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#447413 - 08/23/09 04:48 PM ANOTHER vacuum advance question
StealthWedge67 Offline
master

Registered: 07/19/06
Posts: 3473
Loc: Puyallup, WA
I have been running with no vac. advance for quite a while. Mallory Mag. pickup elec. distributor in a 383 with a 238 / 242 @ .050 cam. No issues, motor runs great. I recently read, however, that vacuum advance can make a huge difference in MPG in part throttle, around town driving. So I decided to give it a try.

Imediately upon hooking it up, the idle raised a bit. I adjusted the idle down with the carb idle screw, and noted that back at the same RPM, it seemed a bit smoother. Took it for a drive. The only real appreciable difference I noted was that now the car now pings horribly when coming up a hill, or when accellerating. (Cant have this). The timing has been set at 24* initial, with 14* mechanical, for 38* total. Obviously, I could back off the initial, and add some mechanical, So with the vacuum, it would be about where it is now. But if thats the case, why bother with the vacuum? What should I do at this point? (my impulse is to remove the vacuum, & go back to where I was, in ignorant bliss of fuel economy)
_________________________
'67 Satellite - full trim street car - 11.81 @ 113.... and tuning. Pump gas 452" low deck in a plain jane 383 wrapper.

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#447414 - 08/23/09 04:56 PM Re: ANOTHER vacuum advance question [Re: StealthWedge67]
D_C Offline
super stock

Registered: 03/10/05
Posts: 1115
Loc: Rancho Cucamonga, CA
From a Mopar Muscle Magazine article:

Quote:

While centrifugal advance varies timing strictly by RPM, 3,000 rpm at wide-open throttle on your local dragstrip's launch pad doesn't call for the same timing as 3,000 rpm on US Hwy 99.

For increased efficiency, vacuum advance is a way to vary the timing in accordance to load. Linked to the distributor's pickup (or breaker) plate, the vacuum advance uses a spring loaded vacuum diaphragm to pull the plate in against the reluctor’ s (distributor cam's) rotation to advance the timing.

On the dragstrip, with your right foot mashed to the firewall, the engine sees full load for the whole 1,320 feet. So, purpose built racecars and racing distributors don't need vacuum advance. However, on a streetcar it is a must for maximum efficiency, plug life and economy.




And this:

Quote:

VACUUM ADVANCE

Only once the centrifugal advance curve is sorted out should the vacuum advance be dialed in. There are two considerations with vacuum advance: How much total and how soon?

How much total is built into the unit. The arm off the diaphragm has a stop built into it, which limits how much travel is available to advance the timing.

There are a bunch of different units, and fortunately most are stamped with the amount of advance in distributor degrees, which is half of crank degrees. Eight and a half degrees will give you 17º at the damper at full advance.

Theoretically, grinding back the two stops could increase the advance “in the can”, but in practice they're very difficult to get to. Advance can be taken "out of the can" by epoxying tabs which bend over the stops, but not, as I've seen suggested, by welding/brazing the stops. That'll cook the diaphragm.

When the vacuum advance kicks in, it can have a significant effect. If it starts at a too low vacuum level, light part throttle acceleration can cause detonation problems. Try to adjust the spring tension on the diaphragm by inserting a 3/32 inch Allen wrench through the vacuum nipple.

Counter clockwise delays the action, while clockwise kicks it in at a lower vacuum level. Most, but not all, diaphragms have this provision, with one full turn typically changing the kick in point by 1 hg. Vacuum.

As with centrifugal advance, to tune vacuum advance requires its curve be mapped. Again, this can be done with the engine used as the distributor machine, only this time using a hand vacuum pump to check the rate of advance for various vacuum levels.

The procedure is simple enough: Pull up vacuum with the hand pump while checking the timing. Note the vacuum pump's gauge point where the advance just begins to kick in and record this.

Move the vacuum level up in 1 inch increments, recording the vacuum level and how much the timing has correspondingly been increased. There's a trick here: Lower the idle speed to well below the RPM at which the centrifugal advance kicks in each time the vacuum pump level is pulled up.

Otherwise, as the vacuum advance brings up the timing, the RPM will increase causing the centrifugal advance to come into play and invalidating your readings.



