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Technical GM HEI Vacuum Advance Mod

Discussion in 'The Hokey Ass Message Board' started by rockable, May 22, 2014.

  1. rockable
    Joined: Dec 21, 2009
    Posts: 2,904


    I see a lot of threads that talk about getting the vacuum advance limited on HEI and other distributors. Most involve or suggest welding and filing the slot. After poking around on the internet some, I came up with this one. It wasn't my idea but I did implement it on my Buick 455 and liked it enough that I thought I'd share it.

    There are a lot of threads on setting the timing on your engines. Most involve modifying the weights and springs and then setting the mechanical advance at some "full in" rpm. I won't try to tell you how to do that. The second part of the modification, for a street driven car, is to dial back the amount of vacuum advance that is built in. Everyone seems to agree that there is too much and usually you only need 10-15 degrees of additional vacuum advance.

    Crane makes a kit that includes springs, new vacuum canister and an adjustable limiter. p/n CRN-99600-1 from Summit Racing - $38.20. You can also buy just the adjustable limiter plate, pn 99619-1 for $3.05.

    The Crane instruction say to install the limiter cam so as to compress the spring in the vacuum canister and limit travel. This makes the spring very stiff as you compress it and the canister that comes with the kit already has an adjustable spring. Plus, it's harder to install and adjust the cam against the compressed spring.

    What I did was tap a 8-32 hole in the bracket and install the adjustable limiter to stop the advance in the normal direction of movement. (Caution. Install it with the adjustable teeth as shown in the picture or you will have interference problems.) Now, I can adjust the total amount of vacuum advance to some reasonable number and hook it to the manifold vacuum port and get a nice smooth idle with about 30 degrees at idle. I also have 30 degrees at full mechanical but about 45 degrees for cruising. The engine seems happy.

    I'm going to do this mod to my other vehicles as I get time. It's easy and it works well.

    Attached Files:

  2. Pretty cool. I've been chasing timing advance issues on my car for years without much success. Spark knock, etc. I ended up unhooking it for a while! I'll have to give this one a try.
  3. rockable
    Joined: Dec 21, 2009
    Posts: 2,904


    Jeff, I think you are right. This is a stock cam and it is for cruising, not drag racing. It is running the best it has since I started screwing with it. I'm ready to drive it now. :)
  4. pbr40
    Joined: Aug 10, 2008
    Posts: 786

    from NW Indiana

    That's some good info. Thanks for sharing.
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  5. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,682

    Bruce Lancaster
    Member Emeritus

    For a good starting point, get a vac can intended for a big truck with HEI. I believe these are the only originals with reasonable advance for engines without EGR.
    Original cans and the aftermarket ones I have seen have total degrees in can stamped on the bracket. From memory, I think passenger car ones are in the low 20's and will ping like crazy at part throttle without EGR, whereas the heavy truck ones are about half that and have worked fine for me.
  6. GrouchySS
    Joined: Mar 24, 2010
    Posts: 101


    I just installed a adjustable vac advance from accel on sbc and bbc they are adjustable with allen wrench through the vacum line port both cars have never ran better. Got them from jegs part #110-31035 for the hei distibutor.
  7. rockable
    Joined: Dec 21, 2009
    Posts: 2,904


    That Allen adjustment only controls the rate or the speed at which the vacuum advance changes. It does not change the number of degrees of advance. Two different things.
  8. ebfabman
    Joined: Mar 10, 2009
    Posts: 648


  9. krooser
    Joined: Jul 25, 2004
    Posts: 4,585


    45 degrees of advance is an awful lot...
  10. lostone
    Joined: Oct 13, 2013
    Posts: 1,202

    from kansas

    I have never had a problem setting up full mechanical advance distributors. I have just bought the kit, rebuilt the distributor to set end play to specs and changed springs and weights to get it where i want it.

    I build pontiac motors mostly and usually run them with around 8 to 10 degrees intial and 34 to 36 total at 2200 rpm. Pontiacs seem to love it and it adds to the bottom end nicely.
  11. rockable
    Joined: Dec 21, 2009
    Posts: 2,904


    No, it is not. Read the article that ebfabman posted. It may not be enough.

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