Eliminating vacuum advance

[Ghoste]

I know it's function and setting it up and the fact that most performance cars do not run one at all, but, what I am wondering was just how much difference does it really make when you eliminate it?
I am talking about highway cruise mileage of course, since that seems to be it's primary function.

[RallyeMike]

In a race application, the load is pretty much constant and the rpm's are all in right off the bat, so vacuum advance is not needed. For a street driven car, vac advance is important because it adjusts advance to varying engine load, improving driveability.

[Ghoste]

But... a LOT of street performance cars are running without it.  Given that, how much are they really giving up?

[dodge freak]

Most likely a lot of headaches. If the light springs are in the dist. so the advance comes in by 2000-2500 and the car has 3.91's or higher - which lots of street performance cars have- I would guess not much. I have to guess cause if I were to hook mine up , it would not run as smooth. Seems like as soon as the motor gets more than 38 degrees or so I can see the motor start to shake at 3500-4000 rpm's. The only reason I can think to use it if the motor pings with both of the light springs in the dist. so the advance is not in till 3000-3500 rpm, then a little vacuum advance would help, just enough to get the advance back to around 38 degrees. I would never run 45-50 degrees of advance even if it is only lite throttle , motors like 38 or so. The best way to increase gas milage is to slow the motor down 3.23 gears  or lower yet and or increase the compression, which if we could run 12 to 13-1 would help out a lot.

[firefighter3931]

The problem with vacuum advance is that it's designed for stock motors which make in excess of 15in vacuum. Once you hop up the engine with a cam, manifold, headers etc...tuning with it enabled causes issues (surging/pinging etc...). If your distributor is curved properly with total advance all in by 2500-2800 then you'll never miss it, inmo. The other issue is detonation...why risk the engine by pulling in extra timing for what is a minimal milage increase...if that.  

Ron

[Ghoste]

That is kind of why I was wondering.  I disconnected mine last year or the year before and I haven't seen the great drop in mileage that I was told I would get on the highway.  My car is pretty screwed up anyway but it still made me wonder.

[Chryco Psycho]

I have seen milage improve with the vacuum disconnected

[Ghoste]

Really?  Any idea why that would be Neil?

[Chryco Psycho]

at higher RPM the dyno I use has proven the timing can or needs to be retarted so adding more vacuum at a stable higher RPM such as highway cruzing may be the wrong direction & by eliminatiing the vacuum advance the timing cannot increse at higher RPM , the last car I dynoed had a retard system & showed loss of power as the retard at higher RPM was reduced 

[RallyeMike]

My problem is that I'm still lost at about 1979 and think people still drive their Chargers every day...      

Firefighter is probably closer to target. This probably is not a stock regular driver.  I still drive a 1971 Mopar every day, with an occasional fling in a 1972 car, and the factory advance set-up is pretty optimal for a daily 6AM ride to work.

[Ghoste]

Well, as Ron and Neil already know, my car doesn't make a whole lot of vacuum.  It is an adjustable vacuum advance which I would think could compensate for it.  CP's point makes a lot of sense though and both those guys know I don't do a lot of highway driving other than the 5 hour trip to Columbus each summer.  The rest of the time is more local and spirited driving.
Since it sounds like it doesn't make a tremendous difference to start with and likely less with my setup, I guess there is no sense trying to reset the thing prior to next weekend.

[chrisII]

in my small block circle tracker i wanted more advance than the stock setup alowed but did not want the vacum adv, if you take different dists apart you find that they have different amounts of mechanical advance, but i could not find enough so i elongated the slots in the mech advance till the weights almost hit the housing. then i used 2 light springs, toped it with the pickup from a lean burn dist (its fixed). I did this beacuase i was runing a limited compreshon 2bbl carb class and i was actually making the best power at about 44-48* @3000 rpm. by playing with the springs i was able to get the 48* and still have it back down around 25-30 so i could start it. nobody even neads to tell me that 48* is WAAAYY too much advance,it did work well for what i was doing tho the engine took it and made gooobs of power. the amount i was able to get would work well on most modified street cars with litle vacume, but it is rough on the springs to stretch them that much.  I have also seen guys remove the vac diaphram and install a choke cable into the drivers area so they can manyally fine tune the advance as they are driving (less for start and idle, more at hwy speed)

[dodge freak]

You can also wire a toggle switch for the ignition. This way you can flip the switch off so there be no spark and you can then crank the motor over, once it is cranking flip the switch on for the spark to start. Now of days most of the MSD boxes can get a start retard wire in, it retards the spark 20 degrees or so until the motor starts.

