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Quadrajet Vacuum Ports

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Blazer_Boy, Jun 20, 2002.

  1. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    I'm confused on this whole vacuum port thing on the Quadrajet. From my own tuning adventures I've found that ported vacuum is always on the throttle body (base) of the carb on the passenger side. There's a little hole by the throttle blade that is covered at idle and when the throttle is opened, it is exposed to vacuum. Meanwhile, there's a port in middle of the float bowl that always gives full vacuum.

    Over at chevytalk.com, we've got Ignitionman screaming his brains out that ported vacuum is evil. He says full vacuum is always below the throttle blades and ported is above them. Then he says if you hook your vac advance up to ported that the more throttle or RPM, the more vacuum and advance. Maybe he's talking about a Holley or something, I don't know.

    Wouldn't hooking up to the port I've described be just like manifold vacuum, only not at very light throttle or idle. It is right by the throttle blade. I've ran full vacuum for my adjustable vacuum advance and it was not fun falling on my face all the time when you let off the clutch, vac falls, and some of my advance.
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    Ported vacuum in a Q-jet is dependent on throttle position. EGR and vac advance are both this way. At idle, no vacuum, no advance/no EGR. (ideally..there might be some "leaking" by, thats why GM still has you disconnect vac advance I guess)

    Vac advance is for nothing except cruising.

    However, if you hook it up to manifold vac, your idle will go WAY up, and you will probably have to re-time the engine to adapt to the extreme amount of timing the vac advance throws in. But once you hit cruise, its the same effect as ported...both are "seeing" manifold vacuum. Vacuum is vacuum, no matter where it is hooked up, if there is no vacuum, vac advance ceases to work. WOT, startup, no vac advance regardless of what port.

    So manifold vac would affect idle and crusise, and ported would affect solely cruise.

    GM did it both ways, but overwhelmingly it was ported vacuum for vac advance.
     
  3. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    He's nuts. You open the throttle vacume drops because your letting more air in past the throttle plates. Any knuckle head with a vacume gage can tell you that.
    I thought the port on the side (passenger side up by the choke assembly) was routed down to the base into the same passage as the PVC connects to. It's the connection used to operate the thermac and it needs uninteruped vac at idle to keep the flap open to draw hot air.
    I thought the timmed port is the one if your facing the carb, on the engine, is just above the base plate on the lower right and that's for the Vac advance It drops the advance when you get out of the throttle to help prevent back fires. Another timmed port for EGR, facing the carb was on the left side towards the top cover. The large pipe in the middle front in the cover is the float bolt vapor recovery connection going to the canister (if so equiped if not it vents at the top in front of the primary's) and on the back is a port on the base for brake booster and one at the top to create a light draw for the hot pipe style choke set up (if so equipped).
    I'm a knuckle head with a Vacume gage. I'm about to rebuild a 79 Quad to put on my 75. Hope to mess with that in a week. I'll look at it then and hook my gage up and see what comes on when.
     
  4. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    Well alright! I know its a simpleton's question, but this guys swears that ported vac is evil. I must be going to hell 'cause thats what I'm using. I know just what your talking about on your '79. Mine's a little different 'cause its '83 and a manual, so it lacks a port to run the lock-up convertor. I don't have the temp controls that turns off vac advance when cold though, it was an unholy mess when I brought home from the tow auction with the old 305. I have the TVS hooked up (off the little vac tree in the rear) to warm the incomming air, vapor canister, and PCV. I can't run EGR because of the vortecs because the way the heads are. I've tried to run every smog piece that I can because I'm not a total a$$, but we don't even have testing up here.

    Comparing my '82 Q-jet to my '83, I have all the same parts except just one port. '82 M4ME I had it overhauled when I first bought it, so the routing was done by the hired wrench. The vac line with the white stripe going to the right doesn't exist on my '83 manual transmission carb. On the '82 it is going the vac advance. The port with the plug on it just above the filter housing will pull vacuum at idle. There is a hose just below the nut on the fuel line that is going to my lockup switch. That maybe backwards with the white striped one. That lower left port will only pull vacuum when the throttle is pulled open. While I had the base plate off on the '83 you can see that hole in the base where it sucks from. Its just covered by the throttle blade at idle. No elaborate passage up to the float bowl, just straight through that base.

    Here is what "IgntionMan" usually says...

    </font><blockquote><font class="small">In reply to:</font><hr />
    PLEASE, not again. Ported vacuum sourcing pulls way too much timing into the engine as it accelerates, if the engine runs better with ported vacuum working the vacuum advance, FIX THE MECHANICAL ADVANCE CURVE, IT ISN'T RIGHT.
    good.wrench has it right. Phat, try to find a nipple on the bottom of the front of the carb to use (vacuum present at idle), but follow the instructions below, please, that does it right.

