I'm confused about distributor advance specs. Also, dual point tech.

[AJ/FormS]

Thank you.

Per the chart, centrifugal advance is all in at 3600rpm.

The compression ratio on the Commando is nominally 10.5:1. I have TRW domed pistons which were supposed to maintain the stock CR, as opposed to the Egge replacement pistons which (I've heard) result in a lower CR. The compression was in spec last time I checked.

At 22.52MPH/1000RPM in top gear, at 60MPH it is only turning about 2600RPM. 3600RPM would be 81MPH. Absurdly, the horsepower peak at 5200RPM would not be reached until 117MPH (!)... which is why I have always felt this was a bit of a silly spec for a street engine. With a bigger rear end and a 4 speed, it would make more sense.

Anyway, in my example scenario, I'm climbing a grade at around 2600RPM, pulling 11 inches of vacuum. This should theoretically equal about 27º of initial + centrifugal advance, plus 16 to 18 degrees of vacuum advance — so around 45º total. Whatever, it seems to be too much, because it's pinging.

First, I suppose I need to confirm the centrifugal advance numbers by driveway testing — it is possible the mechanical advance springs are worn out (240K miles). If that checks out, then I suppose I want to cut back on the vacuum advance.

I don't want to just dial back the initial advance — it really seems to need it at idle and for initial response. Overall drivability is very good.

You also need to get on the same page, in terms of terminology.
Once again;
Your engine has FOUR timing requirements.
Idle timing; sometimes called; static/base
Power-Timing: sometimes called "all-in", or mechanical /centrifical/centrifugal
Part-Throttle Timing,
Cruise-Timing.

All of these are working towards the exact same goal, which is to complete the combustion process, at a specific point in the piston travel, so as to provide maximum thrust, to the crank, at a specific point in the crank's rotation, so as to be most efficiently transmitted.
I most cases, the point is said to be, in the window of 25 to 28 degrees ATDC. All your systems are, from the factory, designed to target that; when you set the Idle-Timing to the factory spec.

But then, 50 years later, along comes a new owner who was told that his engine needs 18* of Idle timing, and he resets it there, and instantly the whole thing is unsynchronized.
If yur gunna change the idle timing, then the entire distributor will need to be "reprogrammed".
If you have an automatic;
the first time the engine cares about timing is at stall, and the second thing it cares about is how-much and when, the "all-in/power-timing" arrives.
The Idle Timing can be just about anything. I mean try it! Just reach back there and tug on the Vcan, little by little, until more tugging does not produce a higher idle-speed. Now read the balancer. I bet you get over 30 degrees, maybe closer to 40*. The engine keeps picking up idle-rpm, because peak pressure is occurring nearer that magic 25* to 28* window.
It is however, impossible to set the Idle-timing there, with no other changes, and still be able to drive it, no matter what octane gas is in the carb; because it will detonate at the slightest provocation.
So then, Idle-Timing has to be retarded.
5* at idle is retarded.
so is 18*, 22, 26, and probably even 30*,
and so is any number less than what produces the highest Idle rpm.
Therefore,
get over it, disconnect the VA, and set the distributor to whatever gets you a max Power-Timing of, in the window of 32 to 36 degrees, in the window of 3200 to 3600; whatever does not detonate under WOT condition, and; Don't bother looking for the fine line, even 3 degrees less than optimum, will not likely be noticed.
AFTER all that is done, then you can start to work with the VA.

BTW, if yur little engine is pulling 11 inches, at 2600, climbing a grade, and the VA is active, IMHO, something ain't adding up.

 

[mvh]

Well, I remembered that the vacuum advance unit I was running was a replacement that I had found on eBay, after the original diaphragm failed, and it occurred to me that maybe this replacement had a different washer pack than OEM. So I pulled out one of my spare original distributors for comparison. But when I went to look at the distributor on the car, as soon as I removed the cap, I saw that the ground strap on the breaker plate was broken. I was like "what the hell?" because I just drove the car the day before. I wouldn't have thought it would drive with the points not grounded.

Anyway, that meant I had to pull the distributor and remove the breaker plate and rig a new round wire. And of course, while I was at it, I compared the vacuum advance washer pack. It turns out they were the same thickness, although one had a single very fat washer and the other had a stack of 4, the total thickness on both was .231". The springs seemed to be the same, as well, so far as I could tell by compressing them by hand.

