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Re: What actually should we gain from a Manual Boost Controller ???
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Posted by Beaudreau (more from Beaudreau) on Sat, 14 Jul 2001 13:16:21 Share Post by Email
In Reply to: What actually should we gain from a Manual Boost Controller ???, Jason, Fri, 13 Jul 2001 21:18:18
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As LDH says, an MBC like the Dawes device will keep the wastegate from seeing any pressure until the set value is reached, which prevents the wastegate from creeping open and bypassing the exhaust gases at relatively low boost pressures.

The Dawes device isn't the simplist MBC though, a bleed is, but a bleed will still allow wastegate creep. Just eliminating creep will give you much improved spool up, you can expect it to happen 500-1000 RPM earlier. You can use the Dawes device in line with the compressor to APC solenoid set at 8-10 psi, provided that you have the fuel management, right fuel, etc to run in the red with your present set up without knock below 12 psi of boost. This still allows the APC to operate normally at pressures above the MBC setting.

My experience in increasing base boost is that, you do get more boost, and you also get less wastegate creep because the spring pressure holding the waste closed takes more boost pressure to make it start moving. The problem is that, if you shorten the rod, you are also changing the maximum amount that the wastegate can open and you reach a point where you no longer have an effective wastegate, because it can't open far enough to relieve the pressure. The stock wastegate works best when set to 5-7 psi.

There are wastegate actuators available that have higher spring values which allow much more boost before opening. The problem here is that you can never set them to a low boost threshold, on a street car this is unacceptable. You never know what that next tank of gas is like until it's too late. There are also dual port wastegate actuators, that have a port on both sides of the diaphragm, these use pressure differential to actuate the wastegate. With the right controller these are cockpit or underhood adjustable from almost 0 to 30 psi. These actuators can be set up to have zero wastegate creep, you can use manifold pressure to hold the actuator closed, then switch it, to open the wastegate at a preset limit.

I hope this cleared it up some.
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