Base timing. Really ?

-
I agree, the problem is not too much timing, but timing coming in too fast.
Just wondering, you have checked your TDC mark for accuracy?
 
I agree, the problem is not too much timing, but timing coming in too fast.
Just wondering, you have checked your TDC mark for accuracy?
yes. and i have the MP timing tape installed on the balancer
 
Glad i stumbled onto this post before i made a new one. On my 440 with 284/484 cam installed 2 degrees advance, 9.4:1 compression, street dominator intake, 3000stall convertor in a 1972 dart, I have the electronic ignition with an FBO limiter plate in it and if i remember right, 1 spring in the distributor. Timing is at 20 initial and 34 total. The timing is all in by 2800. I THINK too light of a spring is the issue and after reading all this i really think it is the issue, but when laying into the throttle at lower RPM's, like under 3500 i guess, it detonates a little, but i dont hear it if i lay into it when the rpm is higher..Thoughts?
What the ignition is doesn't matter too much under 3000 rpm. They'll all get the kernal started within a degree of each other although the HEI might provide slightly better growth if combustion conditions are less ideal. So dont worry about that change as far as its effect on the timing.
The limiter plate and the signle spring are what is making it difficult.
Whether limiting the advance will work or not depends on a number of variables.

20 initial , how about the vac advance ? do you use one ? That may be adding to your problem. I would also add 1 soft spring. Common set up seems to be 1 soft stock spring , ditch the thick stock spring, and install a soft aftermarket one ( mr gasket )
It is a common setup but its not a good setup for street/road uses and with electronic ignition its not the best for drag strip either. That's one of things that Yellow Rose and Bill Baldwin are showing in their videos.

(Add a spring will delay )
Yes. And where it will delay the advance will depend on the details
It depends on the tension force the spring applies to the weight. That in turn depends on the spring length, the spring perch positions and how far out the weight is, if at all.

This was with the mopar ECU. I also had it dialed back to 16 degrees BTDC. Since then i have done the GM HEI module conversion hiding it in the gutted mopar ECU and also installed one of the FBO plates and am in the process of getting it right. I find it like 20* initial better than the 16 but like i was saying, it detonates slightly when nailing it under 3500 rpm and figured the timing needed to be slowed down. I know it comes all in quickly with the spring i have in it. all in before 2200.

Liked 20* better in gear or out of gear?
Out of gear means little, and with an automatic transmission even less.

If you want to stick with the FBO plate, you may have trouble using vacuum advance. Limiting the advance can work, with the right springs in some engines.
If your not married to the FBO limiter and you have the original springs, then it may be easier and better to shorten the slots on the inside.

I've posted about this before. really don't have time right now to repost it. I'll see if I can find some links.
 
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I'll see if I can find some links.

Here's some links:

Post showing adjustments to timing.

By shortening the slot on the inside like Rockable did here, then its pretty much a pre-CAP advance. Everything will work as it should without changing springs. Small adjustments on the start can be done by adjusting the spring perches.
I think how and why is pretty well illustrated with Rockable (360 Tune Up) and further discussed with Mullinax."

Advance curves for performance engines using no vac advance (Race) versus when using vac advance.
Another timing curve question
 
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Since your working with a 440, its worth looking at factory 440 hi performance timing.

This is the range that was acceptable for Plymouth when inital was set at 12.5 BTC (or 5* BTC for a smog distributor).
upload_2020-12-12_18-37-53.png


Many of the electronic distributors have a timing curve that is something like that that '67 CAP timing advance.
These CAP and CAS advances used a fast and long primary advance to get the off-idle timing back where it should be for efficiency. (The late timing at idle was neccessary for more complete burning of HC and reduction of CO.)

The above timing curves work well with vacuum advance. Thst long slow secondary advance also works well with electronic ignitions as it helps offset the slew rate that Yellow Rose posted about.

So when we take early emissions era Chrysler distributors and shorten the inside slots, we get somethng like this.
upload_2020-12-12_18-49-48.png



Now with good fuel and at reasonable elevations, we can often advance that initial somewhat. If doing so increases the mph at the track, then thats what it wants.

