Multics Posted March 3, 2009 Share Posted March 3, 2009 This is a weird one. Car boosts fine on wide open throttle at 0.8 bar. The problem is that I get compressor surge around 0.4 bar and maintaining that pressure for a few seconds... The engine is a stock RB25DET with stock turbo. Mods are stage1 GTST mods (induction, FMIC, full exhaust, coilpacks, sparkplugs, Rising Sun Performance stage1 ECU, EBC running 0.8bar, standard recirculating bov) What could be causing this on a standard turbo where the compressor/turbine are matched to the engine requirements?? This is the best explanation I found concerning ON BOOST compressor surge: --- Surge is caused by stall, which is worsened by surge, its a vicious circle... Imagine the way our turbocharger's compressor is driven, right back to basics: The turbine wheel is spun by exhaust gas, utilising also the energy stored in the exhaust gas as latent heat. This force on the turbine shaft generates torque at the compressor end as they are physically connected. Remember that part as its the key element, Torque Ok, Now lets imagine our turbocharger compressing air, forcing it into the engine, the engine is utilising it.. what happens then? :? Ok, The engine expells it, and our turbine housing pressure goes up accordingly, the torque applied at the turbine wheel increases, increasing the spin speed and compressor efficiency at the other end. Great chain of events! End Result? boost is climbing nicely. Ok, Now imagine this scenario: The turbo begins to spin, as above, but the compressor wheel is generating more air than we can move and the torque required to spin that huge compressor is not being met? Remember that unless the air is processed by the engine, and expelled as hot, rapidly moving air into the turbine housing, our turbine shaft torque will not increase to spin the compressor faster, and it may well diminish.. our compressor is slowing, braked massively by the compressed air its trying to overcome and increase..... Result? Compressor starts to slow a little.. (beginings of a compressor stall) Ok, So now what? The engine is still rotating and consuming air, but the turbo has stopped producing an excess, so our engine has now had chance to consume what excess was available and the inlet pressure is now diminishing, the turbine torque is now increasing again... VERY IMPORTANT: Remember also that at same time, compressor resistance to spin has also dropped due to housing no longer being as pressurised and as a result, the torque required to spin it has dropped massively.. Boom.. Suddenly the Turbine wheels torque massively exceeds the compressors resistance and the turbo spins to speed in an instant!! Sounds great, rapid boost climb!! The compressors acceerating at over 2G BUT: We just hit the same problem again, our huge compressor has made so much boost, so fast, and is trying so hard to push the massive volume or air, that our YB cant use it.... so we start to slow again........ So we went: Fast, slow, fast, slow, or in other words: The turbo is Surging. As you can imagine, it continues this cycle until it hits the revs your engine consumes all the air.. then your away! Downsides to this surge are pretty obvious: We can have serious bearing wear, due to the excessive loads imparted on the shaft supports. Compressors have been known to simply explode!! Worst case scenario, the compressor being slowed rapidly with a turbine still trying to accelerate it can simply do as expected, part company. Quote Link to comment Share on other sites More sharing options...
Tricky-Ricky Posted March 3, 2009 Share Posted March 3, 2009 Sounds a long and over complicated explanation, and IMO would only apply to either badly sized turbos, or serious engine problems. It sounds to me like your BOV is having problems. Is it surge or stall? Quote Link to comment Share on other sites More sharing options...
Multics Posted March 3, 2009 Author Share Posted March 3, 2009 Sounds a long and over complicated explanation, and IMO would only apply to either badly sized turbos, or serious engine problems. It sounds to me like your BOV is having problems. Is it surge or stall? I've tried three different bovs (standard recirc and two aftermarket vta's) and it's exactly the same behaviour. AFAIK, it's surge, it happens on boost (between 0.1bar and 0.5bar). On WOT (0.8bar) the surge disappears. Quote Link to comment Share on other sites More sharing options...
Tricky-Ricky Posted March 3, 2009 Share Posted March 3, 2009 Does it spool up the same as it used to? presuming it has ceramic turbine, it almost sounds like its damaged? Does the GTST have its own boost valve like the 200SX? Quote Link to comment Share on other sites More sharing options...
