[My427stang]

QUESTION: How is the AMOUNT of Dwell that is needed arrived at?

I think you are putting too much concern on dwell. However, consider it something you need enough of for a given RPM.

At idle, dwell is less important but you need enough to saturate the coil to allow the coil to fire. Basically the primary side gets power and creates a field which needs time to build up, when the primary side is shut off, the secondary field collapses and fires.

The issue is "enough" at idle turns into "not enough" at high rpm. That is because the saturation of the coil are determined by time, but the points operate on a fixed open and close point based on cam position that reduces in time as RPM goes up

In other words, points open and close in a certain amount of rotation of the distributor, no matter whether its spinning very fast or spinning very slow, that duration gets shorter at higher RPM, but the coil, like a sponge soaking water still takes as long as it takes.

So one way to look at it is, you need "enough dwell for your highest RPM"

Some say that points don't have stable dwell, they do for a given period, but induced wear changes it over time, that is one reason electronic ignitions are better in the long run.

Now, modern EFI motors with computer control will actually change dwell with RPM to compensate, pretty slick.

With all that, I ask a question that will lead you to an answer.

If the Ford engineers designed an ignition system to adequately saturate the coil at all expected RPM, then provided a dwell setting to do so, do you think you have changed those operating conditions enough to require something different?

I would say in your application, absolutely not. I'd set the dwell toward the top end of the specs, reset the timing, and forget it. If it was an 8000 rpm small block, maybe, but it just isn't that critical for the rpm range a normal street big block runs

Remember, dwell affects timing, but timing doesn't affect dwell, so always set dwell, then readjust timing.

THERE IS A NOTE ABOUT THIS: "To obtain optimum engine performance and fuel economy, the initial ignition timing may be advanced 5 degrees over the "normal" setting. No further improvement in engine performance or fuel economy will be achieved by advancing beyond this point. Advance the timing progressively until engine detonation (spark knock) is evident under actual road test acceleration. Retard the timing until the detonation is eliminated."

Please ignore the numbers on this quote. It is true. However, there is a term called dynamic compression ratio (DCR) that calculates compression of an engine based on the closing point of the intake valve. It is key here.

When Ford wrote this, there was a stock cam & stock compression. Your cam is more aggressive (however minor that may be) and changed the intake closing point. That essentially reduced DCR. Now lower compression seems bad yes, well yes, but cylinder fill isnt only based on the cam and piston, its also affected by all the other components and as you rev higher, the cam makes MORE power.

So yes there is diminishing return, but no you cannot use the same numbers anymore.

Second point, using your ear to time a motor is very dangerous on a motor that will work hard. You cannot hear damaging knock until its excessive. There are tried and true numbers that are good for FEs which I will address in your next question

QUESTION

Have heard a bunch about "recurving" the Distributor to achieve OPTIMUM results.

Here are the relevant specifications from the Shop Manual:
CENTRIFUGAL ADVANCE:
@400 RPM .5 - 1.5 Degrees
@500 RPM 3.5 - 4.5 Degrees
@800 RPM 5.75 - 7 Degrees
@1400 RPM 8.25 - 8.5 Degrees
@2000 RPM 10.75 - 12.75 Degrees

These are in cam degrees, you need to double it to use it at the balancer. So, ignoring the small steps, you have approx 12 degrees at 2000 rpm = 24 you'll see at the balancer.

24 degrees at the balancer PLUS initial at 12 equals 36 degrees total.

That is perfect for a street FE. Much more than that causes detonation, less gives up power for engines with a big combustion chamber and antiquated chamber design.

Some questions though, at 2000 your quote seems to show that advance stops. You could verify that with a light. Lets say it adds 3 more degrees by 3000. Now you are 15x2 PLUS 12 initial = 42 total

That is too much, and THAT is why you would recurve.

