| Not too often that someone carries forward with an explanation. =) Can is open now: My original explanation is a bit simplified. I'll reiterate with emphasis here that the BOV flutter is the primary cause for the noise that people are identifying as surge, but it is not the only cause for said observation. There is another variable that has to be taken into account; BOV preload. This is probably the best opportunity for me to indulge on this topic so its gonna be a long one. Ill start with stock recircs. Stock recircs and BOVs are quite similar in function but they differ in one important aspect - valve preload. They both serve the purpose of preventing compressor surge when sudden throttle closure occurs - they both open during this event to vent airflow and prevent surge. The recirc valve dumps the vented air back into the compressor inlet while the BOV vents the air out to the atmosphere. This difference is actually inconsequential, despite conventional theories that float about this here and on other Z32 forums. (I'll address why this is the case later). Both the recirc valves and any aftermarket BOV system are constructed with a spring-loaded diaphragm that controls the valve body. This diaphragm body connects to the intake manifold via a hose - recircs and all BOVs are built this way. The main difference is within the spring preload. Recirc valves have a lesser spring preload and this causes the recirc valves to open with very little vacuum applied to its control diaphragm. In fact, at idle, the recirculation valves are wide open. This doesn't create a problem for the EFI system though, which should be obvious; it isn't venting air out of the induction system. But BOVs dont behave the same, which should also be obvious. If they are open during an idle condition, they are going to affect the air metering system.
Something you need to know: Even in an idle condition there is enough exhaust gas energy being absorbed by the turbines in both turbochargers to produce a very small positive pressure relative to atmospheric pressure. It is really small, but it is there. What this means is IF you were to punch a hole in any of the induction pipes that are downstream of the turbocharger's discharge, air will be pushed OUT of that hole. Just like with a "boost leak" where it causes excessively rich mixtures while under boost, any large leakage in the induction system will cause the same condition at idle. So, If you were to disconnect the outlet hose on your recirculation valves and set them up as a BOV, the engine would nearly instantly stall out. (obviously you would have to plug/cap the port on the turbo inlet pipes that run underneath the headlamps if you disconnect the recircs' return hoses).... This would occur because the recircs will open up at manifold vacuum during idle and allow metered air to vent out of the system, causing an excessively rich condition quickly followed by a sharp dive in RPM, quick stumble, and then stall.
On the other hand, a BOV is designed to be connected only to the charge pipes. In order for this type of valve to work properly, it MUST remain closed in idle manifold vacuum conditions. So how does a BOV work if it never sees enough vacuum on its control diaphragm to overcome the closing spring? Simple: pressure witiin the charge pipe it is attached to will provide the force required to open the valve and relieve the pressure. This is why BOV flutter occurs, but this is a simplified case of the matter. Consider when the valve spring preload varies. At really high preloads it will require many psi more force applied to the valve's inner face to force it open (the face that is exposed to the charge air in the induction pipes). This will cause the valve flutter to occur earlier in the blowoff event, which should be obvious, but there is also another effect; it will cause the valve to stay closed even when there is a few psi of pressure "left over" in the charge pipes, of which it will not vent. This ultimately will cause compressor surge, the "real" compressor surge. Now before you get all into a tizzy to say that this contradicts my original explanation, you will have to take a step back and weigh this out. Even though the compressor does experience surge in this instance, it is not the severity of surge that will cause rotorbearing damage. You will hear the distinct noise this creates - a buffeting of air - shuh-shuh-shuh-shuh, with a corresponding low-frequency resonant howl at those same intervals. The rotorbearing systems in both dual-ballbearing and journal bearing turbochargers will know no different in these instances. The loads experienced in the bearing systems at high boost produce magnitudes of wear beyond what they experience during this very low pressure surge condition resulting from excessive BOV spring preload. Even with any BOV out there, if the preload is crunk up to its max, it still will not produce a surge condition that warrants any concern for the rotorbearing system. Going back to the OEM recircs - at this point it should also be obvious why the flutter and surge condition does not exist. For the rainman, it is simply because the oem recircs will never attempt to close when you let off the pedal; they will simply open at closed throttle and allow any over-pressure from the turbos to recirculate back into the compressor inlet.....
In short, real compressor surge does exist with BOVs, but "surge" is not an absolute condition of "yes" or "no"; it has scale, degrees, magnitude. You can have "too much" of it or not enough to concern yourself with. You may ask: "How do you know that this amount of surge wont cause harm to my turbos, Ash?" ..... and that would be a very good question. =) And I would tell you: "I know this because of my experience with turbomachinery systems." .... I have experience with these devices that go beyond the scope of what we use them for in our cars. I have more than the single perspective of a "turbo car guru" where the experience with turbos is limited to how they behave when attached to a reciprocating internal combusion piston engine. I have stared into a compressor wheel turning 160KRPM, its shaft orbiting in critical modes as it surges; puffing out gasps of air at my face, and catastrophically grinding to a rotational halt - making screeching and groaning noises you never could have dreamed to hear....... comparing this to the tail end of an over-tightened BOV event is a universe apart.... There simply is not enough energy within the rotating group at the pointmp it experiences this surge condition - it will make a little noise when it does this but it will not cause any damage to the rotorbearing system..... As for the inconsequential condition I spoke of earlier:
There are a lot of people who argue that BOVs will cause the EFI system to dump large quantities of fuel during a BOV event. This is not entirely the case. In the ECU's programming there is a table that defines the maximum fuel delivery per engine RPM. It is a definition within the programming that prevents too much fuel from being delivered to the engine in the event of a very large "boost leak". It is a fail-safe mechanism; it is there to try and prevent fouling of the plugs if a large leak develops. In effect, BOVs are a "large leak" of sorts, but this is only true for short durations; after closing the throttle in a boosted condition. Turbos de-spool very quickly relative to the rate the engine will drop in RPM during closed throttle. Even at high boost pressures, letting out of the pedal at 6KRPM, the boost pressure will fall below 0 in less than a second and the engine will require a few seconds to freely down-rev to idle. At the moment the throttle is closed after a boosted condition, the BOVs open and vent a large volume of air, which is air that has already passed through the MAS and the ECU will deliver a large chunk of fuel as a result. So yes, the mixture does go far rich in that instance, but this only occurs for a very very short duration - a duration lasting only as long as it takes for the turbo to de-spool. This extra fuel is simply blown through the engine with no effect and long after this occurs, as the engine RPM falls to ~1500RPM where the high IACV duty cycle "catches" the RPM deceleration, the BOVs are long since fully closed and no metered air is vented - all is well, the engine stabilizes at target idle speed.... So yes, BOVs do cause a dump of fuel, but the duration of the event is so short that it does not cause idle issues. If your car is having idle issues or issues with easing down to a stable idle, you have other problems in the setup; leaks in the induction system, bad IACV, improperly calibrated idle screw.... .... and speaking once again to the BOV spring preload: you can also have it "too loose"... If the BOV is even slightly open during idle condition, it is going to throw off the air metering. This results in a hunting condition - engine RPM surging up and down. In this case, the BOV creates a feedback loop to the engine; the bov slightly opens, air leaks out, mixture goes rich, engine rpm falls, manifold vacuum falls, bov closes, IACV duty cycle increases and engine RPM surges, manifold vacuum increases, bov slightly opens, air leaks out,... so on and so forth..... BOV systems require an additional measure in the installation process as compared to recirculation valves..... They need to be just tight enough that they dont open at manifold vacuum conditions. Nice thing though, you can't make them too tight; they aren't adjustable enough to create compressor surge to the extent that it will cause rotorbearing damage..... =)
Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the
tailor.
Albert Einstein
|