Multics Posted March 3, 2009 Author Share Posted March 3, 2009 It spooles fine, I haven't noticed any difference in spool time/noise etc... It's perfectly fine on WOT, it's only surging between 0.1bar and 0.5bar when maintaining 3/4 throttle for a few secs. Surge stops when lifting off throttle or booting it. Boost is controlled by an HKS EVC4 Bloody Skylines ... Can it be that the wastegate is not operated properly? Quote Link to comment Share on other sites More sharing options...
edge Posted March 3, 2009 Share Posted March 3, 2009 I thought compressor surge is where the turbo cant physically flow enough air into the engine? As alot of people think surge is compressor stall (the funny pidgeon type noise, a higher pitched version of wastgate noise noises) Would you be able to get a sound clip up of this noise? Quote Link to comment Share on other sites More sharing options...
edge Posted March 3, 2009 Share Posted March 3, 2009 If its like this then this in NOT surge its STALL And if it sounds like this then its surge Quote Link to comment Share on other sites More sharing options...
Swampy442 Posted March 3, 2009 Share Posted March 3, 2009 If the wastegate wasnt operating properly you would get surge I suspect, but that would be when you close the throttle. Surge is complete reversal of the airflow over the compressor Quote Link to comment Share on other sites More sharing options...
Tricky-Ricky Posted March 3, 2009 Share Posted March 3, 2009 Surge is complete reversal of the airflow over the compressor No thats stall, IE when the throttle is closed suddenly, the reversed flow is what causes the distinctive pigeon noise. Surge is when the airflow is caveatted, IE the engine is trying to draw more air than the turbo can supply. Quote Link to comment Share on other sites More sharing options...
Multics Posted March 3, 2009 Author Share Posted March 3, 2009 That's exactly what I'm experiencing: Quote Link to comment Share on other sites More sharing options...
Swampy442 Posted March 3, 2009 Share Posted March 3, 2009 No thats stall, IE when the throttle is closed suddenly, the reversed flow is what causes the distinctive pigeon noise. Surge is when the airflow is caveatted, IE the engine is trying to draw more air than the turbo can supply. No my friend, surge is when the throttle is closed suddenly etc. Believe me im a jet engine technician, compressors have been my job for17 years Stall is a precurser to surge Quote Link to comment Share on other sites More sharing options...
edge Posted March 3, 2009 Share Posted March 3, 2009 Any play in the shaft? Quote Link to comment Share on other sites More sharing options...
jevansio Posted March 3, 2009 Share Posted March 3, 2009 Surge is when the airflow is caveatted, IE the engine is trying to draw more air than the turbo can supply Is that the correct way round? I thought surge was when the engine cannot ingest the air the turbo is supplying (hence it passes back through the compressor when you're at WOT). Quote Link to comment Share on other sites More sharing options...
Swampy442 Posted March 3, 2009 Share Posted March 3, 2009 (edited) Exactly jev. That noise is the air bursting back out of the turbo intake, the compressor loses efficiency, cant compress the air, stalls then the pressure upstream of the compressor 'surges' back past the compressor to atmosphere. Completely back on topic, hope you get it sorted mate, doesnt the 'liner run a recirc/dump valve that has 2 pressure inputs, 1 low and 1 high? Tried on the owners club? Edited March 3, 2009 by Swampy442 (see edit history) Quote Link to comment Share on other sites More sharing options...
Multics Posted March 3, 2009 Author Share Posted March 3, 2009 Any play in the shaft? Haven't taken the turbo out yet. I'm just looking for opinions before I undertake any work but everybody seems to be confused including myself The way I understand what's happening (maybe I'm wrong) is that the compressor is creating more pressure than the engine needs. As this happens at half-throttle, the exhaust gases are not torquing enough the turbine wheel so the compressor spins less fast but still has to deal with the excess pressure that it's producing. The result is that the compressed air is bouncing back and forward between the compressor wheel and the half-opened throttle body. As soon as the throttle flap opens completely (WOT) the surge is gone! Weird. Apparently this happens on mismatched compressor/turbine turbo's but in this case we're talking about a stock nissan turbo... Has the boost controller anything to do with it? Would a different wastegate setting change the behaviour? Quote Link to comment Share on other sites More sharing options...
