HEI vacuum connections

[gmebey]

I'm really confused after several hours of digging for information on factory GM HEI vacuum connections.

My project truck is turning out to be more of a mess than a project. The prior owners did some weird stuff, and to the list HEI and carburetor mess.

First issue, I suspect I have a intake manifold and carburetor mismatch:

The spacer/gasket plate below the carb did not seal the vacuum ports on the bottom of the carb (Elderbrock Q-Jet 1902).....PLUS the intake manifold is from what I think is a post 1974 and has a the EGR passage under the carb. A trip to the auto parts store yielded no solution without using a 1" riser.....either the vacuum or EGR passage is uncovered.

I need a solution, is there an off the self option? At this time I'm leaning to making a plate out of aluminum.


Second issue, the distributor has been switched out for a factory HEI.

I suspect the vacuum is not plumbed right. I'm still lost after reading several articles.

I have two simple questions:
1) Did the original factory HEI use a vacuum canister?
2) Where did the original factory HEI get it's vacuum source. Manifold, not ported at the carb or ported at the carb?

[truckster]

The vacuum pot on the HEI should be connected directly to a ported vacuum.

Later years used the vacuum delay mechanism on the thermostat housing, but I would just make the direct connection.

[Firebirdjones]

First, on the EGR intake dilema, there is a thin plate that is made to go under the carb and block that passage off.
It used to come in kit form with 2 gaskets (top and bottom) and the thin plate is made of stainless, but most autoparts stores wouldn't know what you were asking for, not only that but I doubt any of them would even carry it anymore. It used to be a simple rack item 20 years ago.
I have that setup laying around here, if I can dig it out I'll snap a picture of it. The other option is to drill and tap the intake with a pipe thread tap, and counter sink a pipe plug into the EGR hole. I've done this on a few intakes for other people and it works great. Then you can use any gasket you please.


Second, your HEI vacuum advance from GM (starting in 1975) would have originally been plugged into a vacuum source that was thermostatically opened, generally off of the thermostat housing. That way it would only supply vacuum when the engine was up to operating temp. This was done for emissions purposes. Earlier years (before HEI) used a transmission control spark switch for vacuum. It only supplied vacuum when the transmission was in 3rd gear (auto) or 4th gear (manual). Neither of these setups are ideal however.
You'll find better gas mileage and throttle response by running that vacuum line to a full time manifold vacuum source.

You can try ported manifold vacuum through one of the upper ports on the carburator but that poses a few issues.
First of all, ported vacuum only works when the throttle blades are cracked open, so you won't see any vacuum at idle. Because of this, the engine will tend to run warm while idling in traffic, not enough advance at idle. The other issue with ported vacuum advance, is too much advance when at full throttle.
To explain, at full throttle you should only have your mechanical advance working for you, which is your initial timing and your amount of advance added together. The vacuum advance should be taken out of the equation. The problem with ported vacuum, is at full throttle it's still supplying a vacuum pull on your cannister, so the total timing also has vacuum thrown in the mix. People do this all the time and wonder why they have detonation problems under full throttle, and they want to blame it on other things.
However, with manifold vacuum, once at full throttle vacuum is non existent, which takes the vacuum advance out of the equation.

For example, if my engine makes best full throttle power at 38 degrees total timing, and I set my initial at 12 degrees, then I need to have my mechanical advance dialed in to allow another 26 degrees of timing at what ever rpm best power was found ( say 2800).
Once that is set, then play with the vacuum advance. If you hook up to a ported vacuum source (on top of carb) that vacuum advance never goes away at full throttle. So if you have an additional 15 degrees of vacuum advance, it throws your total up to as much as 53 degrees!!!
That's a ticking time bomb.
Hook the vacuum advance to manifold vacuum, so you have your vacuum advance at idle and part throttle conditions (which is the purpose of the vacuum advance anyway) but when you go full throttle it's completely out of the equation, so you have 38 degrees total as desired at full throttle, but back at part throttle the vacuum advance is working again for better part throttle drivability.

Setup this way gives you the best of both worlds, you get throttle response, cooler operating temps, and better gas mileage while putting around town, and you still have the total advance you need at full throttle.

