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MBC-101: Introduction to Manual Boost Control - page 5

The ball-and-spring type MBC is clearly superior over the bleeder type MBC. The ball-and-spring type does not leak any boost (at least prior to reaching the desired boost level), and then sends an immediate signal to the Wastegate Actuator to open the Wastegate upon reaching the set level. The bleeder type, by design, is actually a boost leak. Typically, for maximum performance, it is best if the charged portion of the intake system has no leaks while it is in a charged state (AKA "under boost") but the bleeder type valve is allowing boost pressure to be released before attaining the desired boost. So, like your Mom or Dad yelling, "We're not paying to heat the outdoors" when you left the door open as a kid, you are in a sense now "paying to boost the outdoors" with a bleeder type MBC. Boost leaks are counter-productive and you pay the price of using a bleeder type MBC by blowing out energy from your closed, charged-air system that would have been better used to cram more air into the intake manifold. Also, Wastegate operation is less efficient with a bleeder type MBC, as some boost pressure is always allowed to reach the Wastegate Actuator. This results in the Wastegate being partially opened at times before the desired boost level, and results in the Wastegate opening more slowly when the desired boost level is obtained.

You may notice that many ball-and-spring type MBCs have, by design, a small hole… either in the case, or in one of its "barbs" or "nipples". In the picture below, the red arrow is pointing to such a hole. This does not indicate that it is actually a bleeder type MBC. Near the start of the last paragraph, it was stated that ball-and-spring type MBCs do not leak boost, "at least prior to reaching the desired boost level". Such a hole is typically on the "Wastegate side", so when boost is building on the charged-air side of the MBC, no pressure can go out that hole until after the desired boost level is achieved and the spring-loaded ball is pushed out of its seat. Then the boost signal rushes past the ball on its way to the Wastegate Actuator. The reason for that hole is to allow the "column" of charged air, or "boost signal", to drain or "bleed" out of the hose that sent the signal to the Wastegate Actuator. Without that hole, the charged air in the hose to the Wastegate Actuator would tend to "get trapped" and would slow the proper closing of the Wastegate. Technically, it is an inefficiency to allow some boost to escape, but it is a very small inefficiency that has a good trade-off. The inefficiency is relatively small so it really is not even worth mentioning… but that hole is a source of curiosity to many, so an explanation was warranted. Some ball-and-spring type controllers may not have such a hole and in those cases, the manufacturer may advise a small diameter "T" fitting or some other means of providing release hole.

the arrow indicates the wastegate release hole
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