Those lean burn pick up plates are great, one piece and cheap. Way to go Mopar.

[Ghoste]

Didn't some of the old super stock guys have "secret" switches installed so they could retard the timing at the far end of the track?

[grouseman]

What is your current set up?  What mileage do you get presently? 

[Ghoste]

If you're talking about mine, the current state is confused and I never check the mileage.  I know it didn't see a big drop when I unhooked it because if I do any long distance driving, it is generally to the same locations and one tank takes me to the same filling stations it always did before.

[myk]

So, when you unhooked your advance how did you have to adjust the engine?  When I disconnect my advance, idle speed drops quite a bit and I'm convinced that the engine won't run smoothly...

[dodge freak]

You have to make sure the timing is set right, might be but I would check , then turn the throttle opening screw till the rpm's are where you want them. You could skip the timing check and just turn the throttle screw-idle screw some call it a 1/4 -1/2 of turn maybe a bit more. You don't need vacuum advance, once the timing is perfect it should run smoother than it is now.

You might even have to put in light spring in the dist. to get the advance to come in sooner, that might be a problem for you. MSD make a good dist. thats easy to change the springs.

[myk]

Ok, so now that my advance is disconnected, it doesn't matter where the little allen screw in the advance was set at, right? 

[dodge freak]

You got it, sounds like that carb. of yours is your main headache right now. Don't know much about eddy's just Holley. My Holley hp 750 runs great but the gas milage isn't , unless you think 11 mpg on a 318 with 3.91 gears is, but it does run fine. Think that MSD 7 box helps out too. Good Luck, don't give up. Might want to jet up 2-3 sizes and see what happens, you always can go back.

[Chryco Psycho]

So, when you unhooked your advance how did you have to adjust the engine?  When I disconnect my advance, idle speed drops quite a bit and I'm convinced that the engine won't run smoothly...

I fthe idle quality is affected you had the advance hooked to manifold Not ported vacuum , so with it disconnected you need to advance the timing a bunch , your power & milage will greatly improve with it disconnected 7 the timing adavnced

[myk]

So you're saying to advance the timing on the distributor and not the vacuum advance, right?  Well, I tried bumping up the timing from 19* initial to about 10* and other numbers in between and I got the off-idle bog/stumble/hesitation from hell.  Anything else you think I should be looking at, or should my motor have not responded this way? 

[firefighter3931]

  Well, I tried bumping up the timing from 19* initial to about 10* and other numbers in between and I got the off-idle bog/stumble/hesitation from hell.   

Myk, going from 19* to 10* is not bumping up....that is "retarding" the timing. Get a vaccum guage on it and adjust for maximum vacuum on the guage. Check the timing and record that number. Readjust your carb idle mixture and idle speed adjutments. Put the light on it and record the advance in degrees from idle to where it stops advancing. Report back with the numbers.


Ron

[myk]

Thanks for your continuing help and patience guys. 
I thought that I was bumping it up, since I'm going from 19* BTDC to 10* ? 
As for your recommended timing procedure, it goes like this:
1. adjust initial timing to get maximum vacuum, document timing number.
2. adjust 'carb and idle speed.
3. adjust initial timing again, this time taking note as to where the motor stops advancing from distributor adjustment-also document the amount of degress between the 1st and 2nd timing numbers.
4. don't run over any cats
5. report back to the forums.

Once again guys, thank you all, very much...

[Ghoste]

You ran over a cat?

[firefighter3931]

3. adjust initial timing again, this time taking note as to where the motor stops advancing from distributor adjustment-also document the amount of degress between the 1st and 2nd timing numbers.

Myk, you can skip this step. Once you've adjusted the timing for max vaccum and readjusted the idle speed and idle mixture then get the light and tach on it for some numbers.....just to see how many degrees it is advancing and at what rpm it stops advancing.