I also suggest an adjustable vacuum advance unit. Most factory units dial in too much vacuum advance. For a heads up, the amount of vacuum advance is stamped into the flat portion of the vacuum unit just behind the cannister.
Most non adjustable units will dial in 20 degrees or more (check yours to see). I prefer an adjustable cannister, so I can dial in an additional 12-15 degrees for part throttle cruising. That's all I've found I needed once the rest of the distributor is setup properly.
Hope that helps.

[Firebirdjones]

Originally Posted by truckster

The vacuum pot on the HEI should be connected directly to a ported vacuum.

Later years used the vacuum delay mechanism on the thermostat housing, but I would just make the direct connection.

I guess I don't type as fast as you do

I went on to explain why a manifold vacuum source is better than a ported source. Sorry for the book.

[truckster]

I guess I was wrong. Here's a thread from the 67-72 truck forum:

http://67-72chevytrucks.com/vboard/s...d.php?t=481245

[Firebirdjones]

Originally Posted by truckster

I guess I was wrong. Here's a thread from the 67-72 truck forum:

http://67-72chevytrucks.com/vboard/s...d.php?t=481245

Ooops, I guess when I read your first post for some reason I thought you said manifold vacuum and "NOT" ported. So I thought you were okay.

I guess I'm getting old and can't read anymore

[gmebey]

Thanks guys for all the advice.

As a short term solution I made the aluminum plate, and plan to drill/tap and plug the intake when I have time to remove it. Or even find one that does not have the EGR path.

After reading a few post on vacuum advanced HEI distributors it is clear that I need to know more about mine. I come by an article by Lars Grimsrud, "SVE Automotive Restoration" that peaked my curiosity. I plan to locate the ID number this weekend and look up the performance specs for it.

What would be a the ideal unit be in terms of pull-in vacuum and max advance angle?

As I mentioned in my earlier post this project is turned into a mess....Oh well that just makes this project a bit longer.

Again thanks,
Gerald

[Firebirdjones]

Originally Posted by gmebey

Thanks guys for all the advice.

What would be a the ideal unit be in terms of pull-in vacuum and max advance angle?

As I mentioned in my earlier post this project is turned into a mess....Oh well that just makes this project a bit longer.

Again thanks,
Gerald

Not sure I understand the question exactly but I'll assume you are talking about the vacuum advance only?

Vacuum advance is the last thing I'll set up on a distributor when tuning an engine.

The initial setting and centrifical is the first thing to attack, the most time consuming, and is the most important.

Without the engine being on a dyno it's hard to say exactly what the engine is going to like because there are a ton of variables out there, cam timing, compression, combustion chamber design, intake manifold runner length, stick or auto, heavy car, light car, rear gears etc.....so it's something you'll have to experiment with.

A baseline I like to start with on most older carbed engines is 14-18 initial depending on the camshaft, more cam likes more initial and produces more idle vacuum and helps the car idle more consistently.
From there I dial in the centrifical of about another 16-20 degrees depending on where the engine makes best power (this is where the dyno comes in handy) or you can use a dragstrip and tune with mph.
Once that number is found (lets say 36 degrees total for example) Only then will I dial in the vacuum advance for part throttle drivability. Again, hooked to a manifold vacuum source.

Now most stock vacuum cannisters usually have too much advance in them, some are 20 degrees or more. The number is generally stamped on the cannister. What I like to use is an adjustable vacuum advance because I only want about 15 degrees of vacuum advance at the most. Sometimes I find 10-12 degrees enough if the engine is finicky on pump gas and part throttle driving. Remember it's only working while idling and light to part throttle driving if hooked to manifold vacuum. At full throttle it's non existent.

Adjustable vacuum cans are available for both points and HEI distributors. Some are just a set screw to set spring tension inside the nipple for the hose connection.
Other more expensive and more precise units also come with a stepped key that bolts inside with the cannister and rubs against the advance plate (or breaker plate) to limit vacuum can travel, the more you limit vacuum can travel, the more it moves the breaker plate for more initial timing.
You can accomplish the same thing with a penny. Cut notches in the penny and drill a hole in it. Mount it the same way and it accomplishes the same thing.

Another tip, on older HEI's I limit the breaker plate advance (centrifical) buy installing a small stop screw in the breaker plate. Most HEI's OVER advance. The hole in the breaker plate is already there, so I just tap it for a #10 screw. I can then file on that screw and fine tune the amount of centrical advance I want in a particular combo. Some advance kits come with bushings, some don't, so I use the screw as a way around it.

Wish you were close by, I'd set it up for you. Hope this helps.