Distributor cap, small dia, brass post style.

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Bill usn-1
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Re: Distributor cap, small dia, brass post style.

Post by Bill usn-1 »

Phasing is particularly important when you finally "Dump the Junk" carb and go EFI with computer controlled timing.

Once the mech and vacuum adv are locked out the phasing has to be adjusted or you may really find out what arc over is.
Most standard ignition phasing has the rotor centered on the cap terminal and the EFI phasing has the rotor at the trailing edge of the cap terminal.
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tgreese
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Re: Distributor cap, small dia, brass post style.

Post by tgreese »

Regarding the larger cap, there is another advantage besides the more-air-lower-ion-density-longer-distance-less-arc-over reason.

The more you increase the diameter of the contact circle, the more accurate the angle between each contact becomes. If you are familiar with vectors, this is easy to see. The rotor angle has an error term that is delta-x and delta-y, not dependent on the contact circle diameter. The longer the rotor is, the less effect on the angle the error has.

So if you are concerned about accurate spark timing, the larger cap will give you a (slight?) improvement in cylinder-to-cylinder timing. I suspect this is a more likely reason that Ford went to the bigger cap - rather than the arc-over reason.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
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Bill usn-1
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Re: Distributor cap, small dia, brass post style.

Post by Bill usn-1 »

It's an interesting discussion point but I'm not sure how relevent it is in this case.
The spark is jumping an air gap to get from the rotor to the cap terminal.
The time it takes to jump due to the difference in cap diameter would be the same because the rotor will be in the same physical relationship to the cap terminal.
If the rotor phasing is out then the rotor tip moves away from the cap terminal as the timing advances. Get too far away and it will jump from the other side of the rotor tip to the previous cap terminal.

Now if you looking at spark timing then you could discuss making every plug wire the exact same length so the timing is the same on all.
1 nanosecond is about 9" long.
Or do like the industry and go COP to eliminate the plug wires all together.

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Re: Distributor cap, small dia, brass post style.

Post by serehill »

Bill usn-1 wrote:It's an interesting discussion point but I'm not sure how relevent it is in this case.
The spark is jumping an air gap to get from the rotor to the cap terminal.
The time it takes to jump due to the difference in cap diameter would be the same because the rotor will be in the same physical relationship to the cap terminal.
If the rotor phasing is out then the rotor tip moves away from the cap terminal as the timing advances. Get too far away and it will jump from the other side of the rotor tip to the previous cap terminal.

Now if you looking at spark timing then you could discuss making every plug wire the exact same length so the timing is the same on all.
1 nanosecond is about 9" long.
Or do like the industry and go COP to eliminate the plug wires all together.
I have to agree with relevance. COP no doubt when programmed correctly is best. The best systems in the world for racing are all small & totally better enginered than all of the HEI kits you see out there. Mallory Magnetos that operate perfectly @ 12K rpm are the small ones some I don't think that is a determining factor. As long as the enginereing compenates then delayed arc would be the only thing that would be a factor that would make the larger cap viable. I've never seen an actual case of this. That doesn't mean there isn't. None of the late model vehicles that are useing COP or coil pac technology are concerned about proximity arcing. Look at how compact the coil pack systems are. With most of the larger systems what has come to be known as HEI distributors have tons of slop in them that allows the larger distributor angles to be moot. I've also found most of the arc issues with the larger distributors to be from the coil to the cylinder wire or rotor. Not from the rotor to an opposing cylinder.
Last edited by serehill on Sat Mar 30, 2013 7:58 pm, edited 1 time in total.
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tgreese
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Re: Distributor cap, small dia, brass post style.

Post by tgreese »

Bill usn-1 wrote:It's an interesting discussion point but I'm not sure how relevent it is in this case.
The spark is jumping an air gap to get from the rotor to the cap terminal.
The time it takes to jump due to the difference in cap diameter would be the same because the rotor will be in the same physical relationship to the cap terminal.
If the rotor phasing is out then the rotor tip moves away from the cap terminal as the timing advances. Get too far away and it will jump from the other side of the rotor tip to the previous cap terminal.

Now if you looking at spark timing then you could discuss making every plug wire the exact same length so the timing is the same on all.
1 nanosecond is about 9" long.
Or do like the industry and go COP to eliminate the plug wires all together.
Mmm. I think you are missing the point. The length of the radius changes the susceptibility to dimensional and positional noise. Think about the rotor in polar coordinates with a radius r and angle theta. Variability in theta changes the interval between each cylinder's ignition. Now add cartesian error vectors dx and dy at the end of the radius. The longer the radius is, the less dx and dy change the value of theta.

This is the same reason why COP systems go to a large toothed wheel to replace the distributor signal - accuracy increases with radius.

