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      04-24-2018, 05:49 PM   #112
BunkerJ
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Okay, so I understand this has been beaten to death for some, so if you don't care about learning more details on the flutter "issue", then read no further.

I finally got back a very detailed response from GFB (Delay was due to the original engineer being out of the office) and it was incredibly detailed. This really helped solidify and confirm the whole debate on harmful vs. non-harmful. If you really don't want to read further, we can again confirm that the flutter is not bad.

Now I'm gonna say this before ilivas comes in and grills me for it; Yes, the engineer does use the surge in many places I would use flutter. So I will take fault for asserting that we classified it as flutter and not surge. I was purely misinformed on wording, but I can say with confidence that "flutter" is an acceptable term to describe the low-pressure surge. So, at least I was 50% correct? Lol.

Without further ado....


*****GFB RESPONSE*****

Hi Justin,

This is Brett, I’m covering Jonathan’s emails today. I can certainly help with a detailed explanation of surge.

Your summary below about the conditions where surge does and does not occur is spot on and the turbo is not in any danger. Broadly, compressor surge occurs when the airflow is low, and the pressure is high – this causes the compressor blades to momentarily “slip” in the air, which briefly reduces the boost pressure, the blades re-grip, pressure goes back up, then the blades slip again and the process repeats and creates the fluttering sound.

Compressor surge can occur during acceleration when the turbo is spooling hard early in the rev range when airflow is relatively low, this condition is quite common with a factory turbo that is running higher boost than usual, and it corrects itself as the RPM and hence the airflow increases, bringing the turbo out of the surge region. This effect is entirely unrelated to the diverter valve, it is a function of engine/turbo combination and the boost mapping. Modern engines and turbos can spool to very high boost pressures at very low RPM compared to turbo engines from 10 years ago, so this issue is more common these days, especially when it is so easy to flash tune to increase boost and performance.

Surge also occurs when lifting off the throttle, which rapidly slows the airflow, and it is the diverter valve’s job to prevent surge under these conditions. This is the point where it is worth explaining the function of the DV+ and the reason for its behavior. The factory diverter is concerned entirely with eliminating any chance of hearing what is deemed an unpleasant noise in the form of compressor surge, and it does this by simply opening 100% at the slightest negative throttle movement, regardless of whether there is boost or not. This behavior is detrimental to throttle response, as the intercooler is completely de-pressurised whenever the throttle is even slightly closed. It is possible to improve throttle response by using the momentum of the turbo to hold a small amount of boost in the intercooler for a short amount of time – upon re-opening the throttle if the intercooler is still at 2-3psi as opposed to zero, you will hit peak boost measurably faster. It also prevents the large “steps” in power delivery when modulating the throttle mid corner as the factory diverter is alternately dropping all boost, then building it back up. The DV+ uses the ECU signal AND boost pressure to determine how far to open. Above 10psi, it opens fully. Once boost has been evacuated below 10psi, the piston will start to close, which helps retain boost pressure longer than the factory diverter.

Under low RPM conditions, sometimes this effect of the DV+ can cause compressor surge, but because it is always at less than 10psi, and typically at low RPM, the forces on the turbo generated by such surge absolutely pale into insignificance compared to what the turbo must survive when the engine is on boost, let alone the effects of running higher than stock boost. Even different driving styles affect the loads the turbo suffers far more than surge at low RPM. Consider the heat, thrust forces, and shaft RPM at full throttle and high RPM, then throw in an ECU tune with 6psi more boost and see how much higher those numbers go and you will see that the turbo has bigger things than surge to worry about.

It is also worth noting that BMW, along with Audi, VW, and Mercedes all have engines now that have completely eliminated the diverter valve (all new BMW B-series engines included), with no detriment to the turbo. They have used other methods to mask the fluttering sound from the public instead.

So yes, compressor surge can inflict additional alternating loads on the turbo, but in terms of severity it is worst when it occurs under full load (because the turbo is already dealing with the heat, shaft speed, and high pressure, with surge thrown on top). If it occurs on lift off at full boost/high RPM it is possible but unlikely to cause physical damage or accelerated wear unless the turbo is already highly stressed – the number of cases of turbo failure that could be attributed directly to surge is very, very small. Surge that occurs at low RPM and boost really is quite irrelevant. If compressor surge at low RPM was a killer of turbos, there would be many tens of thousands of cars affected, and every Nissan SR20DET ever built would have blown the turbo 100 times over by now!

I hope this helps, I’m happy to discuss any of these points in more detail if required.


Cheers,

Brett Turner | Design Engineer
GFB Go Fast Bits
No 2, Norman Street, Peakhurst 2210
T +61 2 9534 0099 | F +61 2 9534 3999
E brett.turner@gfb.com.au | W www.gfb.com.au
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2013 F30 328i: MHI Big Turbo, GFB DV+, Mishimoto CAI, ER TIC/CP/Catless DP, AWE Touring Quad Exhaust w/ Resonated Midpipe, NGK LI Plugs, BM3 with PTF Stage 2 91 AGG Tune, Fuel-it Stage 2 LPFP, Solowerks S1 Coilovers, and DEPO/M-Sport Retrofit
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