In contrast, here's an example of limiting the top end of the advance for a '68 440 manual smogged distributor.
upload_2020-12-12_18-54-24.png

This might have worked good at the drag strip with dual points and no vacuum advance.
Its completely unusable with vacuum advance and if used with an electronic ignition will retard timing at the top end.

upload_2020-12-12_18-59-55.png
 
I am really confused..I do remember when i built this engine and using the stock distributor untouched, it would ping like crazy at high RPM and remember people over on moparts telling me that i would have to limit the slots to around 34 degrees because thats about all it could stand with that cam. So i JB welded the slots and installed the light spring and had to do a trial and error with a file to get the slots right. I ended up with 18 initial and 34 total and she didnt ping so i left it like that for years. This is with the factory mopar ECU. Then i did the GM module upgrade and at the same time, installed the FBO limiter plate and knocked out the JB weld from the slots. I never ran the vacuum advance. You guys are telling me to get rid of the limiter plate but when it goes beyond 34 degrees it pings like crazy. I am just confused.
 
Since your working with a 440, its worth looking at factory 440 hi performance timing.

This is the range that was acceptable for Plymouth when inital was set at 12.5 BTC (or 5* BTC for a smog distributor).
View attachment 1715647869

Many of the electronic distributors have a timing curve that is something like that that '67 CAP timing advance.
These CAP and CAS advances used a fast and long primary advance to get the off-idle timing back where it should be for efficiency. (The late timing at idle was neccessary for more complete burning of HC and reduction of CO.)

The above timing curves work well with vacuum advance. Thst long slow secondary advance also works well with electronic ignitions as it helps offset the slew rate that Yellow Rose posted about.

So when we take early emissions era Chrysler distributors and shorten the inside slots, we get somethng like this.
View attachment 1715647877


Now with good fuel and at reasonable elevations, we can often advance that initial somewhat. If doing so increases the mph at the track, then thats what it wants.

In contrast, here's an example of limiting the top end of the advance for a '68 440 manual smogged distributor.
View attachment 1715647884
This might have worked good at the drag strip with dual points and no vacuum advance.
Its completely unusable with vacuum advance and if used with an electronic ignition will retard timing at the top end.

View attachment 1715647887

so.
For a good starting point for my warm / street 440
In general terms
Let’s say 15 initial
Limit the slots so 35 total mechanical
( 10 degrees at the plate / slots )
Now
Leave the stock springs which will make the timing come in late
Or
Change out the heavy stock spring to a light one to bring in the timing quickly
I will be using a vac can

Thanks everyone for all the great input.
 
I am really confused..I do remember when i built this engine and using the stock distributor untouched, it would ping like crazy at high RPM and remember people over on moparts telling me that i would have to limit the slots to around 34 degrees because thats about all it could stand with that cam. So i JB welded the slots and installed the light spring and had to do a trial and error with a file to get the slots right. I ended up with 18 initial and 34 total and she didnt ping so i left it like that for years. This is with the factory mopar ECU. Then i did the GM module upgrade and at the same time, installed the FBO limiter plate and knocked out the JB weld from the slots. I never ran the vacuum advance. You guys are telling me to get rid of the limiter plate but when it goes beyond 34 degrees it pings like crazy. I am just confused.
If you're not using vacuum advance then shouldn't be issues setting it up with a quick advance. The main refinements will be stable timing at idle rpm, and for high rpm (3500 up) ideally enough advance to offset slew rate (the retard seen in video).

Sounds like something changed in the timing curve from the JBWeld setup to the FBO plate setup.
Without having the timing at various rpms from both setups, its pretty hard to know.
I'd just start from where it is now.
Measure the timing from the lowest rpm until it stops advancing.
Then note what rpm and whether its full throttle, and what gear the pinging is audible.
Adjust as needed.
 
so.
For a good starting point for my warm / street 440
In general terms
Let’s say 15 initial
That seems reasonable for slightly warmed 440, for your engine I'd would start higher.
I'm going to guess 16 to 18* Distributor starting point for a curve
If the combustion chamber conditions at idle have little heat, it will need more and vica versa.

Get it warmed up and see if you can get it to idle with good stability at 18*. Then see if it will run just as well with 17 or 16, or maybe it needs 19*. Whatever timing it needs to run decently without too much throttle opening. Too much throttle opening is idling with the primary throttle plates revealing more than .035 or .040" of transfer slot to the intake manifold.