I'll scan a diagram when I get home.
Tim Reese
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Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
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Bill usn-1
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Re: Distributor cap, small dia, brass post style.

Post by Bill usn-1 »

Yep a picture is always good.
Cause trying to use a delta X and Y with positional noise as it relates to rotor position and actual changes in timing is a little out there.
The tip of the rotor is about 45* electrical wide.
The terminal of the cap is normally about 30* electrical wide. I use electrical because it is different from physical degrees due to the distributor rotating once for each 2 rotations of the crank.
To get an idea just compare distr specs to crank timing specs.

Like i said...good discussion points but i don't think it is really applicable and way more than is required or need to know vs want to know.
Most just want something that works better than what they have and really don't care how.
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tgreese
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Re: Distributor cap, small dia, brass post style.

Post by tgreese »

Well, I'm not sure this will help. Here's a diagram I made earlier.

Image

As R gets longer, delta-X and delta-Y have less and less effect on the value of theta. So a longer rotor will have a more accurate duration between cylinders. It does not matter that the terminals are wide, as long as the conditions are the same for each cylinder. If there is any variation due to the distributor parts moving around or inaccurate construction or mounting of the parts, it will have less and less effect as R gets longer.

It probably will not matter in day-to-day operation of the engine - few people would notice the difference. But the Ford engineers made these changes for some good reason - they would not have refined the design unless there were some incremental benefit. In fact, it would have to be enough of a benefit to justify the expense of redesigning, retooling, and stocking the new-style part. Waiving that away seems a little reactionary to me.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
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Bill usn-1
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Re: Distributor cap, small dia, brass post style.

Post by Bill usn-1 »

Now that you have provided your math basis to justify your position, we need to go back to your original statement.
Regarding the larger cap, there is another advantage besides the more-air-lower-ion-density-longer-distance-less-arc-over reason.

The more you increase the diameter of the contact circle, the more accurate the angle between each contact becomes. If you are familiar with vectors, this is easy to see. The rotor angle has an error term that is delta-x and delta-y, not dependent on the contact circle diameter.
The longer the rotor is, the less effect on the angle the error has.
So if you are concerned about accurate spark timing, the larger cap will give you a (slight?) improvement in cylinder-to-cylinder timing.
I suspect this is a more likely reason that Ford went to the bigger cap - rather than the arc-over reason.
So you are specifically referencing the physical relationship between the rotor tip and the cap terminal.
Correct?
If so then how can you make this statement?
It does not matter that the terminals are wide, as long as the conditions are the same for each cylinder.
The only purpose the rotor has is to bridge the gap from the coil wire to the cap terminal.
If the rotor tip is wider on each side of 0 then the spark has a much wider angle avail to match up with the cap terminal.

Which makes your entire discussion kind of a non issue. Since as long as the spark can jump from the rotor to the cap terminal there is no change in the timing.

I have all the parts on my shelf and I can take comparison photos of each rotor and cap and display the difference in rotor tip width.
In fact...IIRC the tip of the large cap rotor is proportionally larger than the small cap. So if we drew a straight line from the center of the distr shaft out to the tip of the rotor we may just find that both will come out to the same number of degrees wide!

If I get some time i'll do a little comparison.
But it's not like it will make a hill of beans as most who would read all this has probably already made up their mind which they want to run no matter what the reasoning is. :)
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Re: Distributor cap, small dia, brass post style.

Post by tgreese »

Well, I'm trying to explain why Ford chose the longer rotor instead of the existing design. I claim there is an advantage to increasing the cap size, if we can recognize it.

It's purely a geometric problem. I don't see anything I've claimed that is inconsistent.

Bill usn-1, thanks very much for all the contributions you've made to the community. I follow your comments wrt the TBI stuff closely. I see your smiley, and this post is in the same spirit.

However, it seems you are missing what I'm claiming.

The width of the rotor tip does not matter, as long as the approach, arc, and departure of the rotor tip from the terminal is the same for every discharge. All I'm claiming is that the accuracy of the angle between terminals is increased by a longer rotor. If one cylinder fires after 45 degrees travel, then the next 44, and the next 46, the time between those ignitions and the accuracy of the ignition timing will be affected. A longer rotor decreases the effect of any positional error (dx, dy) on the angle.

If all the ignition parts were perfect, and the tolerance between the rotating parts were zero, then the longer rotor would have no effect. But they are not - there is noise. The longer the rotor, the less effect that noise has on the timing of the signal, ie the spark discharge.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
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Bill usn-1
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Re: Distributor cap, small dia, brass post style.

