Hot Rods Eliminating Vacuum advance

What are the advantages and disadvantages of eliminating vacuum advance? I know early vettes relied on mechanical advance only and alot of aftermarket dizzys are that way also. Have some of you done away with it and liked the results? If it is eliminated, there has to be a happy medium for initial timing and total timing after the mechanical advance kicks in, right?? I'm converting to the Pertronix Ignitor and I've purchased a curve kit to lighten up the advance springs. The engine is completely rebuilt top to bottom with factory grind cam. Some have told me that If I have the vacuum to support it to leave it alone but I hate the way it looks and I do not want to drill my NOS 6x2 Edelbrock intake. I'd love to hear from the guys that would know. Thanks.

 

Its mainly there to improve your gas mileage and off idle performance, but with 6 2's and depending on your cam it won't make much difference. It won't affect your initial timing because you block the vacuum advance off when settting your initial and total timing.

 

I run straight mechanical advance on all my rides. Since all my rides use a mallory dist converted to Petronix it is fairly easy. I put a lot on inital advance in and then set the internal advance so that I get 2 degrees below max advance. This allows me to have good throttle response and still get good top end power

 

I used to eliminate the vacuum advance on modified street engine cause that's what everyone did with a recurved distributor.
Now I use the vacuum advance limited to about 12 crankshaft degrees hooked up to full intake vacuum not ported vacuum on the carb.I got the full vac*** tip from a guy who builds hi po HEI type distributors.On many types of carbs,at full throttle ,a false signal can be created as air p***es the ported vacuum opening and pulls the vacuum advance into operation when it absolutley isn't needed causing loss of power from too much spark lead and detonation.
If you engine is really hotted up as mentioned above a vacuum advance may not do anything anyways.

 

I'm curious as to what type of carb adds vacuum advance at full throttle?


Lack of vacuum advance not sourced to manifold vacuum usually leads to overheating.


Lookin' cool at the rod run is one thing, boiling over in traffic . . . another....

 

What kind of carb? A Holley or Edelbrock can do it,maybe any carb.Just hook a vacuum gauge to the ported vacuum on the carb abd try it at full throttle to see if it pulls vacuum at maxium revs.90 percent of engines appear to use a ported carb port for vacuum advance.The port that only sees vacuum after the carb is opened off idle.

 

I did track manifold and ported vacuum with a couple of fairly well matched vacuum gauges and found that, aside from idle and low speeds, each port followed the other fairly well.

I can post the info if you like.

 

Sounds like you're not going to run vacuum advance so I'd set your initial timing at 12 degrees, limit the distributor's advance to 12 degrees (which is 24 at the crank) and you'll have 36 total. Any combination that gets you 34-38 degrees total is fine.

Your lighter springs shouldn't have anything to do with total mechanical advance. They will, however, allow the distributor to attain maxiumum advance sooner.

Less initial timing (for example 5 degrees btdc on a 56 olds) is used by the factory primarily for emissions purposes. FYI some old 50's fords even had devices that retarded the timing to AFTER top dead center at idle for emissions. Your engine will probably like to idle better with 10-14 degrees initial advance.

Peak performance is all trial and error based on gear ratio, vehicle weight, engine characteristics. Takes some fiddling. Have fun!

 

it can help a heavy / high geared cars part throttle driveability
it helps dead spots

 

We advance the timing trying get the perfectly optimum time for combustion during the power stroke. There are many variables that effect how long the complete combustion process takes. So, advancing the timing based solely on RPM doesn't cover all the possible scenarios. But how do you tie those other variables into a mechanical contraption to adjust timing? You see the problem. To adjust the timing ever slightly more, we have an additional device called "Vacuum Advance".
A large factor in the rate of combustion is the density of the fuel/air mix (charge) in the cylinder. A low density mixture burns slower than a high density mixture. So, the spark needs to occur earlier when the fuel/mix charge is less dense which happens at closed (idle) or part open throttle (cruise = flat road) operation. At wide-open throttle (WOT) operation the fuel/air mix is dense and combustion is rapid, so no additional advance is needed.
This additional timing adjustment is made with a "vacuum advance". This is a clever device that ties manifold pressure to a mechanical advance mechanism. At "part-throttle" operation (like cruising on a flat road) the manifold vacuum is high and engine load is low. At full throttle (WOT) the manifold vacuum is low and engine load is high. A vacuum driven diaphragm on the distributor advances the spark even more when the manifold vacuum is higher. SO.... the spark is advanced more during closed or part throttle operation. Example:

Advance at Wide Open throttle = Initial Advance setting + Advance from centrifugal advance only
Advance at Close or Part-Throttle = Initial Advance setting + Centrifugal advance + Vacuum advance

This improves driveability, fuel mileage, and cooler running temp. Many stock distributors will pull as much as 50 degrees total timing under light load, high vacuum!

