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Engine Timing |
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Bandana
AMC Nut Joined: Jul/26/2016 Location: NC Status: Offline Points: 473 |
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Posted: Jun/14/2017 at 8:08am |
I need to get some perspectives on timing set ups for a modified 401ci. The car I bought last year has a full MSD ignition, 6AL box, billet distributor with vacuum advance, coil, wires etc. Early on as I played with tuning I thought the set up was very odd. The car idled ok, started ok and ran ok just tooling around town at low rpm. It had a bad flat spot when I put my foot into it which is what had me messing with it to begin with.
The initial set up had just one spring on the distributor weights and it looks like a medium weight that was crimped in the middle. Initial timing was running around 22 degrees. With the vacuum plugged off I could not get anymore advance at higher rpm. The marks would flicker a bit up around 3000 rpm but settled back at 22-24 degrees. Plugged the vacuum back in and I could get 34 degrees up around 3000-3500 rpm. Very odd to be set up this way and I'm not sure what the PO was thinking. The engine likes the high initial timing, I get that, but since the only thing giving any advance was the vacuum (which goes away at high rpm) the flat spot I was experiencing became evident.. Last evening, I made some changes but because of rain can not get an actual test drive just yet. I twisted the distributor up to 35 degrees all in (no vacuum) and installed the largest MSD (black) bushing which gives the least amount of spread between initial and all in of 18 degrees. I also installed the lightest (blue) springs I had on each weight. Right now I'm seeing around 16-17 initial timing with 34-35 all in. Both readings are kind of shaky hence the ranges (might be my cheap timing light?). Car starts good, idles ok and seems more responsive to the throttle but I need to get it out under load to see what it does for sure. I'm thinking I may need to try to squeeze some more initial timing out of the set up but since there is no larger bushing available from MSD I may need to resort to removing a spring or modifying them. I'm no tuning expert by any stretch and I put all of this up with hopes that some of you guys that have more experience can help me understand what I had and where to go from here. Is my thinking on this original set up valid or is there some real value to the engine being set up that way? Am I on track with the changes I made? How can I best get a few more degrees of initial if I need it? Thoughts?? |
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jcisworthy
AMC Addicted Joined: Jul/23/2009 Location: North Carolina Status: Offline Points: 2805 |
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Try two light silver with a red bushing
If you can get away with it lock it out
Run 34 total in either case |
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Specializing in dyno services, engine building, and cylinder head porting
rbjracing.com Phone Number 518-915-3203 |
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Bandana
AMC Nut Joined: Jul/26/2016 Location: NC Status: Offline Points: 473 |
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Correct me if I'm wrong but if I run a red bushing at 34 degrees total I'll only have6 degrees of initial??? That's even below stock setting? |
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jcisworthy
AMC Addicted Joined: Jul/23/2009 Location: North Carolina Status: Offline Points: 2805 |
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Yes
Try the black bushing with the light silver for max initial and quickest curve. I If you need or want more initial there are places out there that make larger bushings or if it runs without issues lock it, that will be your best bet.
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Specializing in dyno services, engine building, and cylinder head porting
rbjracing.com Phone Number 518-915-3203 |
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ccowx
AMC Addicted Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510 |
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I would suggest that you take this issue on in a logical fashion. First item will be to set the initial and total. I don't know what kind of cam, compression, etc you have, nor do I know if you have the vacuum advance hooked up to manifold or ported vacuum, which will also make a difference. I will go with a few assumptions let's see where you end up.