_________________________
'71 Charger SE - My First Car,Still Have it 440 CI,E-Heads,850 Holley, 292/.509,Dana 60/4:10,ChassisWorks Kit Subframe Connect,Springs Relocated Fresh-Air/'69 440 Scoop,Pan-Evac, TTI Headers,3 1/2" Exhaust Sys,Dynomax Muff

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#447415 - 08/23/09 05:03 PM Re: ANOTHER vacuum advance question [Re: StealthWedge67]
Dougsmopars Offline
top fuel

Registered: 05/03/05
Posts: 2295
Loc: dark side of the moon
you should try another 1 or 2 degree's initial and see how it runs. Big cam and fuel economy do not go together. If it likes no vac advance why fix what doesn't seem broke? Most race dist don't have vac advance. If you wanted fuel miliage why did you build up the motor? Can't have power and milage

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#447416 - 08/23/09 05:36 PM Re: ANOTHER vacuum advance question [Re: StealthWedge67]
dodgeram440 Offline
master

Registered: 02/19/06
Posts: 2574
Loc: Piqua, Ohio
Quote:



Imediately upon hooking it up, the idle raised a bit.




I am by no means an expert on vacuum advance, as is evidenced by my own thread on the subject. However, your comment indicates that you plugged into a full time vacuum source instead of ported vacuum. The guys who responded to my questions say I should connect to a ported vacuum source, which supplies vacuum at part throttle, not at idle. This could be why you are getting the pinging.

Unplug the vacuum advance, bring your idle back up to a baseline, find a ported vacuum source, and try again.
_________________________
1986 Dodge D100 318 "What people say they want and what they respond to are two totally different things." -Sam Young (R.I.P.)

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#447417 - 08/23/09 05:38 PM Re: ANOTHER vacuum advance question [Re: Dougsmopars]
StealthWedge67 Offline
master

Registered: 07/19/06
Posts: 3473
Loc: Puyallup, WA
Thanks for the articles. Those help. I did adjust the canister all the way counter clockwise, we'll see how that works.

As for the why I'm messing with it: because I'm a hot-rodder, and thats what we do; tinker and look for results while trying to understand how the action effects the result. No, it wasn't broke. But for that matter, it wasn't broke when it was bone stock, either.
_________________________
'67 Satellite - full trim street car - 11.81 @ 113.... and tuning. Pump gas 452" low deck in a plain jane 383 wrapper.

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#447418 - 08/23/09 06:19 PM Re: ANOTHER vacuum advance question [Re: dodgeram440]
StealthWedge67 Offline
master

Registered: 07/19/06
Posts: 3473
Loc: Puyallup, WA
Quote:

Quote:



Imediately upon hooking it up, the idle raised a bit.




I am by no means an expert on vacuum advance, as is evidenced by my own thread on the subject. However, your comment indicates that you plugged into a full time vacuum source instead of ported vacuum. The guys who responded to my questions say I should connect to a ported vacuum source, which supplies vacuum at part throttle, not at idle. This could be why you are getting the pinging.

Unplug the vacuum advance, bring your idle back up to a baseline, find a ported vacuum source, and try again.




Funny how sometimes the most obvious things get by me. You were correct. I checked my holley diagram, and relocated the vacuum hose to the port at the metering block. Now, with the canister adjusted all the way counter clockwise, no pinging. I suppose now I can start adjusting more vacuum advance in, with the canister adjustment, until it pings again, then back off a bit. Thanks for the help, guys.

Also, it should be noted that I just never liked the look of the vacuum canister being blocked off. The car has a stock vibe, and even though it runs some hot-rod parts, I like it to have a correct look. This helps.
_________________________
'67 Satellite - full trim street car - 11.81 @ 113.... and tuning. Pump gas 452" low deck in a plain jane 383 wrapper.

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#447419 - 08/23/09 08:07 PM Re: ANOTHER vacuum advance question [Re: StealthWedge67]
Lefty Offline
master

Registered: 09/05/05
Posts: 6462
Loc: Santa Cruz, California
Racers don't use vacuum advance because they don't drive part throttle. My 440 runs way better part throttle with vac advance. Your initial timing is too much imo. This is what I run in the 440 -

http://www.moparaction.com/Tech/quest/SLOW_CURVE.html

Tech Question

bob santarsieri, schaumburg, IL, 71 plymouth road runner 440

I have a msd pro billet distributor and msd 6al box. What would be the best setting for the distributor? Right now it is set at 2 heavy silver springs and silver bushing per the instructions. It starts to advance at 1400 and goes to 5500rpm then levels off. What would you suggest. 440ci, 0.040" overbore, 0.474" purple shaft cam, Hedman headers and 3.23 gears. Just street driven. Thanks for your help.

Bob, assuming sufficient octane vs. C.R., you need a curve something close to this:

Under 1400RPM - no advance - set initial at 15 degrees BTDC
2000 - 10 dergees mech (25 total)
2500 - 20 degrees mech (35 total) - all in.

Plus, at LEAST 15 degrees of vacuum advance under light throttle (50 deg. total, 52-55 better).

If it currently doesn't go to full advance until 5500, you're giving up lots of power.

If it currently has less (or no) vacuum advance), you're giving up LOTS of fuel economy and fouling plugs.

Rick
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Learn the rules so you know how to break them properly - Dalai Lama

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