Hot Rod Forum : Hotrodders Bulletin Board

Hot Rod Forum : Hotrodders Bulletin Board (
-   Engine (
-   -   Port or Full time vacuum (

cool rockin daddy 09-11-2003 10:08 AM

Port or Full time vacuum
Hey fellas. Was hoping you could clear up some confusion around here. Have a debate as to which vacuum source to hook up an HEI distributor to: ported or full time vacuum. I always believed it was ported but a couple of friends say it should be full time. Car is street driven mild small block. Thanks 09-11-2003 10:11 AM

As I understand, the port vacuum is just for some smog Ford engines. 99% of the time you should use the manifold vacuum.

Jmark 09-11-2003 10:32 AM

LOL, well, thats kinda like trying to talk politics with 10 different guys. Once you get past having an all stock engine, timing and how to get timing at different rpms is a matter of what works.

As a rule of thumb, vacuum advances use ported vacuum. Thats the port that is above the throttle plate. It only comes on when the throttle plate is opened.

BUT,,, a few car makers also used maifold vacuum at idle. Its best to ck your manual for the original hose routing for your specific make and model.

Having said that, advance vacuum can also be hooked to a "manifold" vacuum source. That will give you more advance at idle, often helping the idle quality and aid in a little higher idle speed and enhanced cooling at idle.

Ford and Chevy used to use a thermal vacuum switch for changing the vacuum source going to the dist. At cold and normal eng. water temps, the dist. got ported vacuum. At very high water temps, the dist. got manifold vacuum to speed up the engine at idle to try to keep cooling efeciency better. I always remembered how to hook them up by remembering the word "C.A.M." . Top port went to the "carb" vacuum, middle port went to the "advance", buttom port went to "manifold" vacumm.

From what I've read, the best place to start is un-hook the vac advance and set the TOTAL combined initial and machanical advance to 34 degrees at around 2500rpm. Then hook up the vacuum advance to ported vacuum and drive it and ck for pinging and performance. You can buy an adjustable vacuum advance for most apps. and make small changes in the amount of vacuum advance you get under acceleration.

Personally, for my engine, I find that in the winter times, I run my inital advance 4 degrees above what the engine decal states. ( its cooler and doesn't ping.), but in the summer I have to drop it down to the stated setting or a couple degrees lower, depending on how crappy the gas is that year! LOL

With all that said..............LOL...........There will probably be 10 more posts made telling me I'm wrong, but I expect that too, as everyone has had positive or negative experiences with different ways to do it. Once you get past stock, its whatever works. But you need to start someplace.

Mark 09-11-2003 11:05 AM

I admit, I don't work an anything with TPI or more than a PCV pollution control so my comment is couched it that old tech. I don't remember seeing the ported vacuum port showing up on carbs until recent years. It just makes more sense to my feeble mind to be sending the distributor advancing info based on manifold vacuum (engine load info, regardless of speed) rather than speed based info from the venturi port. The distributor already has speed info from the centrifugal advance. That is for old tech engines made B4 1970. Since then I know the engineers have come up with stuff I don't understand, thus the ported vacuum carbs.

troy-curt 09-11-2003 11:18 AM

Nearly all chevys. got there vacuum above the throttle plates, as far back as the mid 60s.

After fuel injection I don`t know much.

65 yesterday,my boys say i`ve forgot 1/2 of what I know????????:(

Troy 09-11-2003 11:31 AM

Then I have been setting up my Chevies all wrong! Maybe I get away with it 'cause I don't use the stock electronic distributors.

Indian 09-11-2003 11:32 AM

I saw that yesterday, hope you had a good one!!!!!:thumbup:

Rodder 09-11-2003 11:53 AM

I am going to agree with the ported vacuum on the early model chevys but I have been known to use manifold vacuum as well.

Like he said whatever works.