Ron

[myk]

Myk, going from 19* to 10* is not bumping up....that is "retarding" the timing. Get a vaccum guage on it and adjust for maximum vacuum on the guage. Check the timing and record that number. Readjust your carb idle mixture and idle speed adjutments. Put the light on it and record the advance in degrees from idle to where it stops advancing. Report back with the numbers.


Ron

Ok guys, sorry I took so long, but here are the numbers:

Vacuum advance connected...
750 rpm @ 36*
1000 rpm @ 38*
1500 rpm @ 50*
2000 rpm @ 51*

Vacuum advance disconnected...
600 rpm @ 18*
1000 rpm @ 20*
1500 rpm @ 33*
2000 rpm @ 34*
2700 rpm @ 37*

What do these numbers mean?  I hope you guys have some insight for me.

I wasn't sure if I was supposed to do the readings with the vacuum advance on or off, so I did them both just so I wouldn't have to stick my head into a hotter than hell motor screaming at 3K rpm again. 

Also, I still don't understand why going from 19* BTDC to 10* BTDC is retarding timing instead of advancing it.  I figured that moving up the scale from 19* to 10*, getting closer to Top-dead-center, is advancing? 

Thanks a bundle, in advance...

Edit: $%@*....I forgot I was supposed to see at what rpm the motor stopped advancing, and at what number of degrees the timing settled on.  Do I have to go run this test again?

[firefighter3931]

Vacuum advance disconnected...
600 rpm @ 18*
1000 rpm @ 20*
1500 rpm @ 33*
2000 rpm @ 34*
2700 rpm @ 37*

What do these numbers mean?  I hope you guys have some insight for me.

I wasn't sure if I was supposed to do the readings with the vacuum advance on or off, so I did them both just so I wouldn't have to stick my head into a hotter than hell motor screaming at 3K rpm again. 

Also, I still don't understand why going from 19* BTDC to 10* BTDC is retarding timing instead of advancing it.  I figured that moving up the scale from 19* to 10*, getting closer to Top-dead-center, is advancing? 

Thanks a bundle, in advance...

Edit: $%@*....I forgot I was supposed to see at what rpm the motor stopped advancing, and at what number of degrees the timing settled on.  Do I have to go run this test again?

Myk, that is damn near perfect...with the vac advance disconnected of course.  I would just leave it where it is and get the light back on it to make sure it isn't advancing any more past 2700 rpm.

Advance is measured in degrees before TDC. The further you are away from tdc (before) the more advanced the timing. Inversely, the closer you are to TDC on the base timing, the more retarded that timing will be, relative to the "optimal" setting. Hope that makes sense.


Ron

[myk]

First of all, I can't believe that I've been looking at this retarding/advancing thing backwards for all of these years.  Secondly, I can't believe you're up this early/late!  Lol...

Well, I'm glad to hear that you approve of the advance rate of my ignition.  So...what if the ignition continues to advance past 2700 rpm?  How is that bad?  And if it does continue to advance past 2700 rpm, what do I do to resolve the condition?  Sigh.  I'll have to stick my head back into that motor again.  Ah hell, I was planning to jet up the primaries one more size anyway.

Oh, and with the vacuum advance hooked up, why does the timing end up at 51*?  How does a setting like 51* affect the performance of the engine?  Depending on how this question's answered will determine if I side with the "Vac-advance disconnected or connected" crowds...

Man, this is great stuff!

[firefighter3931]

I can't believe you're up this early/late!  Lol...

It's not that early out here on the east coast 

Too much advance won't make any additional power and you risk ruining the motor with pre-ignition (ping). Generally, iron headed big blocks make their best power with 36-38* total advance coming in below 3000 rpm. You're at 37* @ 2700 so that's why i recommended you leave it alone. 

The timing is at 51* because the vacuum cannister is pulling in additional timing. The fact that it's pulling in so much spark lead at such low rpm's isn't good. You might have the vacuum advance hose plugged into manifold vacuum and not ported vacuum which is where it should be when using a vacuum advance unit. Based on those results, i would leave it unplugged and just tune the carb.



