I think you are putting too much concern on dwell. However, consider it something you need enough of for a given RPM.QUESTION: How is the AMOUNT of Dwell that is needed arrived at?
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.
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.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."
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
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.Have heard a bunch about "recurving" the Distributor to achieve OPTIMUM results.
Here are the relevant specifications from the Shop Manual:
@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
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
Oh crap you said 36 was perfect and the vacuum advance added MORE LOLVACUUM 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
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
A repeat but seeing you asked specifically, you are incorrectBy 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".
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