You want to keep the 12 initial, but you dont want as much total, so the recurve will limit total

VACUUM ADVANCE:
1000 RPM 2 - 5 Degrees @ 8"HG Vacuum
1000 RPM 4 - 7 Degrees @ 10 HG Vacuum
1000 RPM 5.5 - 8.5 Degrees @ 15 HG Vacuum
1000 RPM 5.5 - 8.5 Degrees @ 20 HG Vacuum

Maximum Advance Limit 8.5 Degrees

Oh crap you said 36 was perfect and the vacuum advance added MORE LOL

Vacuum advance does not work at wide open throttle (WOT) because vacuum is zero. However, when there is low load and part throttle, vacuum pulls more in to help with mileage.

So, it IS tunable, but its really to help you during cruise for MPG not power

So rules of thumb

36 total at WOT = initial 12 plus (2 times distributor advance) 24 Perfect

For a street car, add vacuum on top of that for mileage gains - you are good

By the above specifications, if the Initial Advance is 4 degrees, and once can exceed that by 5 degrees, then Initial Advance should NEVER be more than 9 degrees.

Then if the Centrifugal Advance can be up to 12.75 degrees, would THAT be added to the 9 degrees? If it would then we are looking at 21.75 degrees with the Initial and Centrifugal.

And then if the Vacuum Advance can be up to, but not exceeding 8.5 degrees, would that be added to the 21.75 degrees? And if it would then we are looking at 30.25 degrees "ALL IN". Right?

So on Ms. American's 3.14, we are right now sitting at 10.5 Initial, and with the Centrifugal&Vacuum added provide 32 degrees "ALL IN".

A repeat but seeing you asked specifically, you are incorrect

INITIAL plus DISTRIBUTOR ADVANCE CONVERTED TO CRANK DEGREES = TOTAL (shoot for 36-38 on a street FE)

VACUUM is NOT additive because when you are in hammer mode, vacuum is zero so its not there

Make sense?

 

[My427stang]

With that much total timing, it will probably be very fussy on fuel

I would expect it to ping on a hot day, under load, or at high rpm.

36-38 total, not counting the vacuum advance, is about all these motors can handle, although an older motor with low compression and loose quench may WANT more timing, it cannot bear it

 

[My427stang]

he's got 36*

JC said "The total time is 45 Degrees"

JC - to answer your question

If you add total to your mechanical, you want 36-38 degrees at max advance. That does not include vacuum advance. Put initial to where it hits that.

However, my recommendation would be to measure actual advance up to 4000 rpm, not going by the book. Then you will know what its really doing and when it stops advancing

Also, for the second question quench is the distance from the top of the piston to the quench pad on the cylinder head. In the old days people didnt worry about it, so figures of .060-.080, or even more were common.

Tighter quench causes more turbulence in the chamber and churns the mixture while reducing hot spots making it more detonation resistant, looser quench does that less and makes it fussier on fuel

So tighter quench, needs less advance but can tolerate more. Looser needs more but can tolerate less

With compression ratio, higher compression needs less advance but it also tolerates less

The odd thing, these old motors had moderately high compression but with loose quench, as they age, they want more timing, but the loose quench doesn't play well and they ping.

 

[My427stang]

Glad to hear it, and GZ thanks for catching my mistake on total timing. Certainly didn't want him to pull the timing back due to a misunderstanding!

JC, in terms of mileage, there isn't a lot you can change easily. The swap to the new carb should have helped, as did the advanced timing.

The advice to make sure the vacuum advance is working is a very good one, vac advance will help, and if you sneak up a degree or two of initial it might help a little too

You could run valve lash on the loose side of specs, essentially making the cam appear smaller. In addition, if you really felt like attacking it, you could change the timing chain and gears for a unit that allows advancing the cam, it will gain low end torque and therefore efficiency

However those are going to be very minor gains, most benefits will come from driving style

Something to think about, with a performance engine, specifically looking at cam design, the lowest rpm may not be the most efficient.

My 489 FE actually gets better in mileage with the 4.10 gear than it did with the 3.70s because I am in an rpm range that the motor is more efficient.

