Distributor Vacuum issue

[Bill Spencer]

I replaced my Intake and Carb on my 1955 TBird. I used a later model Intake Manifold and a Holley Slayer Series Carb. with 450 CFM. With the original 292 Y Block intake and the Tea Pot carb, I had a HEI distributor running. There was no problems with this setup. However after replacing the Intake and Carb to the newer setup, There appears to be a Spark Advanced problem. Previous Vacuum to the Distributor was taken from the Intake Manifold. With the New Holley carb, the vacuum to the distributor is taken from the Vacuum Chamber inside the carb.
When driving I get a backfire type noise through the carb. This is what leads me to the timing advance on the distributor.
Should I be using the Manifold Vacuum or use the Vacuum from the Carb?

 

[Digger]

I never changed from original factory intake or carb on my 55 ,but my personal opinion (only) is to stay with the manifold vac, Probably more predictable and stable

 

[64ZCODE]

There seems to be ongoing debate about whether to use ported (carburetor) vacuum or manifold vacuum. I've always understood that ported was the way to go, but because of this thread and after reading up on it, I'm going to try switching to manifold vacuum for the vacuum advance and see how it goes.

 

[64ZCODE]

Interesting post on the subject:

https://www.bangshift.com/forum/forum/bangshift/tech-section/32851-ported-vs-manifold-vacuum-advance

 

[Bill Spencer]

I have Both ports available for Manifold or Carb Vacuum. Will check the Timing with each individual hookup and test drive the car. Will be able to determine which is best after engine warms UP. Will post results..

 

[64ZCODE]

Nice, looking forward to hearing the results, Bill.

Doug

 

[64ZCODE]

In theory, idle speed should go up when vacuum advance connected to direct manifold vacuum. Because, more vacuum means higher advance which should mean more combustion of leaner gas mix under idle condition.

 

[64ZCODE]

Hi all. So my '64 stock 390 engine vacuum advance has been connected to ported vacuum since I got the car a few months back. Indeed, I thought that was the gospel. Today I connected it to manifold vacuum and took a test drive. Here are my "qualitative" impressions: idle speed increased perhaps 100-150 rpms and is smoother, car has better throttle response off the line, and better throttle response when at speed and then getting on the throttle. I think this makes sense because the engine is running leanest when idling or cruising at speed, and that's when the most advanced spark is needed to "light the fire". Under those two conditions, manifold vacuum is at the highest point and so results in the most spark advance. Thoughts?

 

[RT2]

Since you change your manadold and carburetor it time to think about changing the distributor. 1955 Tbird used a much different design for vacuum advance. Good discussion on your previous thread on types of distributors that will work with your setup. You are right that the engine is always getting full advance because of the higher vacuum being seen by the distributor.
Hope this helps!

 

[Bill Spencer]

Configured Dist to Run OFF Manifold Vacuum. However, Prior to switching vacuum over I used a timing light to check timing advancements. When watching timing settings using Carb vacuum, the timing changed very drastically over 20deg with increasing engine speed.
Changed Dist to manifold vacuum.
When watching timing with manifold, there was very little changing of the timing Mark's around 5deg as speed is increased very little from TDC settings.