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Old 11-18-2004, 12:48 AM   #1
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Default help me understand boost

i was driving today back from walmart, on the same road i got my stupid ticket...

ANYWAYS, i was going down a slight hill, cruise set at 74, RPMS just below 2g'* i think....boost was under the -10...

then the road flattened out, RPMS didnt move, but the boost crept up to almost 0....

now, is it the booth control solenoid telling it go higher or what? if i the supercharger is run off a belt, and nothing else, and the motor was staying at the same speed, why would the boost increase? was it just the gauge just showing me that it was not being used when it was below -10? im assuming the supercharger is always spinning at a ratio higher than the engine, prolly like 4x?? do they make a gauge, like a tach for the supercharger...that would be interesting. you could check for slipage then. is there an easier way to check for slipage?
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Old 11-18-2004, 01:10 AM   #2
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What the BCS does is dependent on alot of factors, including your TPS position, which can be controlled by cruise. If the TPS voltage drops, it knows you're getting out of the throttle, so it dumps boost.

Slippage will only be at the higher rpm'*. Do you see your boost flatten out before 8psi? Do you see it drop? Flattening out or 'fluttering' would be slipping of the belt, dropping off suddenly would be the BCS or PCM dumping boost.

By dumping boost when you get out of the throttle, you save fuel. The PCM constantly maintains the proper AFR. By dumping the boost, it doesn't have to deliver as much fuel.
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Old 11-18-2004, 01:18 AM   #3
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i see no fluttering, so thats good...

ever since i had my gauges apart, i had to calibrate all the needles myself, and i think i got the boost on correctly, because it sweeps at the same degree as the others. it is by far the hardest to calibrate, i had to drive almost 10 miles out in the county to get it aligned, going 50 mph...

i am getting a solid 9+ psi right now...which seems about right for my mods, thanks for clarifying this...i dont think i have much slippage
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Old 11-18-2004, 05:41 PM   #4
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For a little clarification, have you noticed the little butterfly valve visible on the bottom of the SC (one removed from liwer intake)? That is the bypass valve. When intake vacuum is present (ie the throttle is closed or not fully open), the BCA uses the vacuum to open the valve and dump boost from the lower intake manifold to the intake side of the blower. When vacuum disappears (ie when the throttle is open), the BCA closes the valve. Now, depending on the throttle angle, vacuum pressure will vary. The BCA adjusts the valve to any position between fully closed and fully open. So you know, the BCA is just a diaphragm in a cylinder. One side is atmospheric pressure, the other is intake pressure (not boost pressure). So, the closer the intake pressure is to atmospheric, the further the BCA closes the bypass valve to stop boost from venting into the intake side of the SC.
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