Limit the slots so 35 total mechanical
( 10 degrees at the plate / slots )
Yes if you know for sure 35* is the maximum advance the engine will want.
For high rpm use and most electronic ignitions, allowing for an additional 2 to 4 degrees in the advance will help in the upper rpms.
In this way we use the mechanical advance to offset the time lost in the electronic switching. (If not racing then not so important.)

Now
Leave the stock springs which will make the timing come in late
Welding up the inside of the advance would make the advance start a little later if everying was stock.
For example lets say the distributor in hand mapped out to have a timing like this
upload_2020-12-13_0-4-6.png

Its a handy one for the example because the factory initial was TDC.
We see by the timing light it has 32* advance. Lets pretend this was a electronic distributor. In which case the advance plate would have something more than 16* in the slots.
Lets say we weld up the inside of the slots enough to get 18 degrees of advance.
If the engine still can idle at 700 rpm, then yes the advance will be delayed.
upload_2020-12-13_0-15-54.png


If you can get your engine to idle that low, great!
If you can get it to idle at 750 or 800, that will be good too.
If its a little higher, don't sweat it.

Regardless, if the advance isn't starting until several hundred rpms above idle, the spring to adjust is the light spring.
(I call it the primary spring because it controls the initial portion of the advance.)

Before going through the trouble of finding a different spring, experiment with reducing the initial tension as shown in this post

That will bring the advance in sooner.
upload_2020-12-13_0-27-13.png


Advancing the initial to 17* at 800 rpm, the curve shifts up.
upload_2020-12-13_0-35-33.png


Change out the heavy stock spring to a light one to bring in the timing quickly
No. The heavy spring has a long loop in it. This spring has no effect on the first part of the advance. Its the secret sauce of the two stage advance. ;)
 
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Guys im confused on why you say im better off without the limiter plate. Dont i need to limit the slots or? I dont use the vacuum advance allthough if it would stop the stinking idle i would love to..just confused. I know Don over at FBO offered to set up the distributor if i sent it to him but not sure how without the engine that it goes in..? I would really like to understand this though and do it myself..
 
Guys im confused on why you say im better off without the limiter plate.
What I was trying to explain in post #59 is that the FBO plate's disadvantages are not your problem:
A. Your not using vac advance.
B. If the JBWeld was on the outside of the slots, then the FBO plate has the same effect on the weight movement as the weld.

My suggestions on what you should do next are in post 59.
I won't be around. I'm already running very late today so can't offer more than this.

If you want to understand the different effects of shortening the advance on the inside vs the outside, spend time looking at the the links above. As far as the heavy secondary spring for racing, do a search for some posts I made about the MP "Tach Drive" distributor.

Different people learn different ways. Another thing to do might be to open up the distributor and move the weights out to see how the spring effects the movement.
You might be best off just following the steps in post 59. Measure and plot out the timing vs rpm up to 3000 or so. Then mark where the problems occur. You can then figure out what to do to reduce the timing under those conditions.
 
I am really confused..I do remember when i built this engine and using the stock distributor untouched, it would ping like crazy at high RPM and remember people over on moparts telling me that i would have to limit the slots to around 34 degrees because thats about all it could stand with that cam. So i JB welded the slots and installed the light spring and had to do a trial and error with a file to get the slots right. I ended up with 18 initial and 34 total and she didnt ping so i left it like that for years. This is with the factory mopar ECU. Then i did the GM module upgrade and at the same time, installed the FBO limiter plate and knocked out the JB weld from the slots. I never ran the vacuum advance. You guys are telling me to get rid of the limiter plate but when it goes beyond 34 degrees it pings like crazy. I am just confused.

Your engine requires exactly one timing, the one that causes the maximum amount of gas pressure to be delivered to the crank at the exact right time. IIRC this is ~28*ATDC. But I have read other numbers.
As rpm, loads ,temperatures, and elevations, change, the BEGINNING of the gas expansion strategy has to be adjusted to achieve that.

