Compressor Surging Explained!!! (in more text than most of you will care to read)

Compressor surging is an often discussed occurrence in the world of turbochargers.

The problem, however, is that the explanations and/or definitions typically presented as “fact” are generally one paragraph long, and are so brief, generic and rudimentary in their descriptions that other common aspects and operational parameters of turbocharged applications are easily confused to be related to or completely misinterpreted to be compressor surging.

With all of these ill-written definitions being used so haphazardly providing more misinformation than factual, I would like to take the time to more thoroughly and rationally explain the concepts behind surging in a way that will shed some better light on the phenomenon and give everyone a better understanding of it and provide an increased level of confidence about undertaking modifications to and tuning of their turbocharged vehicles. I only hope that this text pervasively replaces all of the inappropriate and ill-written explanations cited by your average weekend warrior.

Why are there no photos, diagrams, graphs, charts or anything else aside from 8 pages of text you may ask? Because none of it is needed nor will any of it better explain anything to you. This is all based on fundamental properties of physics that anyone with even the most basic understanding of engines will comprehend,

The simplest definition that I or anyone else can give you is that compressor surging is exactly what its name implies. It is a surging or rapid change or oscillation to the speed or possibly even the rotational direction of the compressor wheel of the turbocharger when under load.

This is caused by one thing and one thing only, of which the origins of will be explain more thoroughly later. Compressor surging is caused by two opposing pressures acting against one another on the impeller and compressor wheels of the turbocharger which share a common drive shaft.

These two opposing pressures are exhaust gas pressure which spins the impeller wheel and then a reversion of boost pressure acting against the normal rotational direction of the compressor wheel which is driven by the impeller wheel.

The opposing pressures act on their respective wheels in pulses which are caused by intake and exhaust valve events (based on cam phasing, timing, and engine RPM). The opening of the exhaust valves within the head of the motor leads to exhaust gas pulses that spin the impeller wheel of the turbo which then drives the compressor wheel which generates boost. The pulses of pressure waves that revert back into the compressor wheel are similarly caused by intake valve events (opening and closing) however the buildup and subsequent reversion of pressure itself is caused by something entirely different. (See the next paragraph)

Though the term surging can seemingly imply many things in the world of turbochargers today, the term was originally derived from a condition in which the turbo (compressor wheel) is actually spooling and generating boost pressure faster than the motor can “ingest” it. In such a scenario, where airflow volume exceeds the rate of consumption at which the engine itself is capable of operating, as the compressor wheel is spooling and boost pressure is building, when it reaches a point at which the motor can no longer accept any more airflow, a buildup of pressure in the intake manifold and charge piping will occur that will cause the reversion of boost pressure (pulse timed by intake valve events) as the turbo attempts to overcome any and all flow restrictions.

The reason that the pulsating reversion of pressure into the compressor wheel is dictated by intake valve events and not by throttle position is that the forces (pressures) that cause surging are only present to act against one another at a high RPM wide open throttle condition in which the throttle plate does not act as a restrictor as it is fully open.

The surging caused as a result of the two opposing pressures acting against one another on the impeller and compressor wheels is manifested by an actual surging or rapid oscillation of the speed of the wheels until such a time that the throttle plate closes or a fail safe devise like a wastegate or bypass valve is actuated. The pulse timing of the exhaust gas pressure acting on the impeller wheel and the pulse timing of the pressure reversion into the compressor wheel and their unique and combined harmonics will determine the frequency of the surging.

(Here, the term frequency implies the frequency of the wheel speed and pressure oscillations. It does not mean the frequency of the occurrence.)

Compressor surging was more of a common phenomenon prior to the introduction of the above mentioned fail safe devices, wastegates and bypass valves, that are now commonly both mechanically and electronically manipulated and controlled by OEM manufacturers and individual tuners/users alike in an attempt to minimize such occurrences and even secondarily to control the boost curve and turbo performance.

Compressor surging is incredibly difficult to empirically measure, however, through using a turbocharger turbine speed sensor (yes, they exist), it becomes possible to directly measure the speed of the compressor wheel under all kinds of different load and boost conditions, but because of the incredibly high speeds at which turbocharger impeller and compressor wheels spin and even gain and lose speed (accelerate and decelerate respectively), the highest sampling rate possible must be used as consistently as possible otherwise inconsistent wheel speed readings will result. Using a turbine wheel speed sensor to measure surging, however is not its primary function and not necessarily the best method which to do so, but it would certainly be the most accurate of any currently available option to measure such a phenomenon.

