Manifold or Ported vacuum source??? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

70GMC396 · 03-05-2007, 03:04 PM

What source do most of you run for your vacuum advance on for Late 60s - early 70s big blocks engines? I have heard both ways - old school saying manifold vacuum on the bottom of the carb and others ported only, doesn't seem to be a lot of opinions in between. Can there be a right or wrong answer here? I have read dozens of threads on the subject but what SHOULD we be running on stock carb/engine setups since we usually had TCS controlling that for us and now TCS is usually non-functional? Is ported more reasonable for a smoother, longer application of advance? Seems like the old muscle car theory of carb manifold was just to smooth out idle/heat during the lower rpms for the streetability and not worrying about vacuum disappearing after that since it wasn't important on a performance set up?
Thx

dmec · 03-05-2007, 03:33 PM

i prefer manifold vacuum. seem like ported vacuum causes pinging under load.

Joeypilot · 03-05-2007, 03:47 PM

Ported vacuum is how the vacuum advance is designed to work. If you go to manifold vacuum, you will have maximum vacuum advance all the time. If you are having trouble with using ported vacuum; something is wrong. Such has the throttle plates in the carb not closed far enough.

67ChevyRedneck · 03-05-2007, 04:01 PM

I'm using the ported vacuum for mine. Truck runs great. I've had it hooked up for over 3 years.

**WorkinLonghorn** · 03-05-2007, 04:10 PM

I think some guys with radical cams need the advance at idle so they hook it up mani vac. Then when you give it the gas the vacuum declines and the vacuum advance gives way to the centrifical system.This would not be correct for a stock or mild cam and your mileage would really suk.

Skip Greaney · 03-05-2007, 06:00 PM

Ported is how it came from the factory...but the advances stick in the distributors or actuator goes bad...that is what starts the problems and alot of things get covered up by goint to manifold vacuum...why would you want full advance at idle unless of course your drag racing??!!!

Skip

ERASER5 · 03-05-2007, 06:28 PM

Well, I for one like to use the manifold vac. All the parts on my dizzy are either new or cleaned and checked. My vac advance is adjustable. At an idle of about 750 to 800 in park, I am able to back my idle screw off nearly a full turn using full vac as opposed to ported vac. I am running about 10° advanced with no vac advance and around 22° with full vac. At idle, my mechanical advance is not engaged, so I am not at "full" advance at idle. If yours is, then you have some dizzy issues to deal with.

The argument for full vac and ported vac will never die. Set yours up both ways and choose the one that runs the best. A nicely tuned motor will have better emissions than any poorly tuned motor no matter how many emmission systems you use.

70GMC396 · 03-05-2007, 09:11 PM

Thanks guys! Great info.
Also...why does the TCS feed the vacuum advance from both manifold vacuum and ported carb sources??? Anyone have a schematic or taken one apart to see when it starts pulling that lower carb line? I seem to remember British cars using both types of carb vacuums simulatneously to advance or retard the engine.

03-05-2007, 03:04 PM	#1
70GMC396 Registered User Join Date: Apr 2006 Location: 93420 Posts: 98	Manifold or Ported vacuum source??? What source do most of you run for your vacuum advance on for Late 60s - early 70s big blocks engines? I have heard both ways - old school saying manifold vacuum on the bottom of the carb and others ported only, doesn't seem to be a lot of opinions in between. Can there be a right or wrong answer here? I have read dozens of threads on the subject but what SHOULD we be running on stock carb/engine setups since we usually had TCS controlling that for us and now TCS is usually non-functional? Is ported more reasonable for a smoother, longer application of advance? Seems like the old muscle car theory of carb manifold was just to smooth out idle/heat during the lower rpms for the streetability and not worrying about vacuum disappearing after that since it wasn't important on a performance set up? Thx

03-05-2007, 03:33 PM	#2
dmec Senior Member Join Date: Jun 2000 Location: yucaipa ca Posts: 580	Re: Manifold or Ported vacuum source??? i prefer manifold vacuum. seem like ported vacuum causes pinging under load.

