Vacuum Advance

[Stingray74CC]

While I was changing out my radiator, I was looking around the engine bay and noticed that there is no vacuum advance connected to the distributor. The car seems to run fine, but then again I really wouldn't know the full potential of a 427 either. I do know that its more power than I've ever driven before. Well, there is one thing that kinda bothers me. When I'm coming to a stop light with the transmission in neutral, the engine's rpm will dip down quite a bit (300 range). Sometimes it'll bounce back up, sometimes it won't and the car will die. Could the vacuum be the problem here? Anyway, if I were to connect it...where should it be connected to and how will it affect the timing?

Thanks for the help. I figured I should fix everything I can while I'm working under the hood anyway.

[RalleyRed]

I'm afraid with the problems your having the best we can do is trade motor's.

Not going for it ,huh?

I wish I could help you but I really don't know. I would think that it should have a vaccum advance, but the big blocks are just a dream to me.

Rick

[Stingray74CC]

Originally posted by RalleyRed
I'm afraid with the problems your having the best we can do is trade motor's.

[1979toy]

The vacuum advance does it's work at cruising speeds. Advances the timing to improve fuel economy. When you have a long duration camshaft the engine doesn't make enough vacuum to operate the advance anyway so most high performance engine builders install a dist. without a vacuum advance or block it off. Takes away one item that may disrupt correct timing. I have my vacuum advance disconnected while I'm tuning the mechanical advance and car. When I've completed those I will reconnect the advance and see if it will do me any good or not.
Hope that helps.

[JohnZ]

Your idle is unstable due to lack of (adequate) spark advance at idle, and the centrifugal advance is "wandering" randomly at low rpm, causing the instability. You need to connect the vacuum advance, which will add 12-18 degrees (depending on the specs of the can) of advance at idle, low speed around town, and at cruise on the highway. It's there for a reason - lean idle, low-speed and cruise mixtures take longer to burn than rich (acceleration) mixture, so you need to "light the fire" sooner to get it all burned at the right point for the power stroke. Vacuum advance improves idle stability, reduces engine temperature, and improves throttle response, driveability, and fuel economy. When you put your foot in it, the vacuum advance disappears immediately (no manifold vacuum to actuate it), as it isn't necessary with a rich mixture, and when you back off, it comes back to do its job. Vacuum advance has no effect whatsoever on wide-open-throttle performance; running without it makes no sense and is like throwing dollar bills out the window, unless you're driving a pure race car.

[Stingray74CC]

What you said makes alot of sense and follows what I'm feeling when I drive the car. Where does it vacuum tie into. I have two vacuum sources available right there. One is the vacuum from the center carb to the front and back carburators and the other is a larger vacuum coming out of the intake manifold (i think). Which should I tie into?

[JohnZ]

The center carb connection that feeds the hoses to the front and rear carb opening diaphragms isn't manifold vacuum - it's air velocity-induced vacuum from the upper part of the venturi; don't use that. You should have a nipple that will accept a 7/32" ID hose on the base of the center carb, just rearward of the right front carb-to-manifold nut; if it's there, it should be plugged (capped) - if it's there and not capped, that's part of your problem (vacuum leak).

Put a vacuum gauge on that nipple and see if it shows manifold vacuum at idle; if it does, that's the one you want to use (that was the production vacuum advance source). The resto parts houses have the original hoses, pipe, and clips if you want it to look original. If it doesn't show manifold vacuum at idle, cap it and use the fitting in the intake manifold on the right side just below the center carb pad. When you connect the vacuum advance, you'll have to re-set your idle speed (the advance will increase it).

You'll notice the difference!

[Stingray74CC]

I should be able to let you know how much it affects the car around town and on the highway too. I think a bunch of the CAC bunch are going to get together down in Virginia this Saturday.

[Stingray74CC]

I think this is what you're talking about. The green arrow is pointing to a screw that I think is plugging the vacuum. I can't get out to the car for the next 5 days so I have to go off the pics that I have. Thanks for the help.

[JohnZ]

Depends on the list number of the carburetor, but that looks like it should be a manifold vacuum source; connect a vacuum gauge to it to be sure.

