Author Topic: Manifold Vacuum Advance and Timing Curves  (Read 17597 times)

LeighP

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Manifold Vacuum Advance and Timing Curves
« on: February 25, 2013, 10:32:57 PM »
Had a friend (who's been into Pontiacs for a LONG time) set up, while I did the watch and learn thing, my car for full manifold vacuum advance.
Basically, the distributor vacuum can is hooked to a full time manifold source, not a ported source like Pontiac and most cars have used since the mid 60's.
The advantages are engine runs cooler at idle (operating more efficiently), and it transitions much crisper (less of the transfer slot in the carb uncovered since the throttle blades are actually not open as much)

First off, he disconnected vac advance, then advanced the distributor at idle in Drive, while watching a digital tachometer. Kept advancing till he reached the highest RPM (approximately 100 RPM gain over my previous 'in gear" setting).
At that point, he read the crank advance....it was showing 33 degrees. This is the most efficient advance as you're basically getting the best combustion from a fixed fuel/air mix, shown as highest RPM.

So, now you deduct your static timing setting, mines 15 degrees.....that leaves 18 degrees of vacuum added advance needed.

He recommends using a Crane adjustable vacuum can as its adjustment doesn't affect the range of movement......the adjustment is to compensate for different sized cams. My dist had an adjustable can fitted to it, and my cam wasn't big enough to need adjusting into the range where the travel is artificially limited....so it worked out OK....two complete revolutions out from the softest setting.

That set, he then did some neat maths calculations to determine the amount of circumferential distance that would equal the 18 degrees of advance needed from the vacuum canister....in my case, that was 5.8mm (we ended up getting 6.0 travel, close enough). He made up a positive travel stop which screwed to one of the vacuum canister mounting screws and stopped the operating rod at the required distance.

My idle now shows a steady 33 degrees of advance after starting, the engine idles smoother as if it had a smaller cam. Both hot and cold starting (which was OK before) is now crisper and quicker.

All in all, its a nice way to set up the ignition. Do some reading on the subject if you're interested, theres a lot about it on the net, and some long discussions on the Performance Years forums.
The big trick to making it work smoothly seems to be in setting the pre-load on the vacuum can's adjustment correctly.....get it wrong, and there tends to be a big change in the advance when you drop the car in gear (auto)....which you don't want. Mine goes into gear smoothly without changing the advance/idle characteristics.

And while we were doing this, I noticed an oil leak....tracked it to an adapter fitting on the remote oil filter. If you're fitting a remote filter, keep an eye on these fittings on the head....mine seemed to have backed out or lost seal on the thread.....easy fix, I used liquid pipe thread sealant this time, not tape.  
« Last Edit: February 25, 2013, 10:34:51 PM by LeighP »
Regards,
Leigh

Sydney, Australia
1969 Pontiac Firebird 400 coupe (project)

Former Firebirds -
1971 Pontiac Firebird 455
1977 Pontiac Trans Am
1976 Pontiac Trans Am
1967 Pontiac Firebird 400 convertible
1967 Pontiac Firebird 400 coupe


LeighP

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Re: Manifold Vacuum Advance
« Reply #1 on: February 26, 2013, 03:25:31 AM »
If anyones thinking about trying this, I just nutted out how he worked out the distance the advance actuator arm had to move......

I guess the maths is out there on the net to figure this out, but this is what I came up with, and the answer matched his.....so I guess its close enough.

Distributor turning at half crank speed, so 18 degrees of crank advance means 9 degrees of distributor advance.

So I guess it works out as -

1. find the circumference of the circle formed by the vacuum advance actuator operating pin
    Measure the radius from the centre of the distributor shaft to the centre of the advance actuator operating pin.
    Multiply by 2 = the diameter.
    Apply the following equation -
    Pi (3.14) times Diameter (C=pD),   so say the diameter at the operating pin is 75mm, then the equation is -        3.14 x 75 = 235.5mm circumference

2. Divide that figure by 360 (degrees in a circle) and then multiply by the number of distributor degrees required -
    235.5 divided by 360 = 0.654   then,  0.654 x 9 = 5.88mm    this is the distance that the vacuum advance needs to move to bring in 18 degrees of crank advance given the HEI in my car.

