Distributor advance
Printed From: TheAMCForum.com
Category: The Garage
Forum Name: AMC V8 Engine Repair and Modifications
Forum Description: AMC-made V8 engine mechanical, ignition and fuel from basic repair to high-perf modifications
URL: https://theamcforum.com/forum/forum_posts.asp?TID=103621
Printed Date: Apr/23/2024 at 1:11pm
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Topic: Distributor advance
Posted By: gnrand
Subject: Distributor advance
Date Posted: Jan/15/2020 at 8:53pm
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The engine tag says to set the initial at TDC with the vacuum off. I assume they using a ported vacuum port to advance the initial timing after TDC is set. 390 motor. I did a search and found posts that recommended the initial timing being set at 10-12 degrees advance at idle with and the vacuum port comes into play a WOT. Jeff ------------- 1968 AMX 1986 Buick GN 1965 Corvair Corsa 1966 V8 Corvair 1969 Scrambler Stock 1969 Scrambler Modified |
Replies:
Posted By: BassBoat
Date Posted: Jan/15/2020 at 9:13pm
| switching the source of vacuum from manifold to ported for vacuum advance was done for emissions. I think you will get better performance with a manifold vacuum source. |
Posted By: ccowx
Date Posted: Jan/15/2020 at 10:45pm
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The original set up from the factory was to use manifold vacuum. If they say "with the hoses disconnected" that is why, since ported won't do much at idle. They did that to give it extra timing at idle, for better smoothness and to be easier on the starter. Your distributor is likely around 32 degrees of mechanical advance. Set at 0 or tdc, that will give a conservative advance at WOT. The vac advance will add around 12 degrees at both idle and at a steady cruise, giving a smoother idle and better mileage on the highway. All of this only applies if you are still using the original camshaft and other major parts. If so, then the factory settings will work, though most people back in the day found that around 4-5 degrees advance (giving a total of around 36-37 mechanical advance at WOT) worked better. If you use the vacuum advance and the stock distributor settings, then I would not use ported vacuum because your idle timing will be too retarded for a good idle. Chris PS: If you are interested in re-curving the distributor, you may get better performance even with everything else stock. |
Posted By: gnrand
Date Posted: Jan/15/2020 at 11:09pm
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Thank you. Good information Jeff ------------- 1968 AMX 1986 Buick GN 1965 Corvair Corsa 1966 V8 Corvair 1969 Scrambler Stock 1969 Scrambler Modified |
Posted By: PHAT69AMX
Date Posted: Jan/16/2020 at 9:54pm
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Not to dispute others posts. Limit the Distributor to only 24 Degrees total potential Mechanical Advance at whatever rpm. Distributor positioning, 'Initial Advance' of 12 degrees goes along with the 24 mech for 36 total. Whatever and however the Vacuum Advance is and does is independent of, and goes on top of that. imho you want to set your total Advance of 36 degrees at rpm with vac adv disconnected. Then whatever it is at idle is whatever it is, it depends on the mech adv weights, springs, and amount. Factory was Ported Vacuum to the Vacuum Advance, no Vacuum Advance at Idle. No, Vacuum advance does or adds nothing at WOT, there is 0 manifold or ported vacuum at WOT. Now there IS venturi vacuum at WOT, and that is what actuates Vacuum Secondary Carbs. Ported Vacuum if working properly has 0 vacuum at idle and will add no vac advance at idle. Although best practice is to disconnect and plug the carburetor side of the vacuum hose. Initial Timing Advance Setting - this is less than what is best to go by, especially without knowing potential total Mechanical Advance. Total combined Initial Mechanical Advance is what matters and what should be used to set the timing at rpm using a Dial back Timing Light or Indexed Dampener, most likely 34 to 36 degrees. What the advance is at idle speed is secondary, avoiding over advance at rpm with mech adv all-in is primary. Vacuum advance has nothing to do with either one, is there for other reasons, and only comes in or applies when there is vacuum, which foot off the gas or steady state cruise. -------------  Link to a http://www.youtube.com/watch?v=MySiKQsmWxU" rel="nofollow - Short YouTube Burnout Video |