Ohio-Dan 09-11-2003 12:28 PM

I vote for manifold vacuum
This is an article I found and saved. It's a little long, but has alot of good info on ignition timing and what is best for your application. Dan

Distributor Vacuum Advance Control units
Specs and facts for GM Distributors

by Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO Rev. B 8-19-02

I’ve been seeing a lot of discussion and questions regarding distributor vacuum advance control units; what do they do, which ones are best, what was used on what, etc., etc. To clarify some of this, I thought I’d summarize a few facts and definitions, and provide a complete part number and specification listing for all vacuum advance control units used by Chevrolet on the points-style distributors. I’m also providing a listing of the specs for all other GM (non-Chevrolet) control units, but without the specific application listed for each (it would take me a bit too much time to research each part number by application across each of the GM Motor Divisions – it took me long enough to compile just the Chevy stuff…!). This latest revision to this paper also includes the HEI listings (the HEI distributors use a longer control unit, so the non-HEI and HEI vacuum advance control units CANNOT be interchanged).

As always, I’m going to include the disclaimer that many of these are my own comments and opinions based on my personal tuning experience. Others may have differing opinions & tuning techniques from those presented here. I have made every attempt to present factual, technically accurate data wherever possible. If you find factual errors in this information, please let me know so I can correct it.

The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response. Once the engine load increases, this “over-advance” condition must be eliminated to produce peak power and to eliminate the possibility of detonation (“engine knock”). A control unit that responds to engine vacuum performs this job remarkably well.

Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.

The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and the distributor’s mechanical advance curve. The following considerations must be made when selecting a vacuum advance spec:

How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection.

How much centrifugal advance (“total timing”) is in effect at cruise rpm? If the distributor has very stiff centrifugal advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit. If the engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance unit can only be allowed to pull in 16 more degrees of advance.

Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times.

Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

Thus, we see that there are many variables in the selection of an appropriate control unit. Yet, we should keep in mind that the control unit is somewhat of a “finesse” or “final tuning” aid to obtain a final, refined state of tune; we use it to just “tweak” the car a little bit to provide that last little bit of optimization for drivability and economy. The vacuum advance unit is not used for primary tuning, nor does it have an effect on power or performance at wide open throttle.

With these general (and a little bit vague, I know…) concepts in mind, let’s review a few concepts and terms. Then it’s on to the master listing of specs and parts…..:

Part Number
There are many different sources for these control units. Borg Warner, Echlin, Wells, and others all sell them in their own boxes and with their own part numbers. Actually, there are very few manufacturers of the actual units: Dana Engine Controls in Connecticut manufactures the units for all three of the brands just mentioned, so it doesn’t make much difference who you buy from: They’re made by the same manufacturer. The part numbers I have listed here are the NAPA/Echlin part numbers, simply because they are available in any part of the country.

Every vacuum advance control unit built by Dana, and sold under virtually any brand name (including GM), has a stamped ID number right on top of the mounting plate extension. This ID, cross referenced below, will give you all specifications for the unit. So now, when you’re shopping in a junkyard, you’ll be able to quickly identify the “good” vs. the “bad” control units.

Starts @ “Hg
Vacuum is measured in “inches of Mercury.” Mercury has the chemical symbol “Hg.” Thus, manifold vacuum is measured and referred to as “Hg. The “Start” spec for the control unit is a range of the minimum vacuum required to get the control unit to just barely start moving. When selecting this specification, consideration should be made to the amount of vacuum that a given engine produces, and what the load is on the engine at this specification. For example, an engine with a very radical cam may be under very light load at 7 inches Hg, and can tolerate a little vacuum advance at this load level. Your mom’s Caprice, on the other hand, has such a mild cam that you don’t want the vacuum to start coming in until 9 – 10 inches Hg. For most street driven vehicle performance applications, starting the vacuum advance at about 8” Hg produces good results.

Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking about these control units, it is important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet, will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees (which is what you “see” with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit.

Max Advance @ “Hg
This is the range of manifold vacuum at which the maximum vacuum advance is pegged out. In selecting this specification, you must consider the vacuum produced at cruise speed and light throttle application. If your engine never produces 20” Hg, you better not select a control unit requiring 21” Hg to work.