 

[My427stang]

JC, you run points right?

If so, it looks as if you have a standard points distributor with the cap adapter off of a Duraspark. The big cap truly isn't needed for a points ignition but the big caps do seem easier to find nowadays.

Rock Auto sells advance cans, if you can remove yours (easy while on the engine) and take a pic, I may even have one here that won't fit mine

As far as testing what you have, I would disconnect it internally and make sure the plate the points are bolted to will move and then check the can disconnected. The plate could be bound up or corroded. I have seen guys drop screws or washers in there and then they bind up

 

[My427stang]

Hey Dusty,
Since the Vacuum Advance Assembly hasn't been doing anything for heaven knows how long, wouldn't it be possible to just remove it, cap the Vacuum Line FROM the 4100, and then, with the Vacuum Advance Assembly IN HAND, drive Ms. American to the NAPA and possibly get another Vacuum Advance Assembly?

No you cannot do that, the plate that the vac advance arm hooks to would move around and change the timing to such an extent that it would be uncontrollable and not run

Also, the thought occurred this morning that the Vacuum Line FROM the 4100 should be tested to see just what value the vacuum is? The Vacuum/Pressure Gauge is here. All that would have to be done is to hook it up to the Vacuum Line and start the 3.14. Then go through the RPM range and see what it is doing.

Yes, but less for value than for presence of vacuum in case it's plugged or kinked somewhere. This goes back to the ported(timed) vacuum versus manifold vacuum.

MANIFOLD vacuum is what you measure, it is negative pressure of the cylinders pulling against a restriction, such as intake and carb. PORTED or TIMED vacuum is vacuum created by air rushing past an orifice in the carburetor, which as the engine RPM increases, timing goes up, it is a function of load and airflow, hard to measure, and really, all you need to do is make sure the line is clear.

BUT, you also need to check the other things

1 - Is the line good and is it clear?
2 - Is the plate free of corrosion and can it move?
3 - Are any of the wires internally binding to keep it moving?
4 - Does the vacuum can work if it's diconnected?

So yes, check all

Mr. Kultulz says that the Vacuum Advance Assembly is "tunable". Meaning?
JC

Get it to move first, but because you will find you need to tune something that is not that critical in your situation, let's review it

Inside the end of most vacuum canisters, you slip an allen wrench into the vacuum tube and you can adjust vacuum opening rate and some have limiters to adjust total throw

Remember, vac advance is for mileage and part throttle power, not WOT power, so once you get it operating, you may want it to come in faster for your style of driving, but you need to get it working first.

Kapeesh?

 

[My427stang]

If a dash mounted Pressure Gauge (like what was called in my youth, a "Motor Minder") were installed, it could be connected to THAT Fitting, couldn't it? And then by keeping the Needle in the "Green", one could get the BEST mileage out of the old Gal.

Absolutely

Can ASSURE you that it is. BUT, don't know about the "Timed Port" itself. Is there anything there other than just an orifice? Is there a way to check to see if it is functioning properly?

Hook your vacuum gauge to the port that feeds the distributor, and open the throttle to a very fast idle, approx 3000 rpm, it should have some vacuum, but don't worry about value, just presence

Wouldn't the fact that the Centrifugal Advance works be an indication that the Points Plate is able to move? Or it THAT a completely different function?

Different function, centrifugal advance changes rotor/point cam assembly position, vacuum advance turns the plate the points are bolted to

Can categorically state that it WON'T. Sucking on the Hose connected to the Vacuum Advance Assembly was like sucking on the hose without it being connected to ANYTHING. The Diaphragm is MOST CERTAINLY busted.

That is certainly a good test, if it was stuck and wouldn't move it would be more of a troubleshooting event, but if it is an open hole, then you know its bad

But as you mentioned previously, with the Crane Fireball Cam, the 3.14 MAY run better at a higher RPM. So the question becomes: How to know where the Vacuum Advance Assembly should be set?

