Aight Bewy.......

Aight @Bewy, you've peaked my interest enough about the manifold vacuum that when I get the engine back in Vixen from fixin the oil leak, I'm gonna try it. So, suggestions? stock bottom end 225 with 175 PSI cranking pressure and that's WITH the 250@.050 .465 cam. 450 QFT VS carburetor. Right now I'm at 18.5* initial and 29* total in by about 2500. Engine has 14hg@ cruise and about 3hg idle. Whaddya think?

3" @ idle?!

rustycowll69 said:

3" @ idle?!

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Yeah.

Chit....I'm gettin old. My bad. 6", not 3. damn. lol

Someone as old and seasoned as you should know the effect or about of running manifold vac .
Huge timing with a drop off when you dip it.
I thought you had your idle all smoothed out?
If it isnt...u can do the manifold timing to keep it cleaner at idle...itll just drop to the initial and advance from there as it does now.

RRR.
I was never able to find a decent method of dialling in MVA so I developed my own many years ago, which follows.
That is a lot of cam for 225 cubes, but the long stroke helps to tame it at lower rpms where big cams are at their worst. I have never tried this on an engine with such low vac at idle, so I am unsure how it will work. The GM type adj VA units work down to about 5" of vac. Here is what I hope will happen: when you start the engine, you normally give the engine a little throttle. That slight rpm increase will pull a little more vac & hopefully pull in the VA plunger..& hold it in at idle speed.
First step is to find what I call the 'sweet spot', the most efficient idle timing. I used to use a digital tach for this, but no longer bother. I just do it by sound & feel.'
- disconnect the VA for now
- warm up the engine & put in gear if auto
- loosen dist clamp & slowly turn the dist to advance the timing. Keep going until you have the highest idle rpm. Toggle dist CW/CCW to make sure you have the highest rpm. Now check the timing. Say it is 40*. That is the 'sweet spot'. Your idle timing can be any combination of init + MVA. The adj VA units usually have a 30* range. With idle timing, a couple of degrees either way is not critical, unlike WOT timing.
- Adjusting the VA unit spring rate. It is imperative that the timing remains steady at idle. Often it varies because of misadjusted VA unit or factory unit used with a stiff spring. Then the comment, 'MVA didn't work for me' when the problem was not MVA...
- turn the AK fully CW to the softest spring setting. You will probably have to fabricate a stop for the VA arm to limit total travel. On the SB & BB Mopar dists, I silver solder the stop onto the arm, near the bend in the arm. Another method that I haven't tried might be a screw through the body that limits the travel of the plate.
- everything hooked up now, start the engine & check timing, in gear if auto. If it is steady, job done. To minimise light throttle ping, do this: turn AK 2 turns CCW & recheck timing; repeat this until timing drops off or becomes unsteady. Then go back & turn the AK 3 turns CW.
- if the timing is fluctuating, it is probably going to be because of the extremely low vacuum. I would prise the VA can apart & see if can find a softer spring that you can use & then repeat above.
Good luck!

I just saw 6" of idle. I think you will be ok then.

MOPAROFFICIAL said:

Someone as old and seasoned as you should know …..

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I also sometimes read or ask a question forgetting how I used to do some stuff. The question is still good though.
I do get annoyed when I totally forget and feel like I have to ask the question.

MOPAROFFICIAL said:

Someone as old and seasoned as you should know the effect or about of running manifold vac .
Huge timing with a drop off when you dip it.
I thought you had your idle all smoothed out?
If it isnt...u can do the manifold timing to keep it cleaner at idle...itll just drop to the initial and advance from there as it does now.

Click to expand...

I do. It's actually running great. I just wanted to try something different.

Also, I'm old enough and seasoned enough to know that not everything always acts like you think it will. I also like asking the opinions of others.