I talk streeter only
so, there are at least four different timing strategies to achieve this;
1) power timing
2) idle timing
3) mid-rpm timing
4) cruise timing
5) auxiliary timing
6) timing from idle to stall


#1) power timing: Of the four, this is the most likely one to break parts so that is the one to nail down first. Thankfully, during the last 7 decades or so, there is a plethora of empiracle data, that says most HotRod engines want very similar power timings, in the range of 34 to 38 degrees, so mostly we shoot for 36* and if it don't detonate then it's done. see note #1

#2) idle-timing; this number falls out of the tune all by itself, during the setting of the transfer slot to mixture screw synchronization. Generally, your HotRod engine will like 14 to 28 degrees, but with an automatic, the only important number is the one at stall. see note 2

#3) mid rpm timing; oh yeah, this is where the hurting occurs; namely from stall-rpm to all-in rpm; somewhere around 2400(depending on your TC), and 3400 (the point you choose). see note 3

#4) the centrifugal advance system can only be right exactly twice, namely; at WOT after about 3600, and at idle. Every where else, at WOT, you can only be close, so don't bust your nuts looking for perfection ....... because, of the thousands of hours your engine will live, ask yourself how many of those might be at WOT from stall to 3600. You get the point? see note 4

#5) auxiliary timing; for guys with manual transmissions.
You guys know that driving below 5 or 6 mph is almost impossible, with a performance cam. see note 5

#6) idle to stall; with an automatic,who cares. see note 6

_________________________


note 1
On the street, your butt-dyno will not know the difference between perfect timing and 2 to 4 degrees not enough, so you ALWAYS aim low.Power timing does not begin until the engine can handle it without detonating. This depends on many factors, but usually falls in the range of 3200 to 3600. Again; aim for the safe numbers closer to 3600.
Why?
Because; ask yourself how often are you gonna be at WOT at 3400rpm, that a couple to a few hp is gonna make a difference, with the tires spinning. I'll tell you; with 27" tires and 3.23s; at WOT, 3400 occurs at ~27mph in 2.45 low/~48 in 1.45 2nd/ and ~77 in 3rd; but, running to redline, only in first gear will it ever actually occur, and if your engine is not smoking street tires at WOT, in 2nd gear at 3400, then there is something wrong with it. Soooo, the true answer is your engine will never care about a power-timing deficiency at 3400! The bigger the engine the less it will care.
There is only one possible situation when it might make a few hp difference, and that is for passing. Here it is;
Again with 3.23s or less, and 27" tires;
Say you are cruising down the road at 50=2000(zero-slip), say 2100 on the tach@5% slip. And you catch up to a slow poke doing 48, but you gotta get around him in a hurry, so you floor it. The trans drops into 2nd and the Rs jump to 48=2800(zero-slip), say3200 on the tach@ 15% slip. Ask yourself how often this situation will ever occur. Then in the blink of an eye, your car will be pulling past his door doing 75mph at 5000. 7hp lost at 3400, will not make a difference.
So ideal power timing at 3400 is not a priority on my combo, I say again; aim safe. You got thousands of miles ahead of you to experiment.

note 2
The only difference in that range (14 to 28) is how smooth the engine idles at; big deal. The more retarded you idle it, the lumpier it will sound. The more you advance it, the smaller the transfer slot will be, and the more likely it will be that you have tip-in problems. Get the tip-in right, by adjusting the transfer slot exposure right, and use timing to set the idlespeed; it's just about that simple. Above all; DO NOT set the timing to some pre-conceived number that you think she should have; instead, let your engine tell you what number is right, by the minimum idle speed required to escape tip-in problems, and that keeps the oil pressure reasonable..

note 3
if you have set the idle-timing too high,you MAY have problems with detonation at WOT, in this range, because it MAY be too high all the way to "all-in".
The only ways to fix this are by
a) reducing the idle-timing, or
b) SLOWING the flyweights down with springing, or
c) by reducing the weight of the flyweights.
Which method you chose, depends on if you have a spring selection, or a bench grinder, spare flyweights, or a limiter-plate, or just back up the idle timing, and mod the cam back to the required power-timing. Any method is acceptable, whatever gets the job done.
I like to get this timing worked out so I can run cheap fuel, cuz over 18 years, a guy can save a boatload of cash.