3. make a positive stop up to prevent the vacuum actuator moving any more than that distance, and you're done!
« Last Edit: November 15, 2017, 12:08:08 AM by LeighP »
Regards,
Leigh

Sydney, Australia
1969 Pontiac Firebird 400 coupe (project)

Former Firebirds -
1971 Pontiac Firebird 455
1977 Pontiac Trans Am
1976 Pontiac Trans Am
1967 Pontiac Firebird 400 convertible
1967 Pontiac Firebird 400 coupe


LilSki

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Re: Manifold Vacuum Advance
« Reply #2 on: February 26, 2013, 08:16:22 AM »
I have always run my ignition this way. The main thing I notice especially with large cams is it lets you keep the throttle plates closed more and make sure you are in the idle circuit. I've made some home made stops for the advance canisters in the past. This is some great info!
Kevin
79 Trans Am: (77 nose), WS6, M-21, 461, Hotchkis TVS, PTFB Contol arms. 
85 Trans Am: Vortec 350 w/LT1 cam/T56, TPI conversion w/Megasquirt II.

Grand73Am

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Re: Manifold Vacuum Advance
« Reply #3 on: February 26, 2013, 08:20:14 AM »
Basically, the distributor vacuum can is hooked to a full time manifold source, not a ported source like Pontiac and most cars have used since the mid 60's.

Actually Leigh, I believe most old cars were already running off of manifold vacuum from the factory, according to my Chevy and Pontiac service manuals from the 50's, 60's and 70's. I think it's a common misconception that ported vac was used. Even my 79 403 TA's specify a direct manifold vac source in the service manual. In the mid 70's they started running the vacuum line thru delays and heat valves and such on some cars, probably for emissions, but it all goes back to manifold vacuum. So, you can bypass the other stuff and just go straight manifold, just like the old cars did, and they'll run better. I've experimented with using ported vacuum, but they ran much worse, so I run my cars on manifold and they run a lot better that way.

I've read where some people prefer ported vacuum, and there may be some modified engines where that works, but with a stockish motor, I believe manifold vac works best.  
Steve F.

ericwy

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Re: Manifold Vacuum Advance
« Reply #4 on: February 26, 2013, 08:29:27 AM »
Great!  Now everybody knows the secret!  :lol:

I would add that the Crane adjustable can should include a advance limit cam. (mine did anyway).

Use of ported vacuum vacuum advance was introduced to lower emissions at idle. Usually not the
best choice for stock and mild cam'd rebuilds. Big cam'd engines may/may not like full vac adv at
idle due to the vacuum signal may vary causing large swings in timing which upset idle stability.
Also, torque converter stall speed can affect load on the engine at idle.
1977 400 TA, 9.5:1CR 351hp 411LbFt,#13 Heads, 1.52 Roller rockers, 1968 Pont intake, Holley 4165 650 DP
Howards .447/.467 IN:288 EX:298 214/224@.050, MSD6A + timing control + H20-Meth
1-5/8 headers + Magnaflow 200CPI Cats, + 2.5 Pypes X, Dynomax STs, 700R4 2200rpm, 3.42 Eaton posi 10bolt, PS, PB, PL, PW, AC, Cruise
13.84@100.14mph 2.18 60ft - 255 60 R15 street radials, mpg : 21.5 hwy 15.2 city

Re: Manifold Vacuum Advance
« Reply #4 on: February 26, 2013, 08:29:27 AM »

tajoe

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Re: Manifold Vacuum Advance
« Reply #5 on: February 26, 2013, 04:11:53 PM »
Good info Leigh. thanx for sharing. I remember back in the day, people throwing away their canisters. "Who needs them when you're racing" was the justification, I used to get. In reality, they just liked the "sound" better, cause it was "radical". Then you'ld hear the same people complaining about how their fuel mileage sucked and would always be running hot. But "man" did it sound cool! 8) (Fools)
"You can sell an old man a young mans car,
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LeighP

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Re: Manifold Vacuum Advance
« Reply #6 on: February 26, 2013, 09:05:55 PM »
Thanks.
Just thought I'd add to the collective knowledge if anyone was researching distributors or advance etc.   :cool:
Regards,
Leigh

Sydney, Australia
1969 Pontiac Firebird 400 coupe (project)

Former Firebirds -
1971 Pontiac Firebird 455
1977 Pontiac Trans Am
1976 Pontiac Trans Am
1967 Pontiac Firebird 400 convertible
1967 Pontiac Firebird 400 coupe


jvmagic

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How do I set ignition timing - curve?
« Reply #7 on: April 04, 2015, 07:28:12 PM »
Hi,

I read that there are three ignition timing settings.

initial timing setting (idle)
Total timing
Mechanical advance (rate of which ignition timing advances)  - it this with springs/weights?  slower curve for stock 403 with 2.73 gears which I assume means heavier springs? 

