• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

How Much Vacuum Advance?

E

EngineerDoug

Well-Known Member
Local time
11:59 PM
Joined
Oct 5, 2016
Messages
321
Reaction score
174
Location
Los Gatos, CA
Folks,

I am running a mild 383 on the street with a Mopar electronic ignition. Compression is 8.5:1.

I have found that the engine likes 18 degrees of initial advance, with 18 degrees of mechanical advance. My question is, aside from checking for ping at light throttle, how much vacuum advance should I run? I have seen some old sources that suggest a total (initial + mechanical + vacuum) advance of 54 degrees for a BB Mopar. That amounts to about 18 degrees in the can; sounds like a lot?

How does that jive with your experience? Thanks.
 
Only thing to do is experiment. On my car I ended up with 20*/15*/10*. Any more and the car started to ping. A lot of guys dont run the vacuum canister at all so there's that to consider also...
 
I think most manufacturers had 14 to 20 degrees in the vacuum advance can, some maybe more. Your 18 sounds about right for stock.
Remember that when those engines were new in the 1960's they had very high octane fuel compared to what is available now and were better able to resist detonation.
Your say your engine is mild, connect it and monitor it. From my experience with the can connected if you have a bit much advance - they ping if you get on the gas or at heavy load as you start to pull up a steep road or gradient.
Unfortunately most cans cannot be tuned as to how much advance they give or when it comes in.
 
Hopefully the OP has something like a Firecore distributor with adjustable vacuum advance...
 
An easy test an old friend taught me. Bring the revs up slowly in neutral, and if begins to missfire slightly, disconnect the vacuum advance and try again. I could not hear slight pinging in my old car (headers, noisy valvetrain) but I could feel what felt like a lean surge above 2500 rpm on the highway. I tried the neutral test and it began to missfire around 2500 rpm (when the centrifugal advance was all in). I disconnected the vacuum hose and no missfire.
 
What size of hex key wrench is it in the vacuum port, 3/32?
 
The method I use is to hook up a vacuum gauge to the ported vacuum and run the hose to the passenger compartment.

Warm everything up, go for a ride, find a flat straightaway.

Cruising in high gear with light throttle pressure, take note of the vacuum reading.

Remove the distributor, connect a vacuum pump to it, draw the vacuum that you noted while cruising.

Adjust the can so the arm is fully retracted at the vacuum reading that was noted.

My stock 67 318 on 87 octane setup is 10 initial, 35 total, 17 inHg vacuum
 
Last edited:
With my '70 383 low CR (8.5 ish) we modified the stock (points) distrib by brazing up the mechanical slot for 12*(24* crank) & set initial at 10 to 12* for 34 to 36* total and used lighter advance spring in place of the heavy one. Worked very well. Wish that had been an electronic distrib, points were a pain.
 
Low compression engines tend to like more timing but the rest of the combination plays a part too. Experimentation is your friend. You didn't supply enough info for anyone to make an 'eddycated' guess. Got a cam that builds a lot of cylinder pressure in the lower rpm ranges? What's your gasoline quality? That's just two questions and there are many more. What is your compression check readings? I've had the pleasure (more like displeasure) of dealing with two engines with low mechanical compression plus compression test numbers in the 120 range that liked 50 total!!! One was a 440 and the other was a 318. The first engine (440) had me doubting my timing light until trying two other lights. The 440 ran mid 12's in a 67 Coronet and the 318 pushed a 72 Demon mid 13's.....
 
Toolman you are right. 3/32'' allen. Most guys don't know that Mopar distributor vacuum cans are adjustable.
 
Mine is close to stock six barrel, it's real happy at 17 / 37 / 52.
Been that way for many years.
 
Cranky said:
Low compression engines tend to like more timing but the rest of the combination plays a part too. Experimentation is your friend. You didn't supply enough info for anyone to make an 'eddycated' guess. Got a cam that builds a lot of cylinder pressure in the lower rpm ranges? What's your gasoline quality? That's just two questions and there are many more. What is your compression check readings? I've had the pleasure (more like displeasure) of dealing with two engines with low mechanical compression plus compression test numbers in the 120 range that liked 50 total!!! One was a 440 and the other was a 318. The first engine (440) had me doubting my timing light until trying two other lights. The 440 ran mid 12's in a 67 Coronet and the 318 pushed a 72 Demon mid 13's.....
Click to expand...

