Vacuum advance - How does it work?

[dshanks]

Can someone please explain how vacuum advance works on my 71. Also, how does vacuum effect the carb?
What should I know before doing an engine swap? Im staying with a Q-jet and stock distributor with Pertronix unit.

Thanks

 

[pgtr]

Basically vacuum is supplied from a port on the base of the carb to the distributor. The diaphragm on the distributor 'advances' the ignition timing when vacuum (suction) is applied. You'll have vacuum applied normally at idle, cruise and deceleration but NOT at wide open throttle or other 'load' conditions. Essentially it's a 'dynamic' load compensator - when there is less or no 'load' it allows the ignition to advance further when the engine can use the extra advance.

Vacuum or 'suction' under the carb at the intake what allows a carb to function essentially.

Unfortuantely 71s really messed it up in stock original configuration: 71s have a convoluted piece of @#$@ 'CEC' set up to shut off vacuum advance in most cases to elevate combustion temperatures and kinda sorta improve emissions. It's that solenoid device on the front of the carb. The short answer is that vacuum advance on 71s doesn't work - at least not much of the time and that's by design. It only allows vacuum under certain circumstances in certain gears - beyond that I don't recall - have to look it up in the manual. They were 'trying' to elevate combustion temps in a lame attempt to improve emissions.

Here's 2 suggestions:

1) Simple and easy bypass of the 'CEC' on your QJet and send the vacuum signal straight to the distributor off the port from the carb.

2) Not quite as easy but when you have the time/opportunity - Recurve the stock 71 L48 distributor mechanical advance (I know you were asking about vacuum advance but...) to behave more like oh, a 69 or 70 L46 or 70 L48 distributor for example. The biggest problem with the 71 L48 distributor is the mech advance is low - only something like 24 degrees.

I've got a plot I did of various stock curves around here - if I can find it I'll upload. What stands out from that plotted set of curves is the 71's mech advance curve was anemic compared to other years especially pre 71s.

WIth the 71 CEC bypassed and providing vac straight to the distributor and a distributor updated with electronic ignition and a decent mechanical curve - you should end up with a nicely optimized set up (presuming the vac can is matched to the cam).

 

[Jack]

What should I know before doing an engine swap?Thanks

Good info from pgtr. I'll add this: Regardless stock motor or swap ... if it's for Street ... and has a Carburetor ... and does NOT have a computerized distributor ... virtually ALL will need a distributor that has both Vacuum & Mechanical (centrifugal) advances. In addition, C3 vettes have lots of other components that depend on vacuum (headlights, hvac controls etc). Those components and Distributor Vacuum Advance depends on motor's ability to produce vacuum. As camshaft duration increases, it decreases the motor's ability to make vacuum. There is no simple answer as to practical limits for cam duration. IMHO, I'd suggest that a cam w/duration more than 235* @ 0.050" lobe lift may not produce vacuum needed for street C3 w/ vacuum advance distributor. Keep the cam in mind as you consider motor swap. BTW, I bypassed/eliminated CEC on both 71 L48 I've had.
JACK:gap

 

[BarryK]

ported vacuum ports on the carb, which were done for emission purposes, negatively affect the performance of the motor.
Another option you can do is connect the vacuum advance can on the distributor directly to the intake manifold rather than the carb if your carb doesn't have a full vacuum port. Some ports on the carbs are full vacuum and some are "ported" vacuum and the "ported" vacuum ports on the carb are their for the emission aspects.
look towrds the rear of the intake behind the carb and you may have an empty fitting that is plugged. If you do it's easy to pull the plug, get a fitting for it, available at vendors such as Paragon, and than run the vacuum line directly from there to the vacuum can on the distributor.

 

[dshanks]

Man, what a wealth of knowledge this site is. My familiarity of vacuum systems was virually nil, except that I knew they powered my headlights and wiper door. Anyway, can some one also add what causes vacuum, what it does, and how/why its used to our advantage?

Also, the car is #s matching, so Im boxing all the dated parts now. That means the carb is being replaced with a remanufactured Q-jet (http://www.carburetor.ca/HTML/gallery/QJets/index.html), set up for my specs, which has an electric choke. So maybe bypassing this BS, uhem, I mean CEC may be easier now??? Sad to hear that the steeper cam may not push my vacuum system. Builder suggested a CompCams Hi Energy 274, so I ignorantly nodded my head like the noob I am. Maybe one of you guys know whether this cam will be detramental to my vacuum system.

By vacuum can, do you mean the thing thats attached to the distributor that resembles the headlight/wiperdoor actuator...that makes sense. Can this be changed to one that suits me better?

Thanks guys.

 

[pgtr]

Man, what a wealth of knowledge this site is. My familiarity of vacuum systems was virually nil, except that I knew they powered my headlights and wiper door. Anyway, can some one also add what causes vacuum, what it does, and how/why its used to our advantage?

I think about.com has a couple of pretty good intro articles on this...?

Also, the car is #s matching, so Im boxing all the dated parts now. That means the carb is being replaced with a remanufactured Q-jet (http://www.carburetor.ca/HTML/gallery/QJets/index.html), set up for my specs, which has an electric choke. So maybe bypassing this BS, uhem, I mean CEC may be easier now??? Sad to hear that the steeper cam may not push my vacuum system. Builder suggested a CompCams Hi Energy 274, so I ignorantly nodded my head like the noob I am. Maybe one of you guys know whether this cam will be detramental to my vacuum system.