Ron

[dodge freak]

I agree-you need to get that motor running perfect. The timing sounds like it is-so long as thats the highest it goes. Get some ear muffs and gun that motor and make sure 37 is the max, it might be if so leave it if not make about 37 the max.

Once you get it to run smooth at all speeds, then play around with that vacuum if you want. I never have used mine and theres no need to, but if you really want a few more degrees at light throttle for maybe a 1/2 mile gain in millage try after you get the motor running right.

[myk]

I'll kill that vac-advance today and see what happens, as the readings I posted with the canister hooked up look scary.  Yup, I always thought that I was supposed to hook the canister up to manifold vacuum.  I suppose if I decide to keep the canister going I can dial back the amount of spark-lead...WAY back..

[grouseman]

myk, the distributor is to be connected to the full manifold vacuum, not ported.  Ported is an artificial reading.  Under light load part throttle lean conditions, fuel burns more slowly than rich, wide open throttle.  So the spark must be ignited earlier in the cycle to get the most out of the fuel charge.  Engine vacuum is the best indicator of the lean/rich condition that SHOULD be existing in the chamber (if jetted correctly).  High vacuum conditions are at idle, and steady state cruising.  Disconnecting the vacuum advance from a street driven car gives you no performance benefit, but instead takes away several advantages.  Using ported vacuum does almost the same job except it reads no vacuum at idle. You can gain a better idle, cleaner plugs, more mpg, cooler running engine, cleaner emissions, etc. using vacuum advance connected to full manifold.  Using full vacuum allows you to close your throttle blades more, which gives you a greater benefit of utilizing more of the idle transfer slots, which will aid driveability off idle and prevent a bog at tip-in. 

And it is strictly race cars that don't use vacuum advance, not high performance street cars.  A drag car only sees wide open throttle and very low vacuum conditions anyway, so the vacuum advance is never activated.  Racers remove it because their extremely narrow use vehicles doesn't require it.  It doesn't GIVE any performance benefit, it simply REMOVES a part that isn't needed. 

If you find that you are getting pinging under light acceleration, simply adjust the vacuum cannister spring using an allen wrench (through the vacuum nipple).  Turning CCW will put more preload on the spring, resulting in less advance for any given vacuum level.  Try a half turn at a time until the pinging goes away.  You can graph your vacuum curve at different vacuum levels as you did for the mechanical advance. 

IF this is a race only drag strip car, remove the advance.  If you drive it on the street, keep it. 

[myk]

I'm not getting in pinging, thankfully.  If I were to keep the vacuum advance, do I want the motor to read 36-38* total timing at 2500 rpm, with the canister hooked up? 

[Chryco Psycho]

if you are using vacuum yes if not time it with it unhooked & leave it unhooked

[grouseman]

I'm not getting in pinging, thankfully.  If I were to keep the vacuum advance, do I want the motor to read 36-38* total timing at 2500 rpm, with the canister hooked up? 

The 36º is initial plus mechanical only, no vacuum.  Unhook the cannister, rev to 2,500, set timing to 36 and rehook the vac line.  Done.

[Ghoste]

If it's an adjustable advance shouldn't you be going through the whole performance of checking cruise vacuum levels and then setting timing at that rpm by putting a vacuum pump on the advance and then turning and resetting until it's right?

[myk]

I'm not getting in pinging, thankfully.  If I were to keep the vacuum advance, do I want the motor to read 36-38* total timing at 2500 rpm, with the canister hooked up? 

The 36º is initial plus mechanical only, no vacuum.  Unhook the cannister, rev to 2,500, set timing to 36 and rehook the vac line.  Done.

Well, I'll still have to adjust the canister thought, right?  If so, how do I know when the canister is adjusted properly?  I'm assuming that the timing will read a particular value at a certain RPM WITH the canister hooked up?  I actually tried that and posted the results a few posts ago in this thread-I didn't like 51* at 2500 rpm.  Shouldn't it be lower?

I tried disconnecting the canister today and I had a bit of hesitation/stumble off-idle.  I'm pretty sure it was less so with the canister connected...

[myk]

If it's an adjustable advance shouldn't you be going through the whole performance of checking cruise vacuum levels and then setting timing at that rpm by putting a vacuum pump on the advance and then turning and resetting until it's right?

