point you in a better direction than any of us on this side of the Pacific.

-Bob

[Careless]

for what it's worth, I saw some greddy hard pipes that were individually sent to the intercoolers to replace the factory hard pipes. However, they did not connect into one large pipe as I saw for some setups.

 

Could it be possible that the hard pipe setup is not allowing the pressure of both turbos to actuate the BOV at the pre-set pressure? perhaps segmenting the hard-pipes in such a way that the BOV does not completely relieve pressure between both turbos at the same time is causing a back-forth surge between the two compressors trying to force air out of the BOV.

 

I've seen hard-pipes that connect at the earliest opportunity and conjoin into a single intercooler inlet, also made from Trust/Greddy (if I remember correctly.

[JustinOlson]

Sound s like compressor surge. I'm sure the port shroud compressor covers will clear this issue up for you.

 

Regards,

Justin

[Bob_H]

Something to add - although somewhat unrelated. Your problem is under boost, so this may or may not be adding to the issue. Many BOV's leak - period. My HKS race pull type leaked when I got it - anything over about 5-8psi.

I don't think that is the cause of your issue, but esp. with the design BOV you have, you will want to check it out.

Find a way to put pressure on the BOV seat from the pipe side,(you may have to remove the BOV and create something custom - that's what I had). Then apply pressure to the valve, (simulating the pipe side), and then apply the pressure to move the BOV itself - you shouldn't hear anything under positive pressure. About 8 out of 10 BOV's I check leak. Some really, really bad. I had to use valve lapping compound on the seat of my valve to get it to seal. Now on the HKS race one, it doesn't have an o-ring like many of those on the market today, (so guys reading this, PLEASE don't use lapping compound on yours if it has an O-ring!).

Another, easier way to check if its leaking is to put your hand over the BOV outlet while on the dyno to see if anything is coming out,(try to seal it up w/your hand). It goes w/o saying to be careful doing that while the car is under power.

[pirate87]

Sound s like compressor surge. I'm sure the port shroud compressor covers will clear this issue up for you.

 

Regards,

Justin

 

Mark, I assume the problem occurs at partial throttle to WOT, and not just when the throttle is released? That being said, is there anyway for compressor surge to occur when the throttle is open? I would think that instead of sending the building air pressure back through the compressor wheel, it would be easier for the set up to force the "boost" into the cylinder head, which is attempting to pump it in anyway.

 

My last setup, a Holset HX35 on an RB25, ran without a BOV so compressor surge was definetely noticable when the throttle was lifted, however the "choo choo" sound never occured during WOT or even partial throttle under booost.

 

 

Also, the HX35 runs a ported shroud, and the issue was still present. I would think the twin setup would offer a different dynamic, but just adding that info to the discussion.

[BirdmanZ]

Here is where it gets difficult. There are two different events being discussed here. The problem being described by Mark is basically the turbos producing more air than the engine can process at a certain RPM. The surging described by Pirate is when you let off the throttle and you get a fluttering noise. Two different events that appear under two different scenarios. I'm sure Tony D or somebody more knowledgeable than I could jump in here and give a more detailed explanation of what is going on when you get compressor surge.

[pirate87]

Ok, I think thats what I was getting at. The issues described are entirely separate, correct? I mean, the compressor surge Mark described is a totally different occurrence than the compressor surge I described. Or is the engine's inability to use the pressurized air, and thus forcing it back through the compressor (what I assume the fluttering noise mark described is), like closing the throttle- which causes the compressor surge I described. In this scenario the engine's capacity acts like the throttle plate in my scenario.

 

I apologize for not offering any real discussion to the topic, I'm still just trying to understand exactly what is happening in mark's situation.

[markrolston@mac.com]

Thanks everyone for all the ideas. The problem is during part throttle. There is no issue when I release the throttle as the BOV seems to work as intended. The ported shrouds should be installed by mid next week and I'll report back the results.

[cygnusx1]

My car does the exact same thing. At part throttle loads, the boost wants to build but there is not enough boost to hold the BOV shut and not enough vacuum to pull it open. It goes into an unstable state that feels like compressor surge. Here are the possible causes and I have thought of a few solutions.

 

Causes

1) BOV too sensitive (I may put some damping grease on the BOV slide)

2) More airflow from the turbo than the engine can use (use a bleed-type boost controller instead of a check ball)

 

Here is an experimental idea to use the boost to keep the BOV shut tighter:

Run the vacuum line from the BOV to a "T". One part of the T goes to the compressor housing with a check valve in it. The other part of the T goes to the intake manifold.

 

There is more boost at the compressor than at the intake. Using the compressor for boost source will push the BOV closed tighter. When you let off the gas, the boost will spool down and the line from the intake will apply vacuum and pull the BOV open. The check valve (or retrictor) in the boost line will prevent a vacuum leak.

 

Anyone understand this? I barely do but it might work.

 

Actually, it won't work unless you use a valve to re-direct vacuum flow when the throttle shuts. It's more complicated than it needs to be.

[pirate87]

haha, I see what you're saying cygnus... complicated, but clever. Basically using the best source possible to control the BOV.

[cheftrd]

Mark, Your Z is one of my all-time favorites. That thing just looks bad-ass. Anyway, it sounds like severe compressor surge to me. The ported shrouds will help some, but may not eliminate it. It’s part of the problem with having high response turbos that are still large enough to make the power… I just tell my customers to stay out of the surge band, but it’s hard to find the surge line when you drop the hammer at 9K!

 

For those who are wondering, surge happens in axial and centrifugal compressors because there is no “stopper†that seals the inlet with a one-way-valve like a supercharger would (sort of). It’s kind of complicated, but the basics are this: When the pressure (head) is too high for the flow rate, the compressor stalls aerodynamically, kind of like an airplane wing would stall. When this happens, flow reverses, moving back through the compressor and it chugs. The compressor blades rely on sound aerodynamics to compress the air; when you see it like this, you can understand why it doesn’t just make more boost at lower flow rates.

[Bob_H]

No need to go to the RB list - the horse has offered his opinion. I think that's about as good as you'll get. Maybe that 3L OS bottom end might help some!

-Bob

[markrolston@mac.com]

thanks again. I should add that I am almost completely sure that this is not the cause of the BOV. I have tried three BOVs on it, a combination of two (one on hot side and one on cold side) and no BOV. The issue continues. So I still wait to try out the shrouds.

 

mark