Ported vs.Non-ported Vacuum Advance - Help the confused - Team Camaro Tech
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post #1 of 7 (permalink) Old Sep 23rd, 00, 09:49 AM Thread Starter
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Join Date: Feb 1999
Location: Vermilion, Ohio
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Well if ignorance is bliss than I must be in heaven on this. I have been reading different threads both here and out on Chevelles.com about this subject. There is so much knowledge out there and I applaud you all for it ...especially I-man but could someone explain ported vs. non-ported vacuum. My guess is that ported is direct of the manifold and non-ported would be off my carb.

I have a situation on a stock L-34 402 engine that is very similar to the posting over on Engines and Drive Train. I found this out on a car when I went to do a tune up 6 months after I bought the car.

*Vacuum line is plugged all the time. Why the previous owner did this I do not know. It was a surprise to me
*At about 900-1000RPM idle my timing mark is at about 12:00 on damper and when I dial the mark back on my timing light from zero I get about 35-36 degrees of timing which I know can't be good
*At about 2000 RPM it climbs to about 40-41
*I did not increase RPM yet to see where it stopped but my guess would be around 50 like the other posting

I want to get it back to stock preferred settings which I suspect should be

* 8-12 BTDC with vacuum disconnected (like it is now
* About 20 with vacuum connected to "ported" vacuum
* and about 36-38 total at increased RPM

On this motor I have a fitting on the back of the stock cast iron manifold with 3 tubes (2 are currently capped). Should I take the permanent plug out of the distributor vacuum line and connect the line to one of those tubes on the back of the manifold and run like this all the time as a ported vacuum set-up???? The factory used the line off the carb I believe asis how it is currently hooked up even though I am getting no vacuum because of the plug in the line. Is this just an improvement on performance from the factory???

Robert Stacho

67 SS 396 Chevelle 350HP 4speed
70 SS 396 Chevelle 350HP 4speed
68 RS Z/28 302 350+HP? 4speed
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post #2 of 7 (permalink) Old Sep 23rd, 00, 02:19 PM
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"Ported" vacuum draws through an orifice in the venturi which is ABOVE the throttle plate in the idle position, so there is NO vacuum at all at idle (emission thing); only supplies vacuum off-idle. Some direct manifold vacuum fittings on carbs don't have any vacuum at all due to using the wrong carb base gasket which closes off the path from the manifold plenum to the vacuum cavity on the bottom of the throttle body - fixed one of those yesterday for a friend who couldn't figure out why he had no vacuum advance.

Connect your distributor vacuum advance canister line to the manifold vacuum fitting you mentioned behind the carburetor - this will give you full-time vacuum advance (plug or cap the "ported" vacuum fitting on the carb). You can use the other available port on the manifold vacuum fitting for your vacuum gauge when you adjust your idle mixture screws (highest steady vacuum reading).

'69 Z28 Fathom Green
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post #3 of 7 (permalink) Old Sep 23rd, 00, 02:26 PM
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If you have read all the threads you know this can be a controverial subject...and I'm no expert but since you ask I will throw in something I discovered on mine....

I bought my 383 from American Speed with a carb and distributor. The carb is a Holley 4150. This engine was run and tuned on a dyno and punches out 435 HP and almost 500 ft lbs of torque.

When I looked at the carb paperwork it describes a "timed vacuum port" on the passenger side of the forward bowl toward the top that they say to hook the distributor to.
There is also a "manifold vaccum port" on the passenger side, on the front, at the bottom.

American Speed hooked my distributor, an MSD billet, up to the manifold vacuum port. So I would say they are a proponent of manifold vacuum.