Yeah, that's what I thought too.  I don't know why the shop manuals and common belief is to unhook the canister, time the engine and then hook the canister back up...

[Ghoste]

Well, it would apply with a non-adjustable vacuum advance I suppose.

[myk]

Oh.  Heh, I just assumed that the original distributor from back then was adjustable like the aftermarket ones we have now...

[deputycrawford]

I am confused here. My Edelbrock carb instructions told me to hook my distributor to ported vaccum. I believe the factory manuals tell us to hook up the distributor to ported. I also believe that engines with low comp and no quench need more lead because of flame propogation. Thats why vacum advance was used. Also, if an Eddy carb is used with anything other than a stock application then throw it out. An Eddy will not use the transfer slots if the vacum is low enough because of the larger cam. The tip in hesitation lessens with the maniflod vacum because you can use the transfer slots for the carb. As soon as the vaccum drops during heavy acceleration, the car stops metering big air flow engines. And since when does a car need full vacum advance at idle? The factory manuals intruct to set timing with advance disconnected.

[Chryco Psycho]

if used dist vacuum advance needs to be connected to ported Not Manifild vacuum
wit bigger cams the idle circuit has less vacuum toi draw in fuel so the idle circuit air bleeds need to be smaller most carbs have no adjustability of rthis , the better Holleys & Proform & demons do though
engines do not need full advance at ilde

[myk]

I am confused here. My Edelbrock carb instructions told me to hook my distributor to ported vaccum.

Hmm...On the instructions for my #1411 Edelbrock it indicates that ported vacuum is for emissions controlled engines, while manifold vacuum is for non-emissions controlled engines, which applies to me and many others here.  I'm confused also.  And what is this full vacuum advance at idle everyone keeps talking about?

[Chryco Psycho]

if you are using the vacuum advance you need to connect it to PORTED vacuum otherwise you get advance at idle that decreases with RPM

[deputycrawford]

Just eliminate the vacum advance all together. What is your cam size? You might have to get rid of that edelbrock all together. I had the same tip in hesitation and had to go to a Holley based Demon carb. Everything is good now. Keep plugging away at it. You'll get it soon enough.

[myk]

Yeah, I'm trying to run the car with and without the advance.  I can't perceive any difference between the two conditions other than a bog/hesitation with the advance unhooked, and that the timing reads 51* at 2500rpm with the canister hooked up (?!).  Obviously, having the advance unhooked isn't merely enough, there's some tuning to be done in addition to that but I'm not quite sure what that is yet.  Is it adjusting the curve of the distributor itself?  With the model of Mopar distributor I have, recurving it won't be easy, as it is the pre-Mallory built type. 

In regards to 'cam or other engine 'specs, I don't know a thing about this motor, which makes me want to tear it down and build it my way.  If it can't run right I can at least know what's in it...

[grouseman]

If the factory Chrysler advance cannister has a hexagonal shape at the nipple end, it's adjustable.  That's what the adjusting plate 'nut' is held in with, while still allowing it to slide. 

The 34 degrees of initial and mechanical only is what your engine needs when it is under a wide open throttle, no vacuum condition.  When you are at a steady state cruise, with relatively higher vacuum levels, this is where the extra vacuum advance comes in.  You don't have reference the vacuum levels at cruise and highway rpm to adjust the timing such that you get 34 degrees or whatever.  The cannister takes care of that (crudely) by sensing vacuum levels, and changing the timing to adjust.  What you could do is adjust it clockwise to 'add more timing' until it pings, and then keep going Counter CW until pinging disappears under all conditions. 

The ported spark was used to change the burn rate, and thus the type of emissions that were produced immediately out of the combustion chamber, so that other aspects of the emissions controls systems would operate on that altered mix.  The initial timing was also drasatically changed; some was even AFTER TDC. 

[daytonalo]

OK , I totally agree that vacuum is needed for a hp street car but , how is external digital , multiple  choice curve ignition interfaced with a vacuum advance ?

[myk]

What kinda' animal are we talking about here?