Bewy said:

RRR.
I was never able to find a decent method of dialling in MVA so I developed my own many years ago, which follows.
That is a lot of cam for 225 cubes, but the long stroke helps to tame it at lower rpms where big cams are at their worst. I have never tried this on an engine with such low vac at idle, so I am unsure how it will work. The GM type adj VA units work down to about 5" of vac. Here is what I hope will happen: when you start the engine, you normally give the engine a little throttle. That slight rpm increase will pull a little more vac & hopefully pull in the VA plunger..& hold it in at idle speed.
First step is to find what I call the 'sweet spot', the most efficient idle timing. I used to use a digital tach for this, but no longer bother. I just do it by sound & feel.'
- disconnect the VA for now
- warm up the engine & put in gear if auto
- loosen dist clamp & slowly turn the dist to advance the timing. Keep going until you have the highest idle rpm. Toggle dist CW/CCW to make sure you have the highest rpm. Now check the timing. Say it is 40*. That is the 'sweet spot'. Your idle timing can be any combination of init + MVA. The adj VA units usually have a 30* range. With idle timing, a couple of degrees either way is not critical, unlike WOT timing.
- Adjusting the VA unit spring rate. It is imperative that the timing remains steady at idle. Often it varies because of misadjusted VA unit or factory unit used with a stiff spring. Then the comment, 'MVA didn't work for me' when the problem was not MVA...
- turn the AK fully CW to the softest spring setting. You will probably have to fabricate a stop for the VA arm to limit total travel. On the SB & BB Mopar dists, I silver solder the stop onto the arm, near the bend in the arm. Another method that I haven't tried might be a screw through the body that limits the travel of the plate.
- everything hooked up now, start the engine & check timing, in gear if auto. If it is steady, job done. To minimise light throttle ping, do this: turn AK 2 turns CCW & recheck timing; repeat this until timing drops off or becomes unsteady. Then go back & turn the AK 3 turns CW.
- if the timing is fluctuating, it is probably going to be because of the extremely low vacuum. I would prise the VA can apart & see if can find a softer spring that you can use & then repeat above.
Good luck!

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Thanks, @Bewy. Got the engine and trans back in after fixing an oil leak and replacing the flywheel ring gear. I'll give it a try after I get it back running. Probably be a few days.

Bewy said:

The GM type adj VA units work down to about 5" of vac.

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Is that where they start to move, or is that all-in at 5"?

The adj VA units usually have a 30* range.

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I'd like to point out that the range of advance degrees is set by the stop on the arm, and the only way to change that is by fabricating a new stop such as you mentioned... what may be adjusted is the start of the advance (from just beginning to move, to all-in). If you screw the adjustment CCW and the start changes from 6" to 8", the all-in vac will also be pretty close to 2" higher also. But it will be the same number of degrees stamped on the arm...

That said, I've been running manifold vacuum for a while, partially because my 950 doesn't have ported vac! But it's a good match for my 272@.050 cam (as described in my thread in the Mopar Perf forum). It has about 8.5" at a 1200 rpm idle, which does not pull the can in all the way even at full CW adjustment (it starts to move around 5-6" but takes 11" for full 16 crank deg advance). The idle is quite stable at 34 deg (mechanical is 24, so the can is about halfway in). So far, no ping.

DrCharles said:

Is that where they start to move, or is that all-in at 5"?


I'd like to point out that the range of advance degrees is set by the stop on the arm, and the only way to change that is by fabricating a new stop such as you mentioned... what may be adjusted is the start of the advance (from just beginning to move, to all-in). If you screw the adjustment CCW and the start changes from 6" to 8", the all-in vac will also be pretty close to 2" higher also. But it will be the same number of degrees stamped on the arm...

That said, I've been running manifold vacuum for a while, partially because my 950 doesn't have ported vac! But it's a good match for my 272@.050 cam (as described in my thread in the Mopar Perf forum). It has about 8.5" at a 1200 rpm idle, which does not pull the can in all the way even at full CW adjustment (it starts to move around 5-6" but takes 11" for full 16 crank deg advance). The idle is quite stable at 34 deg (mechanical is 24, so the can is about halfway in). So far, no ping.

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Correct. To simplify, I always say it's the "when" that's adjustable, not the "how much".

Are you guys talking mopar vac advance?
Thats a screw inside that compresses a spring n diaphram, taking more vac to move it therefore limiting the amount vac could move it aka limiting the total amount of vac advance. Theres a thread i cut one open in for the sake of debate with moper.iirr

Sure, if you overtighten the spring, it goes into coil bind before the stop hits, thus reducing the possible advance. But they are never run that tight in practice.

DrCharles said:

Sure, if you overtighten the spring, it goes into coil bind before the stop hits, thus reducing the possible advance. But they are never run that tight in practice.

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Correct, not with stock applications.
Raged edge compression, sure they can be.

Just for kicks, I relocated the vacuum advance to a manifold outlet tonight. Made zero difference in idle RPM. But with only 6hg vacuum idle, I didn't think it would make much difference if any and it didn't Haven't driven it yet.