note 4
Ok so, what do you need to do, when not at WOT, and from idle to "all-in"?
This is called PT or Part Throttle. At PT, your engine, to achieve the optimum energy-transfer to the crank, will want between two and three times as much timing as the mechanical system can deliver. It is not uncommon to achieve maximum fuel economy with cruise-timing in the mid 50s to early 60s, this being almost wholly dependent on your specific build. This is how early the fire has to begin, to get the max-pressure to occur at the best point after TDC.
If you know your cruise rpm;
you can get a close idea of your best cruise timing, by holding the engine to that rpm, and with the vacuum advance hooked up, just pull in the timing until the engine rpm peaks; and then read the timing . That will be very close. Don't be surprised with what you see; the engine just told you what she wants. Many times, it will not be possible to achieve that, with factory parts. My engine needs a dash-mounted, dial-back, timing delay box, to make it happen.

note 5, part 1
Especially with a hi-compression engine. You have to either slip the clutch or ride the brakes. Have you figured out the why of it? IDK, but I'll tell you how to get as low as your gears will let you go.
timing is the answer.
Typically, with the Transfer slot synced up, your timing is gonna want to be about 12 to 16 degrees, but Ima thinking you guys are setting it much higher. So then, when you try to drive it idling at 700, the engine is so far down the transfer slot, it runs out of power and wants to stall. So you crank up the idle speed and it runs better, idling along in first gear, but the roadspeed with 3.55s is 6mph or more, so you almost never run down there. If you ride the brakes, the engine starts bucking the car, and you end up slipping the clutch.
The reason this is happening is because you have set the parameters to create the max pressure to the crank at or near the optimum point, which, for operation in this mode is just too daymn powerful.and so every power pulse, 4 of them per engine revolution, is hammering down your driveshaft and into the tires, multiplied by all the gearing in-between. Say your engine idles at 80 ftlbs, by the time that gets to the axles, that is multiplied by 80 x2.66x 3.55=755 ftlbs! Imagine some guy back there, with a long bar on your wheel nuts, and whacking it over as hard as he can 6 times per second. Yeah that will drive you nuts.
So what do you do? You already tried idling slower, but the engine just wants to stall. The trick is to cause peak cylinder pressure to occur with the piston further down the bore, which will simultaneously drastically reduce the pressure; problem solved right? And how you do it is to set your T-slot sync right, and then retard the timing until the engine hasn't got enough power to stay running anymore, and then give back just enough timing to prevent that.
By experimentation, my 230* cammed 367 combo that cranks over 180psi on the compression gauge, will idle just fine at 500/550 in gear and rolling, with just 5* advance. I do that with an aux-box. Mine has a 15* adjustability, and I set my idle-timing in Neutral, to 14*.
At 500rpm, you can get your roadspeed down to 4.26 mph with 2.66 x3.55 and 27"tires.
My trans has a 3.09 low in it, so 500=3.7mph, and smoooth sailing... it makes just enough torque to pull itself on flat,hard,level, ground at 500, but needs 550 to climb over a dime.lol.
Now driving this slow may not be important to most of you. But for me, I like Daisy Dukes. They remind me of those early years, in my marriage, 45 years ago, you know what I mean. . I like long reminders. So parading is high on my list of priorities.
Besides, that Hughes cam is finally sounding like something; music to my ears.

note 5, part 2
when your engine is cold, it wants more timing, not just at idle, but at PT, all the time. Mine likes at least 10 over the base idle-timing of 14*. At 24* I can drive away on the coldest September mornings. But three to four miles later, the speed-O has climbed 4 mph, and I know to take those 10* out before I floor it.

note 5, part 3

I use that dash-mounted,dial-back, timing delay box, for other things as well. especially for tuning. It helped me get my PT and cruise curves, to as close to perfection as possible.
And every once in a while I get some crummy gas, or it's just been in the tank too long. I just twist in/out some timing and away I go.
or, sometimes I just want a lil more timing at 35/40 mph cuz I'm gonna be there for a while, and want to eke out a lil more mpgs
or one time I went to Alberta and got into higher elevation, and my fueling was off, so I jut retarded the timing to be safe.

note 6
The only time you should care is if your combo has to cruise in there. With 3.55s your PT mph@stall rpms are;
20@2200, 22@2400, 24@2600,26@2800,28@3000 and so on. At PT your v-can will compensate for any mechanical shortcoming. As you move off PT to Power, stall-timing will takeover.
Only manual-trans cars need to work out the timing in this range.

