Dumping the FBO timing advance plate.
- Thread starter TrailBeast
- Start date
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According to the chart Rob posted on the first page, .490.
TT5.9mag
Two atmospheres are better than one
That’s kinda why I asked. My measurements and the swing I got in my distributor don’t match that chart.
TrailBeast
AKA Mopars4us on Youtube
Keep in mind that all advance assemblies are not the same even between otherwise similar engine combos.
Some have angled slots and some don’t, which must be application specific somehow.
That would change how the advance plate affects it.
TT5.9mag
Two atmospheres are better than one
I was speaking about a factory Chrysler electronic vacuum advance distributor.
Mattax
Just the facts, ma'am
That list is an approximation, but I don't know who first came up with it or what its based on.
My suggestion is to measure everything first; the distance from betwen the slots, the slots themselves, the pins on the weights. Until we have a dozen samples I wouldn't trust it.
Yes some of the governoers have angled slots.
So lets look at how the weight movement moves the timing. It's not relly linear is it?
When the weight is close to the shaft it moves outward with only a small movement in angle.
As the weight gets further out, the relationship between outward movement and shifting of the points/pickup changes.
TT5.9mag
Two atmospheres are better than one
I wish I measured that stuff before putting the whole shebang together. That distributor was in my 318, that I gave away, and now is in my 360 magnum. The breaker plate pivot in that distributor is loosey goosey so I’m replacing it soon with parts from a distributor I bought from @junkyardhero at the fling. When I have it apart I’ll measure the spacing
Mattax
Just the facts, ma'am
This is the most extreme examples I've seen, and they're not mine.
These are both advances for 383s that were posted on moparts some time ago.
The angled slots will cause more advance for a given amount of weight movement.
However we see a stronger spring is on that advance so we don't know what the curve really will look like until we test it or look up the specs.
These are both advances for 383s that were posted on moparts some time ago.
The angled slots will cause more advance for a given amount of weight movement.
However we see a stronger spring is on that advance so we don't know what the curve really will look like until we test it or look up the specs.
Mattax
Just the facts, ma'am
We have a couple of things going on that make me question if the advance in the slots is truly linear.
The spring force component acting to pull on the weights increases as the weight moves out. (This is great because acceleration increases the centripital force exponentially.) I don't think this bears directly on whether degrees in the slots are linear but helps us think about the geometry.
At complete rest, red arrow representing spring force. Even if there is tension on the springs, they provide practically no resistance to the weights moving outward.
Now lets put the governer in, creating the initial tension.
The weight's pin moves a little out and a little bit clockwise.
A small component of the spring force acts to hold the weight in.
Now lets look at the movement.
Install the governer and the pin moves mostly outward, and just a little clockwise
Move the wieghts further out, and the pin moves almost the same amount clockwise as it moves outward.
The further the weights move out, the more degrees are advanced for a given distance the pins move in slot. At least that's how it looks to me without actually drawing it out.
The spring force component acting to pull on the weights increases as the weight moves out. (This is great because acceleration increases the centripital force exponentially.) I don't think this bears directly on whether degrees in the slots are linear but helps us think about the geometry.
At complete rest, red arrow representing spring force. Even if there is tension on the springs, they provide practically no resistance to the weights moving outward.
Now lets put the governer in, creating the initial tension.
The weight's pin moves a little out and a little bit clockwise.
A small component of the spring force acts to hold the weight in.
Now lets look at the movement.
Install the governer and the pin moves mostly outward, and just a little clockwise
Move the wieghts further out, and the pin moves almost the same amount clockwise as it moves outward.
The further the weights move out, the more degrees are advanced for a given distance the pins move in slot. At least that's how it looks to me without actually drawing it out.
