Eliminating vacuum advance

[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?