In order for such opposing pressures to even exist in order to be able to cause surging, the engine will need to be operating at a very high RPM, and the turbo too must be operating at a high RPM to be generating more pressure than the motor can ingest. A 200 RPM change or fluctuation in compressor wheel speed is not indicative of surging, particularly on turbochargers that can sometimes see wheel speeds in excess of 120,000 + RPM. Such a small change could easily be attributed to a wheel speed sensor anomaly based on sampling rate or even fluctuations in the volumetric density and speed of the exhaust gas pulses which are driving the compressor wheel by way of the impeller wheel. Even a 2000 RPM fluctuation in wheel speed is not indicative of surging to any degree. It would take a larger change in wheel speed, and more appropriately, a rapid oscillation of wheel speed more proportional to the maximum compressor wheel speed of the application to result in surging that would be noticeable or even detrimental to the turbo in any way.

Let’s also clarify, too, the distinction between, the “acceleration” of the compressor wheel, it’s “deceleration”, and “surging”. This difference ties into the previously mentioned idea that the term itself will most accurately describe the condition. The acceleration of the compressor wheel, which is obviously a function of an increasing rate of exhaust gas pulses driving the impeller wheel that then drives the compressor wheel, is not surging. Logic dictates that a constant rising rate of wheel speed is not equivalent to rapid oscillations. The deceleration of the wheel too, which is a function of lower or no throttle input that won’t result in combustion that won’t result in exhaust gas pulses that won’t drive the impeller wheel that won’t then drive the compressor wheel, is also not surging. Deceleration does not equal rapid oscillation.

Compressor surging can theoretically occur on ANY turbocharged application, however, in most cases, detrimental and noticeable surging will occur in such a case where the turbocharger is abnormally large for the application, which is rare in and of itself when a turbo is appropriately match to the size of the engine, but it does happen.

In addition to a “flow exceeding consumption” scenario, on an abnormally large turbo, the exhaust gas of the motor can sometimes be insufficient to spool the turbocharger to within its optimal operating speed range, thusly it will already be spinning markedly slower than a more appropriately sized turbo would be on the same application. When a reversion of pressure into the compressor wheel occurs and is accompanied by continuous exhaust gas pressure acting on the impeller wheel, it will be acting against the lower levels of pressure generated by a much slower spinning compressor wheel, thusly more easily causing surging, but at a much lower level than the term normally implies and not dangerously. A more appropriately sized turbo should always be spinning at a higher rate of optimal speed, within its efficiency range of course, which will then make it less prone to surging.

The “surge line”, or point at which any turbo may be prone to surging on any particular application or at a particular boost level, is usually clearly noted on the compressor map of any given turbocharger.

To go off on a small but relevant tangent, even at idle, on virtually all turbocharged applications, a turbocharger is generating positive boost pressure between the discharge end of the compressor cover and the throttle body. Between these two components lies the charge piping often plumbed with an intercooler and a bypass (blow-off/diverter) valve.

The most common incorrect usage for the term “surging” nowadays implies that during an open throttle condition, boost pressure is exiting the compressor cover discharge traveling through the charge piping and the throttle body into the intake manifold and the engine. At such a time that the throttle plate inside the throttle body closes, however, the compressor wheel is still spinning and generating positive boost pressure, yet it cannot enter the engine causing a reversion of boost pressure into the compressor wheel.

This scenario too, can be considered similar to the previously mentioned occurrence of flow exceeding consumption. In this case, however, the amount of boost pressure is not restricted by the airflow capacity of the engine itself, rather it is being manually limited by the flow rate of the throttle body at any given throttle position input level (controlled by your right foot).

This is not an accurate description of compressor surging at all, however, as one of the pressures/forces required for surging to occur is no longer present. As the throttle body has been closed limiting the rate and volume of airflow required for high RPM combustion, the exhaust gas flow is no longer nearly as high as it was when the throttle plate was open which would otherwise accelerate or maintain the speed of the impeller wheel to drive the compressor wheel. As the larger volume and velocity of exhaust gas is no longer present to act against the possible reversion of pressure into the compressor wheel, the entire rotating assembly will just decelerate, not surge. (Remember the distinction?)

Even if the throttle is quickly reapplied, certain fail safe devices will have actuated and there is still a period of time during which the pressures will not be acting against one another thusly allowing the turbo to decelerate not causing surging. Even on some applications in which the throttle is electronically operated and not necessarily closed immediately as your foot lifts off the pedal, the electronic actuation of a fail safe devise or a brief pause in fuel injection and spark is all that is required to slow combustion and to prevent surging.