Since I was in there anyway, I stole one of the washers from the spare and added it to the fat one, making a total thickness of around .29, figuring I wanted to slow down the onset of vacuum advance, so I might as well experiment. It was too late in the day to take it out for a test drive. I will report back later.

 

[RustyRatRod]

Yes, that was pointed out already. But I read it here on FABO, so... That's why I was seeking confirmation.

Well there's lots of wrong stuff here too.

 

[mvh]

You also need to get on the same page, in terms of terminology.
Once again;
Your engine has FOUR timing requirements.
Idle timing; sometimes called; static/base
Power-Timing: sometimes called "all-in", or mechanical /centrifical/centrifugal
Part-Throttle Timing,
Cruise-Timing.

All of these are working towards the exact same goal, which is to complete the combustion process, at a specific point in the piston travel, so as to provide maximum thrust, to the crank, at a specific point in the crank's rotation, so as to be most efficiently transmitted.
I most cases, the point is said to be, in the window of 25 to 28 degrees ATDC. All your systems are, from the factory, designed to target that; when you set the Idle-Timing to the factory spec.

But then, 50 years later, along comes a new owner who was told that his engine needs 18* of Idle timing, and he resets it there, and instantly the whole thing is unsynchronized.
If yur gunna change the idle timing, then the entire distributor will need to be "reprogrammed".
If you have an automatic;
the first time the engine cares about timing is at stall, and the second thing it cares about is how-much and when, the "all-in/power-timing" arrives.
The Idle Timing can be just about anything. I mean try it! Just reach back there and tug on the Vcan, little by little, until more tugging does not produce a higher idle-speed. Now read the balancer. I bet you get over 30 degrees, maybe closer to 40*. The engine keeps picking up idle-rpm, because peak pressure is occurring nearer that magic 25* to 28* window.
It is however, impossible to set the Idle-timing there, with no other changes, and still be able to drive it, no matter what octane gas is in the carb; because it will detonate at the slightest provocation.
So then, Idle-Timing has to be retarded.
5* at idle is retarded.
so is 18*, 22, 26, and probably even 30*,
and so is any number less than what produces the highest Idle rpm.
Therefore,
get over it, disconnect the VA, and set the distributor to whatever gets you a max Power-Timing of, in the window of 32 to 36 degrees, in the window of 3200 to 3600; whatever does not detonate under WOT condition, and; Don't bother looking for the fine line, even 3 degrees less than optimum, will not likely be noticed.
AFTER all that is done, then you can start to work with the VA.

BTW, if yur little engine is pulling 11 inches, at 2600, climbing a grade, and the VA is active, IMHO, something ain't adding up.

I wasn't messing around with idle timing at all — it was set to factory specs. I was just observing that I experienced part throttle pinging. I need to do more testing on that — I was just quoting speed from memory. It's hard for me to find a hill to test on — I live in the flats in a high-density, high traffic area — I don't get a run free from traffic very often, and I can't idle the car in the driveway and fiddle with timing due to HOA restrictions.

 

[RustyRatRod]

I can't idle the car in the driveway and fiddle with timing due to HOA restrictions.

Then it's time to relocate or find a new hobby. I've never understood why car guys move into areas where there are restrictive HOAs.

 

[Dana67Dart]

I don't get a run free from traffic very often, and I can't idle the car in the driveway and fiddle with timing due to HOA restrictions.

Get an exhaust hose and cut a hole in the wall of the garage. Might want to get a CO Alarm too to be on the safe side

 

[mvh]

It would be ******* great if I had a garage... Jesus, don't be so obnoxious. This is a temporary domicile until I can retire.

 

[Mattax]

But when I plot the factory specs, I get this: initial 10º + maximum centrifugal 20º (@1800rpm) + maximum vacuum 22º = 52º.

That's pretty much correct. I see in a later post you caught the mistake about of not converting the 1800 rpm of the dist. to crank rpm.
Only concept you're missing is the cruising rpm is less than the rpm of max advance.
Also keep in mind the tech's had latitude to set the initial a little higher or a a couple degrees lower depending on local conditions, fuel, and customer wants.

