Tom Barber

Those of you that follow this forum religiously will recall that back when all the reports of torn TBBs first started, I hypothesized (LOUDLY) that this would cause the turbos to spin out of control.

Following that posting, there was a reply by Brett of APR saying essentially that no damage can occur because of fail-safe capabilities built into the ECU. (There was also a bit of minor taunting, not from Brett, which is of no importance.)

Now thanks to DaveH, we have some useful diagrams and information about how the boost regulation on the S4 actually works. In the pictures that he gave us yesterday, there are two in particular that reveal how the wastegate regulation works. These are


https://www.audiworld.com/forum/m/s4/msgs/66241.phtml

and

https://www.audiworld.com/forum/m/s4/msgs/66240.phtml

If you study these a little, you will see that the critical component is the "solenoid valve for charge pressure control N75". The wastegate actuator itself is a conventional component where the pressure at its inlet opposes a spring. When the inlet pressure is sufficiently low such that the spring is not compressed at all, the wastegate is fully closed. Once the inlet pressure is sufficient to begin to compress the spring, the wastegate opens gradually from that point on as the inlet pressure increases. The pressure at the inlet to the wastegate actuator comes from the output of the N75 pressure regulator component. The pressure input of the N75, in turn, is direct from the compressed air (intake) side of the turbine. When the solenoid in the N75 is not being engaged by the ECU, the system functions in a default or base mechanical mode, where pressure from the compressed-air side of the turbine is transmitted through the restrictor in the N75 to the wastegate actuator. In this default mode, when the boost becomes great enough to fully open the wastegate, the boost will stabilize at an upper bound of 300-400 mbar.

The only method that appears to be available to the ECU to influence the amount of boost is through activating the solenoid in the N75. The effect of doing so is to cause the outlet pressure of the N75 to be a mix of the compressed-air pressure from the turbine and the outside air pressure. When these pressures are combined and the outlet pressure of the N75 modulated thus, the inlet pressure of the wastegate actuator is reduced relative to what the unmodulated pressure would be, thus reducing the force applied to the spring in the wastegate actuator, and retarding the opening of the wastegate, as compared to what it otherwise would be. It appears that the ECU activates the solenoid in the N75 in a strict open/close manner, and thus modulates the pressure at the output of the N75 by regulating the duty cycle of the solenoid, i.e., the percent of real time during which the solenoid is open. If the ECU does not activate the solenoid at all, the pressure regulation defaults to the basic mechanical regulation, where the spring constant of the spring in the wastegate actuator is the critical component.

It thus appears that the ECU has no means to protect the turbos if there is a failure in the basic mechanical regulation system. All that the ECU can do is prevent itself from increasing the boost pressure above the basic mechanical limit of 300-400 mbar.

If there is a tear in the TBB, there will be a drop in pressure in the intake manifold, which will be transmitted back through the inter-coolers to the compressor-side turbine, and from there transmitted to the pressure inlet of the N75 regulator, and from there transmitted to the pressure inlet at the wastegate actuator, which will prevent the wastegate from opening. It appears that there is no way possible for the ECU to prevent this from happening, i.e., there is no means by which the ECU can force the wastegate to open.

While it may be true that the ECU has some sort of "limp mode" where it uses methods other than boost reduction, such as timing retardation and/or fuel starvation, to prevent the engine from being worked too hard, there appears to be little reason to hypothesize that this is what people are experiencing when the TBB is torn. More likely, what is happening is simply that the boost is lost due to the tear and it is running normally aspirated. Just because the ECU is able to detect that something is wrong doesn't mean that it necessarily shuts itself down through any means at all, or in particular by opening the wastegates. Unless the amount of power that people experience when the TBB is torn is clearly less than what would be expected when the engine is running without boost, then this substantially simpler explanation should be presumed to be correct.

Getting back to the original question, is this condition likely to damage the turbos? Again, just to be clear on a fundamental point, unless someone can explain how it can be possible for the ECU to cause the pressure at the outlet of the N75 to be greater than the pressure at its inlet, then a consequence of a badly torn TBB will be that the wastegate will remain closed. Fortunately, though, this does not necessarily mean that the turbines will spin out of control. As long as the wastegate remains closed, the turbines will spin in accordance with the velocity and the pressure of the exhaust gases. The loss of boost will reduce the velocity and pressure of the exhaust gases as compared to what they will be when there is boost present, and this will help to limit the potential wear and tear on the turbines. The critical question is whether, when the system is functioning normally, the wastegate will begin to open before the intake manifold pressure reaches the maximum level attainable when the engine is running without boost.

When I first thought about this question, my immediate thought was that when the engine is running normally, as long as the intake manifold pressure stays at or below the maximum value that is attainable with no boost, the wastegates should never open. After I pondered the question a bit more, I concluded that (1.) that assumption is not likely valid, and (2.) nevertheless, it still seems reasonable to assume that the occasions during which the wastegate is open while the intake manifold pressure is below the maximum normally-aspirated level would be minimal, and that the reduction in exhaust gas pressure and velocity, that results from the lost of boost pressure, should have the effect of preventing the turbines from spining significantly faster overall than they do normally.

