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  1. #1

    Anybody know vacuum pressure on a 454?

    I am no expert here... anybody know where vacuum pressure should be on a 454?
    Got a friend with a custom 28' cruiser with a 330 h.p. Mercruiser 454, and a TS outdrive that throws a large prop. He doesn't want to take the engine over 2600 r.p.m. and is super cautious about vacuum pressure being low. Long ago the boat was known to do 4000 - 4400 r.p.m. at about 40, but someone said low vacuum pressure was a bad sign, and when he revs, the vacuum pressure falls, so he won't push the r's. So the low rpm and big prop lugs the engine and outdrive, I think to the detriment of the outdrive. The boats gone through prop hubs pre-maturely, and it may be too much lug on the prop/drive.

    Any help much appreciated!

  2. #2
    My Boats:
    1976 Cigarette 28SS
    Join Date
    Oct 2000
    Austin, TX

    Re: Anybody know vacuum pressure on a 454?

    Might get better response in the "technical" forum. But before you go there, define what you're asking about, manifold vacuum? At the risk of oversimplifying, manifold vacuum will always drop on a normally aspirated engine when you open the throttle and then come back up as the engine stabilizes. Vacuum will be different at all RPM's and throttle settings. Reduced vacuum during acceleration and at WOT is what allows the power valve on a Holley carburator to funtion.

  3. #3
    Registered robyw1's Avatar
    My Boats:
    1998 Wellcraft 22' Scarab
    Join Date
    Jul 2003
    Spring, TX

    Re: Anybody know vacuum pressure on a 454?

    No matter what the engine you crack the throttle open the vacuum will drop off. Now at idle the engine you describe should create between 19 to 21 in-hg if it is in good shape. The gauge should also be steady w/o shaking of the needle. When you put that engine under load such as getting on plane then the vacuum is going to drop way down maybe even as low as 0 in-hg depending on how much throttle you're giving it. once your on plane and moving at a cruise speed your vacuum in a cruiser will probably stabilize between 4 to 7 in-hg. Now if your getting low readings like this at idle then yes something is wrong.


  4. #4
    Registered Airpacker's Avatar
    My Boats:
    little cat...BIG MOTOR
    Join Date
    Aug 2001
    Aurora Ontario

    Re: Anybody know vacuum pressure on a 454?

    Vacum at idle should be somewhere between 17 and 21 inches of mercury. Vacum on a loaded engine at WOT should be approximately 0 (zero) as the pressure under the carburetor throttle plates nears atmospheric pressure. Low vacum at idle can indicate a worn camshaft, worn(stretched) timing chain,
    bad valve sealing,worn rings or just mal adjusted ignition timing. A rapidly fluctuating reading can indicate a burnt valve or broken valve spring. Check it at idle and if it ok, tell him to forget about it. Reading 0 under throttle is ok.

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