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Then I would definitely grind so you get the proper arc. You do realize that although he has that chart marked "dist. degrees" that it's actually crank degrees, right? That said, I agree 100% about testing the advance arc yourself. I'd use any chart as a guide. That's exactly why I still like welding and grinding the slots old school style, because no matter what you're working with, you'll get it right. In fact, when I did my slant 6 distributors, I didn't even measure the finished products. I just got it to the advance I wanted and called it done. What's correct on my distributor HERE may be different for some else, elsewhere.
TT5.9mag
Two atmospheres are better than one
So if I understand you correctly you’re saying there is an inherent ratio in the design that could be expressed as degrees of advance per degree of rotation?
Then if all this is accurate.....and it seems it is, the governor slots should really be curved.
70SwingerGuy
Long Haired Freaky Person
So stupid question here, but is FBO a vendor and I just havent heard of them?
They are a vendor, just not one on this site.......that I am aware of. Here is his site.
FBO Ignition - Home of the HRR688
fboignition.com
mopowers
Well-Known Member
He's very pleasant to chat with too.
You'd be a good fiction writer. lolHe's very pleasant to chat with too.
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Mattax
Just the facts, ma'am
I'm not sure I'm explaining this well.
The pin on the weight moves in an arc.
I *think* this usually makes it close to 1:1 relation with degrees when the arc is close to a 45 degree angle to the slots and the slots are perfectly radial like in the above distributor. I *think* when the weight is further in more movement is needed to advance the distributor and when its further out slightly more movement is needed to get a single degree of advance.
Angled slots certainly change the ratio.
Pin diameter and slop can effect the degrees as well. Some think that a little bit of side play between the slot and pin can be an advantage. I don't think these are super precise parts. So I think there is a degree of trial and error testing. No pun intended
Of course they're not super precise. 1920s? technology? lol
Plus, anothet thought I've had regarding the FBO plate is, they do not fit precisely. Look at how much slop they have regarding the center hole. That can give the whole plate room to get off center and that would certainly affect how much limitation the governor has. But of course, yall are havin this discussion without me, so feel free to keep on.
For you math freaks.
Take the plate tower out, measure length of existing slot. Note the degrees on tower. Measure pin on weight.
Subtract pin diameter from slot length. This is the swing. Divide swing by plate degrees= degrees per .***"
Multiply (degrees per) by the number of degrees you want in the distributor (x2 for crank) = slot length
You can check it after altering without having to put everything together. Install tower with advance in place. drop a degree wheel on top of dist. Install rotor and tape a long zip tie to it. Hand twist the rotor and see how many degrees it moves via wheel.
When I do the weld/jb weld deal, I always do the inside. Doing the outer and making the oem start point to new limit, makes things come in faster. I prefer the spring a little more stretched and slower rate (further out for shaft center).
Take the plate tower out, measure length of existing slot. Note the degrees on tower. Measure pin on weight.
Subtract pin diameter from slot length. This is the swing. Divide swing by plate degrees= degrees per .***"
Multiply (degrees per) by the number of degrees you want in the distributor (x2 for crank) = slot length
You can check it after altering without having to put everything together. Install tower with advance in place. drop a degree wheel on top of dist. Install rotor and tape a long zip tie to it. Hand twist the rotor and see how many degrees it moves via wheel.
When I do the weld/jb weld deal, I always do the inside. Doing the outer and making the oem start point to new limit, makes things come in faster. I prefer the spring a little more stretched and slower rate (further out for shaft center).
gzig5
Well-Known Member
I don't think so, based on the pic below. The slots being straight are what cause the rotor to advance clockwise as the weights rotated on their pivot pins. The longer the slot is, the more the rotor can rotate relative the "fixed" position of the drive coming up from the camshaft.
As the wieghts move out, the rotor turns clockwise, advancing with respect to the crank/cam.
Wouldn’t the actual advance be calculated by the dimension of the slot between 2 parralel lines rather than the length of the slot? Straight slots would naturally line up to the 2 parralel lines but an angled slot would be from inside corner to outside corner . It is possible that Mopar did this to change the rate of advance or make it more linear ?
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