I have read a few posts in various forums that have eluded to the fact that turbo life is effected by a boost leak and/or a chipped ECU. I am trying to understand why. Please correct my facts below if you see a mistake.

- Boost leaks do not cause the turbo to work harder. The same amount of exhaust flows through the turbine. The only difference is the back pressure is less. The turbo may spin a little free-er but if driven normally should not overspeed.
- The turbo is not VGT (variable geometry turbo) so the only boost control the ECU has is the BPVs. The ECU demand for more boost does not cause the turbos to work much harder (just more back pressure).
- Turbo failures are most commonly in the bearings. Heat, lack of oil, and overspeeds are the main factors that would cause abnormal wear on the bearings.

Assuming these facts are correct.. please explain HOW boost leaks and chipping the ECU can effect turbo life! Thanks!

 

- Boost leaks do not cause the turbo to work harder. The same amount of exhaust flows through the turbine. The only difference is the back pressure is less. The turbo may spin a little free-er but if driven normally should not overspeed.

You hit it on the head. Overspeed. The turbo shaft speed becomes uncontrollable when there is a boost leak. Turbo shaft speeds have been known to crack 250K RPM under no load situations. This is twice as fast as it was designed to spin. Take a second and try to imagine 250K rpm... I think turbos are the fastest rotating objects in the world.

- The turbo is not VGT (variable geometry turbo) so the only boost control the ECU has is the BPVs. The ECU demand for more boost does not cause the turbos to work much harder (just more back pressure).

The ECU controls boost pressure by modifying the signal going to the wastegate canisters, not the BPVs. BPVs only work when the throttle blade is closed under boost.

- Turbo failures are most commonly in the bearings. Heat, lack of oil, and overspeeds are the main factors that would cause abnormal wear on the bearings.

Turbo failure is generally in the bearings. There are other failure modes, but this one is the most common. The reason chipping causes turbo failure are two chief reasons: 1. Increase in shaft speed, and 2. more importantly, Axial and radial bearing load. The axial and radial bearing load is almost doubled (and in some cases, it is). The bearings are fluid bearings, and are designed with a specific contact load in mind, and when you chip the car, the bearing load is higher then they were designed for.

Assuming these facts are correct.. please explain HOW boost leaks and chipping the ECU can effect turbo life! Thanks!

 

Thank you very much for your response. I can not fathom 250K rpm.

I understand the rotational inertia that, if not countered by back pressure on the compressor side, can push the turbo speed higher than normal but twice as much!?? Wow.

I am still confused on the wastegate canisters. Is there a description of this system on the 2.7T anywhere. I have the Bently manual and have tried to find this info. Are these the charge air regulators? I am having trouble trying to understand how the ECU can demand more boost. There is set amount of exhaust mass-air-flow throught the turbine. I know that the turbos are designed oversized and the extra boost is vented (otherwise the turbo boost would not be variable). I assume the ECU commands the wastegate valve to close if more boost is needed? And this closed valve causes more boost by less diverted intake pressure?

If so I assume the increase back pressure put a greater axial force in the turbine direction. I can not explain the radial force though as the turbo should have less rotational speed with increase back pressure.

Thanks for the help. I am just a curious guys considering chipping my S4.

 

You're a bit confused. The turbine speed is controlled by the wastegate (the silver canister hanging off the side of a turbo), not the bypass valves (dump valves located in the charge pipes). The only time a turbo system should be leaking presurized air is when the throttle blade is closed under boost. Check this out, it will explain it all:

http://auto.howstuffworks.com/turbo.htm

The wastegates have a spring inside them, and a diaphram. When the boost pressure overcomes the spring coefficient, it pushes the actuator rod. The actuator rod opens the waste gate flapper on the turbine housing. The flapper alows exhaust gasses to bypass the turbine, thereby regulating the turbine speed... When the turbo slows down, the flapper closes, exhaust hits the wheel, turbo speeds up, pushes flapper open, etc... you get the idea.

Now, how you electronically control boost is you put a valve inline on the hose running to the actuator. The valve is controlled by the ECU. The spring on the wastegate might only need 5 PSI to open, but, when you chip the ECU, the MAP sensor will tell the ECU to block the boost to the wastegate with the valve until 15 psi is reached, for example. Once the 15 psi is reached, the valve opens, and regulates the boost pressure. This is how you control boost electronically.

 

Oh that makes more sense. Don't let the diaphram/spring see the boost until the ECU desired boost is reached... And more boost = larger forces on the bearings. You have answered both my questions. Thank you very much!

 

Glad I can help. Dont get me wrong, chipping is easily the best bang for you buck, and there are thigns you can do to preserve your turbo life. for example, I dont spool mine the last 2 miles before I get home, and if I spooled them hard shortly before I need to shut the car down, I let it sit and idle a few minutes before I shut it off to cool them down. The other big thing you can do is not spool them until your oil comes up to at least 175*F. I run a 10-40W royal purple oil. The slightly heavier weight helps carry off heat, and I saw a slightly noticeable drop insummer oil temps as a result.

K04's have larger bearings, and better axial thrust protection, so they are more resistant to abuse then the K03s. the K03's really never should have been on this car from the factory. My car has 125K miles on it, and the original turbos, because both myself, and the two previous owners took good care of the car and didnt abuse the turbos. You can ruin ANY turbo in less than 2500 miles by abusing it.

 

this might be a nubie question but how do you not spool your turbos when you are getting close to your house, and before you drive it in the cold do you just let it sit for a couple mins because of the turbo range? i was told by the dealer that the turbo range for the S4 was from 1800 rpms to 4800 rpms so it would be a little hard to drive it with out getting into the turbos when its cold outside. i just want to know so i can take good care of my turbos i have tried my best but i would like more information. My S4 is a 2001.5 with 113K on it. Thanks a lot!

 

the turbos are allways spinning because exhaust is flowing through them but they have an efficiency range and need to be spining a certain speed to create good boost. Maximum boost wont happen at 1800 rpm's, boost will build and then hopefully be maintained and probably drop off a few psi before you hit your rev limiter. If you listen closely you should be able to hear your turbos spooling. The best way to see how much boost you have all across your rpm range is with a boost guage.

By not spooling the turbos hes probably just shifting at a low RPM so he dosnt build alot of boost and not accelerating quickly. You can also buy a turbo timer which keep your car runing for a ew minutes after you have taken out the key to let everything cool down. As far as cold weather yeah you can start it up and let it warm up before you drive it, but most people just drive conservatively (again shifting at lower rpm's and not going boost crazy) untill the oil is about 175*

Basically theres no way to tell how the people before you drove the car, if you make sure to let it warm up and cool down properly you will add a little life to your turbos. However theres no way to tell when they will fail and chances are it will happen. Just be prepared

 

^^ right.

I shift at or less thank 2k RPM to keep the turbos from spooling up. Helps fuel economy too. I average 27 in mine with a 50/50 mix of driving.

 

thanks a lot for that information it helps a lot