79' 403 olds

Thanks in advance.


startat2

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Re: How do I set ignition timing - curve?
« Reply #8 on: May 08, 2015, 09:04:39 PM »
Hi,

I read that there are three ignition timing settings.

initial timing setting (idle)
Total timing
Mechanical advance (rate of which ignition timing advances)  - it this with springs/weights?  slower curve for stock 403 with 2.73 gears which I assume means heavier springs? 

79' 403 olds



Thanks in advance.
This is a topic that really needs a sticky IMO. I generally start by setting my total timing. Pontiacs and olds like a bit more timing.

Start with a dial back timing light.
Remove the vac hose from your vac advance and plug it.
Start reading your timing mark and rev the motor until it quits advancing. (If you can take note of this rpm, this is also your total timing at WOT.)
Set this total timing to 32 degrees.
Let the car idle and take another reading of your timing. ( this is your initial timing)
Hook your advance back up and go for a ride. Try light acceleration, a 50 mph cruise and a wot pull.
Once you get home, let it sit for a few minutes and make sure it still starts easy.
During your drive : Did your car surge? Did you hear any detonation? Did you feel more power up high or low?

If all went well, keep adding a degree or two until you car quits liking it. Your car quits liking it when you hear detonation, or you feel like the power is dropping off.

At this point if it still starts easy, and your total timing was coming in at a reasonable rpm (3000 give or take) you are done to a point.

If your car loves the timing at revs, but not so much at idle you will need a new adjustment.

If your car loves timing at revs, but not so much at cruise speed ( this is when vac advance comes into play) you will also need an adjustment.

vac advance is adjusted with an adjustable vac canister, or can be changed to a degree by changing your vac source.

Mechanical advance is changed by the weights, their springs, and the range of motion of these allowed by the distributor.

If you have a good quality distributor, most people advise against using Chinese made springs and weights as these will often cause more problems than help.

If you don't want to play with the curve, you can always reduce total timing, until the initial is in an acceptable range for the car at idle. A vacuum gauge can be an invaluable tool for determining this.

I'm not an expert by any means, but this is a method that has worked for me.



startat2

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Re: How do I set ignition timing - curve?
« Reply #9 on: May 08, 2015, 09:07:32 PM »
Also, it's not uncommon for many pontiacs and olds to like up to 36 degrees or more total timing, but start low and increase one or two degrees at a time :)

jonathonar89

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Re: How do I set ignition timing - curve?
« Reply #10 on: May 08, 2015, 10:09:16 PM »
Nice write up startat2....agree on the sticky
1979 Trans Am 400 T-top - Platinum Silver, Carmine Interior
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MNBob

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Re: How do I set ignition timing - curve?
« Reply #11 on: May 09, 2015, 06:03:47 PM »
I agree with jonathonar89; startat2 has a great write up on how to do this.
1979 TATA 4spd
Hedman elite; Pypes 2.5; Borla XS; MSD 6A; Edelbrock intake; open scoop; modified Q-jet; Powermaster 150; 4 core radiator/Mark VIII fan; RobbMc mini starter; subframe connectors; solid body mounts; fiberglass rear springs; poly sway bar and link bushings; 81 master; D52’s; Blazer disks; 225/60 & 235/60 17's TrueContact's; relays for PW, PDL, lights; keyless entry

startat2

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Re: How do I set ignition timing - curve?
« Reply #12 on: May 15, 2015, 04:08:09 AM »
This is really just a basic go by guide. There are many others on here that are likely more knowledgeable about the subject and even different ways to achieve the same. I know a guy that removes the springs from his dist while setting total timing so that he doesn't have to rev the motor while he dials it in. If a sticky is requested I can add info and clean it up. :)

oldskool

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Re: How do I set ignition timing - curve?
« Reply #13 on: May 15, 2015, 06:53:21 AM »

 ...I know a guy that removes the springs from his dist while setting total timing so that he doesn't have to rev the motor while he dials it in...


That's a GREAT idea. I've never thought of that. Some stock springs may not allow the dist to advance fully til the engine is past 4000rpm. Somehow, when I'm standing over an engine, with the fan and everything else turning, I don't feel real good about going past 4000. Under 3 seems to just FEEL safer somehow.

