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Proflo XT vacuum port help

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  • Proflo XT vacuum port help

    We have a 1967 Mustang coupe equipped with a 302 Windsor helped by a Novi 1200 supercharger. We swapped from the blow-through carby arrangement to an Edelbrock Proflo XT with Haltech engine management, cam position sensor and crank trigger. Being EFI newbies, we could really use a knowing head on MAP and vacuum port use on the manifold in a boosted application.



    The Proflo manifold, was simple to install and flows much better than the previous set-up. It ships with an attached MAP sensor. Unfortunately, the Haltech is configured to use a three BAR sensor but the supplied MAP is smaller than this. Fortunately, the Haltech has a built-in MAP sensor that connects via a tube to a port on the Proflo XT to calculate MAP. We have tried the brass connection fitting on the manifold and compared it to a port before the throttle body; very different vacuum readings. What is more confounding (to us) is the brass fitting appears to deliver constant vacuum even when we rev the charger into clear positive pressure. We suspect the venturi effect is delivering constant vacuum; good for brakes but not for a boost signal for the ECU of blow off valve.



    We wold really appreciate some help on where we pick up vacuum for:

    1. ECU

    2. Blow off valve

    Cheers
    from Oz


  • #2
    The throttle blade is what creates vacuum when the engine is running as it gives the engine something to "suck" against. If the throttle blade is closed and the engine is running, there is vacuum. As the throttle blade is opened, vacuum decreases and a MAP sensor will eventually report atmospheric pressure (or eventually boost if forced induction) as the restriction that the engine was sucking against to create vacuum is reduced. There is no vacuum ahead of the throttle blade. The only time post-throttle blade MAP and pre-throttle blade pressure will ever be the same (naturally aspirated) is when the engine is either at WOT or if the engine isn't running. Your ECU will want to reference proper absolute manifold pressure which is always post-throttle blade.

    Your Novi centrifugal supercharger isn't likely to make any measurable boost by simply revving the engine with no load especially if a blow off valve is plumbed in. If you were to put the car on a dyno and load it up, you'd see positive pressure come out of that vacuum port. There's no physical way for a vacuum to be produced when there's nothing but positive pressure in the manifold.

    Since you're admittedly unfamiliar with EFI, I presume that you'll eventually be taking this to an experienced tuning professional for full tuning. They'll likely be able to walk you through any other setup/configuration questions you may have but this MAP and vacuum port setup is pretty straight forward. You'll have vacuum when there's vacuum and boost when there's boost.

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