    Now, as far as full intake manifold vacuum sourcing for vacuum advances, yes this is the right way to do it, but the vacuum advance HAS TO HAVE A STOP MADE SO IT DOESN'T DELIVER TOO MANY DEGREES OF TIMING.

    Once again, here's how to do it. Make a stop out of thin steel or aluminum, .035/.040 thick, about .400 inch wide and 1.00 inch long. Put two through holes in it on one end, .140 each. place the plate on the bar that mounts in the distributor, between the pin that pulls the points plate or pickup coil and the diaphragm end of the unit, and at a distance from the pin of .085 inch. mark the centers of both through holes, drill and tap the vacuum advance mounting bar with 6/32 thread, mount the stop plate to the just tapped bar, make sure the screws are trimmed off on their threaded ends, so they don't stop the pull pin from free movement on then underside of the mounting bar, and mount the vacuum advance back on the distributor.

    .085 inch pin travel will give 8 crankshaft degrees of timing from the vacuum advance, .095 will give 10 crankshaft degrees. No matter what the initial idle timing is, you want between 18 and 20 running idle degrees of timing. 12 initial and 8 vacuum degrees gives 20, 10 and 10 give 20, etc. Adjust the stop plate to give the right number of degrees by filing the through holes into being slots.

    Time the initial idle engine timing as normal, vacuum advance disconnected, usually around 10 to 12 degrees at a low idle speed, then connect the vacuum advance to the full intake manifold vacuum sourcing. The engine running, out-of-gear timing should go up by 8 crankshaft degrees over the initial idle timing, this is the right way it should be.

    Anybody have a parrot they will give me, so I can teach this to him, so he can repeat this for all those who just don't get ported vacuum advance sourcing doesn't help engine performance and/or idle quality at all.

    I just got done with a Ford 429 guy on the phone, who just couldn't grasp why ported vacuum advance and 55 degrees timing on acceleration would cause pinging. he relicgtantly built a stop, and connected the sourcing to full intake manifold vacuum, and was just flabbergasted to find the engine now accelerates correctly and doesn't ping anymore. He is now at the same 10 initial degrees, 24 mechanical degrees he was before, but now isn't adding the ported 21 degrees he was before, he now adds 8 crank degrees at idle, no and light load cruise rpm operation, and it works. It also works on all other makes and sizes of engines without EGR systems, not just the Ford I outlined here.


    [/ QUOTE ]

    He uses that same recipe for EVERYTHING!
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    He's a dumba$$. Can I say that? Good : )

    I can't stand it when people take their theory and act like its law or something.

    The vacuum port on a Q-jet IS TIMED. Dammit, if he'd look in the primary venturi wall on that side (or the other, EGR is timed too) you can SEE that the opening is ABOVE the throttle blades at idle, and even a bit off idle as well.

    Vacuum is NOTHING at even about 1/4 throttle and trying to accelerate. I bet Grim will say the same thing, you start climbing a hill, and crack the throttle 1/2", and vacuum will drop by 10" at LEAST. I can pull something like 22" at cruise, but as soon as I start climbing a hill, and try to maintain the same speed, vacuum drops INSTANTLY...and I'm talking 7-9" at 35mph, and VERY light throttle.

    You can completely eliminate the advance mechanism if you want to run your engine at one speed, but I've only seen circle track guys try that. (it seems to work ok)

    If ported vac advance was so evil, GM wouldn't have done it on almost every single carbed motor from the 60's (or earlier) until CCC. (1981 for most cars) That is ALL you need right there. If it didn't work, GM would not have CONTINUED to use it for that period of time.

    If he can say exactly how much advance ported vac pulls in at 35MPH and 9" of vacuum, I'll be real surprised, and so will he, because its not much.

    What happens if you run too much timing??? Go ahead, advance the distributor 15* at idle and try to climb a hill. Exactly, it pings like crazy, and would do so with vac advance *if vac advance didn't DROP as the throttle is opened* But then, you'd have to have vac advance run off a source other than engine vacuum, and thats not the case now is it?

    This guy probably tested this with his vehicle in park, when he couldn't drop the vacuum down below 20" if he tried. Probably thinks the vac secondaries OPEN by vacuum too, and that they never work correctly when he "tests" them in park. : )

    Don't waste much time with him, because he's wrong, an you'll probably never get him to admit it, even when he finally understands it.
     

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