more coming hit refresh,
ok I better quit for now
 
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with the engine combo i have, should i be running vacuum advance? It does stink like hell at idle,,otherwise it does idle great in gear and doesnt come close to trying to stall..it only drops 100 rpm from park to drive, if that..this is at 18 or 20 *btdc. I would like to get the most out of the thing i can and if running vacuum advance will help it, then i wouldnt mind getting it going..
 
with the engine combo i have, should i be running vacuum advance? It does stink like hell at idle,,otherwise it does idle great in gear and doesnt come close to trying to stall..it only drops 100 rpm from park to drive, if that..this is at 18 or 20 *btdc. I would like to get the most out of the thing i can and if running vacuum advance will help it, then i wouldnt mind getting it going..


Fix the carb first. Then work with the timing.
 
To answer your question: yes, you should always run a vacuum advance unless it's a pure race car that only operates at idle and WOT. For a strip-only car it's one more thing to stick or malfunction.

Depending on the size of your cam, it may not have enough vacuum at idle to pull in the can. And a lot of idle advance will make the rpm drop in gear worse.

More advance and proper carb tuning (including setting the idle mixture screws) will help clean up that stinky idle. My [email protected] doesn't stink (much) :p but I have a four-speed so rpm drop in gear isn't an issue.

Big cams do like richer idle mixtures though, especially with automatics that put a load on the engine when engaged... how loose is your converter again?
 
I don’t use the VA to fix idle issues. What carb are you using and what’s your cam timing?

Im going to have to see if i can find my old notes on when i built this thing because i cant remember for sure, but i do know it has the original grind MP 284/484 cam that if installed straight up was 108*, but i installed it either 2 or 4 degrees advanced but again i cant remember if it was 2 or 4..the more i think about it i believe it is 4 advanced. It has a race prepped thermoquad by demonsizzler that was on moparts back then. it does great on the combo! Like i said, there is no issues with it idling in gear whatsoever. the cam is 3000 stall. I have a friend that used to have a very nice chevelle with a cammed up 350 with automatic that wanted to die at idle in gear..this doesnt do any of that, i got it right with the idle on the carb and mixture screws and timing at idle. Can sit at a 10 minute redlight in gear idling just fine as if it were in park..stomp it and it will care the crap out of you! It does great except for stomping on it from a cruise and it pinging slightly. It has so much torque that when cruising at say 40 mph, you can tromp down on it without HAVING to drop down to 2nd gear and thats when she will detonate a little..not a lot but i dont want any! The timing is all in at that speed when tromping it and like we discussed, i believe slowing that timing down a little will stop that ping..If i drop the timing down to 16 at idle, it sets it all back enough where it doesnt ping, but it also takes away performance that i can feel..
 
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That seems reasonable for slightly warmed 440, for your engine I'd would start higher.
I'm going to guess 16 to 18* Distributor starting point for a curve
If the combustion chamber conditions at idle have little heat, it will need more and vica versa.

Get it warmed up and see if you can get it to idle with good stability at 18*. Then see if it will run just as well with 17 or 16, or maybe it needs 19*. Whatever timing it needs to run decently without too much throttle opening. Too much throttle opening is idling with the primary throttle plates revealing more than .035 or .040" of transfer slot to the intake manifold.



Yes if you know for sure 35* is the maximum advance the engine will want.
For high rpm use and most electronic ignitions, allowing for an additional 2 to 4 degrees in the advance will help in the upper rpms.
In this way we use the mechanical advance to offset the time lost in the electronic switching. (If not racing then not so important.)


Welding up the inside of the advance would make the advance start a little later if everying was stock.
For example lets say the distributor in hand mapped out to have a timing like this
View attachment 1715648080
Its a handy one for the example because the factory initial was TDC.
We see by the timing light it has 32* advance. Lets pretend this was a electronic distributor. In which case the advance plate would have something more than 16* in the slots.
Lets say we weld up the inside of the slots enough to get 18 degrees of advance.
If the engine still can idle at 700 rpm, then yes the advance will be delayed.
View attachment 1715648090

If you can get your engine to idle that low, great!
If you can get it to idle at 750 or 800, that will be good too.
If its a little higher, don't sweat it.

Regardless, if the advance isn't starting until several hundred rpms above idle, the spring to adjust is the light spring.
(I call it the primary spring because it controls the initial portion of the advance.)

