Vacuum advance to manifold vacuum?

I've read a LOT here about people saying (for a variety of reasons) to connect the distributor vacuum advance to manifold vacuum rather than ported vacuum. I haven't worked on every engine made or every distributor used, but from what I've experienced, this just doesn't make sense. Here's my understanding, so by all means set me straight (with an explanation) to cure my ignorance.
When an engine is put under load, additional advance is necessary to compensate for the flame front in the combustion chamber slowing down and added resistance to spark jumping the plug gap. When a load is applied to the engine and additional throttle is applied, vacuum DECREASES. All the distributors I've dealt with have a vacuum advance that moves OPPOSITE of the rotor rotation, meaning that when vacuum is applied it effectively ADVANCES ignition timing. In this instance, if you have manifold vacuum to the dist, you have maximum advance (not including mechanical) with little to no load (cruising/idle). Adding a load would actually RETARD the timing.
Are there that many distributors with the exact opposite action, or am I missing something really simple?

 

For performance and economy reasons, you want manifold vacuum. Ported vacuum has been used on pre-emission motors but not too often; it's become common on emission motors as it helps with reducing emissions, but nearly all I've seen used a temperature-controlled vacuum source to switch between manifold and ported vacuum. Ported vacuum advance can make the motor run hotter.

Explanation? Ok, here goes...

Under light-load conditions (small throttle opening, high manifold vacuum) you have a slower-burning, leaner mixture so starting the ignition burn sooner allows all the fuel to ignite. Apply a load and/or more throttle, the mixture richens/burns faster and you retard the timing so it doesn't all ignite before reaching TDC; it if does, you get knock.

 

Took what you posted and did more research to get decades of emission control "technology" out of my head. All my performance apps have been mechanical advance as those vehicles normally are all in by a set rpm. Vacuum advance as you explained it makes sense as I confused the burn rates.
Thanks for setting me straight..... I have work to do this weekend

 

Best explanation I've seen to date. Thanks, Steve.

 

Yeah, I learned my lesson about dual-point, no-vacuum-advance distributors on street cars a looong time ago. Real racey, but the gas mileage sucks....

 

Mr. T -

If you are asking these questions, looking to decide what to do to your engine....since it's all reversable...give everything a try...yourself..!
This is the way toward learning.
And yes, your thinking is correct.

Just remember, that any given way of doing the -
1. Full vacuum advance
2. Ported vacuum advance
3. No vacuum advance
The initial advance will need to be readjusted to get the full picture of your adjustments. JUST...altering the vacuum method, will not give you the full answer to your questions, you need to adjust the system as a whole.

What the "best" way, partially depends on the combination of parts in "your" engine, partially on "your" driving style, and the area you drive mostly (flat lands vs. mountain driving yes, people live in mountanious areas of SoCal).

For what it's worth, most of my past and my current cars engines, run with no vacuum advance..! But with more initial advance. My next combination will run a crank trigger with adjustable advance after the engine starts and no vacuum possible.

Test away (remember, you can change things back if you don't like them), but you need to test to learn.
Have fun.

Mike

 

I have one vehicle in particular I'll be playing with this weekend. Never got the gains I expected after a cam change, now I have a plan......

 

Keep in mind there's a bunch of variables here; different vacuum advance canisters will have different total amounts and rates of advance. You can still get adjustable canisters for some distributors allowing you to 'tune' the vacuum advance, much like recurving the mechanical advance.

 

Under light-load conditions (small throttle opening, high manifold vacuum) you have a slower-burning, leaner mixture so starting the ignition burn sooner allows all the fuel to ignite. Apply a load and/or more throttle, the mixture richens/burns faster and you retard the timing so it doesn't all ignite before reaching TDC; it if does, you get knock.[/QUOTE]
That's the opposite of what I've learned. Advance and a lean mixture causes knock. Lean mixtures burn hotter which is also why you retard timing on nitrous motors. You want advance to start the combustion process sooner as the RPMs increase. The faster the engine spins the less time there is for complete combustion.

 

Check out post #21 best info yet on the subject.
http://www.jalopyjournal.com/forum/showthread.php?t=740699&highlight=hotrodmyk&page=2

 

Hotrodmyk - Thanks for Link. The real Thanks goes to Bigchuck with his post.

 

You are welcome. Yes, Bigchuck set me straight.

 

One big variable is how much distributor advance you are running and how much initial.

With 6 initial and 32 in the distributor and 16 degrees vacuum advance you would definitely want manifold vacuum. That would give you 22 @ idle.

With 18 initial and 20 in the distributor with a 20 degree vacuum advance you would have 38 advance at idle without ported vacuum and that's likely not the best.