Be sure to consider, however, that even when a throttle body closes, it never closes completely and it will still offer a given amount of airflow into the engine to maintain idle. This is often accomplished either by actually keeping the throttle plate open to a certain percentage or by actually closing the throttle plate completely but allowing airflow past it through some type of perforation (hole) that is sized accordingly to maintain a given idle RPM for the application.

To briefly touch on some of the fail safe devices used in turbocharged applications, the wastegate, either built “internally” into the exhaust housing of the turbo, or incorporated “externally” typically on the exhaust manifold prior to the turbocharger, exists in the system to bypass exhaust gas away from the impeller wheel of the turbocharger (which drives the compressor wheel) in order to prevent the turbo from continuing to generate boot pressure beyond the pressure level it has reach at the time the wastegate opens and begins bypassing exhaust gas. Additionally, if a bypass valve is present in the charge piping system to any degree, whether it is setup to recirculate into the intake side of the system or to vent to the atmosphere, and it is both sized and tuned appropriately to the application, it is still relieving this residual amount of pressure that needs to be vented at throttle lift, thusly it is preventing a buildup and reversion of boost pressure altogether.

As the wastegate is used to bypass exhaust gas to limit the increase of boost pressure and indirectly prevent surging, and a bypass valve is used to prevent pressure reversion to prevent surging, it becomes even clearer that the two forces together are required for surging to occur. Exhaust gas pressure alone will not create surging and a reversion of boost pressure alone will not do so either. They must occur in unison for surging to result. If the above mentioned, incredibly effective fail safes are used to independently limit and even prevent both contributing factors to surging, it is altogether infinitely more unlikely it will happen than if neither fail safe is incorporated at all.

Back to bypass valves, though. As mechanical style valves operate based on a pressure differential, they will open until such a time that the pressure differential (between the intake manifold and charge piping) is equalized. Electronically controlled valves are manipulated in accordance to prerecorded and predetermined pressure differential control variables that try to mimic mechanical valve operation as closely as possible under every possible condition. These control variables are stipulated in code in the ECU that can then be tied to other variables such as throttle position which will actuate the valve when certain predetermined conditions are met.

Under certain circumstances, even when a bypass valve is utilized, a reversion of boost pressure into the compressor wheel MAY still result, however, it is often as a result of some other underlying cause, such as improper tuning of the valve or a failure of the valve to operate altogether, but as mentioned above, this reversion of pressure alone will NOT result in surging.

If a valve does not offer enough flow volume to discharge the entire amount of residual boost pressure that needs to be discharged, either because it is too small for the application or it is adjusted too stiffly to allow enough flow volume, whatever volume of air that cannot be vented will remain in the system and MAY cause a pressure reversion if and only if that remaining volume of pressure is such that it is not forced through the partially open or perforated throttle plate by the constant pressure being generated by the compressor wheel. A very small pressure spike in the charge piping may occur, but for it to cause a pressure reversion and then surging, as mentioned before, it will need to measurably slow down and, together with the exhaust gas pulses, create an oscillation to the incredibly high rate of speed at which the compressor wheel is already spinning.

If a valve is somehow not operating properly or it is not venting at all, or similarly if a valve is not present in the system at all, clearly the residual amount of pressure that needs to be vented will remain in the charge piping either partially or entirely and a pressure reversion may result but if and only if the failure of an incorporated valve is such that it is prevented from venting any amount of pressure at all and such an occurrence measurably slows the speed of the compressor wheel.

Consider that even if a vacuum reference to a mechanical style valve is leaking or blocked in some way preventing the valve from operating properly, certain valves may still open and discharge pressure when a spike occurs in the charge piping, thusly preventing pressure reversion. In this regard, improper valve operation will not always and in fact rarely result in pressure reversion.

This is where a distinction between “pull type” and “push type” mechanical valves can come into play. “Pull type” (ie; HKS SSQ) valves are solely operated by vacuum, in that the plunger of the valve which seals the charge piping is opened solely in response to vacuum acting on a sealing surface that is completely enclosed in a separate sealed chamber. This means that the pressure in the charge piping acting on the plunger will not open the valve until the vacuum reference of the intake manifold is such that it can overcome the pressure in the charge piping needing to vent that is acting on the plunger holding it closed.

“Push type” valves do not have independent chambers in which the sealing surface and the plunger are located, thusly allowing both the intake manifold vacuum reference AND pressure in the charge piping to open the valve either together OR independently of one another. At throttle lift, both the residual pressure within the charge piping, AND the return of the intake manifold to vacuum will both "push" and "pull" the sealing surface of the valve open thus releasing the residual pressure from the charge piping.