 

Initial timing is set for low speed operation and starting. There is a bit of compromise there. Sometimes, if you can't get a decent low speed operation without kicking back on the starter, a manifold vacuum signal is used. More on that later.

Centrifugal advance is to compensate for the reduced time at higher RPM to start the burn. If you want the flame front pushing hard at 10 degrees after TDC (think about pedalling a bike), you start the flame front earlier when the engine is spinning faster to get it to hit at the same (right) time.

Those 2 will make the car run pretty good. It won't be optimized for street use, but it will work.

Vacuum advance is load related. Less load, more vacuum. More load, less vacuum. This allows an even closer to optimum timing curve, since initial and centrifugal do not change with load. This is especially useful during part throttle driving. The reasons race cars and the guys that drove hot street cars did away with vacuum advance are:
1. No part throttle vehicle operation, on or off
2. Engine changes that favored only high RPM operation, which tended to kill low RPM vacuum. This meant that the vacuum signal no longer mirrored engine load.
3. After setting the dist. initial timing and centrifugal advance, they did not want to introduce another variable to fail (KISS)
4. Since the vacuum advance helps MPG, this was not a concern for the intended use. The average daily driver complains about MPG much more.

Of course like a lot of race stuff, some guys saw that and had to copy it. Others that had big cams ended up disconnecting it due to No. 2 above. More copycats and guys that couldn't afford a new vacuum advance unit or were unable to really dial in the timing followed.

BTW, Chevy in particular used to connect to manifold vacuum instead of ported vacuum. If you set the timing up to where it's happy at idle, it will kick back on the starter when cranking. It also caused some tuning issues. If you set the idle in neutral then check it in gear (PG), the idle would drop more than the usual 100-200 RPM, due to the decrease in vacuum (more load). On some tired engines, you could see a 400-500 RPM drop!
This would require a higher idle, getting into the range where dieseling (especially on an older engine) can happen.

 

Well said guys!

 

Yes,more information is always better. The situation I'm talking about is actual experience from several guys and myself running full intake vacuum on mild- moderate tuned SBC's.With no changes to the mechanical advance,running the vacuum advance to full intake vacuum gave better low speed response,a harder pull at top end and less detonation on some engines.Again,the vacuum advance was limited to 10-12 crankshaft degrees,mechanical advance total of 32-36 degrees with initial timing of 12-18 degrees depending on who's engine is was.Obiviously the vacuum advance was disconnected when setting the timing.

 

On many OEM engines that little pod was simply a 'load compensator' that removed timing when the throttle was opened using manifold vacuum.

 

If your engine is even remotely a street engine, it needs vacuum advance.

The guys that successfully run without vacuum advance fall into 3 categories-
#1- High compression race engines on the street
#2- Supercharged
#3- Stockish engines & owners that are oblivious to what they're leaving on the table.

good luck!

 

I run vacuum advance on my blown Hemi, When cruising on the hi way light light load 16 in. the vacuum works great. If I were you I run it.


Ago

 

Here's part of a timing article I wrote:

"I got curious about a comment I heard about manifold and ported vacuum going to zero at WOT (Wide Open Throttle) and ran a little experiment.

The car - 32 roadster - weighs 2400#, engine is an overbored 455 with 462 cid, 9/1 compression ratio, Edelbrock Performer intake, Carter 750 cfm compe***ion carb with electric choke added later and a Crower Compu-Pro #1 cam which has about 262 & 266 degrees advertised duration intake and exhaust with 112 degree lobe centers. It’s a smooth cam and the car when warm idles @ 19" vacuum.
The dash carries a large (2 5/8") S-W vacuum gauge which indicates the same as my vacuum/pressure test gauge.

Advance is 8 degrees initial and all in at about 2600 rpm with a total of 32 degrees. Vacuum advance is about 16 degrees and sourced from Manifold Vacuum (MV).
The car runs very well on 87 octane in summer and winter and does not overheat in traffic.

Firing the car from dead cold and on the elec choke, MV reads 18-19" and idle is around 900-1000 rpm.
Ported Vacuum (PV) read 12" on startup.

Once the engine warmed up, MV reads 18.5 - 19" and PV reads zero at about 500-600 rpm.

Cruise at 40 mph with a light throttle setting on a flat road gives you 18.5 - 19" MV and just about the same on PV.
Rolling the throttle in about half way shows 8 - 10" of vacuum on both MV and PV during light acceleration.

Once at 60 mph MV read 18 - 18.5" vacuum (keep in mind this is a very light car) and PV read
10".
Flooring the throttle at 40 mph or 60 mph brought the MV down to 1" or so and PV to zero.

At idle with a fully warm engine, MV reads 18.5 - 19" and PV reads zero.