Hard to have a universal answer. 28 advance at idle is usually great, 38 less often works best.

Hoop

 

A +1 on what Hoop said...

But as a general rule, you want as much advance as you can get without the motor knocking. That will give you the best performance and mileage every time. Maybe not the cleanest emissions-wise (I think NOX goes up), but the best running. Every motor is different; some will tolerate more advance than others, what fuel are you running, etc.

One thing you do have to be careful about is too much initial advance is hard on the starter; that's why even the race motors have some sort of 'mechanical' advance, even if it's done with electronics.

 

If you have a HEI you can use a relay and a 5 pin module to get a cranking retard.

5 Pin EMR Example

Example:
.
Base timing:-->10 degrees
Mech Adv:--->24 degrees
Total Adv:--->34 degrees
.
Modified with EMR
.
Base timing:-->20 degrees
Mech Adv:--->14 degrees
Total Adv:--->34 degrees

 

Wow I just learned a whole bunch about vacuum advance.
Got some experimenting to do.

 

Swapped the mule over to manifold vacuum and the idle and mid-range has improved. It's easy to tell if there's a noticeable difference since it's only a 2.3 four cylinder.
I'll play with initial advance this weekend, as well as cam timing.

 

The original "Rule"

Vacuum Advance Principles and Applications
By Duke Williams, MSME
The basic rule for vacuum advance control (VAC) selection (henceforth referred to as THE RULE):
THE VAC SHOULD PROVIDE FULL ADVANCE AT NOT LESS THAN 2" LESS THAN PREVAILING IDLE
VACUUM AT NORMAL IDLE SPEED WITH APPROXIMATELY 24-32 DEGREES TOTAL IDLE TIMING.
This is a system engineering rule of thumb, and total idle timing should be in the upper half of the range for "big"
(high overlap) cams and the lower half for "mild" (low overlap) cams. With a 16 degree VAC this is achieved with
8-12 degrees of initial timing for mild to medium cams and 12-16 degrees for medium to big cams. Based on
overlap, the "300HP cam" is "mild", 327/350 and all BB cams, except L-88/ZL-1, are "medium", and all SB
mechanical lifter cams are "big". L-88/ZL-1 cams are "REAL big".
Idle vacuum in neutral is an inverse function of effective overlap, and the range on C1/C2 engines is the very high
overlap 30-30 cam, which only pulls 10"@900 to the low overlap base engine SB cams (which were also used on
some optional engines) that pull about 18"@500. All others are in between, except L-88/ZL-1, which are pure racing
engines that were never intended for street use so they were not equipped with VACs. In all cases, typical idle
vacuum is affected by both idle speed and total idle timing. Higher idle speed increases vacuum and, up to a point,
so does increasing total idle timing, which is why high overlap cams need both higher idle speed and higher total
idle timing.
Initial timing should also be established to keep maximum WOT timing in the 34-40 degree range for SBs and 36-
42 degree range for BBs, and WOT detonation may dictate the lower end of these ranges depending on compression
ratio, cam, and operating conditions such as ambient air temperature and altitude. Higher ambient temperatures
promote detonation as do low altitudes where average air density is higher.
Higher overlap increases exhaust gas dilution at idle and cruise, which slows flame propagation speed, which
increases the timing requirement. Insufficient total timing at idle and low speed cruise increases EGT, which will cause cause more heat to be absorbed by the cooling system, which can result in high operating temperatures and, in
extreme cases, overheating to the boilover point, even if all cooling system components are within their original
performance range.
Using THE RULE, one of the following three NAPA/Echlin vacuum cans should be appropriate for all C1/C2 OE
engines, including those converted from ported to full time vacuum advance and C1 engines that are converted from
the non-vacuum advance dual-point distributor to a single point vacuum advance distributor.
VACs for modified engines (such as cams that alter OE idle vacuum characteristics) should be selected using THE
RULE.
My system engineering "best fit" for all OE engines is also listed including those not originally equipped with
VACS, but a few "best fit" VACs (396/425, 427/435 and '63 327/340,360) are significantly different than OE, due to
either a poor match to engine idle vacuum characteristics i.e. don't meet THE RULE ('63 327/340, ’65 396/425) or
will not meet THE RULE when converted from ported to full time vacuum advance (427/435, '63 327/360). My
"best fit" for 327/350 is also different than OE, which I discuss below.
VC1802 (stamped "B22") 0@8" 16@16" (283/220, 230, 245, 250, 275; 327/250, 300)
VC1765 (stamped "B20" or “B26&#8221 both 0@6", 16@12" (327/350, 396/425, 427/390, 400, 425, 435)
VC1810 (stamped "B28") 0@4", 16@8" (283/270, 290, 315; 327/340, 360, 365, 375, L-88/ZL-1)
All these VACs are now manufactured by Standard Motor Products, and they can be cross referenced to other
brands (Standard, BWD, Neihoff, Wells, Delco) at the other brands Web sites. The alphanumeric code stamped on
the mounting bracket (B22, B20, B26,or B28) is the code that denotes the specifications regardless of the brand
name/part number.