Although my intuition is that the second of these conclusions is likely correct, it is nevertheless entirely possible that with a torn TBB, the overall rate at which the turbos will spin will in fact be significantly higher than normal. My gut feel is that there is not likely to be much excessive wear if you get a torn TBB replaced in a reaonable period of time, perhaps days or weeks. But by no means would I suggest that it should be safe for you to continue driving with this condition indefinitely.

If you have a tear in your TBB, or even suspect that you might, whether you do or don't have a chip, I encourage you to either replace the TBB or have it checked ASAP.

Tom Barber

JDS

SORRY But your wrong wrong wrong. Not only in theory but the fact is there is a "limp" mode of 6psi or about .45bar and this is clearly noted in the shop manual. As soon as the ECU "sees" run away boost such as occurs with a torn boot it puts the car in this mode. When you shut off the car the ECU resets and you can get one or two blasts of full boost before it returns to limp mode. The exception to this is in the early stages of the "tear" in the boot you can lose power without going in to limp mode. However, in those cases you will not over boost anyway. We race our S4 and have studied this problem. We are working on finding or making a stronger boot. So far we just replace on regular basis.

Buzzo

The ECU uses vacuum to assist the spring in holding the wastegate shut. It produces the vacuum using the assembly that is located with the N75. It does this by bleeding boost off into into the atmosphere. The air rushes by the control opening and creates vacuum pressures depending on the velocity of the air passing by. The solonoid is closed when programmed boost is acheived and reduces the velocity of the airflow in the N75's module which reduces vacuum assist on the spring. this allows the exhaust pressure to push open the wastegate and bypass the turbine.

The mechanical boost is acheived with just the spring's resistance.

I assume there is a valve (maybe the restrictor) that does not allow for positive pressure to build up on the normally vacuum side of the spring/diaphragm unit.

Tom Barber

The diagram of the N75 does give the appearance that it operates on Bernoulli's principle as you suggest. However, it is not obvious to me that this is the only mode in which it will function, or what the important consequence of the distinction might be.

Isn't it possible that when the ECU disengages the solenoid entirely when the maximum allowed boost is achieved, that the flow of pressurized air through the N75 to the air intake (downstream from the air box) will be blocked entirely, and that in this situation the pressure at the input of the N75 will be transmitted directly to the wastegate actuator? If this is the case, it will still function properly, won't it? As the compressor pressure increases, the wastegate will be opened a little further. In fact, this is a much more simple and direct operation than the Bernoulli mode, which, without the active control of the velocity of air through the N75 by the ECU, would have an effect that is the opposite of what is desired.

Regardless, the pressure that is presented to the wastegate actuator is a control signal that is obtained by modulating the compressor output pressure.

The important point is that the ECU has only the ability to force the wastegate to close further than it would otherwise be closed. The spring in the wastegate actuator regulates a basic level of boost, and there is no means for the ECU to override or defeat that basic level of boost.

The question of potential damage to the turbocharger due to a torn TBB is of course influenced by whether the compressor output pressure is in fact transmitted to the diaphragm chamber in the actuator when the ECU is not actively permitting pressurized air to flow through the N75. If so, then the loss of that pressure will have the effect of closing the wastegate further than it would be closed normally, and some amount of incresed wear on the turbocharger will result. On the other hand, if it is the case, as you propose, that one-way valves or something similar in effect block the compressor pressure from reaching the actuator whenever the ECU is not actively allowing the pressurized air to flow through the N75, then clearly no additional wear on the turbines should result.

And I do sincerely appreciate your insight.

Tom Barber

Tom Barber

SORRY, but you are rude, rude, rude, rude. Aside from that, you have evidently made the mistake of writing a response to someone else's post and then posting it erroneously in reply to one of mine. This is evident because I never said things that are strongly implied by your response. If in fact it was your intention to reply to my posting, then I take issue with your liberties and your implicit accusations which are patently false, and I must ask for your apology. If your are incapable of reading, then by all means offer that as an excuse and you will be forgiven on that basis. What you have written strongly implies that I denied the existence of a limp mode. I did no such thing. Rather, I provided a detailed explanation of why it is that all the ECU is capable of doing is withholding the incremental boost, and allowing the boost to default to the basic level that results from the mechanical regulation, which is largely determined by the spring in the wastegate actuator, and which is 300-400 mbar, according to the information that was printed on the very informative diagrams that DaveH provided. By the way, since you evidently did not figure it out, .45 bar is the same as 450 mbar. The ECU does not "put" the car into this mode, it merely ceases to intervene.

MOREOVER, IF YOUR TBB IS TORN, the specific boost associated with the "limp mode", or the method through which it is caused, are not even relevant, because there is a big stinking leak in your intake just in front of the manifold and the intake pressure will be determined more by the size of the hole than by whatever the attempted boost happens to be, duh. Whether or not the spin rate of the turbine is affected by the tear is not particularly relevant to the question of what the actual intake manifold pressure will be, but if you happen to be interested in that question, you can go read the other follow-up that "Buzzo" wrote and my follow-up to that.

Rude, rude, rude, rude, rude.