The lack of additional timing at idle is what creates an overheating problem in the GM engines.
It takes time to burn the lean idle mixture and additional advance is required to get the process underway early and avoid overheating.
Exactly the same thing (overheating) would happen with the timing severely retarded in an engine under load at a higher rpm level.

There’s a lot of confusion out there about timing, both centrifugal (mechanical advance) and vacuum advance as well as the vacuum sources to use.
The key thing is to realize they are two different systems that work together to give optimum spark advance for a particular condition and key on rpm as well as load.

To my way of thinking perhaps there would be less confusion if the vacuum advance cannister was called the vacuum retard cannister.

I’ve been amazed at the lengths some go to, to cure an overheating problem that can be solved in most cases simply by selecting the correct vacuum source.
Granted, most of my experience has been in cars with small engine bays and many times not the biggest radiator in the world, but I note, the bigger cars have the same amount of timing and overheating problems as the small car guys do and for some reason many car owners avoid doing something as simple as swapping vacuum sources to cure overheating and prefer to throw money at the problem.

As far as spinning up a little experiment, I’m not trying to prove anyone wrong here, just got curious, had some free time and those are the results I came up with.




An additional note; for those of you who live at a higher al***ude than where these tests took place, you’ll find that your vacuum levels at no-load (idle) rpms will read lower.
To the tune of a 1" vacuum loss for every 1000' of al***ude.
The tests took place at 350' al***ude and manifold vacuum at idle read 18.5 - 19".
After moving to Sunny Arizona and ending up at 3300' al***ude the manifold vacuum now reads 15.5".
Highway figures and under load vacuum levels remain the same.




This combined article was written from research on timing figures and real world experience.

You can learn a lot by taking the time to set up some inexpensive instrumentation and taking the time to run a few simple tests.
What you’ll gain is a better understanding as to what’s going on with your engine and gain a small bit of education about the particular thing you’re researching.

Learning from books is one way to do it and there’s nothing quite like taking advantage of what smart, experienced and educated people have done and written down for your educational pleasure.

What I’m talking about is the stubbornness and unwillingness to learn from those who’ve been down the road before us.
It’s amazing sometimes to talk to an individual who thinks factory engineers don’t know much.Thing to recognize is that factory engineers know a helluva lot more than we do and get into sophisticated areas that the great majority of us know nothing about.

Keep in mind too, factory engineers are constrained by the bean counters, the necessity to build a vehicle that is useful to the majority and seldom are let loose to pursue a dream or even an interesting idea.
When they do get the freedom to investigate particularly interesting areas, the results can be astounding.

The name, "Zora-Arkus Duntov" should ring a bell...."
__________________

 

A small addendum.

GM developed ported vacuum so that the engine would run hotter on the low end.

I believe it helped the Nox situation.

Engines that ran this system also had a - usually - black plastic internal thermostat controlled vacuum switch.

The engine normally ran ported vacuum to the distributor advance can.

When engine temps got high enough it switched to manifold vacuum, temp came down and it returned to ported vacuum.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some of the confusion about ported/manifold vacuum advance sources came about - in my opinion - due to Carter and later Edelbrock carburetor instructions that point to the ported vacuum bib as the place to install vacuum advance.

Doesn't work well on the average hot rod.
Average meaning only one hose to the vacuum advance system.

 

OR:
#4-Someone who has a properly set up (curved) distributor to elimate the need for vacuum in a engine that has little or no useable vacuum due to cam profile, multiple carbs or both.

 

Can i just say "amen" ? Good take on a system often misunderstood!!

 

This all relates directly to what I've been going thru with the Cad 472 in my '36 roadster. It ran hot initially, and some poking around revealed the diaphram in the va***e advance (retard?) was faulty, so I had the distributor way retarded.

So, today I'm replacing the bad one with the new one. The 472 had the black plastic thermal switch you mention in a water jacket. I've tossed that long ago, and was going to simply hook the vac. line to the port at the carb base, the stock Quadrajet as I'd done with that engine before in another car. Would I be better off to use manifold va***e? The last thing this now needs is help to run hotter...

 

Multiple carbs, huge cam = race engine. Refer to #1

But this thread did shake out somebody using vacuum advance with a blower, and I'd sure like to know more about that!

 

Sounds like you may be connecting the vac advance source to manifold vacuum.

I note that the late Carter/Edelbrocks have bibs at the front of the carb.
P***enger side (USA) is ported.
Drivers side (USA) is manifold.

Best bet is to stick a vacuum gauge on it and see what you get.


Shifty's comment on big cams etc. is a good one.

Fwiw - a 280* advertised duration cam idles about 10"-12" vacuum.
A stock or very mild cam - 260*-266* intake and exhaust duration cam will idle at 17"-19".

A big cam - the 280* cam mentioned above - will reach vacuum levels of 17"-19" not too far up the rpm range from idle.
Somewhere around 1200-1500 rpm in most cases.

