Vacuum Advance Line

[bluestringer]

On a high rise manifold with plenum and 2 carbs where would the vacuum line from the distributor advance be connected. The manifold, plenum, one of the carbs? This is on a 455 Buick engine.

[espo]

Usually, the vacuum line would go to the base of the primary carb. on the primary side. 

[MeatMan]

On 3/26/2023 at 11:01 AM, bluestringer said:

On a high rise manifold with plenum and 2 carbs where would the vacuum line from the distributor advance be connected. The manifold, plenum, one of the carbs? This is on a 455 Buick engine.

You mention a plenum, are you referring to a tunnel ram manifold?

[bluestringer]

20 hours ago, MeatMan said:

You mention a plenum, are you referring to a tunnel ram manifold?

This is the manifold, I guess it would be a tunnel ram. It has a plenum that goes on top, then the carbs. I've got a distributor that I will wire up and it has the vacuum advance on the side and so I thought I would run a hose off it.

[Ace-Garageguy]

32 minutes ago, bluestringer said:

This is the manifold...

The most stable vacuum source on that would be the center of the balance tube between the two tall runner segments.

You need a stable source to get a decent signal to the distributor or electronic ignition box, if it's one that can vary timing.

Taking a vacuum signal from anywhere else on that would result in a pulsing signal...useless.

NOTE: A drag-racing competition-only car would very likely not have any vacuum advance, but a street-driven car most likely would.

Edited March 28, 2023 by Ace-Garageguy

[NOBLNG]

Grabbed these two pics off the Camaro forum. First one shows a vacuum advance tube I believe, and the second one shows a connection for the brake booster.

 

Edited March 28, 2023 by NOBLNG

[Ace-Garageguy]

11 minutes ago, NOBLNG said:

Grabbed these two pics off the Camaro forum. First one shows a vacuum advance tube I believe, and the second one shows a connection for the brake booster...

Good pix...and it's debatable whether a setup like that would even draw enough vacuum to drive a brake booster.

How much cam it was running would be the determining factor.

More cam with more duration and overlap = less low RPM vacuum.

Electric supplemental vacuum pumps can be used in cases like that, or a "hydro-boost" system that runs off the power steering pump, using pressurized fluid and a special master cylinder, can be substituted for the vacuum boost.

[bluestringer]

3 hours ago, Ace-Garageguy said:

The most stable vacuum source on that would be the center of the balance tube between the two tall runner segments.

You need a stable source to get a decent signal to the distributor or electronic ignition box, if it's one that can vary timing.

Taking a vacuum signal from anywhere else on that would result in a pulsing signal...useless.

NOTE: A drag-racing competition-only car would very likely not have any vacuum advance, but a street-driven car most likely would.

Yea, after doing more research it seems most agree that manifold vacuum will give better performance than ported carb vacuum. Not really important on the build, the distributor has the advance and since I'm wiring it, I though I may as well run a line off it. I'm building it as a street custom per the kit. I'll run it to the manifold/tunnel ram.

[MeatMan]

On 3/28/2023 at 8:12 AM, Ace-Garageguy said:

The most stable vacuum source on that would be the center of the balance tube between the two tall runner segments.

You need a stable source to get a decent signal to the distributor or electronic ignition box, if it's one that can vary timing.

Taking a vacuum signal from anywhere else on that would result in a pulsing signal...useless.

NOTE: A drag-racing competition-only car would very likely not have any vacuum advance, but a street-driven car most likely would.

Correct, and thus my question about the manifold type. Any car using tunnel ram would have a fairly gnarly cam, which wouldn't provide enough vac for accurate distributor advance in my experience..

[Ace-Garageguy]

4 hours ago, MeatMan said:

Correct, and thus my question about the manifold type. Any car using tunnel ram would have a fairly gnarly cam, which wouldn't provide enough vac for accurate distributor advance in my experience..

While I agree with you in general, it's important to remember every car is different. And manifold vacuum at part throttle cruise, even with a healthy cam, is much higher than manifold vacuum at a cam-induced lopey idle.

I've seen perfectly street-drivable high-rise equipped cars that were built for the "look", running moderate cams and smallish carbs (2X500 CFM 4-bbls or the occasional 3X2 setup), with manual gearboxes or well-matched converters in automatics, that benefitted from a vacuum advance.

[ctruss53]

On the few real engines I have built, I run the vacuum advance to the intake manifold.

It has been covered, but on this style of intake, I would run the vacuum advance to that balancing "duct" between the two risers.

Whenever there is not a good place already on the intake, like an old single plane Weiland intake I used, I just drilled and tapped a hole, installed a nipple, and attached the line to that.