What are the parts needed?
Is it a ford v8 cap & rotor,hopefully someone will know.
Big Cap to Small Cap
Started by
gtrboyy
, Jun 03 2006 07:54 PM
18 replies to this topic
#1
gtrboyy
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Posted 03 June 2006 - 07:54 PM
#2
_Yella SLuR_
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Posted 03 June 2006 - 08:43 PM
Presumably a small base as well, which just clips off like the cap. There is a post here somewhere which mentions which specific model Ford, try a search.
#3
_Herne_
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Posted 03 June 2006 - 09:08 PM
This has been covered here before (and not too long ago) as Yella points out. Damned if I can recall what the thread was called though.
I may be wrong here but something in the back of my mind tells me it's a Commodore cap with a Ford rotor??????
I am almost certain Bruce told me he used Commodore VL/VK??? in my car but I don't recall him mentioning the rotor button and I didn't ask 'cos I assumed it was all Commodore parts - bad to assume
Anyway I am sure someone here will point you in the exact direction.
Cheers
Herne
I may be wrong here but something in the back of my mind tells me it's a Commodore cap with a Ford rotor??????
I am almost certain Bruce told me he used Commodore VL/VK??? in my car but I don't recall him mentioning the rotor button and I didn't ask 'cos I assumed it was all Commodore parts - bad to assume
Anyway I am sure someone here will point you in the exact direction.
Cheers
Herne
#4
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#5
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Posted 03 June 2006 - 09:22 PM
Dude told me the reason for the large caps the other day. With the higher voltages of the electronic system, and the high resistance modern leads, there is a tendency for them to spark across terminals (path of least resistance). Never knew that.
My large cap is a tight fit, and if you totally pull the dizzy out, you have to remove the base to get it in and out. Reminds me have to do that soon to get No. 1 180 degrees about.
My large cap is a tight fit, and if you totally pull the dizzy out, you have to remove the base to get it in and out. Reminds me have to do that soon to get No. 1 180 degrees about.
#6
rodomo
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Posted 04 June 2006 - 09:30 AM
This is interesting. I wondered when I first saw one of those "flying saucer" dissy caps, why so big? The spark from one of those systems can jump about 30mm. The gap between the electrodes in a small cap would only be about 20mm. or less? Then you have got mechanical and vac. advance to contend with. As the posts above have already pointed out, leads and plugs would have to be spot on and timing critical to reduce "cross firing" at high rpm.
#7
Peter UC
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Posted 04 June 2006 - 05:45 PM
Of course this only occurs because of the crappy resistive factory leads, will proper quality leads this no longer is a problem.
#8
_Yella SLuR_
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Posted 04 June 2006 - 06:49 PM
I don't believe so. High quality leads have higher resistance to give a more powerful spark. Weird shit hey? Reverse psychology. I'm currently running 11mm Eagle silicon leads, not a bit of wire to be seen.
Edited by Yella SLuR, 04 June 2006 - 06:50 PM.
#9
rodomo
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Posted 07 June 2006 - 12:03 AM
Well it works on Fords with small caps? Doesn't it? Is the cap and rotor that fits off an electronic dizzy?
#10
_devilsadvocate_
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Posted 07 June 2006 - 12:46 AM
Not too sure about any of the theories ^.
My thinking would be that the most likely scenario is that the spark leaks from the centre electrode to any of the terminals of the circumference of the dizzy, rather than from one outer terminal to the next. There should be significant half circular shields around each terminal on the underside which should reduce the straight line contact area between the outer terminals and that a"short' is only likely if a layer of conductive grime is present. A short also being more likely on the top where the spark can leak around the seals of leads on the male terminals of the cap.
Of course in a 6, the dist from centre to outside = distance between outside terminals, so to create the same dist between outside terminals in an 8 the cap needs to be 8/6 bigger in radius.
Resistance of the leads will not affect the voltage of the spark across the plug electrodes. Straight wire can be used(with good insulation), however, the interference that is produced will see the guys that control illegal radio signals(whatever they are called) trace down you car, as you will cause interference to everything within 100m) From my measurements the resistance of leads has not changed greatly in the last 35 years, some higher performance leads have lower resistance.