[Stingray72]

Hold on, vacuum advances are usually not supposed to be connected to manifold vacuum. They are connected to "ported" manifold vacuum, which will provide no vacuum at idle. A vacuum advance is intended to provide additional advance ONLY at part-throttle operation. To acheive this, the "ported" vacuum opening in the carburetor's venturi is located just above the throttle plates when they are in idle position. When the throttle is opened slightly, these ports are uncovered, exposing them to manifold vacuum while you're cruising. When you've got the pedal to the floor, manifold vacuum drops overall and the vacuum advance does not play much of a role.

In engines with a large cam and a rough idle, you can experiment with connecting the vacuum advance to regular manifold vacuum. This might help smooth out the idle and make a radical cam more streetable, or it might not (especially if the cam is so radical that idle vacuum is virtually non-existent).

[JohnZ]

Not quite - "ported"vacuum was invented as an emissions strategy in the days of A.I.R. pumps, before catalytic converters were invented (I was there, and was part of it). It was done to retard timing at idle (which was where emission testing was done in those days), which significantly increased EGT (exhaust gas temperature), in order to make the air injection into the exhaust manifolds more efficient - the injected air combined with the higher EGT gases and created a hotter "secondary afterburning", to more effectively consume the unburned hydrocarbons in the exhaust stream (and get by the idle emission tests of the day).

The secondary effect, of course, was to dramatically increase the exhaust stream's heat input into the exhaust ports, which raised coolant temperatures in the water jackets around them (which is why the temp senders were relocated from the intake to the cylinder heads, between the exhaust ports), as "ported spark" engines ran hotter at idle due to the retarded spark timing.

99% of all engines with "ported spark" will run better with full manifold vacuum to the distributor, assuming the calibration of the advance diaphragm and its travel is correct and the centrifugal advance curve is set up properly, and they'll run cooler at idle as well. "Ported spark" wasn't done as a performance or driveability enhancement - it was a stop-gap measure implemented to get carbureted cars to pass idle emission testing until more effective means of reducing emissions were developed - catalytic converters, and, later, electronic fuel injection and computerized engine controllers.

[Stingray72]

Thanks for the info, JohnZ. That was enlightening. How does using ported vacuum to the vacuum advance result in retarded timing? Is the air pressure just above the throttle plates (at idle position) in the carburetor actually above atmospheric air pressure at idle?

Thanks

[JohnZ]

With ported spark, initial(base) timing is the same static as it is with the engine running, as the vacuum pickup orifice in the venturi for the vacuum advance is above the throttle plate at idle, thus not exposed to manifold vacuum; when the throttle is opened even slightly, that orifice is then exposed to manifold vacuum and the vacuum advance comes into play.

Base timing spec on all ported-spark Chevrolet engines (1969, for instance) was between TDC and 4 degrees BTDC; the only exception was the L-88 (which had no vacuum advance at all), at 12 degrees BTDC, and several 327/210's in Camaros and full-size Chevrolets were actually spec'd at 2 degrees AFTER TDC. This put all of these engines (except the L-88) WAY retarded at idle, and with ported spark, they had no help from the vacuum advance at idle.

To make up for the retarded idle spark, most of the distributors on these cars provided between 24 and 38 degrees of centrifugal advance, to bring total timing back where it belonged for high-rpm performance. Later cars (when catalytic converters and HEI came on the scene) had base timing bumped back up closer to the 10-14 degree level, and the distributor centrifugal advance mechanisms were cut back to 20-24 degrees, keeping total timing at about the same level.

Until catalytic converters and electronic fuel injection came along later, it was a constant game of calibrating base, centrifugal, and vacuum advance spark timing to match up to the Federal emission testing procedures; California procedures were different (and still are today), so California engines were calibrated differently than 49-state engines, just to pass the California tests (remember the 305 LG4 California Corvettes in 1980 that got a $50 credit?). Remember the infamous "electronic Q-Jets" with the duty-cycle solenoid-operated metering rods?

Lots of weird things were done to carbureted cars to get them to pass emission tests, and "ported spark" was one of them.

[maxrevs85]

Hello John,
I was cheching out your profile. Talk about been, there done that, very impressive. How about some pics of that dream garage?