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

[mvh]

I was trying to plot a distributor advance curve for my Prestolite dual point 2642242 IBS-4013A (273 Power-Pak w/o CAP) using the specs in the 1967 Plymouth FSM.

I realize that the FSM lists the specs as "distributor degrees at distributor RPM". I also realize that the distributor rotates at 1/2 the crank speed, so you have to double the RPM figure to get the reading from your dwell tach. What I am confused about is if I also have to double the listed degrees to get crank degrees? The posts I have read on this subject suggest that I do.

The reason I am asking this is because when I plot the curves using that assumption, the total advance all-in seems to be extreme. Most discussions I have read suggest that the total advance (initial + centrifugal + vacuum) should be in the 35-40 degree range. But when I plot the factory specs, I get this: initial 10º + maximum centrifugal 20º (@1800rpm) + maximum vacuum 22º = 52º.

I wouldn't question this if it seemed to be working in real life. 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. Full throttle seems to be fine, but I can't drive this thing at WOT all the time and keep my license <lol>.

For context, this is a factory Commando 273 with a Crower Baja Torquemaster hydraulic cam (very similar to the factory cam profile), bored .020, with fresh mildly ported heads. The distributor is the original 240K mile unit, with new points. 3.23 rear.

My thought was I could use a little less vacuum advance, and the vacuum advance on this distributor is theoretically adjustable — 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.

 

[Dana67Dart]

vacuum advance on this distributor is theoretically adjustable — 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

The spring determines the rate the advance advances. (Light spring, the quicker the advance reaches max )The washers determines the vacume at which the advance starts

 

[Dana67Dart]

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

The vacume advance is load dependent, so put a vacume guage on the ported line to the vac advance to see what it's getting under various conditions.

 

[1MeanA]

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

I've never read that. Generally, total advance without vacuum advance should be in the 35 degree range.

 

[AJ/FormS]

Dwell time, read on the meter, is NOT doubled.
The only thing that gets doubled is the distributor rpm, to get crank rpm.
"All-in"/Power-Timing, is measured with your timing light directly.
For all the SB Mopes I have ever worked on, the "all-in" Power-Timing, should happen in the window of 3000 to 3600.
VA is never included in the "all-in" power timing.
VA is added to the mechanical and helps to get Part-Throttle efficiency.
Every engine seems to have it's own needs. Here's mine.
I like to run a two stage mechanical-curve, very fast from Idle to about 2800, and then I like to slow it down until it's in mid-window. This allows me to run a modest amount of Idle Timing, and to tune the carb, to get the rpm way down, so I can drive my manual-trans car, very slowly.
After that, I like the VA to come in hard and fast, cuz it makes my engine very snappy at Part Throttle. And, my engine likes a boatload of advance for cruising on, over 56 degrees.
To get a handle on these things, you needy to map out what you currently have.

BTW;
Detonation is caused in part, by too much heat or pressure, in the combustion chambers. Anything you can do to reduce the heat, will alleviate the detonation.
Unfortunately, heat makes pressure, makes torque and times rpm is power. So, when you reduce that heat, you will lose power; there's no getting around that.
Taking timing out, will reduce the pressure AND the heat, a double whammy for low-rpm powerloss.
Your camshaft, may be the root cause of your detonation.. I highly recommend a Compression test.

 

[mvh]

I've never read that. Generally, total advance without vacuum advance should be in the 35 degree range.

Oh, OK. The post I just looked up (talking about a 440) put it that way. But even so, this distributor would be at 40º without vacuum advance.

 

[mvh]

Dwell time, read on the meter, is NOT doubled.

I never said it was. Dwell wasn't really under discussion.

 

[mvh]

The vacume advance is load dependent, so put a vacume guage on the ported line to the vac advance to see what it's getting under various conditions.

So I have a vacuum gauge in the dash. What I don't have is a tachometer. When testing the timing in the driveway, I can't really simulate load to see what happens at different vacuum readings. When road testing, I can observe the performance at different vacuum readings, but I don't know the RPM. I guess I could calculate it based on road speed and axle ratio.

 

[mvh]

The spring determines the rate the advance advances. (Light spring, the quicker the advance reaches max )The washers determines the vacume at which the advance starts

That's helpful. So, say the advance cuts in at 10" of vacuum (per the chart). Would adding an additional washer make it not cut in until a higher reading? Or would you remove a washer?

 

[AJ/FormS]

I can't really simulate load

Yes you can;
Disconnect the VA from the carb, then Tee a vacuum gauge on the VA, and a long hose that reaches out to the radiator. With your timing light installed, engine idling; just suck on the VA until the timing changes. Then suck harder and read the timing again; and repeat until more sucking produces no more timing changes.
I like to suck the gauge up about two inches at a time.
Then I plot the vacuum and the timing change, on a graph. Now I know exactly how much my timing will change, at any rpm, with varying load.
If you have a mity-vac, use it, cuz when you get to 15>18 inches, sucking on that hose gets to be a chore.


As to adding washers,
that increases the spring-pressure, which then requires more vacuum to achieve an increase in timing. So then, at 10 inches, the timing will be LESS than listed.

 

[mvh]

Thanks. In theory, it's maxed out at 12" of vacuum.

I suppose, then, that substituting a lighter spring in the vac unit is what you do after you have removed all the washers, and still want MORE vac advance. Not that I want to do that.