Multics Posted March 3, 2009 Author Share Posted March 3, 2009 Completely back on topic, hope you get it sorted mate, doesnt the 'liner run a recirc/dump valve that has 2 pressure inputs, 1 low and 1 high? Tried on the owners club? Thanks for your help guys! Yes it has a MAF so I'm using the stock recirculating BOV. It only has one pressure/vaccuum feed from the plenum (RB25DET). I've tried both So.com and gtr.co.uk but no joy. I posted on here as there's alot of people running single turbo's and maybe someone had experienced that before. Quote Link to comment Share on other sites More sharing options...
Tricky-Ricky Posted March 4, 2009 Share Posted March 4, 2009 No my friend, surge is when the throttle is closed suddenly etc. Believe me im a jet engine technician, compressors have been my job for17 years Stall is a precurser to surge In that case me and a lot of other turbo car enthusiasts have got it all wrong:blink: So when the throttle is suddenly closed, and the already compressed and moving stream/column of air thats in the pipework is halted in its direction of travel, then becomes reversed as it where, and slams into the compressor turbine,slowing and stopping it, thats not stall but surge? Quote Link to comment Share on other sites More sharing options...
Swampy442 Posted March 4, 2009 Share Posted March 4, 2009 (edited) ""Centrifugal (turbo compressors) and axial (like on an airliner) compressors will surge when forward flow through the compressor can no longer be maintained, due to an increase in pressure across the compressor, and a momentary flow reversal occurs. Once surge occurs, the reversal of flow reduces the discharge pressure or increases the suction pressure, thus allowing forward flow to resume again until the pressure rise again reaches the surge point. This surge cycle will continue until some change is made in the process or compressor conditions."" It doesnt slam into it, as you said the pressurised air has nowhere to go so it stops moving. The compressor loses efficency as it cannot squeeze any more air into the system, pressure build up round the compressor, it slows down. This reduces the output pressure so the higher pressure in the system can overcome it. This will stall the compressor first, then surge past it to atmosphere. Edited March 4, 2009 by Swampy442 (see edit history) Quote Link to comment Share on other sites More sharing options...
Tricky-Ricky Posted March 4, 2009 Share Posted March 4, 2009 (edited) OK not being funny,as i'm trying to work out if everything i have learnt/ been told is wrong. But in real terms, is what i described (badly) and you explained in detail, stall or surge? Yes i understand that your saying that the secondary effect is surge, but for the sake of pedantry humour me:) As i was always lead to believe that the distinctive sound AKA pigeon noise was the sound of the no longer moving pre compressed air slowing/stopping the compressor blades whilst still trying to be driven by the exhaust flow on the exhaust turbine. PS Sorry Multics, for the hijack, hopefully there will be more theories and ideas on getting to the root of your problem:) Edited March 4, 2009 by Tricky-Ricky (see edit history) Quote Link to comment Share on other sites More sharing options...
Swampy442 Posted March 4, 2009 Share Posted March 4, 2009 What you described buddy was surge, the noise is surge. Cant put it any more simply than that The air is already compressed which causes the problems in the first place. As I said its the stall that leads to the surge Quote Link to comment Share on other sites More sharing options...