But, I don't know about removing both springs. That might cause damage by producing full advance at a very low rpm. Not sure.  Might be safer to either just remove one spring, or use the 2 weakest springs from an advance curve kit. Then, once you know the total mechanical advance, you can add heavier springs, one at a time, until you reach total advance by 3000-3500 rpm. I've read that you should never run the weakest spring in the kit. But it may be useful when determining the amount of mechanical advance your dist has.

http://www.summitracing.com/parts/msd-8428?seid=srese1&gclid=Cj0KEQjwstaqBRCT38DWpZjJotIBEiQAERS6_Ci-kGz298Bd0tz9LNXIRIOsAxgFxTVuJ6fHPp6g7pUaAqY28P8HAQ

This could also reveal that the dist has too much mechanical advance. That will require some sort of positive stop, to limit total mechanical advance. Let's say your engine likes 12 degrees initial advance at idle. But when set at this 12 degrees initial, your total advance is 38 degrees, and your engine prefers 32-36 degrees, then a positive stop is needed.

I've read of several ways to rig up a positive stop. One way, especially on an HEI, is to install a hardened, self tapping screw, of the correct size, into one of the holes, located just inside the threaded holes used to attach the rotor. This can be adjusted by grinding a flat on one side of the screw, to provide more advance than when using the full screw head.

Of course, when discussing this subject, it is always worth mentioning that lots of old HEI's (and most ARE OLD) you need to inspect the advance weight pivot pins for wear. After they get deep grooves in 'em, they can cause erractic fluctuations in timing. Also badly worn dist shaft bushings, gears, and excessive vertical play can cause fluctuations. The vertical play can be fixed with hardened shims, made for the purpose. Bad bushings must be replaced. And of course, if the gear is badly worn, it too must be replaced.  :-)



« Last Edit: May 15, 2015, 07:02:15 AM by oldskool »

firebirdparts

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Re: How do I set ignition timing - curve?
« Reply #14 on: May 16, 2015, 07:51:21 PM »
To keep the sticky list short, we could merge this into the vacuum advance sticky, which also was about total timing.  I thought that there were pictures in that one at one time, but maybe they pictures were in LeighP's build thread or something.
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Joe Bays
1977 Y82 4-speed (red interior)
1978 Skybird 350
1978 Redbird 305
1979 Redbird 301 4-speed
1992 Lincoln Mark VII LSC/SE (red)
1970 Datsun Pickup
1953 Chevy pickup (converted 4x4)
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Re: How do I set ignition timing - curve?
« Reply #14 on: May 16, 2015, 07:51:21 PM »

Brashanic_IM

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Re: Manifold Vacuum Advance
« Reply #15 on: November 14, 2017, 05:13:10 PM »
If anyones thinking about trying this, I just nutted out how he worked out the distance the advance actuator arm had to move......

I guess the maths is out there on the net to figure this out, but this is what I came up with, and the answer matched his.....so I guess its close enough.

Distributor turning at half crank speed, so 18 degrees of crank advance means 9 degrees of distributor advance.

So I guess it works out as -

1. find the circumference of the circle formed by the vacuum advance actuator operating pin
    Measure the radius from the centre of the shaft to the advance actuator operating pin.
    Multiply by 2 = the diameter.
    Apply the following equation -
    Pi (3.14) times Diameter (C=pD),   so say the diameter at the operating pin is 75mm, then the equation is -        3.14 x 75 = 235.5mm circumference

2. Divide that figure by 360 (degrees in a circle) and then multiply by the number of distributor degrees required -
    235.5 divided by 360 = 0.654   then,  0.654 x 9 = 5.88mm    this is the distance that the vacuum advance needs to move to bring in 18 degrees of crank advance given the HEI in my car.

3. make a positive stop up to prevent the vacuum actuator moving any more than that distance, and you're done!
im trying to do this right now as i need to limit my vac advance.

for clarification as to instruction #1 do you mean to:

1. find the circumference of the circle formed by the vacuum advance actuator operating pin
    Measure the radius from the centre of the shaft (Distributor shaft?) to the (center of?)advance actuator operating pin.

thank you so much for this formula
1979 W72 WS6 true barn find survivor

LeighP

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Re: Manifold Vacuum Advance and Timing Curves
« Reply #16 on: November 15, 2017, 12:06:55 AM »
Yep, you have it right....the radius from the centre of the distributor shaft to the centre of the operating pin that the vacuum advance pulls on....
Regards,
Leigh

Sydney, Australia
1969 Pontiac Firebird 400 coupe (project)

Former Firebirds -
1971 Pontiac Firebird 455
1977 Pontiac Trans Am
1976 Pontiac Trans Am
1967 Pontiac Firebird 400 convertible
1967 Pontiac Firebird 400 coupe


Re: Manifold Vacuum Advance and Timing Curves
« Reply #16 on: November 15, 2017, 12:06:55 AM »
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