On the subject of bypass valves, there is a common attribute to their individual operation that is often misinterpreted or confused to be a problem not only to the valve’s operation, but it is also often confused to be related to or even to actually be compressor surging.

This occurrence is “valve fluttering”, and it should be fully understood that it is NOT an indication of compressor surging occurring, nor does it “cause” compressor surging in any way.

Bypass valve fluttering will occur under various circumstances, so please consider under what situations you are experiencing valve fluttering before you presume that a pressure reversion or even compressor surging is taking place at all, or more importantly, before it is assumed that a problem with the valve even exists at all.

Valve fluttering under a wide open throttle or full boost throttle lift condition typically means that a valve is tuned or adjusted to stiffly, and while this MAY potentially lead to pressure reversion and even more rarely, compressor surging, if equal and opposing exhaust gas pressure pulses happen to be present as well, if the issue is corrected quickly, no significant pressure reversion nor surging will occur. It would only be after prolonged use of a valve in an improperly tuned manner that reversion and surging MAY result. This will still require that all-important presence of equal and opposing exhaust gas pressure acting on the impeller wheel for a measurable oscillation in the speed of the compressor wheel to occur in order for surging to result. If no such oscillation or change is measured, it cannot immediately be presumed that a pressure reversion or surging is occurring at all just because a valve is fluttering under these conditions.

Valve fluttering under a partial throttle or partial boost throttle lift condition, on the other hand, is a completely normal occurrence and IS NOT an indication of pressure reversion into the compressor wheel by any measure whatsoever as it is often presumed to be. Virtually 100% of the time, at partial throttle, the turbo is not even spinning nor generating boost at a rate or level which is near to the surge line of the compressor map.

Partial throttle or partial boost valve fluttering is solely an indication that the valve is directly responding to an inconsistent and rapidly changing (oscillating) pressure differential on either side of the throttle body and valve sealing surface. This consists of pressure in the charge piping acting on one side of the sealing surface of the valve and rapid changes between pressure and vacuum in the intake manifold which will act on the other side of the sealing surface of the valve (through the vacuum reference).

An internal combustion engine naturally creates a vacuum effect during the intake stroke of any given cylinder. When boost pressure is built from the turbocharger, it will reach a certain level inside the charge piping, but as it enters the intake manifold through the partially open restrictor that is a throttle body, it is almost instantly reduced by a given amount of vacuum created by the intake stroke of the engine, thusly resulting in a marginally lesser amount of boost pressure inside the intake manifold compared to inside the charge piping. Since all mechanical bypass valves see references from both of these sources, the sealing surface of the valve, be it a diaphragm or a piston, will directly respond to these differences and rapid changes in pressure and vacuum, as minor or severe as they may be. This sealing surface response of the valve is what is creating the fluttering effect at partial throttle or partial boost conditions.

In essence, this is actually a positive attribute of mechanical valve operation as it is a clear indication that the valve is working properly, directly responding to changes in pressure and vacuum as it is designed to do. If this fluttering effect were not occurring, this would be an indication that the valve is not responding to changes in pressure as it is supposed to and this may indicate a functional problem with the valve’s operation that, over time, may lead to other issues.

This occurrence may be more pronounced on some applications than others based on engine size, airflow volume and spring tension, and will vary in its severity based on a number of other factors for a given application, but while it will always be present to a small extent with any mechanical style valve, it is not a problem for the vehicle in any way whatsoever and is NOT compressor surging as is commonly believed.

Now, if you are still able to determine that your application is, indeed, experiencing compressor surging, lets take a more in depth look at what effects may result from such an occurrence.

Since we know that compressor surging results in a dramatic and rapid oscillation of the compressor wheel while at speed, the effects this will have may seem obvious, but to say that this will cause immediate, permanent and irreparable damage to the turbocharger is a stretch to a large degree.

Surging will manifest itself through a seeming end to the power band of the engine as boost pressure and flow volume surpasses the volumetric efficiency of the engine while at speed. A severe hesitation and even bucking of the motor will occur as it and the turbo try to overcome the airflow volume restrictions if a wastegate and bypass valve have not already intervened by the time the surging begins. The boost pressure, as measured usually from the intake manifold, will stop increasing, but also fluctuate rapidly in response to the surging. A chattering sound emanating from the turbo will also result however it is not the same as bypass valve fluttering. Valve fluttering is just a sound without any corresponding drivability related concerns, whereas compressor surging will result in both an audible sound AND a significant detriment to the drivability of the vehicle. Even if surging does occur, you will assuredly notice it immediately and hopefully be wise enough to remove your foot from the accelerator pedal before prolonging the occurrence.