The following listing (Non-HEI) is as follows: The first two part number listings are the two numbers that are most commonly used in a Chevrolet performance application. The “B1” can is the most versatile and user-friendly unit for a good performance street engine. As you can see, it was selected by GM for use in most high performance engines due to its ideal specs. The “B28” can was used on fuel injected engines and a few select engines that produced very poor vacuum at idle. The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. The Chevrolet applications are listed first by application, followed by a complete listing of all of the units used on any GM product (all GM units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do).

Non-HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC680 B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18
1964 Corvette exc. FI
1964 Impala, Chevy II
1965 396 High Perf.
1965-67 283, 409
1966-68 327 exc. Powerglide
1967-68 All 396
1969 Corvette 427 High Perf.
1969 396 Exc. High Perf.
1969 Corvette 350 TI
1969-70 302 Camaro
1970 400 4-bbl
1970 396 High Perf.
1970 Corvette 350 High Perf.
1973-74 454 Exc. HEI

VC1810 B28 1965 409 High Perf. 3-5 8 @ 5.75-8
1965 327 High Perf.
1966 327 High Perf.
1964-67 Corvette High Perf. FI


VC1605 B9 1965 impala 396 Exc. High Perf. 7-9 10.3 @ 16-18
1965 327 All Exc. FI
1969 327 Camaro, Chevelle, Impala
1969-70 Corvette 350 Exc. High Perf.
1969-70 350 4-bbl Premium Fuel
1970 350 Camaro, Chevelle, Impala High Perf.
1971-72 350 2-bbl AT
1971-72 307 All

VC1675 B13 1968 327 Camaro Powerglide 9-11 8 @ 16-18
1968 327 Impala AT
1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy II
1970 350 Camaro, Chevelle Exc. High Perf.

VC1760 B19 1969 350 Camaro, Chevelle, Impala 4-bbl 5.5-8 12 @ 14-18
1969-70 350 2-bbl

VC1765 B20 1965 396 Impala High Perf 5-7 8 @ 11-13
1966-67 Corvette Exc. High Perf.
1966-67 Impala 427 Exc. High Perf.
1966-68 327 Powerglide Exc. High Perf.
1969 307 All
1969-70 396, 427 Camaro, Chevelle High Perf.
1970 400 2-bbl
1970 307 MT
1973 Camaro 350 High Perf.

VC1801 B21 1971 350 2-bbl 7-9 10 @ 16-18
1971-72 400, 402
1971-72 307 AT

VC1802 B22 1971-72 350 4-bbl 7-9 8 @ 14-16

Other Part Numbers & Specs:

VC700 B3 8-10 11.5 @ 19-21
VC1415 M1 6-8 10 @ 13-15
VC1420 M2 5-7 11 @ 16-17
VC1650 B12 8-10 10 @ 15-17
VC1725 B18 8-10 12 @ 13-16
VC1740 A5 6-8 12 @ 15-17.5
VC1755 A7 8-10 12.5 @ 18-20.5
VC1804 B24 6.5-8.5 10 @ 12-14
VC1805 M13 6-8 12 @ 14.5-15.5
VC1807 B25 5-7 8 @ 13-15
VC1808 B26 5-7 8 @ 11-13
VC1809 B27 5-7 9 @ 10-12
VC1812 B30 5-7 12 @ 11.75-14

The following listing (HEI) is as follows: The first four part number listings are the 4 numbers that are most commonly used in a Chevrolet performance application. The “AR12” can is the most versatile and user-friendly unit for a good performance street engine. The AR 15 and AR23 are almost identical, with only slight variations in their “start-stop” specs. The “AR31” can is the HEI equivalent to the “B28” Hi-Perf can used on the early engines: The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. All GM HEI vacuum advance units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do.

HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC1838 AR12 1975 350 Buick 7-9 7 @ 10-12

VC1843 AR15 1977 305 All Exc. Hi Alt, Exc, Calif. 3-5 7.5 @ 9-11
1974 400 All w/2-bbl
1977 305 El Camino
1976 262 Monza Exc. Calif
1976 350 Vette Hi Perf, Incl. Calif
1975 350 Z-28
1977 305 Buick Skylark

VC1853 AR23 1976 350 All Calif. 5-7 7.5 @ 11-12.5
1976 350 Vette Calif., Exc. Hi Perf
1976 400 All, Exc. Calif
1975 350 4-bbl
1974 350 All w/1112528 Distr.
1978 350/400 Heavy Duty Truck, Exc. Calif, Exc. Hi Alt.

VC1862 AR31 2-4 8 @ 6-8 09-11-2003 01:00 PM

That article is very informative but has left me still confused. He talks of vacuum pots taking their signals from manifold (below throttle plates) vacuum and wanting a pot that is all in when manifold vacuum is high (idle, partial throttle cruising, etc.) All of this is in line with what I have always understood and how I have always set up my engines. Where does the use of ported vacuum (above throttle plates) come in then? Since it is desirable to have max vacuum advance @ idle and part throttle cruising, it appears that ported vacuum can perform properly in neither case. Rather, it would pull maximum advance @ WOT, higher speed, and accelerating condition which is exactly when you DON'T want extra advance over initial and centrifugal advance.:confused: :confused:

JUSTINH 09-11-2003 01:19 PM

On my 65 Buick 401. I tried full vacuum but got a rough idle. I went to ported idle and it smoothed it out.

So my advice is to just try the 2 possibilities. Since you don't use it for initial timing, all you do is move the vacuum hose. 09-11-2003 01:52 PM


Originally posted by JUSTINH
On my 65 Buick 401. I tried full vacuum but got a rough idle. I went to ported idle and it smoothed it out.

So my advice is to just try the 2 possibilities. Since you don't use it for initial timing, all you do is move the vacuum hose.

Does your vacuum pot advance or retard the distributor as vacuum increases?

JUSTINH 09-11-2003 02:41 PM

Advances the spark

KeithB123 09-11-2003 03:53 PM

My edelbrock Manual says use the port side and if you have a lumpy cam to use the full or manifold side. That sums it up.

I got a decent cam and I find the full is MUCH better.

Ohio-Dan 09-11-2003 04:01 PM

Early on in the emissions game, it was found that if you delay the spark, some of the fuel would then burn in the exhaust system and reduce the bad things Big Brother was looking for. This was at a loss of fuel mileage and power mind you. It also caused the engines to run hotter. If you have problems running manifold vacuum, you may have an "Emissions" distributor. In the case of the 65' Buick, Was it a California car? 1971 was the first year for an across the board emission's reduction for all 50 states. No dought, the factories where working on the problems, trying to prevent Big Brother from stepping in quite a bit earlier in the game. California was always first in line for that sort of thing anyways. I know the smog pumps and such showed up several years earlier in California than the rest of the states.
Most "Smog" distributors that I have seen don't have much mechanical advance at all. (A distributor with worn, rusted, or seized weight would act this way as well). A high number would be 20* at the crank. Then they have real stiff springs that need a whole bunch of RPM before you get full advance. If you have the initial set at 8*, you may only see 18* @ 2500 RPM, 23* @ 3500, and 28* @ 4500. Not near enough for any combo I can think of. In this case the ported vacuum would help your performance by bringing in more timing. Thing is, you crutched the problem and didn't fix it.
You will also see guys and gals who advance the initial timing to the point that it is hard to start. And the motor still wants more. Lighter springs are then added to allow the mechanical advance to come in quicker. You then run into the stumble, surge, and miss problem from having way to much timing advance. The motor runs much better at low RPMs with the added timing, but at higher RPMs, the timing is still being piled on by the vacuum can.

You read posts of timing woes quite often. You also read of those who feel Ported vacuum is the best. In my eyes, manifold vacuum is the only way to go. More power, better fuel mileage, and cooler running..........Dan

All times are GMT -6. The time now is 06:48 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999 - 2012. All Rights Reserved.