Before going through the trouble of finding a different spring, experiment with reducing the initial tension as shown in this post

That will bring the advance in sooner.
View attachment 1715648093

Advancing the initial to 17* at 800 rpm, the curve shifts up.
View attachment 1715648094


No. The heavy spring has a long loop in it. This spring has no effect on the first part of the advance. Its the secret sauce of the two stage advance. ;)

Great info !
Just a note. With my cam I would say it will idle higher than 800. Prob 900 or a touch more.
My distributor is prob a late 70s smog job. /. MSD box on the inner fender.
So
Base timing , we are close. 15-18 ish
Leave springs alone , or lighten the primary spring
Don’t touch the secondary spring
Limit the plate to ,say 35 ish. ( 8-9 degrees at the plate )
Yes ?
I know this is taking a lot for granted, but I would like to be as close as I can to start. You guys have a ton of experience. Gold.
Thanks
 
Great info !
Just a note. With my cam I would say it will idle higher than 800. Prob 900 or a touch more.
My distributor is prob a late 70s smog job. /. MSD box on the inner fender.
So
Base timing , we are close. 15-18 ish
Leave springs alone , or lighten the primary spring
Don’t touch the secondary spring
Limit the plate to ,say 35 ish. ( 8-9 degrees at the plate )
Yes ?
I know this is taking a lot for granted, but I would like to be as close as I can to start. You guys have a ton of experience. Gold.
Thanks
so even with our cams we are running, we should retain the factory elongated spring and change the other one out for a lighter one?
 
Yes that would be the approach.

I don't have any good info on late smog Chrysler distributors.
You may find the smog advance curve isn't quite as extreme as the '68 440 with M/T
It may have other differences too but it will still be a two stage advance.
I know with AMC Jeep V8s, in the early 80s they were able to run more normal initial timing and advance. I suspect this was because they were able to advances in catylitic converters and other tweaks to offset slightly higher idle emissions.

When take the distributor apart enough to get to the advance plate you can see how much advance is in there. The plate (aka governer) will be stamped with the approx degrees. Also take all the measurments with a caliper so you have your own reference.

You can also play with the moving the weights using needle nose pliers to get sense of how many degrees of movement before the long looped spring engages. A few guys have used a degree wheel and a homemade pointer to measure movement (distributor degrees) on the bench.

I know this is taking a lot for granted, but I would like to be as close as I can to start
Sure thing. Might as well take advantage of the research and testing Chrysler did for their engines and vehicles.
The most difficult part in the disassembly is removing the spring under the felt.

If you think idle is going to be around 900, probably best to start without weakening the tension on the weight.
If the weights start moving out at 750 or 800 rpm and the engine is idling around 900 rpm, timing will change with every adjustment which then changes the vacuum which effects the idle circuit and in turn the rpm. Its like chasing a moving target.
 
the one that causes the maximum amount of gas pressure to be delivered to the crank at the exact right time. IIRC this is ~28*ATDC. But I have read other numbers.
As rpm, loads ,temperatures, and elevations, change, the BEGINNING of the gas expansion strategy has to be adjusted
That's the crux of it isn't it.

Whether the piston is 28* or 25* into the power stroke probably depends on the crank arm and rod and stroke ratios. Need to think about that or run some examples. Thats one of those little details that makes each engine a little different. But it really doesn't matter, the concept is the same.

Reposting for those that may be not following why we're talking about pressure in the cylinder. It's all about maximizing leverage on the crank.



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Mechanical or Centrifical Advance
Compensates for the reduced amount of time (miliseconds) available for the burn as rpm increases. Its not directly proportional because combustion efficiency increases with rpms. This means decreased burn time as rpms climb. A 'low performance' 318 usually has good lower rpm efficiency, but does not gain as much efficiency with increasing rpm. A 'high performance engine' is usually the opposite, poor - slow combustion at low rpms, but more efficient and quicker pressure development at higher rpms.

Vacuum Advance:
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from Ignition System Analysis (Session 259) from the Master Technician's Service Conference
 
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That's the crux of it isn't it.
Reposting for those that may be not following why we're talking about pressure in the cylinder. It's all about maximizing leverage on the crank.
I'm sorry I can give you only one like,lol. I'd like to give at least 10 times as many just on this one post.
 
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