Sounds like my 440-average of 130 psi in all cylinders...
 
EngineerDoug said:
Folks,

I am running a mild 383 on the street with a Mopar electronic ignition. Compression is 8.5:1.

I have found that the engine likes 18 degrees of initial advance, with 18 degrees of mechanical advance. My question is, aside from checking for ping at light throttle, how much vacuum advance should I run? I have seen some old sources that suggest a total (initial + mechanical + vacuum) advance of 54 degrees for a BB Mopar. That amounts to about 18 degrees in the can; sounds like a lot?

How does that jive with your experience? Thanks.
Click to expand...

Those are almost my numbers on my 440. Actually the combined went off the scale, past 55*. My mechanical was too much and had to limit that. I've adjusted my vacuum can to limit it as far as it will go. It's good now but I might still do some tinkering internally to limit it a bit more. Don't want to go much more than 50* - 55*
 
If I remember correctly the adjustment relates to the amount of vacuum necessary to start the advance, not the total amount. To restrict that you need to somehow limit the arm movement.
 
The vacuum cans do have different part numbers along with different ratios. The ratios are marked on the cans arm.This relates to how much movement thus advance.
 
I've been fooling around with the vacuum advance a bit and have noticed that the idle likes to hunt around a bit. I have read all the posts about ported versus manifold vacuum, and which one to use. I have been using manifold vacuum, which adds another 18 degrees timing to the 18 degrees initial. This added timing increases the idle speed - the engine likes the added timing.

In gear my idle speed is about 800 rpm, with manifold vacuum about 14". The adjustment on the vacuum canister shifts the amount of vacuum needed to move the advance arm. Adjust it all the way CW, and vacuum advance kicks in once the vacuum climbs above about 13". If you start to adjust the canister CCW, it takes more and more vacuum before the arm starts moving.

OK so here is my theory. The vacuum advance and engine RPM constitute a feedback loop. My manifold vacuum falls right in the range over which the vacuum canister operates (note whatever the preload setting, the advance comes in over a range of about 5" vacuum). So if the RPM falls a bit, the vacuum falls a bit, and then the advance falls a bit. They reinforce each other, and that goes for RPM increases as well. The engine will hunt around for a range of vacuum and RPM.

This might explain why some combinations like ported vacuum, some like manifold vacuum, and some just don't like vacuum advance at all. My combination just happens to generate enough vacuum that it will dither the vacuum advance around. So I am going to try a couple of things:

1) Adjust the canister so that advance kicks in well above my idle vacuum reading; that should yield no advance at idle.
2) Try ported vacuum so there is no advance at idle.

We'll see if this yields any results.
 
Yes getting the idle right with vacuum advance and a lumpy cam can be a problem. In neutral there is enough vacuum to bring up the idle speed, then you put it in gear and the vacuum falls off and the advance is lost and it stalls out. Double whammy.
 
Try the ported vacuum. 36 degrees at idle is too much for me.
A lot of those engines that had manifold vacuum on the advance unit had 8:1 compression and were lean as **** and could not fire the mixture unless it was advanced to hell.
Forgot one thing - they also had only a few degrees of initial timing as well. Typically only 2 to 5 degrees.
 
Last edited:
You must log in or register to reply here.

Similar threads

funknut
Help me pick a distributor!
2
Replies
22
Views
2K
XCELLR8
X
Road Grabber
Distributor Advance Expertise Needed
2 3
Replies
45
Views
7K
biomedtechguy
biomedtechguy
Kern Dog
Electronic distributors and tuning them.
4 5 6
Replies
106
Views
4K
Kern Dog
Kern Dog
D
Timing mark jumping around
Replies
19
Views
2K
Black_Sheep
Black_Sheep
6
Understanding Distributor Mechanical and Vacuum Advance Springs
Replies
18
Views
3K
67 GTX
6
Auto Transport Service
Back
Top