Your carb should also be numbers matching too. If you pull that it's part of the 'numbers matching' equation. Having said that - if it's the original carb it would have that CEC thing off teh front of it - it's simple to bypass and leave it installed so it looks or appears to be correct/numbers matching if that's important.

If you just want to go with a new carb - I'd recommend a QJet which would be more or less correctly configured but with a different part# like you are considering - they are very good carbs IMHO and awesome for a street driven application.

One caveat re the carb you referenced - your 71 is NOT set up for an electric choke - it's got a spring mounted right on top of the intake. It can be converted easily enough. Also verify the linkage - I think 71s still have a ball stud for the throttle, not sure on the kick down for automatic. Another option might be: http://www.carburetor.ca/HTML/gallery/QJets/V247/V247.html or find another carb supplier that offers pre 1974 Rochester QJet 4MVs.

By vacuum can, do you mean the thing thats attached to the distributor that resembles the headlight/wiperdoor actuator...that makes sense. Can this be changed to one that suits me better?

Thanks guys.

Yes - it's a little round metal 'can' on the side of the distributor. It has a diaphragm inside and when vacuum (suction) is applied - it pulls an arm inside the distributor that advances (or retards) the ignition timing. Normally a stock vacuum canister is fine unless someone has changed to a more radical camshaft that produces considerably less vacuum at idle (sounds like you have a fairly standard cam).

A couple of invaluable tools to consider:

A vacuum guage, a factory shop manual and a 'generic' auto service text book or manual that covers a lot of general or basics. The Haynes shop manual is not as good as the factory manual by any stretch but it does introduce a number of basic items that are 'presumed' by the factory manual.

 

[JohnZ]

There should be at least one port on the base of your carb (besides the PCV port) that provides full manifold vacuum at idle, which is what you want for your vacuum advance; the stock arrangement only provides "ported" vacuum (no vacuum at idle), which was an emissions aberration. If you can't find a port on the carb that provides full manifold vacuum, you can get it from the fitting on top of the intake behind the carb.

The TCS system on your car shuts off vacuum to the distributor completely unless the transmission is in high gear (or is still cold or is overheating); most folks bypass it.

 

[Jack]

Sad to hear that the steeper cam may not push my vacuum system. Builder suggested a CompCams Hi Energy 274, so I ignorantly nodded my head like the noob I am. Maybe one of you guys know whether this cam will be detramental to my vacuum system.By vacuum can, do you mean the thing thats attached to the distributor that resembles the headlight/wiperdoor actuator...that makes sense. Can this be changed to one that suits me better?Thanks guys.

I don't know of CC "HiEnergy 274" ... I do see Xtreme Energy 274 ... it has 230* intake duration @ 0.050" lobe lift ... that's a little steep but will probably work OK w/ manual trans (no convertor). That cam needs a good bit of compression too ... my best guess is minimum 9.5:1 static CR. Better choice for 350 vette might be one step down to XE 268. There's been a lotta complaints about Comps cams failing prematurely ... I dunno if root cause is cam or install. If you have a chance to back up ... please consider Howard's cams ... no significant $ difference. http://www.howardscams.com/ maybe Howard's P/N 110041 .465"I/.490"E 224*/234* dur @ 0.050". jsteely@howardscams.com .
Yes you can get a different vac can for distributor ... there are articles archived here by Lars Grimsrud that list the many part numbers and their specs ... highly suggest that you read up.
JACK:gap

 

[dshanks]

I don't know of CC "HiEnergy 274" ... I do see Xtreme Energy 274

You are correct sir! it is a Xtreme Energy 274...

 

[glen242]

You are correct sir! it is a Xtreme Energy 274...

Have a XE274 in my 383, had a HE268 in my 350. IMHO, XE274 is too much for a basically stock 350. In the 383, it has , what the kids call, a lopey idle. Good thing I have a 4 speed. Vacuum is 13 - 13 1/2" at idle. Power brakes work fine. C.R. is about 10.2.

 

[lnirenberg]

Only slightly off topic, my base engine 71 has a new edelbrock carb w/ electric choke. When I moved the vacuum advance line from the ported connection to full vacuum the idle would go up about 200 rpms to a 900 rpm idle after the car warmed up and if I backed off the idle it would stall on start up (summer months w/ no choke engagement). Moving it back to ported it settles at about 700 rpms. I have a mild cam and lots of vacuum, what am I doing wrong? I think it actually ran better on full vacuum but the high idle was driving me nuts.

 

[JohnZ]

Even in moderate weather, you still need choke activation and the fast-idle system to get a quick start and good cold driveability, at least for the first 30 seconds or so. When you change from ported to full vacuum, you need to re-adjust the idle speed and idle mixture to compensate for the added advance at idle; this is totally separate from the choke adjustment.

Electric chokes require 12V power all the time in order to remain fully open; how did you set up the choke for "no engagement"?

 

[dshanks]

Hi Guys, Many of you have been back and forth with good answers on this issue, as well as my expansion tank problem. We discussed how this will not work for the purpose that I need it for.
So since some of you think my cam may not produce enough vacuum to push my headlight/wiper doors well, can I modify this expansion tank to be used as a vacuum reservoir? Im a fairly resourcefull handyman...