Even if, by some twisted turn of fate, surging occurs, it will not necessarily result in any damage to the engine or turbo. It is possible, however, but without getting into too much detail concerning the materials used in the construction of turbocharger compressor and impeller wheels, often aluminum, steel and titanium alloys, inconel, and even composite materials, they are all designed and manufactured to operate under the highest stresses, both working loads and temperatures, and within tolerances that exceed the working loads that the application will see. It will typically require the introduction of a foreign object into the blades of either wheel while they are at full speed in order to cause any sort of damage to the blades. Not only are the materials used often of the highest quality, but their construction involves manufacturing processes that strengthen the metals even further. Forging, cold forging, flow forming, each used appropriately depending upon the application will further strengthen the materials to a higher level than otherwise less specialized manufacturing techniques.

Both the compressor and impeller wheels themselves are attached to each other via a drive shaft that is also made of very light weight yet high strength alloys. It too is designed and engineered to operate under stress and load levels that will far exceed the working load levels ever seen on virtually all OEM automotive applications which will typically incorporate safety features such as wastegates and bypass valves too. The same theories can similarly be applied to the materials and construction of the engine itself.

There is also an ever prevailing argument that the stresses applied to the compressor and impeller wheels when compressor surging occurs actually has the effect of putting extra stress on the impeller/compressor drive shaft bearings (either journal bearings or ball bearings). While this may be possible under very extreme conditions, the tolerances under which turbochargers are assembled does not permit enough shaft play that will allow any sort of immediate or permanent stresses be applied or damage caused to the bearings either.

Additionally, even in such a case where the compressor and impeller wheels may stall or even rapidly change their direction, bearings are not one-way devices that will not permit such a stalling or reversal. As the bearings will allow the shaft and wheels to spin in either direction, any opposing pressures that may potentially cause surging will only be acting against the pressure already being made by the compressor wheel and the exhaust gasses acting on the impeller wheel. This reversion of pressure will not be acting against any mechanically limiting features of the design or assembly of the turbocharger itself nor its bearings.

The case can be made that the weakest point in the system will be the point of failure, however, as we are dealing with a pressurized system where airflow will always take the path of least resistance, and the materials used in the construction of the turbo and the engine are designed to operate beyond a level at which surging will be immediately problematic, the weakest points in the system will be the partially open or perforated throttle plate that will allow airflow into the vacuum of the intake manifold, and fail safes such as a bypass valve that should be appropriately setup and tuned to be capable of venting any residual pressure in the system preventing reversion, and a wastegate designed to bypass exhaust gas, both doing their job before such a time that the compressor wheel will ever “surge”.

In short, compressor surging is a phenomenon that most of you will never experience on your car. The turbocharger in your engine bay is undoubtedly appropriately sized to your application so that it will never generate more boost pressure than the motor can handle thusly never causing surging. Additionally, many of the things that you have been lead to believe are surging, like bypass valve fluttering and almost every throttle lift condition, are unequivocally not surging at all and are not even detrimental to your engine in any way. Also, at a minimum, at least two fail safe devices are clearly incorporated as well only further limiting the possibility for surging to occur.

I will be the first to advise everyone to take everything that you read with a grain of salt and learn to selectively apply it to your situation as you see fit, but use some common sense and don’t take every singular paragraph explanation of something as fact just because it seems too simple to possibly be incorrect.

Right about time, it took me 2 days to examine it after your email. Good information. Thanks Mike

I've added and revised a few things since you've last seen it, so it might be worth going over again! :thumbsup:

Very informative, thanks for the read Mike :thumbsup:

wow...very detailed! i don't have time to read it now, but i'll def. read it when i have a chance! thanks for taking the time and putting it together for us

Good job Mike. I'm glad you didn't rush your explanation. Hopefully this will dissolve some of the confusion. I know I learned something.

Forge FTW!

Cliff Notes??:bellyroll:

Wow Mike, thanks a bunch for such a great in depth explanation.

Hkysk8r07 said:

Cliff Notes??:bellyroll:

Click to expand...

Its worth reading

Boonedock82 said:

Its worth reading

Click to expand...

Yep it really was worth the read, Thanks Mike for a very detailed explanation.

Boonedock82 said:

Its worth reading

Click to expand...

why do you think i added this :bellyroll: :wink:

Great read Thank you MIke..

good deal not only did i learn alot from Mike.... but i also managed to waist 2HOURS at work....f-ing great man well written and easy to follow along with