Post by Bill usn-1 »

I understand it may seem frustrating trying to get your point across on the web.
However, it seems you are missing what I'm claiming.
But, I completely understand what you are trying to say.
I claim there is an advantage to increasing the cap size, if we can recognize it.
This is great and we have discussed a few points as to "why" they changed, to include the push for better emissions by increasing plug gaps up to .060 which increases the resistance the spark must jump to complete the path to ground.
I don't see anything I've claimed that is inconsistent.
Maybe not but maybe we need to look at this a little closer.
All I'm claiming is that the accuracy of the angle between terminals is increased by a longer rotor.
This is an accurate statement and I have no issue with it.
As the arm gets longer the distance between degrees increases.
The width of the rotor tip does not matter, as long as the approach, arc, and departure of the rotor tip from the terminal is the same for every discharge.
.....
If one cylinder fires after 45 degrees travel, then the next 44, and the next 46, the time between those ignitions and the accuracy of the ignition timing will be affected.
This is where I have trouble understanding how the length of the rotor actually has any affect on the actual timing.
As mentioned earlier, the rotor is fixed to the shaft and only provides a means of transferring the energy from the center post of the cap to the end of the cap.
There is no timing function involved in the rotor, only phasing. Which places the rotor in the correct position in reference to the cap terminal.


Let me show a couple things before i get to the rotor itself. We will discuss electronic timing instead of mechanical since it's easier to visualize.

Here is the AMC firing order and rotation.
AMCFiringOrder.gif
Now for timing we need to ensure the reluctor phasing is correct. This sets the position of the reluctor in relation to the shaft that is directly connected to the cam/crank and is referenced to the #1 piston at TDC. So your timing mark should be on 0*

Each tooth of the reluctor...as close to the shaft as it is, is still ~15* of timing for it's full width.
So in this pic we are way retarded.
AMC DS (4).jpg
This is where we want the reluctor for TDC.
AMC DS (21).jpg


So to ensure the rotor is correctly positioned in relation to the cap we also need to set the rotor phasing.


With the firing order CW rotation, if the timing is ADTC or retarded then the rotor will arc over as the rotor is at the trailing edge of the cap terminal.
AMC DS (2).jpg
As the timing increases it fires sooner and sooner so the rotor tip will sweep across the cap terminal. Total timing (base + vacuum + mechanical) is normally around 40*.
DSC01481.JPG
If we go back to the reluctor we said the width was ~15* wide. So if it is .060 wide and then the rotor is twice as long then the same 15* should equal .120 correct? I'll refer to your math skills to verify that one. It's really just for demonstration purposes.
So if we need 40* then the rotor tip would need to be at least .320 wide to still be in direct relation to the cap terminal. But we also have a wider cap terminal so that helps.

If the rotor is increased in length say to 3 time the distance of the reluctor then the tip width would increase.
15*=.180"
30*=.360"
40*=.480"

Note that when we change the timing, the position of the rotor to the cap does not determine or affect the actual timing.
It is the reluctor or points lobes in reference to the shaft and the piston at TDC that affect the actual timing.
When we grab the distr and rotate it we are changing the relationship(phasing) between the baseplate and the pickup or points rub block to change when it fires(timing).
So does the length of the rotor affect the actual timing or timing accuracy?

HTH
Bill
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tgreese
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Re: Distributor cap, small dia, brass post style.

Post by tgreese »

Doh. You're right. I'm mixing up phasing and timing.

Sorry to take so much of your time.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
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Re: Distributor cap, small dia, brass post style.

Post by husker77 »

Well after installing the adapter and large cap I can say I think the idea of it being loose and shifting doesn't really seem to be a problem. Mine is tight and solid, no movement what so ever.


I believe one reason for the larger cap has to do with the ionizing of the air. I am more familiar with outboard motor ignitions then distributors anymore.
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Bill usn-1
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Re: Distributor cap, small dia, brass post style.

Post by Bill usn-1 »

tgreese wrote:Doh. You're right. I'm mixing up phasing and timing.

Sorry to take so much of your time.
NP

Dang,
I just went out and took pics and measurements of the rotors too. ;)
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Re: Distributor cap, small dia, brass post style.

Post by carnuck »

The bigger cap is because the higher output coil can jump a bigger gap and on smaller caps, the contacts are closer, ergo more likely to cross fire. I wonder how well our rigs would run with a wasted spark system?
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Bill usn-1
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Re: Distributor cap, small dia, brass post style.

Post by Bill usn-1 »

Carnuck, That's the typical myth that is spread all over the web.
I don't think you read the entire thread.
If that was true than why did they go back to the small cap with even better ignition?


I've done wasted spark with my EFI.
The megajolt has also been done.
And i remember seeing a COP setup on IFSJ.
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