Tricky-Ricky Posted March 4, 2009 Share Posted March 4, 2009 Just been doing some searching trying to get my head around it, still rather confused, as what your saying is that stall is very transitional, and very quickly leads to surge which causes the noise. But according to this, there are two different types of stall:blink: Anyway it may help others get the jist, although contradictory. There are two types of compressor stall: [edit] Rotational stall "Rotational stall" is a local disruption of airflow within the compressor which continues to provide compressed air but with reduced effectiveness. "Rotational stall" arises when a small proportion of aerofoils experience aerofoil stall disrupting the local airflow without destabilising the compressor. The stalled aerofoils create pockets of relatively stagnant air (referred to as "stall cells") which, rather than moving in the flow direction, rotate around the circumference of the compressor. The stall cells rotate with the rotor blades but at 50%-70% of their speed, affecting subsequent aerofoils around the rotor as each encounters the stall cell. Stable local stalls can also occur which are axi-symmetric, covering the complete circumference of the compressor disc but only a portion of its radius, with the remainder of the face of the compressor continuing to pass normal flow. A rotational stall may be momentary, resulting from an external disturbance, or may be steady as the compressor finds a working equilibrium between stalled and unstalled areas. Local stalls substantially reduce the efficiency of the compressor and increase the structural loads on the aerofoils encountering stall cells in the region affected. In many cases however, the compressor aerofoils are critically loaded without capacity to absorb the disturbance to normal airflow such that the original stall cells affect neighbouring regions and the stalled region rapidly grows to become a complete compressor stall. [edit] Axi-symmetric stall or Compressor surge "Axi-symmetric stall", more commonly known as "compressor surge" or "pressure surge", is a complete breakdown in compression resulting in a reversal of flow and the violent expulsion of previously compressed air out through the engine intake, due to the compressor's inability to continue working against the already-compressed air behind it. The compressor either experiences conditions which exceed the limit of its pressure rise capabilities or is highly loaded such that it does not have the capacity to absorb a momentary disturbance, creating a rotational stall which can propagate in less than a second to include the entire compressor. The compressor will recover to normal flow once the engine pressure ratio reduces to a level at which the compressor is capable of sustaining stable airflow. If, however, the conditions that induced the stall remain, the return of stable airflow will reproduce the conditions at the time of surge and the process will repeat.[1] Such a "locked-in" or self-reproducing stall is particularly dangerous, with very high levels of vibration causing accelerated engine wear and possible damage, even the total destruction of the engine. Quote Link to comment Share on other sites More sharing options...
Swampy442 Posted March 4, 2009 Share Posted March 4, 2009 Axi-symmetric stall or Compressor surge Yes ok its a type of stall, but its normally called a surge The ones in the passages you quoted refer to axial flow compressors, and dont really apply to centrifugal ones I can regale you with stories of my earlier years ground running Jaguars and experienceing locked in an pop surges if you like lol. Shall we keep this debate to PM to stop spamming this thread? Quote Link to comment Share on other sites More sharing options...
Tricky-Ricky Posted March 4, 2009 Share Posted March 4, 2009 Yes ok its a type of stall, but its normally called a surge The ones in the passages you quoted refer to axial flow compressors, and dont really apply to centrifugal ones I can regale you with stories of my earlier years ground running Jaguars and experienceing locked in an pop surges if you like lol. Shall we keep this debate to PM to stop spamming this thread? Sorry! i thought it was relevant to multics problem:) Maybe we could take it to tech, as i'm sure a lot of people would be interested, as i suspect quite a few people would be thinking along the same lines as me, could be interesting an educational, if you're game for imparting some of your experience? Quote Link to comment Share on other sites More sharing options...
Multics Posted March 4, 2009 Author Share Posted March 4, 2009 Just for an update I called Turbo Dynamics today to get a quote for a hybrid and also explained the problem. The chap (very helpful) said that it's highly unlikely that the stock turbo surges at part-throttle (on boost). It must be due to a boost leak somewhere all the way from the exhaust manifold, IC piping up to the throttle body. It could also be due to a bad part throttle mapping, my ECU has been remapped by a Skyline tuner so I'll be taking it back on the dyno to have it all checked next Tuesday. I will keep you informed guys but please carry on posting information as it's very interesting. Some of the content should definitely be posted in the FAQ section as a technical article or something... Cheers Stav Quote Link to comment Share on other sites More sharing options...
Swampy442 Posted March 4, 2009 Share Posted March 4, 2009 Lol Ricky I dont mind at all, if Stav doesnt mind his thread being filled with techno babble The mapping is a curious one but makes sense when you think about it Good luck. Quote Link to comment Share on other sites More sharing options...
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