Only limiting factor is if the resistance of the leads is too high, then either coil will not deliver enough voltage for spark to be able to jump plug gap and go through lead or spark will take easier path, perhaps from one terminal to another which isnt connected by such a high resistance lead to its plug.
My thinking would be that the most likely scenario is that the spark leaks from the centre electrode to any of the terminals of the circumference of the dizzy, rather than from one outer terminal to the next. There should be significant half circular shields around each terminal on the underside which should reduce the straight line contact area between the outer terminals and that a"short' is only likely if a layer of conductive grime is present. A short also being more likely on the top where the spark can leak around the seals of leads on the male terminals of the cap.
Of course in a 6, the dist from centre to outside = distance between outside terminals, so to create the same dist between outside terminals in an 8 the cap needs to be 8/6 bigger in radius.
Resistance of the leads will not affect the voltage of the spark across the plug electrodes. Straight wire can be used(with good insulation), however, the interference that is produced will see the guys that control illegal radio signals(whatever they are called) trace down you car, as you will cause interference to everything within 100m) From my measurements the resistance of leads has not changed greatly in the last 35 years, some higher performance leads have lower resistance.
Only limiting factor is if the resistance of the leads is too high, then either coil will not deliver enough voltage for spark to be able to jump plug gap and go through lead or spark will take easier path, perhaps from one terminal to another which isnt connected by such a high resistance lead to its plug.
Edited by devilsadvocate, 07 June 2006 - 12:53 AM.
#11
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Posted 08 June 2006 - 12:18 AM
[quote name='devilsadvocate' date='Jun 7 2006, 12:46 AM'] Not too sure about any of the theories ^.
My thinking would be that the most likely scenario is that the spark leaks from the centre electrode to any of the terminals of the circumference of the dizzy, rather than from one outer terminal to the next.
I reckon your rotor button would have to be in big trouble for that to happen.
My thinking would be that the most likely scenario is that the spark leaks from the centre electrode to any of the terminals of the circumference of the dizzy, rather than from one outer terminal to the next.
I reckon your rotor button would have to be in big trouble for that to happen.
Edited by rodomo, 08 June 2006 - 12:21 AM.
#12
TerrA LX
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Posted 08 June 2006 - 12:29 AM
are you saying female caps are better for this than male ones. i always felt that the male one was going to give me a better connection.A short also being more likely on the top where the spark can leak around the seals of leads on the male terminals of the cap.
i hear holden went to the large cap to stop hi energy misfire (internal, one to the next). ford used the same dizzy with the small cap. can any fordies enlighten of any probs.
#13
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Posted 08 June 2006 - 02:06 AM
Definitely think that the female terminals other better protection with the plastic surrounding the terminal, however, it is more the mechanical mating of the terminals that causes problems here in the female connectors(soft circle inside another soft circle) whereas the soft circle on hard connector seems to provide better contact.
If youve seen the scorched terminals of a female dizzy cap youll know what I mean.
Leakage from the centre terminal to any of the outside terminals via the top of the cap is always a possibility, possibly leaking(misfiring) to the plug that is experiencing the least compression, or just beginning the intake stroke(possibly requiring far less voltage than the desired plug at the top of the compression stroke), which may not be the terminal thats next in the firing order on the outside of the dizzy.....too late to work that out atm.
A high performance and or compression engine will require higher plug voltages at full power, so its reasonable to suggest it will need a dizzy with wider spacing all round so the spark doesnt jump via an easier path
If youve seen the scorched terminals of a female dizzy cap youll know what I mean.
Leakage from the centre terminal to any of the outside terminals via the top of the cap is always a possibility, possibly leaking(misfiring) to the plug that is experiencing the least compression, or just beginning the intake stroke(possibly requiring far less voltage than the desired plug at the top of the compression stroke), which may not be the terminal thats next in the firing order on the outside of the dizzy.....too late to work that out atm.
A high performance and or compression engine will require higher plug voltages at full power, so its reasonable to suggest it will need a dizzy with wider spacing all round so the spark doesnt jump via an easier path
#14
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Posted 14 June 2006 - 10:59 PM
mate i have this exact setup on my 308 torana, i used a v8 holden points cap( so i didin't have to change the lead ends) and a ford v8 rotor. cant remember the part number but call burson auto parts(03 97295522) ask for campbell and tell him you are converting from large cap to small cap and he will give you exact bosch part number, he looks it up through my account all the time.
