Compressor surge (The bad kind)
 

I'm having compressor surge problems, and not the surge you get when you take your foot off the gas and the turbo make that sound. My surge is the full throttle type, when the turbo spools under medium/ high loads and the boost guage rapidly bounces between 0-7psi causing the car to buck. When I slam the throttle to the floor at high rpm the car pulls really hard and no issues, its the low rpm stuff that causing the surge. I need some help with this, any ideas would be welcome. I'll answer all questions as fast and as accurate as I can.

Set up:
S4 stock motor
S5 Stage 2 BNR turbo
Tial 50mm BOV 11LB spring (to stiff)
FMIC
E85 4x 1600cc injectors
Dual walbro 255lph pumps
E6K Haltech
RB 3" DP and silencer, side exit exhaust. (very loud and open flow)

I've about had it with this set up, and if a solution can't be found I'm gonna part this bitch out.

Thanks for your feedback.

Have you tried removing one of the springs in your BOV?

Yes, the BOV only has one spring and I've changed it from a 7lb spring to an 11lb spring. It has had no effect on the surge at all, if anything it made it worse. now with the 11lb spring it does the woo woo sound when you just free rev it.

Well if you have surge, I think it'd make sense to put lighter load on the bov not a stiffer spring. What about the wastegate?

If you don't have the mechanical ability to tell the difference between surge and flutter, you don't have the patience to learn, and you are not willing to take the car to a professional, then just sell the car and buy a new one with a warranty. Tinkering with cars isn't for everybody.

I have taken this car to a professional, and had him tune it, and he made me aware of the "surge" problem. I've since then had the the turbo rebuilt with a clipped turbine, ported wastegate, and even removed the boost controller, and still no change. I've consulted a turbo expert and he told me to change the bov spring to a stiffer spring and with no change except for the negative.

I've built this car from a rolling chasis up, so I know a thing or two. My willingness to learn can't be questioned, as I've been reading and consulting professionals for the past four months. all I get from them is that the turbo is to big and the engine can't swallow the boost the turbo puts out. But how can that be, I know people who run much larger turbos on the same stock S4 block.

Expalin the difference between surge and flutter, I'm willing to learn!

Yes, thats what I thought, but then I wondered if it was to light, and opening once the turbo got into boost, causing a flutter of the BOV. So I took some advice and put the heavier spring in and no change except for the negative.

I was thinking about the wastegate, and wiring it open and see if it stops the "surge". Slowing the boost response may be a way to work around the issue, but slow boost responce is not what I had in mind when putting this turbo system together, as this is my track day car.

thanks

Post a video up if you can one of these days.

You need a professional turbo mechanic, not a tuner. The mechanic needs to be able to fix the problem, not just tell you that something isn't right. As in most cases, rotary engine knowledge is not necessary.

Engineering for Millennials: Google search, LOL
What is wastegate flutter? - Yahoo! Answers

I don't need a professional turbo mechanic, I just need the knowledge and good suggestions from people like you. I like "tinkering" with my car, I was just stumped and needed a fresh perspective. I'm going to check the wastegate accuator arm to check for slop, and I will tighten it a little to see if that helps fix the issue.

Thank you ;)

try adding a helper spring on the wastegate arm to help keep it closed. all BNRs should come with one

but if it is comp surge (compressor pumping more air than engine can use), your hot side is too small and you need a bigger turbine wheel. clipping the wheel can help too. another quick fix is opening up a boost leak between the turbo and throttle body. this eliminate surge, but may also cause turbo to work too hard at higher rpm (common problem with hybrids). you could wire an extra PWM output to a solenoid to this "boost leak" to open at low rpm, and close above a threshold

Thanks, the helper spring is a great idea. I am going to first adjust the arm on this wastegate actuator to preload it. If that doesn't solve my problem then I will install a spring to provide even more pressure keeping the WG doors closed. The solenoid is something I will do if these don't work.
Again thanks for your great suggestions.

a BNR stage 2 is NOT too big of a turbo. a video would be beneficial. has the wastegate diaphragm been rebuilt/rebplaced? maybe that is the culprit.

The BNR Stage 2 turbo is brand new, has about 30 miles on it. I knew it wasn't to big, I had this same turbo about 5 years ago, it worked great then.