[daytonalo]

472 hemi , 600hp , 373 rear , tko 600

[Ghoste]

If vacuum advance is mainly improving mileage during a cruise condition, well, you know where I'm goin'.

[daytonalo]

I agree , all the above. My concern is the crane digital box has multiple curve all with a max advance of 34 degrees around 2500 . Again how will the vacuum advance operate with this system , and wont it give me more total advance ?

[Ghoste]

Not if you adjust it to only pull in under the higher vacuum load.  If you are using that overdrive trans, then you should see a lower rpm during cruise (high vacuum) conditions. 
Do you already have a distributor picked out for this?

[daytonalo]

I have a stock dist with a crane conversion , I want it stock appearing

[Ghoste]

If it were me personally, with a setup like it sounds you are going to have I would run a vacuum line to the distributor but up there at the carb I would block it off and just dispense with it's participation altogether.  It sounds like performance is the number one consideration in your case.

[daytonalo]

I still want good highway economy

[dave571]

Depending on build, the vac advance won't make much difference to economy, if any, much as neil has described.  A 509 cammed 440 in a B body will make no better economy with vacuum advance.  A stock cammed unit  will.

As for the ported/manifold debate, this comes up often, but the answer is always ported.
The manuals say so, and so does all know timing theory.  I don't know why the guys at edelbrock think the install of thier carb should affect the way a motor is timed, but it doesn't.  Cam and compression, are much bigger factors.

The vacuum advance IS for economy only.  If properly installed, even the guys who run it, will notice NO performance difference at all with it disconnected.  If there is a difference, it wasn't right to begin with.

The thing that seems to not be mentioned enough in this thread is total timing.   

The best way to set ignition timing BAR NONE, is to set the total mechanical timing.  On most open headed cars, 34-36 degrees. with the vacuum off, at whatever rpm your timing is all in at.

If that makes your initial too low, then you need to shorten your advance slots so you can boost the initial as required. 
Too high, and the curve is too short.

[deputycrawford]

Dave 571. For just having 2 posts on this site, and no credibility what so ever, I agree with absolutely everything you posted. Thats a good continuation to the reliability of the information on this site. Welcome, and I hope we can assist you in the future as well. There are many here with much experience. Oh, and we all goof around a bit too.

[Chryco Psycho]

Depending on build, the vac advance won't make much difference to economy, if any, much as neil has described.  A 509 cammed 440 in a B body will make no better economy with vacuum advance.  A stock cammed unit  will.

As for the ported/manifold debate, this comes up often, but the answer is always ported.
The manuals say so, and so does all know timing theory.  I don't know why the guys at edelbrock think the install of thier carb should affect the way a motor is timed, but it doesn't.  Cam and compression, are much bigger factors.

The vacuum advance IS for economy only.  If properly installed, even the guys who run it, will notice NO performance difference at all with it disconnected.  If there is a difference, it wasn't right to begin with.

The thing that seems to not be mentioned enough in this thread is total timing.   

The best way to set ignition timing BAR NONE, is to set the total mechanical timing.  On most open headed cars, 34-36 degrees. with the vacuum off, at whatever rpm your timing is all in at.

If that makes your initial too low, then you need to shorten your advance slots so you can boost the initial as required. 
Too high, and the curve is too short.

I agree
usually you will see a performance Gain with the vacuum disconnected as ti will allow more initial advance at idle where ti really helps

[Chryco Psycho]

Dave 571 is Knowledgable

[Duey]

Dave 571 is Knowledgable

I did what Dave (and Ron and Neil) recommends about shortening the slots in the distributor (with my MIG welder) so my timing is 16* at idle and hits 37-38* by 2800-3000.  Once I get all the new bits on the motor, we'll see how it like it.

Cheers,
Duey

[dave571]

Dave 571. For just having 2 posts on this site, and no credibility what so ever, I agree with absolutely everything you posted. Thats a good continuation to the reliability of the information on this site. Welcome, and I hope we can assist you in the future as well. There are many here with much experience. Oh, and we all goof around a bit too.

Thanks man. 
This one is post 5, so I'm working my way up...LOL

Thanks to Neil for the kudos too.

[Chryco Psycho]

you are welcome Dave , I know you have spent a  lot of years working on these older Mopars as well