You wont find magical mpg with vacuum advance, ANY will help, and odds are your style will only adjust for rate, not limit. So you want as much as you can at cruise, without it pinging. However, I will tell you thins, and please mind the advice, just bolting it on and having it work will probably be as close as you'll ever be able to tell. Road conditions, weather, etc, will have a greater variable than vacuum advance timing rate

And you know what? THAT could get EXPENSIVE! The reason that THAT is said is because: In order to check to see what the Gas Mileage is, one HAS to FILL up the Gas Tank (The FSM says the Gas Tank holds 20 gallons. At $3.15 per Gallon for 87 Octane Swill, that's $60.00!), and then drive for a while the way one drives. Then REFILL the Gas Tank and calculate the MPG. THEN, make an adjustment to the Vacuum Advance Assembly, and again, drive for a while the way one drives. Then REFILL the Gas Tank and calculate the MPG. Doing THAT routine till one finds the BEST adjustment, at today's gas prices could turn into a significant amount of money!

Yet another reason to not overthink vac advance, typically it needs adjusting for very high compression motors that need it limited, not relatively mild motors that want a lot of advance.

Truthfully...you may not heed this, but I wouldn't lose sleep over it. Get it working, and you will be 99.325% there The remaining is almost unmeasurable

 

[My427stang]

Aren't you running a much smaller primary jet?

Maybe it's too lean. Pull a plug out and see if they are hot and white looking. They rarely have color with todays gas, but very white with a points type ignition is unlikely too

 

[My427stang]

I would set the timing to 12 degrees and forget it, fix the bad vacuum advance, then see how it runs

The reason I asked about jetting was because IIRC, you said that those 47 primary jets were significantly different than what you thought the carb should have in them

Before you change them though, you need to make sure the two carbs you are comparing are truly the same. Jet size is primarily determined by carburetor venturi size and booster design, so if the Ford cab in the book was a bigger version, the jet comparison would not make sense

 

[My427stang]

JC you are getting a LOT of good stuff here.

It is of course a fire hose of info.

I don't want to simplify an important post but there are some points I think I need to hit that are very simple

1 - You need to get the vacuum advance canister replaced. Although you can run it without it, you are losing the benefit of having it there, therefore much of the tuning you do will be incorrect once its adding advance

2 - You need to resolve why you changed jets so severely. An engine NEEDING more than a few single digit numbers either way from factory usually indicates an problem, but my hunch is, they were new ones that you swapped just because they were new. That will make some tuning challenges to include a motor that WANTS more timing, but is unable to HANDLE more timing

- If this carb is different than the factory application, it may use a smaller jet with more power valve fuel enriching the circuit under load, however, if it is the same carb, it really shouldn't be that different It will take some research to resolve that

3 - You need to use the correct terms for ignition timing and hold others accountable or at least ensure they can explain what they are recommending

Initial or base timing - what you set at idle, without vacuum advance usually.
Centrifugal or mechanical advance - the amount of mechanical advance that comes from the spring and weight combo. Often referred to in distributor degrees, if so, you double it to get crank degrees to be added to the initial for common curve discussion
Total timing - Initial plus centrifugal (the reason why I am typing this section) also called "ALL IN" An earlier post you were concerned about 50 degrees "all in" , as you pointed out, not realistic if using the correct terms, BUT, it is realistic for a lean engine at low loads when you add the vacuum advance too. However, generally that is not the way the term "all in" is used. Matter of fact, "all in" is usually used to discuss an RPM point that you hit your total timing. Example..."I have 36 degrees total, "ALL IN" by 2800 rpm"
Vacuum advance - Generally not discussed a lot in degrees, usually its "manifold or ported" first.