-First up is to set the initial and total. 34 total mechanical sounds like a good starting point, though an extra degree or two may be possible when you fine tune it. If you want to have a high initial timing, then go with a bushing that gives around 18-20 degrees advance. That way you can set it around 16-20 and get the correct total advance. This assumes you have it hooked to ported vacuum. -Set the advance rate to come on at about 3000-3500, once you have the initial and total close to where you want it. Then you can start speeding up the advance rate a few hundred rpms at a time until you get the feel how you like it, without detonation. My guess is that it will come in around 2500-3000-ish. Faster seems to work best with an automatic, slower with a stick. Start slow and work down to it, less risky for the engine! -Once you have those two set up, then you can see about vac advance at cruise. -Another option, especially if you find that with all done as above you are getting detonation in the mid range coming off idle, is to use manifold vacuum. That way you can have it at around 10-12 initial and 20-24 degrees mechanical. At idle the vac advance can be set to add 10-16 degrees if you need it to clean up the idle, but it falls away as soon as you put your foot in it. -Something also to bear in mind is that you can limit the vac advance. Maybe 10 degrees at idle and cruise is enough, rather than the usual 16 or so. You could have the dissy start the mechanical at 12 initial, but use the manifold vacuum to add some degrees at idle. It will idle where you want/need it to but at WOT it will drop back to something reasonable for when it is pulling hard, avoiding detonation. These are just suggestions, but if you use the mechanical advance and vacuum right you can tailor it to do just about anything. I hope that helps! Chris PS: You made a comment about using lighter or cobbled/crimpled springs to make up for the initial. That sounds kind of like the mickey mouse stuff causing the trouble in the first place. Bear in mind that the advance total, rate of advance and vacuum are all separate systems, each to be adjusted independantly. I would also suggest to you that the distributor you have is designed to cover a very wide variety of engine needs, stock and otherwise, so if you have having to modify parts to get it to do things it is not intended to, perhaps you are covering other issues with MM bandaid solutions. PPS: The numbers I have tossed out are just rough examples. Your initial could be as little as the 6 you mention or as high as 25-30+(with vacuum), depending on what the engine needs. 10-14 seems popular for initial and 34-36 seem most common for total, but neither is cast in stone! As JCIS worthy says, some even lock it out, though that is not my personal preference on a street car. Edited by ccowx - Jun/14/2017 at 12:31pm |
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Bandana
AMC Nut Joined: Jul/26/2016 Location: NC Status: Offline Points: 473 |
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A bit more info I should have added. My cam is an Edlebrock hydraulic with 234/244 duration @.5" and .520.544 lift with 112 degree separation. Balanced rotating assembly with a 0.030" overbore, forged pistons and a 9.6:1 compression. Carb is a Holly 700 DP on a Performer intake. Vacuum is connected to the carb. Car is a 4 spd.
My first question is why would a supposed knowledgeable engine builder set up the timing as I outlined in the first section? They must know that the advance gained from the vacuum disappears under throttle? I keep wondering if I am missing something with that set up but my limited knowledge tells me that was wrong. While I haven't done a lot of experimenting, the engine does seem to like high initial timing. Pretty sure I need to stay in the upper teens to low 20s for initial. If the engine will live with the 16-18 degrees of initial from the black bushing then how fast the curve comes in and what role the vacuum plays will be the variables. Right now I'm set at 17/35 degrees with the black bushing and the blue springs with no vacuum advance. I thought the blue springs were the lightest since they have fewer coils but maybe I should try the silver springs? No idea at this point what effect the vacuum has on anything as its disconnected. |
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BDCVG
AMC Addicted Joined: Dec/19/2007 Location: Endless Mtns. Status: Offline Points: 992 |
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Here's My setup which is similar to Yours except Comp. ratio. 390 10-1 comp. 234/244 cam 1 lt silver 1 heavy silver Blue bushing. 14 deg initial + 21 deg advance = 35 deg. total all in by about 3200 rpm. No vacuum advance. You maybe be able to have a quicker curve than I do with less Compression. With My 5 spd. I am at 3200 rpm very quickly but cruise 70 mph at 1920 rpm so I opted for a little slower curve to avoid ping when passing in 5th gear.
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1970 AMX 390 5 spd full Control Freaks front and rear suspension
2014 E63 AMG-S wagon 1965 Austin-Healey 3000 MK III |
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ccowx
AMC Addicted Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510 |
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Your vacuum is ported if hooked to the carb, which means that you are not seeing any effect at idle, but you will get additional timing at a steady cruise.
I would suggest that BDCVG is probably right around where your engine should be, though each engine is always a bit different of course. I tend to like a slower curve as well, but other knowledgeable guys go faster. See what your engine likes! My only real suggestion is to start out slow and then speed it up until you get ping, then back off one step. Safer that way! Chris PS: As for why did the PO do it that way, I can only say that many knowledgeable engine BUILDERS are not necessarily great engine TUNERS. |
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tufcj
Supporter of TheAMCForum Supporter of TheAMCForum Joined: Jul/10/2007 Location: Watkins, CO Status: Offline Points: 4064 |
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On my 69 AMX 390, similar specs to yours, I'm running 18* initial advance, another 20* mechanical for a total of 38*, all in by 3000 RPM. No vacuum.
Bob tufcj |
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69 AMX
74 Javelin AMX 67 Rogue If you need a tool and don't buy it... you'll eventually pay for it... and not have it. Henry Ford |
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Bandana
AMC Nut Joined: Jul/26/2016 Location: NC Status: Offline Points: 473 |
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Based on what you fellows are running having 17 initial and 35 all in may not be a bad place to start. Maybe I'll try driving it without the vacuum connected first to compare with and without the assist.
I was thinking that the blue MSD springs were the lightest as they have less coils and looked about the same gauge as the light silver but after the comments and checking I think I was wrong. |
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