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, Tue, 26 Jul 2005 18:21:52
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A MBC or MBC+A or SMBC can also reduce boost lag. So if one has an issue with loss of knock control, then a SMBC can be used to set the base boost desired. The boost will come on faster. The adjustments are easier than tweeking the WGA rod*. "Power delayed is power denied."
Stiffing up the WGA adjustement does have the effect of increasing the tendancy of a MBC to create pressure overshoots aka spikes. This is not the MBCs fault. The problem is that the MBC cannot flow enough gas to the WGA when boost comes on fast. A softer WGA spring would allow the WGA slew rate to keep up with the feedback demand. The MBC+A was designed to overcome this problem by use of a pneumatic anticipcator* that creates a regulated precharge in the WGA before the MBC starts to flow. The MBC flow can then get straight to the business of moving the WGA instead of a flow duration that is simply bringing the WGA pressure up to the lift off point.
* The anticipator is implemented by an air pressure regulator (APR). This needs to be one designed for a low pressure range. One from a hardware store will -not- work right. And it has to be one that is not a relief type. Jeff did all of the hard work on finding the right APR for the job... its a special order component.
The MBC+A design, when fully implemented, eliminates the boost pressure droop of the MBC. It does this by taking the feedback pressure from the intake manifold or TB instead of the turbo compressor scroll case hose barb. That latter arrangement is what causes boost droop as the pressure at the TB is then the boost pressure less the pressure drop** in the IC and ducting. The MBC+A thus eliminated boost delay (so the result is the turbo's native turbo lag), eliminates boost droop, and eliminates pressure spikes and fuel cuts. It is a really cool solution, but not for every application and definately not for any T7 setups.
** The pressure drop across the IC increases as the square ~ cube of the flow rate, so at high CFMs this becomes a real problem. With the APC function, the BPC valve simply increases the boost during a run to hide this effect. When you go to a MBC and discon the BCP, you get this problem. The MBC+A with its pressure tap in the right location also increase boost to overcome the IC flow resistance.
The boost lag improvement can be much greater in a stick shift vs autobox as the autobox torque converter slip allows the RPMs and gas flow to develop much faster. With the stick shift the engine RPMs are tied to road speed and that really can be a problem. This effect is very strong comparing the two tranies on the 2002+ 9-5 5spd automatics to the manual gear box. Can't speak with experience about other models.
, Tue, 26 Jul 2005 18:21:52
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