 

[mvh]

Dwell time, read on the meter, is NOT doubled.
The only thing that gets doubled is the distributor rpm, to get crank rpm.
"All-in"/Power-Timing, is measured with your timing light directly.
For all the SB Mopes I have ever worked on, the "all-in" Power-Timing, should happen in the window of 3000 to 3600.
VA is never included in the "all-in" power timing.
VA is added to the mechanical and helps to get Part-Throttle efficiency.
Every engine seems to have it's own needs. Here's mine.
I like to run a two stage mechanical-curve, very fast from Idle to about 2800, and then I like to slow it down until it's in mid-window. This allows me to run a modest amount of Idle Timing, and to tune the carb, to get the rpm way down, so I can drive my manual-trans car, very slowly.
After that, I like the VA to come in hard and fast, cuz it makes my engine very snappy at Part Throttle. And, my engine likes a boatload of advance for cruising on, over 56 degrees.
To get a handle on these things, you needy to map out what you currently have.

BTW;
Detonation is caused in part, by too much heat or pressure, in the combustion chambers. Anything you can do to reduce the heat, will alleviate the detonation.
Unfortunately, heat makes pressure, makes torque and times rpm is power. So, when you reduce that heat, you will lose power; there's no getting around that.
Taking timing out, will reduce the pressure AND the heat, a double whammy for low-rpm powerloss.
Your camshaft, may be the root cause of your detonation.. I highly recommend a Compression test.

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.

 

[mvh]

FYI, here is the curve diagram I created, based on the spec sheet.

 

[Newbomb Turk]

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.

The way I read the table you posted is they want 16-20 degrees at the crank at 1800 rpm. I can believe that. I would suppose that if we extrapolate the numbers out you would be somewhere near 35 total at 6k or so.

So two things.

The fuel is markedly different today than it was in 1967. So that means (to me anyway) you can take several grains of salt with those numbers. The SHAPE of the curve will be close but the numbers like rpm will be different.

Secondly, you’ll need to limit the total amount of advance the vacuum can adds in. In 1967 you could run more timing because the fuel was different. EFI made carb (and ignition) tuning much harder.

I would work on a curve that makes the engine happy and not get to married to those exact numbers.

 

[mvh]

No, remember, the table doesn't include the static timing. So it's 26 to 30º at 1800RPM, but it's all-in at that speed.

Yeah, I know, fuel is the problem here. And that's what I was thinking — I need to limit the vacuum advance somewhat.

 

[Newbomb Turk]

No, remember, the table doesn't include the static timing. So it's 26 to 30º at 1800RPM, but it's all-in at that speed.

Yeah, I know, fuel is the problem here. And that's what I was thinking — I need to limit the vacuum advance somewhat.

If that’s how to read it (I’m not doubting you on that) then that curve is way too fast for about anything today running pump gas.

26-30 is a LOT of timing at 1800. In my opinion anyway.

 

[mvh]

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).

 

[Dana67Dart]

. Would adding an additional washer make it not cut in until a higher reading? Or would you remove a washer

Add a washer will raise the vacume needed to move the diaphragm

 

[mvh]

Hmmm. I happen to have an NOS (never used) CAP version of this same distributor (2642358, IBS 4013-B). I was wondering if it might be a good substitute. But then I looked at the specs. The vac advance curve is exactly the same. But the centrifugal curve is even steeper — it is + 24º to 28º @ 1560RPM. I suppose this is because the static timing was retarded, at 5º ATC. I don't have the CAP widgets, so I don't think it would be happy at that timing, and the curve problem would be worse.

I also have two examples of 2642461 IBS-4013-AS, which is not listed in my sourcebook, so I have no idea what the spec is. It is visually identical.

Oh, I forgot — Cuda Al explained this to me once. The final "S" just means it was sold as a part without cap & rotor. So these two items should be stock replacements for mine.

 

[Dana67Dart]

. I suppose this is because the static timing was retarded, at 5º ATC. I don't have the CAP widgets, so I don't think it would be happy at that timing, and the curve problem would be worse

I have 2 CAP 67 distributers. I run both at 0 to 5 before. The nice thing is the curve is essentially the same but it comes in faster. But look at the all in point, it's about the same.

Mine is a tired 273 2 bbl and runs on 85 octane without issue, 85 at 5000 feet is equivalent to your 87 at sea level.

 

[mvh]

I can't run 87. I have to run 91 with the CR in the Commando (if I could get 93 I probably wouldn't have any problem). I think that fast curve would be worse in my case, and it wouldn't idle worth a bean at 0º.

 

[RustyRatRod]

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

You're reading the wrong stuff. TOTAL advance is initial, plus "whatever" mechanical is in the distributor. Vacuum advance is not considered part of total. It's not uncommon to see 60 plus degrees at part throttle cruise when vacuum is high and the vacuum advance has all the vacuum advance pulled in, but you do not include the vacuum as part of the total advance. You need to get your nose in the Mopar Performance engine book.

 

[Bewy]

If the dist is original, there is a good chance the springs [ for the weights ] have stretched. That will bring in the advance quicker. If you have a spare parts dist, you could try removing metal off the weights. If the posts for the springs can be moved, you could try bending them to stretch the springs slightly. Both will delay the advance curve.

 

[RustyRatRod]

If the dist is original, there is a good chance the springs [ for the weights ] have stretched. That will bring in the advance quicker. If you have a spare parts dist, you could try removing metal off the weights. If the posts for the springs can be moved, you could try bending them to stretch the springs slightly. Both will delay the advance curve.

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

 

[mvh]

You're reading the wrong stuff. TOTAL advance is initial, plus "whatever" mechanical is in the distributor. Vacuum advance is not considered part of total. It's not uncommon to see 60 plus degrees at part throttle cruise when vacuum is high and the vacuum advance has all the vacuum advance pulled in, but you do not include the vacuum as part of the total advance. You need to get your nose in the Mopar Performance engine book.

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