Thumbs up!

Does the smaller cap have a vent?  I seem to recall that crossfire often was caused by ionized air inside the cap, and if you could vent it the issue resolved.  It is interesting that both the GM HEI and Ford DS/TFIs all used large caps.  However, GM eventually went to a small cap HEI to make packaging easier (that's what they get from cramming the distributor against the firewall).  The two distributors are interchangeable with some minor wiring modifications.  The small cap also used a divorced coil instead of the coil in the cap.  I did this swap on my '86 IROC.  Seems the Ford swap is even easier since the diameter of the distributor didn't change, just the cap and shield. 

Daze wrote:

TKOPerformance wrote:

Does the smaller cap have a vent?  I seem to recall that crossfire often was caused by ionized air inside the cap, and if you could vent it the issue resolved.  

Seams like a good theory, but I don't buy it.   The larger cap IS vented and if venting was all that was required Ford' bean counters would save $.02 per distributor on less plastic.  Yes ionized gas inside could crossfire so venting is good but I think their decision to go bigger has more to do with cheap spark plug wires and crossfire outside the cap.  If small caps were prone to misfire inside than all the MSD and other aftermarket caps would be bigger as ignitions have only gotten hotter.  Why can the aftermarket run smaller caps, because of aftermarket wires.
 

A sound theory, and I agree Ford wouldn't spend a penny they didn't think they needed to, but therein also lies an important point: "think".  The best engineers in the auto industry have often been put to shame by racers with no formal education who are just better at figuring stuff out.  So possibly Ford sought to tackle the problem with a bigger cap initially and found it didn't 100% solve the issue, leading them to make a minor alteration to the mold and install a vent once they'd already changed over to the large cap and weren't going to change plug wires, etc. again.  The question would be was the large cap always vented?  Or did that come later? 

Another thing to consider is the possibility of spark jumping from the coil wire to a plug wire was an issue for Ford.  The HEI had a large cap originally, but they also put the coil in the top of that cap, necessitating the larger arrangement.  HEI caps were never vented.  The coil wire didn't exist, so it couldn't jump spark outside the cap, only inside where it doesn't seem to have been an issue.

So, without knowing why they made the switch its all just speculation on our part. 
 

Daze wrote:

TKOPerformance wrote:

So, without knowing why they made the switch its all just speculation on our part. 
 

That!! that right there, exactly my point
 

EZ answer would be..........Axe Greg B if he has had any crossfire situations pop up.....
6s6
 

Ford went cheap on materials used to make distributor caps…my guess.

I remember seeing a video in the past about phasing a distributor.  (maybe a few memory flaws)  What I remember was that an inspection hole was drilled into a cap to use as a viewing port while phasing the distributor to be at the center of the rotor tip.  The video included a discussion of the large caps having a broader rotor tip thus allowing more tolerance to poor distributor phasing.  This means a person's experience with spark jumps using a small cap will be more dependent on proper distributor phasing to prevent the jumps.  ...this is what I remember.  I don't know what video that was.

TKOPerformance wrote:

Rufus68 wrote:

I remember seeing a video in the past about phasing a distributor.  (maybe a few memory flaws)  What I remember was that an inspection hole was drilled into a cap to use as a viewing port while phasing the distributor to be at the center of the rotor tip.  The video included a discussion of the large caps having a broader rotor tip thus allowing more tolerance to poor distributor phasing.  This means a person's experience with spark jumps using a small cap will be more dependent on proper distributor phasing to prevent the jumps.  ...this is what I remember.  I don't know what video that was.

Well, that actually makes a LOT of sense.  The tip on the large cap rotors is quite long (width wise) whereas the small cap rotors are not.
 

Explain what this is for, why matters?

Rufus thanks for pointing out the difference in rotor tips.  You helped me bust the myth that ford went to a bigger cap to avoide crossfire INSIDE the cap.  