Mark M.
Red 67 Convertible
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post #4 of 7 (permalink) Old Sep 23rd, 00, 03:06 PM
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Iíll give this a shot because Imanís idea sounds good to me and I will try it on my car.
You have two advance systems built into the dist. (vacuum and mechanical) that operate independently of each other. The vacuum canister rotates the plate that the points are screwed down to by using manifold vacuum. The vac canister has two variables 1) the amount of its travel (rotation) and 2) the amount of vacuum HG it takes to make it rotate the points plate and thus advance the timing.
Mechanical advance uses weights and springs (centrifugal force) to advance and retard the timing and it also has two variables 1) the amount of rotation travel (in deg.) and 2) at what rpm it reaches total advance (controlled by spring tension)
One more thing has to be considered - the vacuum source (touchie subject)
Ported vacuum is taken above the throttle valve so no vacuum at idle
Non-ported has vacuum at idle.
Some say ported vacuum is used to improve the emissions by causing higher exhaust gas temp with a retarded spark at idle so some carb books tell you to use it - some engines may run better depending on the way the vacuum and mech advance is set up. Iman says use nonported.
Now the fun part; you get to play with the 6 variables woohoo , 1) ported vacuum, 2) non ported vacuum, 3) amount of vacuum (HG) needed to operate the vac canister,
4) amount of travel of the canister in deg.,
5) amount of mach advance travel/rotation (deg) and 6) how soon or late your mach advance comes in (springs)
Now how do you make the adjustments?
A stock canister is fixed - for example a 1965 Corvette FI canister opens at 8-HG vacuum
Some stock cans open at 15 HG but you can buy an adjustable can with a setscrew in the vacuum nipple-sexy huh? This is needed if you have low vacuum with a big camshaft
To limit the travel of the canister - you put in a little adjustable plate (see Iman's pics) that adjusts (limits) the travel of the can . The rotation of the mechanical advance is changed by installing a brass bushing on a pin that rides in a slot or by brazing up the slot to shorten it. (Its under the rotor) How soon your advance comes on is adjusted by swapping the springs.
Now you need the numbers (tricky part) no one can tell you what is best for your engine, trans, rear gear, fuel, and driving conditions.
To get started ( your numbers will differ) forget vacuum for now . I think Iman recommends to set your mechanical travel rotation at 24 deg (fix the pin and slot gismo under rotor) put in the springs that brings in advance by 3500 rpm , set initial at 12 deg. This gives total mach advance of 36 deg. Now limit your vac canister travel (little plate in the pics) to 10 deg of advance at full non ported vacuum. Now with vacuum connected you have 22 at idle and 46 at 3500 rpm. The beauty of this Iman set up is the engine will adjust its own timing as its vacuum changes. Now re-read Imanís posts and adjust as needed.
Am I close on this explanation of your set-up Iman???

[This message has been edited by JOE58 (edited 09-23-2000).]

[This message has been edited by JOE58 (edited 10-11-2000).]
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post #5 of 7 (permalink) Old Sep 23rd, 00, 03:06 PM
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Most older Chevys used manifold vacuum on the advance, smogger engines used port or timed vacuum. For all around street use I like the port source, but if you have cooling problems or like a smoother idle (?) the lower connection might be best.
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post #6 of 7 (permalink) Old Sep 23rd, 00, 04:41 PM Thread Starter
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Joe, John, Mark, & Tom

Wow...thanks for all the informed input. The picture is a little clearer now and I wonder if some of what JohnZ said might be contributing to my situation. As the Chevelle (sorry guys this little rat is in my 70 Chevelle not my Z) sits right now I have no vacuum advance at idle because there is a plug in the hose from the carburator preventing this occurence. The motor is a stock 350HP 402 motor that a previous owner took off the Rochester in favor of a 780 Holley (List3310 dual feed - vacuum secondary model) In order to fit this carb on a spread bore stock cast iron intake an adaptor was needed. I wonder if my venturi is covered so no vaccum available. Plug was inserted and timing was set with what was available... mechanical advance only, although I don't understand why the manifold vacuum ports were not used that are available behind the carb. Maybe they thought this would be incorrect and to much vac or whatever. Who knows what went through the minds of previous owners.

What started my pursuit of a tune-up was this car pulls fine with a heavy foot on the throttle but when cruizing at constant speed it appears to have a slight hesitation...almost like a big gust of wind is constantly hitting the front end. I pulled the plugs and they all looked good...brownish tan burn on all of them and no carbon or oil present. Plugs are not fouling. Set the dwell at 030 with new points, cap, and rotor. Left timing where it was at after checking it and still have the hesitiation at cruize speeds. So this started me down this path of corectly setting the timing before I look at the carburator for the problem. Just curious though, with the vacuum line plugged and venturi maybe covered by the adaptor plate, where does the carb get the vacuum to open the secondaries when I put my foot down???

When I can get my buddy over here to give me a hand with all this, it looks like I will have my hands full with some things to try. When I get to work on Monday I am going to print all these threads out for reference

Thanks for your input

Robert Stacho (a.k.a. Mytmouse)

67 SS 396 Chevelle 350HP 4speed
70 SS 396 Chevelle 350HP 4speed
68 RS Z/28 302 350+HP? 4speed
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post #7 of 7 (permalink) Old Sep 23rd, 00, 08:54 PM
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The carb gets the vacuum to open the secondaries from an internal source. Take a picture or make a sketch of what you have now so you can put it all back if your experiments make it run worse, then try a few things. You can put a "T" in a vacuum line to get a non ported vacuum sourse. It sounds like you are getting a surge condition without the vacuum advance but if you hook it up you may have to limit your vacuum advance travel to about 10 deg. The carb adapter is not a good set up, so you may want to switch to a spread bore carb or change the intake to one designed for your Holley.
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