Funny the factory made millions of cars with a whole team of guys figuring out the best timing curves, but you're one of the few who have taken the time to study them.

 

[Mattax]

you unscrew the end of the pot and change the washer pack and/or the spring (possibly). But I can't find any references to what that actually does — does the thickness of the washer pack affect the total amount of motion of the breaker plate, or does it affect the amount of vacuum needed to move the breaker plate? I have a couple of spare distributors that I can source washers from. I don't know where you would source the springs, or find out the specs for them.

Its in the Master Tech Conference. I'm not sure if the link in my how to article is still valid but the citation is. The sessions are available at mymopar.com in pdf or the on-line imperial club.

Here is the curve diagram I made, as a JPEG image. Again, these are the factory specs, not field observations. Yes, you can see it is very steep initially.

But I misstated the amount of timing — at 1800RPM, it is about 21 to 25º. So that's not as extreme.

You can tell the table is not including static timing because it starts at 0º at 325-475 rpm (distributor).

View attachment 1716481290

Nice job!
Correct. The distributor information is the specfications for the advance. It does not include initial, and it may not even be the maximum advance. It just provides the limits so we can check it on a tester or on the engine (doubling it all and subtracting the initial). The info for the initial timing and slow idle rpm is in the fuel sections.

Most Mopar spring posts are adjustable. No need to bend.

The spring perches in Chrysler's points distributors get bent, the perches in the magnetic pickup distributors are on eccentrics, except for the Mopar Performance Race distributors.

 

[Mattax]

Most discussions I have read suggest that the total advance (initial + centrifugal + vacuum) should be in the 35-40 degree range.

No. Go with the factory info. High speed cruising with an iron headed LA will be around 50 degrees when using vac advance.

But running CA 10% ethanol 91 octane premium, I am getting noticeable detonation at partial throttle under load; for example, climbing the grade on 680 at Sunol at 60mph, with the vacuum gauge showing 10-12 inches.

The springs seemed to be the same, as well, so far as I could tell by compressing them by hand.

The way to test this will be measuring the timing at some convenient rpm and then using a vacuum pump (eg Mityvac) to measure the timing with vacuum applied to the vacuum advance. RPM has to be noted at each of these new measurements because obviously mechancial advance of the new prm will have to be subtracted to get the vacuum advance. But even without doing that, you can confirm whether vacuum contributes to the advance starting at 6"Hg or 8"Hg and stops adding at 10" or 12" or whatever.

. I was just observing that I experienced part throttle pinging. I need to do more testing on that — I was just quoting speed from memory.

You can try delaying the vacuum advance. OTH being octane limited (91) the more appropriate fix may be is to drop the initial two degrees. The reasoning is that the initial and mechanical timing is for full load situations. That's when the engine can get hurt by too much timing. So take the timing out of the whole curve and leave the light throttle alone. Now that's theoretical. If you have the engine hot and there is no pinging in 2nd or 3rd geat at full throttle, then that theory is proven wrong.

Clearly not having a tach is one of the challenges. Without the tach there's no way to confirm the advance is within spec. I assume you have a low rpm tach (up to 1200 rpm) which should let you document the beginning of the advance. So that's probablky where I'd start.

 

[RLF Cuda]

My 340 with Sealed Power +20 pistons (10.2:1 measured) and Eddy heads likes ~35 deg total advance and 22 degrees vacuum advance. Dist advance is 11 degrees dist for 22 degrees mechanical. Initial advance is 12-13 degrees which works well with the 284/484 MP cam. Starting is instant and no pinging on premium in Colorado (5800 ft and 89 octane fuel).
Each engine combination must be tuned to what the engine wants, not what any specs say as FSM specs are conservative to allow any driver to have a pleasant experience but not to maximize performance. Total timing (not with Vacuum advance) is best determined by timed speed trials (Drag Racing but another name) with top speed being the goal. Vacuum advance is then set if possible by steady state hill climbing to make sure there is no detonation. Another method for VA is to measure fuel economy on a known route and adjusting for maximum FE without detonation.
But all this needs to be done as part of a programed tuning regime including initially getting the carb spot on. This should result in an engine that starts without prolonged cranking and has excellent response while delivering good FE and accelleration.