The conventional wisdom that Ford went to a bigger cap to avoid crossfire inside the cap is a myth and like all good myths it is based in a sound logical concept but I just discovered proof that it is in fact a myth. Yes the terminals are further apart and logically that would indicate less unwanted spark jumping because the spark has further to go, BUT I went out to my garage and compared the smaller cap rotor to the bigger cap rotor and the bigger cap rotor has a much bigger tip to account for it moving faster. I took the distance between the cap contacts, subtracted the width of the corresponding rotor tip width and divided by 2. The number was THE SAME. That means inside the cap, from a spark point of view there is no difference. From this information it is obvious Ford made the change to the bigger cap fore some reason OTHER than crossfire inside the cap. Not to mention that if crossfire inside the cap was an issue there would be 20 years of Internationals sitting on the side of the road.

Daze wrote:

Rufus thanks for pointing out the difference in rotor tips.  You helped me bust the myth that ford went to a bigger cap to avoide crossfire INSIDE the cap.  

The conventional wisdom that Ford went to a bigger cap to avoid crossfire inside the cap is a myth and like all good myths it is based in a sound logical concept but I just discovered proof that it is in fact a myth. Yes the terminals are further apart and logically that would indicate less unwanted spark jumping because the spark has further to go, BUT I went out to my garage and compared the smaller cap rotor to the bigger cap rotor and the bigger cap rotor has a much bigger tip to account for it moving faster. I took the distance between the cap contacts, subtracted the width of the corresponding rotor tip width and divided by 2. The number was THE SAME. That means inside the cap, from a spark point of view there is no difference. From this information it is obvious Ford made the change to the bigger cap fore some reason OTHER than crossfire inside the cap. Not to mention that if crossfire inside the cap was an issue there would be 20 years of Internationals sitting on the side of the road.
=13px

CORRECTION I measured hastily this morning and when I came out to work, I double checked my measurements there is about a .075 inch difference in the space on either side of the rotor contact on the bigger cap from the tip of the rotor when the rotor is centered. I don’t want to put out false information so I’m retracting what I said above. Ultimately, this has worked for a lot of ignitions. It may not work for all of them. A person is just going to have to try it and possibly adjust if they’re getting crossfire.


 

You could modify the cap easy enough by enlarging the index slot until you got the correct phasing and then fill in the unused portion with epoxy or that super glue/powder stuff they sell to repair plastic (works pretty well). 

At a normal RPM in a normal engine it doesn't seem worth it though.  We make a LOT about spark, but I remember reading where Smokey Yunick spent months working on various ignition systems he thought would improve power only to find no power in any of it.  In the end what he said was that so long as the mixture ignited when it was supposed to how it got ignited was pretty much meaningless.  So unless distributor phasing is causing some kind of an issue in the engine its unlikely altering it is going to be of any benefit.  It would be like indexing the plugs on a typical street engine.  I spent a lot of time trying that years ago.  It made absolutely no difference. 
 

Like I said before I only think it is critical in extreme situations.  Boost or NOS above 6000 RPM.  In those situations one degree either direction can cause detonation in an intake or to much advance in the cylinder.  Both results can be catastrophic but with a normal every day engine its not going to be an issue.  A few years back I was fighting an issue on a 351w on my run stand. Engine wasn't running poorly but I had two header tubes not getting as hot.  I was playing with EFI in self tune and it kept going rich.  Problem ended up being the 5 and 6 spark plug wires had gotten switched so one was way advanced and the other was way retarded.  Point being even though far from perfect the engine was still running okay and if a stock(ish) motor can be off by that much without major issues the one or two degrees of phasing is not going to effect it at all.

TKOPerformance wrote:

Daze wrote:

Like I said before I only think it is critical in extreme situations.  Boost or NOS above 6000 RPM.  In those situations one degree either direction can cause detonation in an intake or to much advance in the cylinder.  Both results can be catastrophic but with a normal every day engine its not going to be an issue.  A few years back I was fighting an issue on a 351w on my run stand. Engine wasn't running poorly but I had two header tubes not getting as hot.  I was playing with EFI in self tune and it kept going rich.  Problem ended up being the 5 and 6 spark plug wires had gotten switched so one was way advanced and the other was way retarded.  Point being even though far from perfect the engine was still running okay and if a stock(ish) motor can be off by that much without major issues the one or two degrees of phasing is not going to effect it at all.

Or convert to a gear drive,