#15
rodomo
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Posted 14 June 2006 - 11:17 PM
Does the ford rotor have a narrower arm than the holden one?
#16
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Posted 14 June 2006 - 11:32 PM
ooohhhh cant remember and at the moment i cant get the cap off my dissy to check( slight issue with the manifold spacer touching it). the shaft diameter etc and cap diameter etc are same from ford to holden, a ford 6cyl rotor will not work as it is rather wide, im pretty sure the rotor is same dimensions as the points one (aside from the shaft size and locater inside the rotor)for a holden v8 but actually the ford electronic version. as i said try campbell, tell him james said he will help you(97295522)
#17
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Posted 15 June 2006 - 10:49 AM
Hi Guys.
When Ford & Holden went to the Bosch HEI system for their V8s, Ford stayed with the old style (smaller) cap & Holden went to the larger diameter one. One of the reasons for this was that Holden went to the super wide 1.5mm spark plug gap while Ford remained at 0.8-0.9mm. Holden also went to 8mm Packard leads. They used the same ignition coil, so Holden obviously had a problem or wanted to reduce the risk of crossfire.
When Holden introduced their EFI V8, the coil output remained the same as the Blue/Black HEI system, but the spark plug gaps were reduced to 1.1 mm & they went back to the smaller cap.
To answer the question on which rotor to use, the best one would be the XD/XE Ford V8 which has the same dissy shaft size as the Holden HEI, but the smaller cap, therefore the rotor is a perfect fit. I believe the EFI V8 rotor would also fit, but have never tried it.
Just remember to reduce your spark plug gap when using the smaller cap. I always use a maximum of 1.3mm gap on any Holden HEI system(6 or V8) in anycase, you get less misfire & longer spark plug life.
Dr Terry.
When Ford & Holden went to the Bosch HEI system for their V8s, Ford stayed with the old style (smaller) cap & Holden went to the larger diameter one. One of the reasons for this was that Holden went to the super wide 1.5mm spark plug gap while Ford remained at 0.8-0.9mm. Holden also went to 8mm Packard leads. They used the same ignition coil, so Holden obviously had a problem or wanted to reduce the risk of crossfire.
When Holden introduced their EFI V8, the coil output remained the same as the Blue/Black HEI system, but the spark plug gaps were reduced to 1.1 mm & they went back to the smaller cap.
To answer the question on which rotor to use, the best one would be the XD/XE Ford V8 which has the same dissy shaft size as the Holden HEI, but the smaller cap, therefore the rotor is a perfect fit. I believe the EFI V8 rotor would also fit, but have never tried it.
Just remember to reduce your spark plug gap when using the smaller cap. I always use a maximum of 1.3mm gap on any Holden HEI system(6 or V8) in anycase, you get less misfire & longer spark plug life.
Dr Terry.
#18
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Posted 15 June 2006 - 04:09 PM
When I have done this conversion I have always dropped the plug gap (to .9) and used teh HOLDEN dizzy cap. To use the holden cap you need to use the FORD 6 cylinder rotor. if using the v8 rotor you need to use the ford v8 cap. (this is all from memory and a few years ago.) In the end I found it was better to just run the bigger stuff as it hides alot of mess. 
Also look nice painted.. hehehe
Cheers
Also look nice painted.. heheheCheers
#19
_Toranamuk_
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Posted 07 July 2006 - 11:21 PM
The rotor button pt no is GB 752 to enable you to use a standard Holden V8 small dissy cap.
A quick tip from many years in race & dyno testing is to drill a 1/8th hole approx half way down the cap (& at the rear of the cap) between 2 posts.
This lets some air flow through the distributor & helps to discharge the "charged" electrons from the cap/rotor area & eliminate cross firing.
This may sound strange, but check out any of the top "name" brand performance distributors & caps & you'll see the holes.
It's very subtle but extremely important.
A quick tip from many years in race & dyno testing is to drill a 1/8th hole approx half way down the cap (& at the rear of the cap) between 2 posts.
This lets some air flow through the distributor & helps to discharge the "charged" electrons from the cap/rotor area & eliminate cross firing.
This may sound strange, but check out any of the top "name" brand performance distributors & caps & you'll see the holes.
It's very subtle but extremely important.
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