mbrown24

Where does the distributor vacuum advance need to be connected, to ported vacuum or not? At the base of the carb or somewhere else, i am confused as to the difference, i heard that at wide open throttle you want the vacuum advance to go away so which connection is that?

Fevre

Most times it is connected to a ported source which is located above the throttle plates and should have no vac signal when the throttle is closed. You can run it to an unported source (below the throttle plates) if you want more adv at idle like if you have a long duration cam and want to smooth out the idle a little. Either way when you drop the hammer neither one should have much if any vac going through it so the vac adv will drop to 0.

Boofers

Manifold Vacuum apparently gives you better fuel economy, smoother idle, better driveablity, better engine efficiency resulting in lower under the hood temps and cooler exhaust temps. I have no proof to back this up its just my own conclusions from reading about this topic a hundred times since somebody posts this question about once per week. Try a forum search and hopefully these articles will help you out as well.



https://www.corvetteforum.com/techti...=115&TopicID=3
https://www.corvetteforum.com/techti...=109&TopicID=3

stingr69

Manifold vacuum IS better but you will likely need a different vacuum can and you will need to reset the carb mixture and idle speed.

If you are not a whiz at this sort of thing, you can hook it up to the ported tap on your carb like it was originaly and just drive it. Basicaly, ported is a no-brainer while manifold vacuum is a little more work for a little improvement. The emissions are a little worse hooked up to manifold vacuum but everything else is an improvement.

-Mark.

SpyderD

Yeah, this, along with "how do I bleed brakes" is a weekly topic. We should have seperate forums just for these topics.

One point left out is that, when the throttle is opened, ported and manifold vacuum are the same and it makes no difference. One negative about using manifold vacuum that was hinted at is that you need to tune the carb and vacuum advance carefully - if you don't you can get erratic timing at idle, a too fast idle speed, and an off idle stumble. Personally I never understood the use of manifold vacuum to raise timing at idle (thus improving economy, idle, and reduce overheating). The no brainer alternative would be to simply raise your initial timing at idle at the distributor, stick to ported vacuum, and achieve the same results without the negative aspects (although then you have tune the distributor to limit your total timing so it still gets that 36 degrees at 2,800 range).

mbrown24

sorry, didnt realize it was such a recurring question, im new. but thanks for the reponses and i promise to not ask it again.

Avette4me

While both of you tried to help in the rest of your posts, why do you feel the need to add this kind of thing. It’s a peeve of mine. A new member hasn’t seen it asked, the search function doesn’t work well, and why not just answer an honest question rather than try prove you were here first. The reason I’m bitchin is this…

There’s no need to make a new guy apologize for askin’

Flame away if ya wish…

btw mbrown24, I think it should be on a ported source

cardo0

Have heard on the forum that the Chiltons car manuals have some carb vac line drawings for different years - $20 from PAW or JC Whitney or maybe at local pub library for free. Just too many different applications to know each one and you didn't even post your year or model and equipment. I know where mine are on my '74 sb but that doesn't do us much good unless I can see your carb.
Good luck and good night. cardo0

stingr69

More to the question,
The (engine) vacuum will go away at WOT no matter where you run the advance hose.


SpyderD,
The engine will idle smoother and cooler using "manifold" vacuum as we have established. The problem with adding additional initial timing and reducing the centrifugal a coresponding amount is that you can experience hard cranking with a lot of initial timing. With the vacuum advance hooked up to manifold vacuum, you do not have to "crank against the engine" when starting and the timing at idle will pick up additional vacuum timing as soon as the engine starts. As Martha would say: "this is a good thing". Try to visualize an engine slowly cranking over with the spark plugs firing at 25°-30° BEFORE the piston gets to the top of the compression stroke. That engine is going to be hard to start and the starter nose cone is probably going to break sooner or later. Less initial helps cranking and once it is running, more initial + vacuum helps the idle.

If you are running a more radical cam, you realy notice a big improvement by switching to a well designed manifold vacuum setup. I ran my DZ 302 both ways and I can tell you, once you have it set up right and drive it that way, you will not want to go back to ported.

Keep in mind that manifold vacuum was the design of choice before emissions came into the picture. Emissions was the reason we have "ported" advance. Vettes built back before the late 60's used manifold advance from the factory.

Bottom line, if you have a fairly stock engine you can run it ported and enjoy it. If you want to try to improve your idle, you can change out the vacuum can and set it up for manifold vacuum. You can not just switch the hose connection and expect it to work for you.

The car will not be any faster either way.

-Mark.

Ybnormal

When I put my new cam in the paperwork said to run ported vacuum. Well I have never been a fan of ported vacuum so I sent them an email to ask why and this was the reply:

"It is a matter of personal preference, some people like to have the vacuum advance hooked up all the time but the correct way would be to have it hooked up to the ported vacuum source due to when as more gas is introduced into the carb the timing advances to compensate for it, as opposed to having the timing advanced all the time it would bog until the excess fuel burnt off."

I'm still running manifold vacuum

YBnormal...drive a vette

SpyderD

Very good point. But those are extreme cases. I have my initial set at 16 degrees of timing with my relatively big cam and high compression, and I was refering to those people that have 4 or 5 initial and use manifold vacuum to raise the timing to 16. It's like, why bother? -tune that into the initial at the distributor. I would think it would also mess up your curve (and thus the performance) to have 16 degrees of timing at idle, go off idle and have it immediately drop down to 4, and then have it advance again.

mbrown24

Well, to confuse things a little more. I have a pretty hot LT-1 engine with serious cam, 850 double pumper. I added an adjustable advance and heavy springs and with the static timing set at 6-8 i get amost 55 deg. at 3000 rpm, dont know if the balancer markings are off or what, but i screwed the adjustable can all the way out to reduce advance and no change. the car does not detonate at all and runs pretty good. the can is hooked to the carb. for what's it worth.

Paul L

mbrown,
Just to confuse matters, "hooked to the carb" can take on different meanings. This is the Edelbrock version. The inlet on the left (plugged) is ported and that on the right (being used) is manifold. One is above and the other below the throttle plate. This is a pre-emissions engine. A vacuum gauge will tell the true story for you.

Fevre

On most adj vac adv cans the adj you made just changes the rate of adv not the total adv, on the crane adj vac adv can their is a supplied stop that needs to be installed to limit the total adv. If the can was on the car when you bought it pull the cap and look at the vac adv arm coming into the dist and see if there is a stop installed, the Crane stop is kinda odd shaped with about 5 teeth to select from, it will increase your intial timing by about 2 degrees for each tooth so you have to subtract it out to make get the correct adv reading.

Boofers

:o Hey no apology necessary... Sorry, it wasn't my intention to be negative I guess I just came across that way.

Point taken.

mbrown24

just for the record, im new to this forum, ive been on many other types of forums and this one is one of the most active, friendly and helpful that ive ever run across. must be something about owning a vette that brings that out in people. thanks for the help

Avette4me

thanks man, sorry if I sounded like a grandmother...

Paul L

mbrown,
I have been around here for five years this month. There aren't too many questions I haven't asked or seen. And yes they can get repetitious with time. But the population is transient and there's always room for newcomers. And there are always new twists on old tricks. So welcome aboard!

Boofers

I thought about it and realized that the best way to remember something is to read it multiple times. So all new members post away and we all benefit!