Posted By: 69 ambassador 390
Date Posted: Jan/16/2020 at 10:59pm
Phat is 100% correct. AMC did not use manifold vac to the distributor at idle EVER. The later cars had a TCVS that connected the distributor to ported vac under normal conditions and would switch to manifold vac if coolant temps got above a set high limit. This was done to reduce further heating by advancing the timing, thus raising the idle and also reducing EGTs to help cool the heads. Running manifold vac under light load and lean cruise can actually raise NOX emissions by increasing combustion pressures and temps in the chamber. In short, You would normally run ported, just like AMC did. ------------- Steve Brown Algonac, Mi. 69 Ambassador sst 390 84 Grand Wagoneer 69 Cougar XR7 65 Fairlaine 500XL 79 F-350 Super Camper Special |
PHAT69AMX wrote:Posted By: Steve_P
Date Posted: Jan/17/2020 at 10:31am
| The engine will idle and run much better with 5+ degrees of advance (no vacuum to distributor when you set initial timing). The TDC setting was for emissions. As said, limit the mechanical and vacuum advance as necessary to prevent pinging. if you run too much initial advance the engine may be slow cranking depending on how good your starter is; so that's another variable |
Posted By: ccowx
Date Posted: Jan/17/2020 at 10:46am
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Fair enough, I got thrown off on the manifold vs vacuum! However, I do have to say that in fact it is not true that AMC never used manofold vacuum, they did on the dual advance distributors. It was used for the idle retard on some early 70 distributors and I believe others as well. Mine still has the hook up for it, though I am not using it at the moment. Chris
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Posted By: gnrand
Date Posted: Jan/17/2020 at 11:30am
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Thank you. I will play around with it to find the sweet spot. Jeff ------------- 1968 AMX 1986 Buick GN 1965 Corvair Corsa 1966 V8 Corvair 1969 Scrambler Stock 1969 Scrambler Modified |
Posted By: Heavy 488
Date Posted: Jan/17/2020 at 11:47am
| Vacuum advance connected to manifold is more of a bandaid to other issues. |
Posted By: Red Devil
Date Posted: Jan/17/2020 at 11:47am
... on my '73 and '74 AMCs, it used Manifold vacuum at idle when cold (to improve driveability when cold, according to the TSM) and ported when warm, switched by CTO valve. Also had TCS that limited vacuum advance in high gear / above certain rpm. All emissions related.   Ref:http://www.oldcarmanualproject.com/manuals/AMC/1973/Service/Chapter4Ahtml/browser.htm Some engines like vacuum advance connected to manifold vacuum to get advance at idle ... but can cause timing instability without the right vacuum advance unit. Once throttle blades are opened enough (just off idle), manifold and ported vacuum is the same. Use what the engine likes. Hope this helps,RD |
Posted By: Heavy 488
Date Posted: Jan/17/2020 at 11:50am
| I always felt that vacuum advance connected directly to manifold is more of a bandaid around other issues. |
Posted By: PHAT69AMX
Date Posted: Jan/17/2020 at 1:46pm
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One 'reason' or 'scenario' that I have run manifold vacuum to vacuum advance that I have used and it worked well is with a big cammed 'hot rod' engine to increase idle vacuum, minimize primary throttle blade idle opening, and reduce static initial spark advance to make hot starts easier. But this is a somewhat 'far to one end' situation that did not seem to fit the original posters situation. Ah, and thank you for posting about the dual-vac-adv exception on some cars and on later cars in some case with TCS etc that it calls for it. I always get stuck thinking of only a narrow window of application. Thanks everyone for their opinions, I like seeing all sides of a situation, and seeing most thing are not necessarily a single simple straightforward answer. Keeps up thinking and learning and that is good. gnrand - LOL - yes, you can increase your timing at idle and it may run better, just be careful not to end up with to much when the mechanical advance comes in. ------------- Link to a http://www.youtube.com/watch?v=MySiKQsmWxU" rel="nofollow - Short YouTube Burnout Video |