Manifold vacuum ADDS to initial when idling, and when vacuum drops, it goes away. Therefore, it affects initial/base settings, but at WOT it does NOT affect Total. Typically good for bigger camshafts with low vacuum, or very high compression motors that do not like to crank. I do not feel the design or use of your engine needs it

Ported does NOT add to initial, but as airflow increases through the carb it starts coming in. This is how Ford delivered your car.

With that being said, you can adjust the rate of either and you can build limits to stop how much vacuum advance comes in, but at cruise with a light load, it is perfectly fine to have total timing + vacuum nearing 50 degrees

Again though, stabilize the patient by fixing the known broken piece, then see if you can figure out why you should or should not have the lean primaries jets in the carb, then I would guess the supertuning can begin

 

[My427stang]

I was referring to the fact that at 900RPM you were seeing the combo of initial(static) advance + cent. advance. Remember, it starts coming in at 400 RPM. At 900 RPM it is over half done (8 degrees or so).

His values are in distributor revolutions not crank revolutions. You are right that at 900 rpm engine idle, it looks as if he may have some advance coming in, but at that time the distributor is only turning 450 rpm, not 900

Remember, cam turns 1/2 speed of crank which drives the distributor.

 

[My427stang]

JC stick around for 5 minutes, I am doing some math and want you to read this

 

[My427stang]

What needs to happen is for me to acquire the proper sized Jets to meet the specifications in the FSM and install them.

It is NOT "exactly" the same Carburetor. The EXACT Carburetor would be a: C3AF-9510-BY. This one is a: C4AF-9510-DG.

Yes you do, here is why, a lean motor needs more advance but cannot handle as much advance in terms of octane tolerance. In addition, bucking, power loss etc, are all indicators of a lean condition at cruise.

Now with the possibility of being too lean on the mains, you need to find out UNDOUBTEDLY what that C4-DG carb came with from the factory. THAT is a smarter course of action than using what the manual says.

Now get this, fun with math.

Calculating the area of a .047 jet comes out to .0017349 sq in
Calculating the area of a .055 jet comes out to .0023758 sq in

When you compare the two, the increase of fuel from jet 1 to jet 2 to is 36%. THIRTY SIX PERCENT

Consider how much difference that is to the combustion process, jet changes are done in single numbers. Although those may not be your exact numbers, jet changes are VERY dramatic, even with small changes

You could be starving Ms American, but jets are better matched to the carb than the engine, so recommend researching what THAT carb came with

Be advised that 90% of my tuning repairs on carbs of any kind consists of UNDOING someones supertuning, then starting with original, usually its very close.

in a recent post Mr. B&F has revealed that the Centrifugal Advance starts coming in at 400 RPM. And the 3.14 is idling at 850 RPM. Which means that some of the 12 degrees of Initial Timing is partially Centrifugal.

Respectfully, he is wrong

The distributor turns at 1/2 speed, so 800 idle RPM = 400 at the distributor. So indeed at 900 rpm you may have advance coming in, its not as severe as he said, you are actually at 450 distributor RPM

This is because the cam turns half speed from the crank, otherwise, think about it, the rotor would fire the #1 plug twice, once at TDC and once on the exhaust stroke

After setting the Initial Timing at 12 Degrees. Then revving up the 3.14, the Timing Light showed 32 Degrees. I was assuming that the additional 20 Degrees was Centrifugal, but it may be that the Initial 12 Degrees was partially Centrifugal, and if that is the case, the Centrifugal Advance is in excess of 20 Degrees. But I don't know how to determine how much of the Initial 12 Degrees is Centrifugal and how much is "PURE" Initial Timing.

Idle it lower while you watch the timing light, if you are getting advance, as you drop the rpm toward 800, advance should drop, if it doesnt drop then your centrifugal is at zero at idle

What led me to be incredulous about 50 Degrees being realistic was that without any Vacuum Advance on the 3.14, and at the time, there was 16 Degrees Initial and 36 Degrees including the Centrifugal Advance being shown by the Timing Light, And on the road, at between 50 and 60 MPH, there was this intermittent/random hesitation which I was assuming was too much Advance. But NOW, am coming to understand that the intermittent/random hesitation might be fuel related caused by the Jets that are in the 4100.

