Should I Disconnect And Plug My Vaccum Advance On My Stock 454 JET
And Why?
It serves no purpose.
The vaccume seccondary provides advance at part-throttle, low RPM load to improve efficiency.
The mechanical advance of a jet-boat engine should be fully in by arround 3,000 RPM. When is your engine opperating under a load, below this RPM? Never.
Cap it off and forget about it.
You will not even find it on marine distributors.
Jet boat or not don't matter, stock 454 put 18 in the distributer and have it all in by 2800-3000, set total at 36 it will start easy and run smoothe, forget about the vacuum advance!..
Rather or not you can use the vaccum advance depends on your engine and how it's loaded in your boat.
If you're showing 10+ " of vaccuum at a cruise speed the engine is showing a light load and it can use a lot more advance.
If you're showing low manifold vaccuum meaning the engine is heavily loaded then the vaccuum advance is not going to move anyways.
My jet boat showed 10.5" of vaccuum at a 3000 rpm cruise speed and I let the advance go up to over 42 degrees. High manifold vaccuum = slow burn = ignition advance over 40 degrees. Low manifold vaccum near or at 0 = fast burn = max advance near 34 degrees.
The ignition advance wants to be optimised for what your engine and setup needs. Just locking it to one setting is not optimising.
I ran those exact timing figures with my old flattie V-Drive. Worked awsome. If you do some research in an old Delco parts catalog you can even pick total vac. adv. values and at which point the vac. stops advancing. I know some of the old HD trucks (1975 454 3/4 ton w/ tow package or whatever) had very low values of advance (10-12 deg. ) and would not be in effect with less than 7.5 inches of vacuum. There are tons of different ones, finding them is another story.
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