What I have discovered though, in driving it the last few times is, it has a light spark knock right at cruise tip in. It stops immediately after going on into the gas. Even lugging it hard in high gear at low RPM only produces a slight rattle at tip in, but nothing when I really plow into it. My first thought was the vacuum advance. Disconnected it and no change. I haven't driven it since I changed to full manifold vacuum at the can, but I see no reason why that would make it stop.

The change I made that I THINK caused it......I had a 1.5" thick carburetor adapter on the Offenhauser manifold. I got the correct 1/2" thick Offenhauser adapter and that's when Kitty noticed it was doing it. I'm really hard of hearing in the higher frequencies, but once she pointed it out, I heard it.

I did go from the 3.5 power valve back to my 9.5 power valve thinking it was a lean tip in, but no change. Now, Kitty and I talked about it and I asked her if she thought it was possible it's been doing it the whole time and she said she didn't think so. It was the first time we drove it after I changed the adapter that she noticed it. I can give it tip in and hold it and as soon as the car speed picks up and matches the extra pedal, it stops doing it. If I bend the floor board with the gas pedal, it does it for a split second and then stops. I've tried going from 23.5* initial down to about 17.5* initial with no change. I have a friend getting me a couple of gallons of 114 race gas just to see if that stops it. If it does not, I will rule out it being spark knock. I'm almost to that point now. It doesn't sound like any spark knock I've ever heard. It's very high frequency. Kitty pegged it when she said it sounds like a glass jar full of marbles being shaken for a second. And that's what it sounds like. It's not metallic like spark knock....but I know it can sound different in every engine so.......I'm open to suggestions. I'm thinking maybe even trying a size or two up on the primary jets. I don't want to back off the initial anymore because 1) I don't think it's the initial timing, since that didn't change it and 2) I don't want to start losing power backing the timing off.

Dr.C.
For the GM adj units, I have the chart for an Accel unit.
The points type starts to pull at about 4.5", HEI at about 6". The chart is really confusing because it shows the vac starting from the max of 9 turns [ points dist ] 7 turns [ HEI dist ] starting from CCW [ the stiffest spring position ].


As the plunger starts to pull in, even at low vac, the vac is going to increase slightly, & help pull in the plunger even further. This is going to vary with each combo.

RRR.
The VA made no difference? Did you check the timing with VA connected to see if the VA actually added any timing? To tell if VA is going to add anything, better to do the rpm test in post #5.

To all.
Both the Accel & Crane adj vac units have a sleeve inside the spring that limits spring travel. A most stupid idea because you can get a situation that the VA locks up, just goes solid. I prise these units apart & remove the sleeve. I use a lot of Chinese HEI dists here, never had a problem; they come with adj VA & do NOT have the sleeve. I can buy a Chebby HEI here for $80 that has the good VA unit, Crane adj VA unit would be about the same price!!

I installed a mechanical oil pressure gauge under the hood today, because I am just curious about the oil pressure. 70 PSI idling cold and it doesn't rise one pound when I rev it, because I put the MP big block/slant 6 high pressure spring in it, so it's relieving pressure at 70 PSI as designed. Once hot, it idles at 60 and rises to 70 when I rev it. I knew it had good oil pressure, but dammit. lol

On to the "noise" that seemed to be spark knock. It's not. As I was revving the engine noting the oil pressure, I heard the noise right under my nose. It's the carburetor sucking air. Kitty said "That's the noise RIGHT THERE!" and she's right. Again. So no spark knock. I haven't had a chance to adjust the timing like you said. Kitty had a pre op doc appointment for upcoming surgery so we were gone pretty much all day. She has a stress test tomorrow with her heart doctor to make sure she's cleared for surgery, so tomorrow's a wash, to. Thursday, I'm gonna get on it and mess with the timing and adjust it like you recommend and see where it is after that.

@Bewy I had a little time after we got home from Kitty's dr appointment to mess with it. I pulled in timing until the engine stopped gaining RPM, dropped timing until it lost a little, then pulled in enough to gain that back. We're right back at 23 degrees. Right where I had it. Advance can attached to manifold vacuum shows NO added timing at idle. It is pulling about 6hg at the port at idle. It's just not enough to pull and in the can. The can does add some with RPM. I'm seeing "around" 25-30 extra on top of mechanical at RPM, so the can does work, it just gives nothing at idle. I'll drive it tomorrow and see if there's a difference.