I think so too, but we cannot be sure until we know what THAT carb delivered with, you could just try the original jets, but I think it would be better to see what the C4 carb came with from the factory

What is going to the 3.14's Distributor is Ported Vacuum. From what I've read, it is coming from ABOVE the 4100's Throttle Plates. Am going to put the Vacuum Gauge on the Vacuum Line FROM the 4100 to see how much vacuum is being pulled by that Line.

Again, how much doesnt matter, remember the post from a few days ago? It will change with load, RPM, etc. Just be sure its

1 - Not alive at idle
2 - Alive when you crank the throttle a bit

Alas, NOW Mr. B&F is suggesting that maybe the Centrifugal Weights and Springs may be worn out. THAT would put a severe crimp in things for a number of reasons. First: The budget! But THAT'S always a problematic factor. Second: We'd be into another virgin territory for me. But, what the heck, what else is new? What concerns me most is that it might entail getting a NEW Distributor, and THAT might be out of the question budget-wise. But we'll just have to wait and see. Time will tell.

JC

I believe that you are concerned about nothing. An improperly jetted carb by almost 36% AND a broken vacuum advance...

ONE bite of the elephant at a time, you have significant things to focus on until your data supports all of the fancy stuff you are trying to contemplate

Remember, you don't TUNE something that is broken, you FIX it, and then TUNE it.

You have a broken advance and probably an incorrectly assembled (wrong jets) carb. You must research and FIX that, then start the super tuning

 

[My427stang]

Hey Ross,
Am going to take the time here to list the Specifications from the FSM about the Distributor:

Centrifugal Advance
(Distributor RPM and Degrees):
400 = .5 - 1.5
500 = 3.5 - 4.5
800 = 5.75 - 7
1400 = 8.25 - 9.5
2000 = 10.75 - 12.75

Vacuum Advance
(Distributor RPM and Degrees and Ported(?) Vacuum):
1000 = 2 - 5 @ 8"hg
1000 = 4 - 7 @ 10"hg
1000 = 5.5 - 8.5 @ 15"hg
1000 = 5.5 - 8.5 @ 20" hg
[Maximum Advance Limit = 8.5]

There is a question mark behind "Ported" put there by me because I don't know what Vacuum Source is used. On the actual 3.14, it would be Ported Vacuum from the 4100, not from the Intake Manifold. BUT, the FSM values are from a "Test Stand" and NOT from an actual Engine.

So as was stated previously, am going to do the procedure outlined by Mr. B&F, and am going to read the Damper at 12 Degrees Crankshaft, which would be 6 Degrees Distributor. The FSM says that it can be between 4 and 9 Degrees Distributor, which would be between 8 to 18 Degrees Crankshaft.

Anyway, have finished burying the stray cat. Am going to have something to eat, and then start on removing the OLD the Vacuum Advance Assembly and installing the NEW one.

Will keep you all updated.

JC

I think mapping the curve is a perfect idea

The FSM gives you given values for the distributor that you can of course check, but the window of what is correct for the motor is WRONG now. You changed the cam, it changes cylinder pressure at EVERY rpm

I do agree you need to plot it like B&F says, but once you do that, let's get out of the "dist degrees vs crank degrees discussion and just talk about what you see with the light

Your motor, in your car, with your combo will run GREAT at

10-12 initial, 34-36 total, all in by 2800 or so.

Then ported vacuum on top of that.

Again though, until you resolve jetting and the vac advance, don't disassemble or change anything

 

[My427stang]

JC

Regarding the carb, I am not an expert on these carbs to know minute differences per part numbers.

