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Thoughts on MBC & cold weather
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Posted by Dean (more from Dean) on Wed, 9 Oct 2002 12:40:39 Share Post by Email
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Many of us are all too familiar with the ball-spring MBC's and fuel cuts in cold weather. The MBC is blamed, with the implicit implication that the MBC has changed. But that is obviously not right either. So what is going on?

This is the understanding that I am comming to:

When the air is cold, the turbo is much more efficient. So to produce a given level of boost, it requires less power from the exhaust turbine. So to produce a given level of boost, the wastegate has to be more open * . And as the engine is more powerful in cold air, there will be more exhaust gas energy for a given level of boost. So there is another reason that the wastegate would need to be even more open in cold air.

* when the WG is open more, less gas goes through the turbine, and there is less pressure and velocity in the gasses going through the turbine.

Now the MBC interacts with the wastegate actuator and for a given pressure, it will interact with the actuators spring and produce a fixed opening of the wastegate. When its cold, that amount of opening is not enough, and the pressure is higher.

And in the cold air that pressure gets higher very fast. When the pressure goes above the set point of the MBC, there will be some extra flow, which will then open the wastegate further. The correction may or may not be a decent pressure correction. But such a correction must lead to some increase in the boost presure, however small that might be.

But the increased pressure may be hitting the fuel cut before any corrective action can occur. So if you were close to the fuel cut in warm weather, the MBC will not be able to control the pressure when its cold. But it will try to open the wastegate to the same extend as in the warm weather, but that is not enough to do the job. And the rate of change of boost in cold weather just makes this all the worse. Some of the problem is that the MBC cannot get the wastegate to open fast enough. Part of that could be solved with a softer wastegate setting. So an electronic controller could easily deal with some of these issues better than a MBC. But a $300 to $500 electronic boost controller is close to the cost of a modified ECU, so perhaps not what folks are after.

So the MBC is doing its job OK, but the requirements change and are not static. The result is not ideal. The whole situation is more complex than one would expect.


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