My job requires me to travel, so once I get back I'll get a video and post it up. I'll be back on Thursday.

Is this a genuine BNR turbo, or a hybrid that's similar to one? Brian does a ~7 degree clip on the turbine wheel to prevent compressor surge.

Allow me to chime in. I literally deal with "real" compressor surge on a regular basis at my day job.

Where you can really see compressor surge is on an engine dyno. With the wastegate closed, If I hold the rpm fixed using the dyno loading I can gradually dial in surge. So set a fixed speed of 1500rpm. Gradually close the throttle. As the compressor outlet pressure gradually increases, you can literally see backflow in the dyno lab as the facility air supply tube connected to the compressor inlet pipe starts pulsating. Other things that show up are a fluctuation of manifold pressure, and a fluctuation of the crank angle resolved combustion chamber pressure trace on the oscilloscope. You can call this "soft surge." You can also hear a "chug-chug-chug" sound on some turbos when they hit "hard surge." The compressor outlet pressure just hits a wall and won't go any higher, but in that kind of test with fixed rpm you do not see massive swings in boost.

"real" compressor surge has very little to do with the wastegate or the turbine, not directly at least. You can still have boost oscillations related to the turbine side that are not compressor surge. Surge typically occurs as low speed high load, maybe 2500rpm or lower. It's common on 4 cylinders with small turbos that run a lot of boost.

One way for you to test would be to put the engine on a loading dyno at low speed (under 2000rpm) and hit the gas. If that's not possible, put the engine into 5th gear, up a hill, at low speed (about 1000-1500rpm). Basically lug the engine and nail it. Take a log with the Haltech and if possible get a video. You might hear the sound at that point, but with your very loud exhaust it could obscure the "chug chug chug" noise.

From what you describe though it could be a hotside issue. Adjust the preload of the wastegate arm (make it longer) and it will be stuck closed. If the wastegate is coming open (I don't care for the term "wastegate flutter"), you lose turbine power and the wheel will slow down, dropping boost. You would need a turbo speed sensor to see that. It's sooo much easier to diagnose turbo problems with a turbo speed sensor.

In terms of the root cause, there's a big difference between insufficient turbine power to drive the compressor and compressor backflow aka "real compressor surge."

Thanks for the technical insight, I will do what you say and take a video of it. Before I do that I'll preload the wastegate actuator and see how it responds.

Thank you.

you'd have to shorten the rod to preload it more.

and i would also suggest checking for boost leaks, sometimes a small gap in a intercooler hose will only present itself at certain boost levels(boost and bleed).

I understand the shorting of the rod to preload.
I was also told that I need to check the BOV diaphragm for leaks, so I'll check that too.

Any idea for the hardware to use so that I don't get leaks from the end caps I put on the IC plumbing. I may have to get creative at the Home Depot!

Thanks

nothing wrong with home depot for plumbing couplers to make a turbo cap. just measure the turbo inlet size and get the appropriate PVC plumbing cap and a rubber coupler with a few hose clamps.

you can either put a stem in the cap or leave it closed and pump air into one of the intake vacuum lines to pressurize the engine/intake(BOV port or pressure sensor port).

this should tell you if the BOV is leaking under pressure. a well sealed system should hold upwards of 15psi with minimal leaks and take about 10 seconds to leak down assuming the engine is in good shape.

So I shortened the wastegate accuator arm by 5mm, this puts alot of preload on the wastegate door, and took it for a test drive. Nothing has changed and its still doing the fluttering at high load low RPM.

So if i disconnect the wastegate vacuume line from the accuator, the wastegate will fail to open at all, this may tell me if its wastegate related at all. I have a feeling its going to flutter just the same.

I tried to take video, but with no power steering and no passenger it was damn near impossible. maybe tomorrow.

I have run this type of turbo before with no issues, the one thing that is different is that I am currently using a vacuume/ boost distribution block for the map sensor, BOV, Gauge, FPR, and Boost controller(currently disconnected). Do you think this has anything to do with it?

Is there ever a good kind of compressor surge?:biggrin:

Ever thought about going with a different bov altogether? Synapse engineering maybe?