However, assuming your carb came of a 352, I would not expect it to be the 1.12 venturi size, therefore I would expect it to use the smaller jets

However, if it is indeed the larger size, then, it is more likely to take the larger jets

So its a bit of a conundrum

Lets walk through this

Basic jet range is determined by:

Venturi and base plate size (how much air is flowing)
Booster design - how sensitive is the booster to the air that is flowing
Power valve restriction size (PV fuel plus main jet = total fuel)

and just for reference

Air bleed size - (doesn't affect jetting, changes timing and responseiveness of the signal, only put it there because someone would have said something)

So, IF the venturi are the same size, there could be differences in the PV circuit that would affect jetting by a number or two, but not enough to make THAT much of a difference in jets IMHO

Booster design is probably no different between the carbs

So I cant resolve the question, Do you have a smaller carb with the C4 version, or does the variant of the C4 you have use bigger jets?

Question - What happened to the ORIGINAL jets in the C4 carb before you rebuilt it? and if you know, what size were they?

 

[My427stang]

Hey Ross,

Here are various quotes about the jet size made by JC...

Even with that, I have no idea what the original C4AF-9510-DG was jetted at.

I'll do some research on my own too

 

[My427stang]

Well the FE forum is saying that carb came with 48s and 55s, using Ford jets, not Holley numbering

I am not sure if Holley jets fit or how they crossover, but it does not seem you have as big of a jetting issue as I once thought

 

[My427stang]

Don't get hung up that a bigger motor needs bigger jets if that's what you are alluding to

For the most part, there is a fixed amount of air able to flow through a venturi, regardless of whats under it

So jets match venturi and boosters, not cubic inches

It's pretty simple if you think about it, it's air/fuel ratio. So for any given amount of air, it needs X amount of fuel. The size of the carb venturi doesnt change, so the jet size wont significantly change (although there are some minor changes for tuning, its not huge)

Now, the specific amount of air/fuel together a 390 needs will happen at a lower RPM thsan with a 352, but that doesn't mean you change jets, because as the RPM goes higher, it pulls on the boosters harder and gets more fuel at higher RPM

The bigger question is, how did it run with the vacuum advance working?

 

[My427stang]

Hey Ross,

After installing the Vacuum Advance Assembly, I hooked up a hose to it and sucked on the hose to see if the VAA would move the Contact Points and Condenser Plate. It would, but only about 1/8", and it took a LOT of suction to do it. Was waiting to see if THAT was normal, and to get any recommendations about adjusting the VAA.


JC

If the circumference of a circle with a .75 inch radius, which I am estimating to be the distance from the center of the distributor to the edge of the point cam, is 4.72 inches, each degree would be .013 inch of change of the plate.

Therefore, your 1/8 of travel which would equal 9.6 degrees (.125 of estimated movement /.013 inches per degree )

Please RAISE THE BS flag because this is the kind of baloney people make bad decisions with :tup:

So figures never lie, but liars love to figure....so recommend you measure it.

Map your initial and centrifugal like you wanted to, then at idle, connect the vac advance to manifold vacuum, see how much it changes initial, that's your answer of how much vacuum advance you are getting

What I COULDNT find is how much vacuum the average retired Texan could pull on a 1/8 hose, so I will leave that up to you today in front of the TV with your new gauge

 

[My427stang]

How much the "All In" (Initial + Centrifugal + Vacuum) advance is.

JC

Loved it until this comment which again reared it's ugly head, as I posted the other day, "All in" is not a term that measures degrees, it will only confuse things

Initial + centrifugal = Total (as you said)

"All in" is generally RPM point where your total stops increasing.

"I have 36 total, all in by 2400 rpm"

The reason you don't define the combination (Initial + Centrifugal + Vacuum)is because it will almost never happen in a repeatable format. You would have to be WOT above the max centrifugal point in a high vacuum situation, then a slight wiggle of the throttle or a shift and it changes.

So, the way most of us discuss timing curves is:

"I have 36 total, all in by 2400 rpm, with another 8 degrees of (manifold or ported) vacuum

However, your plan is sound, I just need to get you to terms that you can readily discuss with anyone