Highway speed vacuum levels will be about the same for most any cam within streetable reason.


Note that the "big" 280* cam is a streetable cam in most cases.
Especially if you're running a lot of cubic inches.

The 284*-292* cam in my 462" Buick idled at about 11"-12" at 600 rpm with dual quads.


I'd with Shifty and would like to hear a little more about the vacuum advance install on the blown engine.

Seems like you could choose a vacuum source below the throttle blades and above the blower and it would work ok.

And tapping into the manifold proper below the blower would also (IMO) result in a vacuum advance system working properly.

 

some old 50's fords even had devices that retarded the timing to AFTER top dead center at idle for emissions
Those would be the 1950s Fords built in 1967.

little or no useable vacuum due to cam profile, multiple carbs or both
So, your power valves are always open, metering rods fully retracted, the engine runs full rich all the time?
The manifold vacuum curves for race cam + big carb vs. small cam + 180° manifold are very different at idle, but begin to get very close at 60 mph - which is when gas mileage matters.

 

I'm sure you're correct about that,however,I've worked on a stock 40-50's GM light trucks and that have have a ported vacuum for the distributor.I doubt there was any concern about Nox then.
Also switching from ported vacuum to intake vacuum will advance the timing at idle and bump up idle speed a few hundred rpm's.So maybe that's what's it's about?

 

No. If it runs on manifold vac it's a 'load compensator' to back off timing when the throttle is opened. Too much advance will detonate or backfire through the carb.

I think available vacuum has more variables than duration and carbs. My 477 race motor had 18" of manifold vacuum at idle (1100RPM) with a Comp Cams roller that had 286 deg/duration @.050. It was very easy to tune but had no relation to the vac advance gig here...I ran 36deg total at all times with a 22deg retard box for starting. Initial was actually 38 but would pull about 2deg out at 3500RPM, which I always attributed to "mechanical take-up" of all the tolerances from the crank to the distributor.

Ignition can be as simple as it can be complicated.

 

True, idle speed does go up, but it's easily cranked down with the carb idle speed adjuster.


Where was the ported carb bib located?

Some carbs have a bib up on the body of the carb and supply maifold vacuum there.

So the simple test for manifold or ported vacuum being below or above the throttle blades is a good one, but doesn't work in every case.

Which is one good reason to have a vacuum gauge in your toolbox.

Better yet is to run one in the car.

You can catch some things going wrong with a vacuum gauge before they fail completely.

 

It's far better to run manifold vacuum in my opinion.

It's a freebie in most cases and far better than springing for a new radiator, fan etc. which I've seen a few guys do rather than try a small experiment.

The 462" Buick in my 32 roadster would run hot when it was initially on the road.
Traffic on hot days or even on cool summer California mornings would have it start to overheat in 5-10 minutes.

I swapped to manifold vacuum and the slow speed/idle overheating problem went away and it was no problem to idle through the In & Out Burger take-out line on hot days.
Still true today, although it runs about five degrees warmer than it used to.
The Walker radiator is circa 1985 and put in service in 1993....

 

Good post guys !!! I have taught these systems for over 30 years and i can tell ya that each engine desisgner is different from the others.
Motor Manuals has made a big yellow manual for years with tons of pages on each and every timing advance and retard including all the emission devises applied to indivudal vehicles.
EACH METHOD IS WORKABLE PER ENGINE.
Try it with vacuum and then try it with out. Then use the one that works!
We typically ship our conversions "without" based on the fact that few have the equipment and the patience to tune a vacuum curve in the igntion after any engine modifications. However those that do and have certaintly seen a increase in overall performance.!!

 

After today, I'm waiting for the proof of the pudding so to speak.

My long time pal rings me up this morning and tells me his 302 SBF with carb and Unilite distributor overheated in a parade a few days back.
102* F fwiw . . . that always brings out the shortcomings.

So he's thinking he needs a bigger fan and an aluminum radiator.

We've had this conversation before - in fact about his two previous hot rods which ran SBC's, no vacuum advance and they'd overheat in traffic on warm SoCal days.

So he's in Sunny Arizona now, moved here a month after we did.

I run the roadster up there about 1100 in mid-90's heat.
Which is ok, cuz I'm somewhat acclimated to it and the rear window which opens up for lots of airflow really helps.

My thinking was, I'd demonstrate the frickin' vacuum overheating to him.

Plug in ported vacuum, fire up the roadster, let it get hot at idle and then swap back to manifold vacuum and watch it cool down, but he demurred.

I think he's in love with the ***y looking hot roddy distributor and doesn't want to blow $20. on a junkyard distributor to find out.

We went into the nice cool house and I see several catalogs laid out to the radiator sections.

Looks like he's gonna blow $500. or so and end up with the same problem.

Seen it with other guys before and I still can't believe it when it happens.

I mean, I ain't that smart, but I ain't dumb either....