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I know this was discussed a while ago, on here:

http://sr20forum.com/showthread.php?t=45154&page=2&pp=20&highlight=fuel+pressure+rise+boost

But what I was wondering...is how does a vacuum regulated fuel regulator actually regulate fuel when you have positive pressure applied to it? Is coach right on the second page? I am not trying to prove him wrong, I was just having this discussion with my friend/mechanic who has a turbo miata, and he was saying his regulator works under boost and vacuum. Thanks in advance.

Justin
 

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Well both vacuum and boost regulate fuel pressure. In a 3 bar setup fuel pressure at idle should be 43psi without vacuum, if ur engine can pull say 18in/hg at idle fuel pressure should be around 8.8psi lower than whatever it is whith the vacuum disconnected, this is because 18in/hg is equivelant to to 8.8psi, so this particular engine should see about 34.2psi at idle. The FPR constantly adjust from the point of vacuum up to atmospheric pressure to above and beyound it. The more you can drive in vacuum the better ur gas milage will me. Hope this helps.

How it works internaly:

Its has a spring, diaphram and vacuum/boost port. When the engine is in vacuum it pulls up against the spring thus allowing more fuel to be bypassed from the fuel rail back to the fuel tank, i.e. that is how less fuel is injected into the engine.

When possitive pressure i.e. boost psi is seen it will act upon the diaphram thus closing the gates of fuel return back to the fuel tank at a 1:1 ratio thus increasing fuel pressure by that amount. Thats how it works
 

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^Charlie has the solutions to everything! :)
 
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