03-05-2007, 03:47 PM	#3
Joeypilot Registered User Join Date: Feb 2007 Location: Everett, WA Posts: 8	Re: Manifold or Ported vacuum source??? Ported vacuum is how the vacuum advance is designed to work. If you go to manifold vacuum, you will have maximum vacuum advance all the time. If you are having trouble with using ported vacuum; something is wrong. Such has the throttle plates in the carb not closed far enough. __________________ Joey '69 K10 396 '72 K10 454 2001 K10 Silverado

03-05-2007, 04:01 PM	#4
67ChevyRedneck Hittin E-Z Street on Mud Tires Join Date: Nov 2004 Location: Greenville, SC Posts: 23,090	Re: Manifold or Ported vacuum source??? I'm using the ported vacuum for mine. Truck runs great. I've had it hooked up for over 3 years. __________________ Jesse James 1967 C10 SWB Stepside: 350/700R4/3.73 1965 Ford Mustang: 289/T5-5spd/3.25 Trac-Loc 1968 Pontiac Firebird: Project Fire Chicken! 2015 Silverado Double Cab 5.3L Z71 2001 Jeep Wrangler Sport 4.0L 5spd 2020 Chevrolet Equinox Premium 2.0L Turbo 2011 Mustang V6 ~ Wife's ride American Born, Country by the Grace of God 1967 CST Shop Truck Rebuild! My 1967 C-10 Build Thread My Vintage Air A/C Install Project "On a Dime" Trying my hand at Home Renovation! 1965 Mustang Modifications!

03-05-2007, 04:10 PM	#5
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,861	Re: Manifold or Ported vacuum source??? I think some guys with radical cams need the advance at idle so they hook it up mani vac. Then when you give it the gas the vacuum declines and the vacuum advance gives way to the centrifical system.This would not be correct for a stock or mild cam and your mileage would really suk. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, 2nd owner, Work-Truck supreme. Best $300 I ever spent.

03-05-2007, 06:00 PM	#6
Skip Greaney Junior Member Join Date: Jan 2007 Location: Land O' Lakes Florida Posts: 103	Re: Manifold or Ported vacuum source??? Ported is how it came from the factory...but the advances stick in the distributors or actuator goes bad...that is what starts the problems and alot of things get covered up by goint to manifold vacuum...why would you want full advance at idle unless of course your drag racing??!!! Skip

03-05-2007, 06:28 PM	#7
ERASER5 Registered User Join Date: Jun 2006 Location: Indiana Posts: 4,859	Re: Manifold or Ported vacuum source??? Well, I for one like to use the manifold vac. All the parts on my dizzy are either new or cleaned and checked. My vac advance is adjustable. At an idle of about 750 to 800 in park, I am able to back my idle screw off nearly a full turn using full vac as opposed to ported vac. I am running about 10° advanced with no vac advance and around 22° with full vac. At idle, my mechanical advance is not engaged, so I am not at "full" advance at idle. If yours is, then you have some dizzy issues to deal with. The argument for full vac and ported vac will never die. Set yours up both ways and choose the one that runs the best. A nicely tuned motor will have better emissions than any poorly tuned motor no matter how many emmission systems you use. __________________ '70 GMC C1500 LWB Power disc brakes. WooHoo! Posi 6 Lug Dana 60

03-05-2007, 09:11 PM	#8
70GMC396 Registered User Join Date: Apr 2006 Location: 93420 Posts: 98	Re: Manifold or Ported vacuum source??? Thanks guys! Great info. Also...why does the TCS feed the vacuum advance from both manifold vacuum and ported carb sources??? Anyone have a schematic or taken one apart to see when it starts pulling that lower carb line? I seem to remember British cars using both types of carb vacuums simulatneously to advance or retard the engine.



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