 

Guess I still haven't learned my lesson. I don't run vacuum advance distributors, and my 327 powered Austin, with twin 450's on a tunnel ram, and 3.73 gears, still gets 16 mpg on the highway.

 

I'd be willing to bet you a grand plus expenses I can get you 10 per cent better!

 

I've never had much luck with running vacuum off the manifold. It seems like it runs worse through the rpm range.

I'm getting a lot of hard starting when hot, I assume it's because of too much initial advance. But if I retard the distributor, it's too little. I'm starting to suspect it's because the cam was not put in straight up.

 

wife's '50 Chevy PU has a Chevy ZZ4 350 crate motor. ended up not using any vacuum with factory distributor and it runs great. before would ping under hard acceleration, especially at high altitude.

 

100% read post #2 you will find your answer.

 

With some manifold vacuum, and some tweaking on that light of a car you ought to make 20 mpg pretty easy.

 

If the question is manifold vs ported you have to answer a few other questions.

1. What is my initial timing?
2. What is the lower limit on my vacuum advance.
3.What is my idle vacuum?

We are shooting for 24 -32 degrees at Idle.
Our best mechanical curve advance comes first.
We will probably want 16 degrees of vacuum but may have to limit that for sustained part throttle knock.

Example SBC;

12 degrees initial, 24 distributor, 16 degrees vacuum @ 2 " below idle vacuum is going to be very close to ideal.

So let's say you have 20 initial and 16 in the distributor and a 20 degree vacuum advance, that would give you 40 at idle which i likely too high.

Fuel economy will definitely pick up with a properly setup vacuum advance and power enrichment system, probably 20 - 30 pct over having neither of them.

 

Duplicate

 

You can't just throw a distributor in a motor and expect it to be ideal. You have to know your total timing, amount of mechanical advance and at what rpm, how much vacuum advance and what vacuum it comes in at. I think the people having trouble with vacuum advance need to tune the distributor. If it pings with vacuum advance you have to limit the the advance or get a can that drops out at a lower vacuum.


Ago

 

I've continued tweaking my tune to see if I can get better performance or mileage out of this. Just so it's not assumed this is a SBC and the normal parts can be thrown at it, this is a '74 Pinto wagon with a tweaked 2.3 & auto.
I swapped from ported vacuum to manifold vacuum and picked up a smoother idle, mildly increased pickup in low and mid ranges. I've lost some power in higher rpm, so I gave it a few more degrees of initial advance as it liked more advance in higher rpm before. I'll be playing with the initial advance this weekend to sneak up on it either pinging on accel, or cranking hard. I have yet to find an adjustable advance can for a 2.3 dist, so this won't be an optimum setup, just a "best compromise". I got a consistent 17mpg on ported vacuum, so if I can increase the mileage and have close to the same power I'll be happy.

 

Another name for Ported Vacuum is Timed Vacuum. The vacuum source is "timed" to the throttle opening. As soon as you open the throttle it is manifold vacuum, so ported/timed vs manifold vacuum only affects idle, as son as the throttle is opened they are equal.

Way before we had Smog there was ported or timed vacuum advance. Short duration, low overlap cammed engines may idle fine on less timing. As soon as we get more radical on the cam timing they definitely like more timing.

Try it both ways and whichever one idles best, use.

Ported vacuum may help you pass an idle smog test, manifold vacuum may cool the engine at idle.

No one size fits all but as you get a healthy cam it is very likely you will want vacuum advance to get you up around 24-32 adv at idle.

Finding the best compromise is the art of tuning.

 

I know I'm coming a little late to this thread, but help me walk through the process of setting the timing with manifold vacuum. Let's just say that I wanted 24° Advance at idle (initial+vac cannnister).
So if I set my initial timing to 6 degrees (without the vacuum hooked up to the distributor), then when I hook up the vacuum to the distributor I'm wanting to see 24 degrees so then do I use the Allen wrench adjustment inside of the canister to either increase or decrease Advanced until I get that 24 degrees (at idle), which is the initial 6 plus the canister (18) to make 24?