Distributor: How many millimeters are 1° of advance ?
 

Hi,I did a check of the degrees of advance with my timing light and found that it was at 12 degrees.
Later, when I wanted to turn the distributor, the chinese timing light failed, so I said, if the distributor has 360 degrees and "x" millimeters in circumference, how many millimeters do I have to turn the distributor clockwise, to retard the advance by 6 degrees?


So, I proceeded to measure the diameter of the cap of a used distributor in good condition, using the measurement between 2 opposite pins, to then multiply by the number pi (3.1416) and obtain the circumference, and here I got the big surprise: the distances between the opposite pins of a distributor are not the same. They vary between 113.2 and 114.9 millimeters.


Then, the measurement of the circumference of the pins of a distributor can be between 113.2 x 3.1416 and 114.9 x 3.1416, that is, between 355.6 and 361.0 millimeters, which implies a possible total difference of 5.4 millimeters, or 0.68 average millimeter difference between adjacent pins (5.4 / 8).
Now, if the circumference of the distributor pins is 360 degrees and measures between 355.6 and 361.0 millimeters, that means that in order to retard the advance by 6 degrees, I have to turn the distributor exactly between 5.9 (0.99 x 6) and 6.0 (1.00 x 6) mm.

Please, give me your comments about this "theory".

Thanks





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Re: Distributor: How many millimeters are 1° of advance ?
 

In what country was this cap manufactured? Does its name start with C?

Meanwhile, just set the timing to where the car runs the best, and don't worry about "degrees". What THE ENGINE really wants is probably different from whatever YOU THINK it should get.

Re: Distributor: How many millimeters are 1° of advance ?
 

There's nothing lame about that. It's cool you took some time to do some critical thinking. I don't see any major flaws with your theory. But with the tolerances of that cap you really can't be certain to be 100% exact. Sofakingdom gave the best advice in this post.

Re: Distributor: How many millimeters are 1° of advance ?
 

Yep, timing light to get it in the ballpark, and then by ear to get it right on the money.......

Re: Distributor: How many millimeters are 1° of advance ?
 

In defense of all random countries whose name starts with C that might be the source of dist caps: (Canada? Cameroon? Colombia?)

Look at how wide the contacts in the cap are; and how wide the contact on the tip of the rotor is; and how sloppy the "index" tabs of each to their respective metal parts, is. Do you REALLY think that 1°, or 0.1mm, is going to make ANY difference in all that? Numbers like that are WWWWWWWWWWAAAAAAAAAAAAAAAYYYYYYYYYY "down in the noise". (yes I'm an electronics engineer by trade, though not by education) Measure with micrometer, mark with chalk, cut with axe. If all you've got in your hand is an axe, it doesn't matter too much whether your micrometer's calibration certificate is current down to the last 0.01mm. ;)

Similarly, as both centrifugal and vacuum advance (or in the case of EFI, the ECM's equivalents, which though not generated in exactly the same way, have the same effect... the spark occurs at some highly variable coincidence, or not, of rotor and cap), wander over a 40 - 50° range, will the tiny variations in the cap contact location REALLY matter?

I agree, your approach to this using logic and arithmetic and reason, is FAR BEYOND what most people in this hobby would ever attempt; but it's a bit misguided. It's, shall we say, not the right question to ask, even if we can come up with the "right" answer. Kind of like, what's the optimum number of gears to equip a bicycle for a fish with, and we pull out all sorts of fluid dynamics, biomechanics, etc. kind of facts, and come up with the brilliant answer that a fish bicycle needs 7.83 gears for an "average" fish like say a cod, or perhaps as many as 8.25 gears if the fish is a flounder, and maybe 0.65 ± .176 extra gears in either case if this is a mountain bike compared to a highway racing bike. OOOOOKKKKKAAAAAYYYYYY!!!! Great work! Let's all go get a beer now. :D ;)

Adjust it to where it runs the best, and put the question of the "numbers" out of your mind. The engine does not care what "numbers" YOU come up with; it runs the best when the spark occurs at the right point in relation to piston movement. It KNOWS without looking at "book" "mark" "spec" "light" "tab" or anything else. That's IT.

Re: Distributor: How many millimeters are 1° of advance ?
 

Sounds about as accurate as hitting a tire with a hammer to say that there's X amount of tire pressure in it.

Buy a new timing light.

But even then, the accuracy of the ignition timing is not exact unless the camshaft has been dialed in with a piston stop and a timing wheel and you know that when the piston is exactly at TDC that the timing pointers on the balancer/front cover are also pointing to TDC.

Re: Distributor: How many millimeters are 1° of advance ?
 

Here's another way to look at this - look at the clock (analog).
1 hour is 60 minutes. 360 degrees in a circle. Therefore 360/60 = 6. Therefore 1 minute = 6 degrees.
Can you visualize what 1 minute looks like at your distributor cap?

Re: Distributor: How many millimeters are 1° of advance ?
 

I was thinking of using that analogy also however 1 minute of rotation measured at the base of the distributor is not the same distance as 1 minute of rotation measured around the distributor cap. Since he's asking about distance of rotation, I don't think anyone can give a precise answer. Due to how many variable tolerances there are in the timing system, I highly doubt a distance measurement will be anywhere close to accurate.

Re: Distributor: How many millimeters are 1° of advance ?
 

O.K., I understand,but the reason for setting the distributor to 6 degrees btdc is because I'm going to migrate to the arap bin, which has an aggressive spark advance curve, so I want to avoid possible knocks.

Re: Distributor: How many millimeters are 1° of advance ?
 

I'm not sure of your modifications. But I would not run a unmodified ARAP bin on a stock 305. Things need to be modified. It's been a long time. But I believe there is some fan setting that needs to be changed in the ARAP bin to run in an fbody. My best advice would get yourself some tuning equipment so you can modify the ARAP to run best on your setup.

Re: Distributor: How many millimeters are 1° of advance ?
 

Oh, yes,

The ARAP bin must be modified in injector flow rate (single fire and double fire) from 350 calibration to 305. If not, it will run with black smog.



I've scan + prom burning equipment.