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So I have been messing around with adding sensors to the MX2E digital cluster as some of you may know.
I drive mostly like a grandma because 1. I have two cars on collector insurance so i do not want to get a ticket and 2. the ladies cannot admire me if I am flying by at 100 mph. lol.
so I notice with my newly installed boost gauge that I am mostly pulling vacuum in the intake manifold.
only when I floor it do i get into positive territory, like +4 PSI.
so what is the supercharger doing when it is not producing boost?
is it clutched so it doesn't actually do any compressing unless the throttle position demands it?
isn't a supercharger always spinning no matter what as long as the engine is running?
is it just at slower engine speeds or gradual throttle inputs its not really making much boost?
I am totally new to FI so sorry if this is a dumb question.
L
 

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2017 Lotus Evora 400 manual, 2009 Rossion Q1, 2019 Jeep Cherokee Trailhawk
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Interesting question, look forward to replies from those smarter than me to answer.
 

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2005 Lotus Elise
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05' Project Fennec
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Short answer is the supercharger most kits and OEM use have an internal bypass valve that is plumbed up so that it is open at idle and low loads allowing air to flow between the low pressure side (throttle body) and high pressure side (Intake manifold) freely. Although the supercharger still spins it cant overcome the boost leak that the bypass valve introduces. This helps smooth out idle and improve low load fuel efficacy as compared to a system without the bypass valve.
 

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Yup, the boost is only there for full throttle. The supercharger otherwise just spins for fun.

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Supercharger is always spinning at a ratio to the RPM according to the pulley sizes even when it's not making boost.
It has it's own vacuum actuated butterfly valve at the inlet that let air bypass the compressor blades and into the intake manifold.

Under low load/high vacuum, the butterfly valve is actuated fully open - bypassing the compressor blades.
As you get on the throttle, there is less vacuum and the butterfly valve closes - giving air no place to go other than into the compressor blades to make boost.

When it's getting bypassed, the compressor blades aren't being put to work so it won't be robbing much power from the engine. There will still be parasitic loss through inertia and friction, but the power loss through that would be negligible compared to when it's actually working to compress air, so its impact on fuel efficiency would be minimal.
 
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