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Manifold vacuum or ported vacuum to the distributor..help??



Ported or manifold vacuum from carb to distributor?? whats the scoop, anybody know? Im getting mixed messages about this,... anyone?
Thanks in advance, (pun intended :L )

The vacuum connection to the distributor is connected to the timed port on the carb for emission controlled engines, and to the manifold port on the carb for non-emission controlled engines.
manifold vacuum or ported vacuum

You got me thinking on this one.. i just ran out to my vette to see how i had mine set. According to specs in the manual that came with my crate motor it said to set the vacuum hose to the ported side. But I think that it's a matter of which variation reponds the best on your engine. Try setting your initial timing, then plug into manifold vacuum- adjust your idle. Run the car for a couple miles. Then try the same procedure when you plug into ported vacuum. See which performs better. If someone else out there has a sure fire way of figuring this one I'd like to even know. Hope this helps you out Tom.

Darryl aka Sarge :w
sounds like a good plan..thanks Sarge
Quote by :70zz3
Being an original owner of a 70 my car came from the factory with TCS. First modification I did to the car was bypass the the TCS and use full time vacuum to the vacuum advance. It is still that way today. My 68 also runs full time vacuum.Have used Holley double pumpers with mechanical secondaries on both since 1973.
Lars Grimsrud: "For stock and near-stock street-driven cars, I normally recommend running ported vacuum to the distributor. This tends to produce more stable idle characteristics (since vacuum advance will slam right back to "zero" every time the throttle is closed), and most engines will produce better off-idle throttle response in this configuration.
On cars with radical cams and manual transmissions, direct manifold vacuum can be used with success if a lot of timing advance is needed at idle due to the cam profile. This is normally not the case on stock or mildly modified cars.

The ported source on a Q-Jet can be either the driver's side, forward port located in the middle of the fuel bowl, or it can be the port in the lower throttle plate, passenger side, sticking out the front at a 45-degree angle. You just have to hold a finger over the ports at idle to see which one is not producing much, if any, vacuum at idle."

Quote by Steve G: You want ported vacuum that is zero at idle then comes on when the throttle blades are cracked open off idle.
On my Q-Jet, GM hooked the distributor up to a full time source. Lars set me straight and hooked it to the correct spot. On my Q-Jet, that is front, passenger side of the carb.

Steve G.


Quote by Duke: Ported manifold vacuum advance is an emission strategy. By eliminated vacuum advance, EGT is increased, which lowers emissions by enabling oxidation of HC and CO in the exhaust system. The higher EGT will also cause more load of the cooling system, which will make the car tend to run hot at idle and low speed.
All pre-emission cars ran full manifold vacuum advance except for those few engines that did not have it. Ported vacuum, and TCS are both emission control devices, and these systems were typically accompanied by slow, "lazy" centrifugal advance and more vacuum advance than pre-emission cars, so just disabling TCS and rerouting the vacuum can signal line to full manifold vacuum is not necessarily the way to go. If you don't have to be emission tested your engine will run stronger, cooler, and get better fuel mileage if you install a pre-emission centrifugal and vacuum advance system.


quote by Ken: My vaccum advance was hooked up to full manifold vacuum for a long time. The car ran ok but the gas mileage was way down. The timing was at 12deg (according to spec w/o vacuum advance hooked up) and I started to play with the timing to get all centrifugal advance in by 3000rpm, and ended up with about 16deg at idle. Sent the Qjet to Lars to find a ported vacuum source and he fixed a port so it was. Put the carb back on and connected the vacuum advance to the new ported source. Still had 36deg at 3000rpm (no vacuum can) and about 50-51 deg at 4500 rpm (with vacuum can).
One thing I did notice was that my gas mileage improved for highway driving. Up about 5-6 miles/gal. I did not see that much of a change around town with the vacuum connected to a ported source, but then I did not have it on the road for very long last year. I do think that having all the advance come in early was a little harder to manage in stop-and-go traffic. So I'm convinced that for all around driving comfort and performance I like the advance connected to the ported source rather than full manifold source.

Now I'm getting ready to swap out the Qjet for a Holley (and other changes as well) so we will see if my assumptions still hold true when it's on the road again.


Ken - '75 L82/M21/FE7 & modified

quote by Mororhead: This timng and vacumm really has me thinking, here is what I have done --> I have run my car with no vacuum advance and intial timing
of 21 degrees BTDC, it runs fine. I have connected the vacuum advance to ported vacuum and it runs fine. I have connected the vacuum
advance to manifold vacuum and it runs fine. I have changed the intial timing to 10 degrees BTDC and conected to both vacuum ports and it runs fine.
It is really amazing the you can make all these adjustments, when theoretically they should have a dramatic affect on performance etc.
and you can hardly notice any difference

PS I have a timing light and all new parts ( distributor, Crane adjustable can, carb ) and I can see the different timing curves I am getting.

[Modified by MotorHead, 11:32 AM 10/25/2001]
Owww...my freakin' head!

I'm getting mixed messages about this
Now, I am too! Thank God that I won't be pulling off my distributor and intake for awhile. Maybe by then, we can agree which method is better, if any. Too bad Smokey Yunick is no longer around.--Bullitt

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