[LOCKJAW]

Recently I upgraded to a MSD distributor w/ vac advance sdtup. The instructions that came in the box clearly stated to connect the vac adv line to a timed or "ported" source rather than a below the throttle plate constant vacuum source. Been driving GM cars mostly for 55 years. I remember always routing to constant vacuum port.

Yesterday I retimed my spark from 8 degrees to 12 degrees on the crank with 22 more in the centrifugal side for a total of 34 degrees, all in @ 2500. With the vac adv now on the ported side the idle is much more deep and agressive soundkng. After a full warmup lap around the neighborhood I reset all the idle controls to a slightly" fat " 900 rpm.
It seems to like this as well this way. It may reduce my highway mileage a bit because of reduced vacuum signal. With these double pumpers mileage is a sore subject.
Anyway, I am going to drive it like this for avtrial period then read plugs.
I have had no pinging at all but I am running a m20 muncie and I never lug my motors.
The motor seems to like the rapid advance curve. At a 3000 roll in first gear it will boil both big 10" tires just by mashing pedal GO pedal.

I know...people my age aren't supposed go do stuff like smokin rubber. To thaf I say : Bullfeathers

 

[69 SS396]

Cool James, smoke em if ya got em.

 

[LOCKJAW]

I am trying to see for myself if one way is better. I have read discussions about this. Never did understand the conclusions.

Sean, I went iut thursday to a cruise in @ hardees in Weeki Wachee. Not much action. I hsd the only cruck. Lot's of lste 40's. Fords. I was hoping to run into another necoa membr.

 

[BillyJack]

Subscribed for your final evaluation.
In the 70's, I would always move my vac advance to a manifold vacuum port. It really only affects the amount of advance at idle, since the ported advance comes in just off idle. Some of the 80's non-computer truck carbs wouldn't idle down low enough, so I had to move from manifold to ported vacuum. I always felt that manifold vacuum gave a bit crisper response off idle, but it certainly wasn't a huge difference. I'll be curious to hear your thoughts after trying it both ways.
BTW, 34-36 degrees @ 2500 rpm sounds like a real good place to start for your total timing. IIRC, 2500 rpm was what you got with the middle springs in the old Mr. Gasket 3 spring kits. As long as you used decent gas, it worked well on a bunch of near stock street SBCs I tuned over the years.

Bill

 

[michaelj]

Subscribed for the hell of it. Would love to hear what happens with your experiment.

 

[Guest]

The instructions that came with my zz4 said not to use the vacuum advance, to plug it. Mine works best without it. I have tried it both ways.

 

[steelybill]

Having only the weights for advance would limit total advance of course. Mileage would be the most affected without the vac canister, to bring the timing up to optimal at highway cruise.
The last car I bought new with manifold vac on the distributor was a 1959 Pontiac. Back then I thought it was strange !

 

[Jack_K]

I initially had ported vacuum on my newly rebuilt 305. I later changed it to manifold vacuum and I'll never go back. Starts and runs much better. Of course I had to slow down the idle.

 

[mopowa]

Yea I'm curios too. I'm running full vacuum now with the MSD pro billet distributer as well but was considering trying ported.

 

[mopowa]

Sorry about the second reply but I forgot to mention in my first post that I've also heard that full vacuum helps the small block (I think more on the 400s) run a little cooler. Keep an eye on temp.
Ang.

 

[CANTED]

vacuum advance

Mechanical advance changes ignition timing based on engine speed, period. The benefit from vacuum advance is that it makes adjustments to timing based on engine load. The vacuum source that changes based on engine load is manifold vacuum. There is a great article on the subject written by a GM engineer that developed these systems. It can be found in several forums on the web and I have referenced it here, when these discussions have come up in the past. Here it is from a Nova site.
http://www.stevesnovasite.com/forums/showthread.php?t=11689
It's a bit of reading but a lot good info.

 

[LOCKJAW]

Jim, thanks for the link. I can't wait to srudy it later.
So far the car seems content @ 12 static / 22 crntrifugal total of 34degrees in @ 2500. I made a beer run an hour ago and siezed the opportunity to turn up the wick. I ran a marked 1/4 on a backwoods road with no traffic. Motor sang to 6000 every shift. I might add two more degrees static to it yherr was no complaint from my mini starter. The strangest result is the really nice change to the exhaust note and rumble. I run long tube Hookers with two chamber flowmasters with 31/2" exit just behind rear wheels. I will run a mileage test soon asap.
Performance wise I don't feel any change as I shouldn't because the vac adv is moot when @ WOT. I still have no pinging at all with a costantly changing blend of 89/93 alleged octane. And my pipes are now snarling!

 

[mopowa]

Mechanical advance changes ignition timing based on engine speed, period. The benefit from vacuum advance is that it makes adjustments to timing based on engine load. The vacuum source that changes based on engine load is manifold vacuum. There is a great article on the subject written by a GM engineer that developed these systems. It can be found in several forums on the web and I have referenced it here, when these discussions have come up in the past. Here it is from a Nova site.
http://www.stevesnovasite.com/forums/showthread.php?t=11689
It's a bit of reading but a lot good info.

I read the info once tonight and will read it again to digest more. It was informative. It was always my understanding that the ported vacuum was more for emissions than for optimum engine performance. I have read similar info on the subject and tend to be in the full vacuum school. My motor likes it and as they (who ever they are) say if it ain't broke. Real world/time results are always interesting and good to see and hear.
Ang.

 

[michaelj]

Awesome read, Jim. Thanks!

 

[steelybill]

Thanks again Jim. I remember that article when you posted it a while back.

 

[LOCKJAW]

CONCLUSION: I have gone back to my traditional setup. After noticing loss if fuel mileage and reading this great technical article Jim posted I am convinced that manifold vacuum is what I should be using, not potted ir timed vacuum. With fuel prics so high and nothing to gain I am back to where I started. I set the static spark @10 with 22 in the centrifugal side. Runslike a top!
Thanks, Jim for the tech support.

 

[mopowa]

CONCLUSION: I have gone back to my traditional setup. After noticing loss if fuel mileage and reading this great technical article Jim posted I am convinced that manifold vacuum is what I should be using, not potted ir timed vacuum. With fuel prics so high and nothing to gain I am back to where I started. I set the static spark @10 with 22 in the centrifugal side. Runslike a top!
Thanks, Jim for the tech support.

Hey Lockjaw, besides the fuel economy stuff how did it feel with the seat of the pants dyno. Did it feel or act different on a cold start, acceleration ect...
Ang.

 

[LOCKJAW]

I saw no difference in performance at all. I liked the exhaust note, but that was the only real differrnce. Highway mileage down by almost 2 mpg.

 

[mopowa]

I saw no difference in performance at all. I liked the exhaust note, but that was the only real differrnce. Highway mileage down by almost 2 mpg.

Thanks Man,
I might try it for a weekend just for sh##s and giggles. Two miles less per gallon is significant especially with no gain.
Ang.

 

[LOCKJAW]

Ang, I gotta admirable the motor really seems to like the vacadv. Smoother throttle transition when shifting and it seems more eager to take input changes with the manifold vac source.
Final final conclusion: If you have vac adv...leave it!:beer: