Manifold vacuum

[Doug&Deb]

I’ve read several threads where manifold vacuum of 18-20 inches is normal for these flatheads. I’m only able to get 16 inches of vacuum. Timing is correct and dwell is within spec. Idle speed is 475 rpm’s. There are no vacuum leaks that I can find. The car runs fine with no drivability issues. Am I overthinking this or is there anything else I need to check?

[Bryan]

Maybe measure cylinder compression on all six?   The pistons are what produce the vacuum..

[Sniper]

I get 20 inches.  But I am running 5 degrees advanced and bumped up my idle to get my charging system to charge at idle.

 

Altitude can play a role, I am under 2000 ft.

 

Idle vacuum is an interrelationship between idle timing, dwell, idle speed AND idle mixture.  All three need to be checked and adjusted in a loop until everything settles down to a maximum.  Sometimes that maximum is not 20 inches.   There are many possibilities as to why.

 

https://www.motor.com/magazine-summary/mastering-the-basics-reading-a-vacuum-gauge/

[kencombs]

3 hours ago, Doug&Deb said:

I’ve read several threads where manifold vacuum of 18-20 inches is normal for these flatheads. I’m only able to get 16 inches of vacuum. Timing is correct and dwell is within spec. Idle speed is 475 rpm’s. There are no vacuum leaks that I can find. The car runs fine with no drivability issues. Am I overthinking this or is there anything else I need to check?

I'm weird about specs for idle speed, timing etc when using a vacuum gauge.  I set all the 'setable' things to get the max vacuum while just ignoring all the specs.  Test drive and maybe fine tune.

 

I start with idle speed, then timing, then mixture, repeat in a circular fashion as mentioned by Sniper.   I usually end up with a slightly higher idle speed than recommended as I like it for driveability reasons.

 

[Doug&Deb]

I normally would do the same except that I have the M6 trans that is idle sensitive. When I set the timing with the vacuum gauge I had to back off to stop the detonation. I’m probably just overthinking this. It’s a bad habit of mine lol. It’s running good and it ain’t broke so I’ll leave things alone.

[busycoupe]

What port are you using to tap the vacuum?  I use the connection for the vacuum wipers and get 20 inches of vacuum.  If you use a port for the vacuum advance or transmission (on some cars) you may get something different.  

[Dodgeb4ya]

If it runs nicely...go with it.

Vacuum gauges are over rated as to the perfect number for all engines.

[Doug&Deb]

I use the port on the intake. Electric wipers on my Coronet. 

[soth122003]

One of the other members was talking about this and said that the vacuum setting takes into account the wear on the engine, distributor and such to help make up the difference for the standard settings such as with the timing light, dwell and idle setting. Slop in the timing chain, wear of the piston rings, age and wear in the carb that can't be taken into account with just the manual setting alone. That being said 18-20 inches is just an average. I get about 21-22 inches, but my idle slightly is higher to offset the worn throttle plate bushings.

 

Joe Lee