1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Manifold vacuum question

Discussion in 'Engine & Performance' started by motohunter4, Dec 8, 2019.

  1. motohunter4

    motohunter4 Junior Member

    Age:
    42
    Posts:
    13
    Likes Received:
    9
    Joined:
    Oct 28, 2019
    Location:
    US
    First Name:
    Hunter
    Truck Year:
    1983
    Truck Model:
    Jimmy
    Engine Size:
    5.7
    Can I run the distributor advance and transmission modulator off the intake tee?

    Sent from my LGLS755 using Tapatalk
     
  2. bucket

    bucket Super Moderator Staff Member

    Age:
    38
    Posts:
    20,230
    Likes Received:
    5,261
    Joined:
    Aug 3, 2010
    Location:
    Usually not in Ohio
    First Name:
    Andy
    Truck Year:
    '77, '78, '79, '84, '88
    Truck Model:
    K5 thru K30
    Engine Size:
    350-454
    Yes. GM ran the trans modulator vacuum line to the tee as well. Although opinions vary on running the vac advance on full vacuum or ported vacuum.
     
  3. SirRobyn0

    SirRobyn0 Full Access Member

    Posts:
    76
    Likes Received:
    38
    Joined:
    Dec 6, 2019
    Location:
    In the woods in Western Washington
    First Name:
    Rob
    Truck Year:
    1984
    Truck Model:
    C20
    Engine Size:
    350
    You should be just fine with that, provided that you want to run the distributor on manifold vacuum. Personally on a stock engine I always try ported first for vac advance because that was the way it was setup from the factory. Unless I have drivability issues I leave it like that. Whatever you decide to do, never hook the modulator for the tranny to ported. That must be manifold.
     
  4. ali_c20

    ali_c20 Full Access Member

    Posts:
    149
    Likes Received:
    131
    Joined:
    Sep 20, 2016
    Location:
    Austria
    First Name:
    Alexander
    Truck Year:
    1974, 1979
    Truck Model:
    C20, K5
    Engine Size:
    350, 350
    Both works.

    Found this in an amc forum:

    This is a controversial topic in some circles. The factory used ported vacuum. I.e. the factory used the carb's spark port. In off-idle conditions there is actually no difference between the two. The difference is just at idle. At idle you have no vacuum advance with ported vaccum. That's because the spark port is just above the throttle blades. So when the throttle is closed, the spark port sees no vacuum. However, as soon as you open the throttle, even just a little, the spark port sees full manifold vaccum. With a carb manifold port, the port is just below the thottle blades so that port sees full manifold vacuum at all times. As far as how your engine runs, with ported vacuum the distributor supplies initial mechanical advance only, at idle. With manifold vacuum, the distributor supplies intial plus full vacuum advance at idle. Which should you use? Well the factory always used ported vacuum and Holley and MSD reccomend using ported vacuum too, so if for no other reason I'd say go with ported vacuum at least with a typical street motor that pulls good vacuum at idle. With a really big cam that doesn't make much vacuum, it probably doesn't make any differnce.

    Just to be a little more specific. The factory actually used both manifod and ported vacuum (at least during the '70s). If look at a TSM showing the vacuum hose routing, you will see that the factory used a spark CTO (coolant temperature overide switch) in the thermostat water outlet. There were two vacuum lines into the CTO. One supplied manifold vacuum and one supplied ported vacuum. A third line ran from the CTO to the distributor vacuum advance. What the spark CTO did was supply full manifold vacuum to the vacuum advance upon startup and until the CTO reached 160 degrees. Once the CTO reached 160 degrees, the CTO switched from manifold to ported vacuum and supplied ported vacuum to the distributor vacuum advance thereafter. The factory did this because the extra advance at startup makes the engine run hotter and hence warms it up quicker.
     
    Turbo4whl and Snoots like this.
  5. 75gmck25

    75gmck25 Full Access Member

    Posts:
    735
    Likes Received:
    382
    Joined:
    Oct 13, 2016
    Location:
    VA
    First Name:
    Bruce
    Truck Year:
    1975
    Truck Model:
    K25
    Engine Size:
    5.7
    I prefer manifold vacuum because the extra advance at idle brings the idle speed up and it runs smoother. You can also usually keep the idle screw set a little lower, which keeps less of the transfer slot exposed. Once you come off idle it doesn't really make any difference.

    Bruce
     
    idahovette and ali_c20 like this.
  6. motohunter4

    motohunter4 Junior Member

    Age:
    42
    Posts:
    13
    Likes Received:
    9
    Joined:
    Oct 28, 2019
    Location:
    US
    First Name:
    Hunter
    Truck Year:
    1983
    Truck Model:
    Jimmy
    Engine Size:
    5.7
    Thanks guys, I'm trying to go for the cleanest, least visible wires/ vacuum hose setup. The P.O. already scrapped all the emission stuff.
     
  7. Turbo4whl

    Turbo4whl Full Access Member

    Posts:
    78
    Likes Received:
    117
    Joined:
    Oct 30, 2019
    Location:
    Downingtown, PA
    First Name:
    Wayne
    Truck Year:
    1974
    Truck Model:
    Jimmy
    Engine Size:
    350
    Generally the distributor vacuum is ported vacuum. This is vacuum that is drawn above the throttle plate, as stated by others. Your 1983 had what was called "trapped vacuum spark". This is the system described by ali_c20 above. When the engine was started cold, manifold vacuum was applied to the distributor. As the engine warms up then the vacuum to the distributor moved to ported vacuum. This gives you the best cold run/lesser emissions and fuel efficiency. GM didn't waste the money adding "emission stuff" to the engine. All was there for a reason.

    A couple years later, fuel injection and electronic spark control took over. The engine compartments looked much cleaner for a little while.
     
  8. motohunter4

    motohunter4 Junior Member

    Age:
    42
    Posts:
    13
    Likes Received:
    9
    Joined:
    Oct 28, 2019
    Location:
    US
    First Name:
    Hunter
    Truck Year:
    1983
    Truck Model:
    Jimmy
    Engine Size:
    5.7
    I understand that the emissions equipment served a purpose. I am stating that it has been removed and discarded by the PO.

    Sent from my LGLS755 using Tapatalk
     
  9. SirRobyn0

    SirRobyn0 Full Access Member

    Posts:
    76
    Likes Received:
    38
    Joined:
    Dec 6, 2019
    Location:
    In the woods in Western Washington
    First Name:
    Rob
    Truck Year:
    1984
    Truck Model:
    C20
    Engine Size:
    350
    "GM didn't waste money adding "emission stuff" to the engine." I couldn't have said it better myself. For me, on my truck my vacuum system was in bad shape so I did unfortunately end up having to eliminate some of that stuff, but you can bet my air pump and catalytic converter are still on the truck. Why simple, because I've worked around cars so much I can't stand the smell of strong exhaust. With a good cat, air pump, properly tuned carb and distributor vacuum hooked to ported, the exhaust is no worse (at least in smell) than a modern car. All things to consider, and to the OP sorry about hi jacking the thread.
     
    Turbo4whl likes this.

Share This Page