vacuum system

[tscott9330]

Alright, here goes. I have a 78 with a 350 that has that wonderful vacuum system that i don't know anything about. I know it works the bead light actuators and the cruise control but what i need to know is how it works with the whole vacum advance system.
1. how's the vacuum advance system work ont the distributor?
2. how important is it?
and 3. What do i do if i want to eliminate the vacuum system?
( i dont need cruise control, and i plan on converting the headlights to electric)

4. Can i replace the vacum advance with mechanical advance of some kind and if so, how?

Tom Scott

 

[Jack]

Alright, here goes. I have a 78 with a 350 that has that wonderful vacuum system that i don't know anything about. I know it works the bead light actuators and the cruise control but what i need to know is how it works with the whole vacum advance system.
1. how's the vacuum advance system work ont the distributor?
2. how important is it?
and 3. What do i do if i want to eliminate the vacuum system?
( i dont need cruise control, and i plan on converting the headlights to electric)

4. Can i replace the vacum advance with mechanical advance of some kind and if so, how?

Tom Scott

Your 78 came with a big cap HEI distributor that is probably the best distributor design GM has yet to offer. It has both vacuum and mechanical (centrifugal) advances. Unless running VERY radical motors, virtually all carbureted street cars need BOTH vacuum & mech advances ... very important for both advances to be intact & working properly ... if either or both are not up to snuff, power/economy or both will likely suffer. There are very good distributor papers by Lars archived here within CAC knowledgebase ... lookemup. They'll explain much better than I can ... how the advances work & why both are important.

Chances are, your car has power brakes. If so, they rely heavily on vacuum.

Dunno a 78 very well, but chances are its defrost-heater-AC doors that direct air flow are actuated (at least in part) by vacuum.

Your engine produces manifold vacuum (source for all vacuum operated accessories/components). This is a normal & necessary function of any normally aspirated gas motor.

I'm all for the electric HL motors ... but you're probably gonna need to keep the rest of your vacuum system in good working order.
JACK:gap

 

[Stallion]

Alright, here goes. I have a 78 with a 350 that has that wonderful vacuum system that i don't know anything about. I know it works the bead light actuators and the cruise control but what i need to know is how it works with the whole vacum advance system.
1. how's the vacuum advance system work ont the distributor?
2. how important is it?
and 3. What do i do if i want to eliminate the vacuum system?
( i dont need cruise control, and i plan on converting the headlights to electric)

4. Can i replace the vacum advance with mechanical advance of some kind and if so, how?

Tom Scott

Tom, the way I always saw it is when the distributor is called a "mechanical" distributor, they are indirectly referring to the vaccuum advance on it. In other words, when the crank is at a higher RMP, the vaccuum is greater therefore advancing the ignition and timing for the distributor and ignition system.

Unfortunately in my case, my advance is disconnected from my distributor and I have yet to find out how my ignition advances. I will have to work with a timing light and maybe some other tooks to find degrees and if I have no advance on it.

Good luck!!

 

[Jack]

Re: Re: vacuum system

the way I always saw it is when the distributor is called a "mechanical" distributor, they are indirectly referring to the vaccuum advance on it. In other words, when the crank is at a higher RMP, the vaccuum is greater therefore advancing the ignition and timing for the distributor and ignition system.

No. Although the two discrete advances work in concert with one another ... they are separate functions and work independantly.

The mechanical advance consists of a pair of springs & flyweights that increasingly swing outward from centrifugal force. This motion begins approximately upon startup and continues as rpm increases ... until flyweights hit a stop ... ideally at or near cruising speed. This swinging motion translates to a rotation (advance) of the points’ cam or HEI’s reluctor. This mech advance motion can be recurved using different springs or flyweights or both. Note the mech adv motion is entirely a function of shaft speed; not load nor manifold vacuum.

The vacuum advance consists of a vacuum diaphragm (vac can) connected via a short rod to a rotating plate whereupon the points or pickup (HEI) is affixed. As engine load decreases, manifold vacuum increases. As manifold vacuum increases the diaphragm pulls on the rod which in turn pulls on the rotating plate; then the affixed points or pickup are rotated (advanced) in relation to the cam or reluctor. When at cruising speed with little or no load, there will be high manifold vacuum and vacuum advance is ideally fully-advanced. Given same condition, but driver stabs accelerator to floor; throttle plates open wide (load increases) and manifold vacuum drops dramatically ... then vacuum diaphragm-rod-plate returns to zero/near-zero degrees vacuum advance. This vacuum advance motion can be recurved using a different vac can, an adjustable vac can & by adjusting the rod stroke with a bushing. Note the vac adv motion is entirely a function of load and manifold vacuum; not shaft speed.

Some late C3 (about 81-82 I’m not sure) use similar big cap HEI; although it has NO vacuum advance can. With those models, I understand the vac can advance function is replaced with a computer-driven or electrically-driven function. Not simply a matter of GM tossing a needed advance function; the function was replaced but not deleted. Again, I’m not too familiar with late C3.

Lars archived papers on setting ignition timing and recurving are highly recommended. His are comprehensive, clear & concise; replete with part numbers & instructions.
JACK:gap

 

[Stallion]

Okay, thanks for correcting me, Jack!! I know now. Is changing the distributor a big job? I wouldn't change mine, because I want to keep her all stock (or as much as possible), but what about the rod going down with the threads at the end, that looks hard to get at.

 

[tscott9330]

The reason I asked was that i plan to replace the carb. with an edelbrock efi system and i am not sure if it will be set up for any vacuum but with the system you describe the efi system would have to include place to plug in vacuum hoses.

has anyone installed one of these edelbrock systems, if so how is it?


Tom Scott

 

[Jack]

but what about the rod going down with the threads at the end, that looks hard to get at.

I am puzzled at what you decscribe ... please clarify

The reason I asked was that i plan to replace the carb. with an edelbrock efi system and i am not sure if it will be set up for any vacuum but with the system you describe the efi system would have to include place to plug in vacuum hoses. has anyone installed one of these edelbrock systems, if so how is it?
Tom Scott

I believe Chris (69myway ) may have eb efi ... I seem to recall that it uses a distributor unlike typical C3 . I've no experience w/ eb efi. I'd ask Chris or eb.