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Old 02-06-2009, 07:32 AM   #1
WesChapman
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Default Prestolite with Holley parts

1) vehicle model. Scout terra
2) production date/or year model.July 1978
3) two wheel drive or four wheel drive.4x4
4) type of transmission (and transfer case if applicable).t-19 close. Dana 20
5) if 4x4, which axles front and rear.Dana 44 x2
6) which engine variation.345 presumed original
7) which ignition system variation.prestolite
8) which fuel system variation.2bb swapped for 4bb thermoquad
9) which brake system variation.oem
10) non-oem modifications present which May impact a response.relocated coil to fire wall


Reading other posts in the forum, I gather using a Holley cap & rotor on a prestolite distributor is not recommended. A po of my truck has put Holley parts on the prestolite and I replaced with same because I didn't know any better. (sometimes I leap before I look) my truck seems to run ok, but what would be some issues caused by intermixing parts?
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Old 02-06-2009, 09:27 AM   #2
Michael Mayben
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Default Re: Prestolite with Holley parts

Neither cap seats properly if not the proper dedicated part. One will be way too loose for "indexing" purposes if improperly applied, the other will be so tight that it has to be forced into place and can result in cracking...and won't index properly either.

And terminal "phasing" is different by approximately five degrees, that is very difficult to see with your eye. I discovered that years ago by drilling observation ports in each cap and measuring the offset difference with a caliper inna redneck fashion...that method makes it very obvious!

Some folks don't ever realize they have a phasing issue, and simply live with a poorly performing ignition system, which when mixed with typical worn distributor internals, they attempt to "fix" by installing a pertronix or some other conversion trigger system.

You are right...the engine can "run" but not correctly since the ignition timing will be highly unstable at any rpm point if the distributor is worn and the wrong cap is used!

Thanks for posting your details so nicely! That is exactly what we need ta see to be able to help folks most effectively!
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Old 02-08-2009, 09:02 AM   #3
WesChapman
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Default Re: Prestolite with Holley parts

Thanks for the info! I'll replace with the proper components before I start driving it again. I'm amazed how high tech, this low tech stuff is. From this site I learned plug wire routing is important also. Looking back, it makes sense based on what tiny tiny bit I know of ac electrical and inductance, but I needed someone else to point it out. I'm so glad you "old-timers" (respectfully) take the time to school us kids.
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Old 02-08-2009, 09:05 AM   #4
WesChapman
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Default Re: Prestolite with Holley parts

Oh, and, what is phasing?
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Old 02-08-2009, 10:12 AM   #5
Michael Mayben
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Default Re: Prestolite with Holley parts

"phasing" is a term I use kinda loosely...in relation to what I described, I'll try and explain more clearly!

Any distributor, for any conventional "distributor" ignition system, has distinct alignment points for all critical components.

The distributor "drive" must be properly aligned with the cam/crank drive, the point lobes on the breaker point cam must be properly indexed to the piston position through the cam/crank drive, the distributor rotor must be properly attached to the point cam so that it's tip aligns at a distinct point with the individual terminals inside the distributor cap. The distributor cap must be correctly indexed/aligned with the distributor body.

If the distributor is an electronic trigger model such as what you are dealing with, the same holds true, the reluctor/trigger wheel/whatever must properly align with the pickup module at a distinct point in order to "fire" the spark plug at the proper moment in time.

So "phasing" is a term I (and others in the ignition industry) use to describe the relationship of all the above "critical points"! If all components are original and unmolested, and/or the aftermarket parts are the correct version and design of the oem stuff, then all this "alignment" or phasing" stuff is a given...just install the fresh parts, make any necessary adjustments (like point gap or dwell or reluctor air gap, etc.), then verify base ignition timing and drive on!

But when performing modifications to this kinda stuff (like boneyardin' in distributor guts form one electronic distributor to a points distributor, or installing an aftermarket replacement trigger system), this phasing gets all outta whack, even just one or two degrees of "slop", misalignment, or plug cable terminals not positioned correctly is hugely magnified when the engine is running or even attempting to start.

In relation to your situation...the actual plug terminal (all of the eight terminals) in the prestolite cap, are "slightly" out of index (or phasing) as regards the engineered position that the reluctor/pickup provides to "trigger" the coil to fire the secondary side (spark plug). How much??? I don't know right now as I don't have the actual data or photographs I did about 10 years ago when I "analyzed" this particular issue!

At that point I also experienced the same thing you have found with your rig...common wisdom on the I-net at that point in time was those caps were interchangeable...if that was actually the case, then all aftermarket manufacturers would have certainly taken advantage of that detail...but...the "wisdom" was/is wrong!

I vastly prefer the male terminal distributor cap model (such as the prestolite or the delco hei)...I think it's superior in all areas to the female terminal system. But back in the day...those were certainly not in the mainstream of ignition design and most likely due to the proprietary nature of such as far as patents, manufacturing license, etc. They are also much easier to "treat" for the prevention of moisture/contaminants over the long haul, that keeps corrosion/oxidation of the terminal connections to a minimum.

So...guess it's time to run ta town and snake a prestolite cap, drill it full of "observation ports", and do an analysis again so we can prove or disprove this item. I've got tons of Holley caps (new and takeoffs), and points distributor delco caps (both sv and I-4 apps), but not a prestolite on the place right now!

This is a great topic you have brought up, this is the kinda stuff we hope stimulates thought around here and makes us put up or shut up! And we always back up our "ideas" with testing and report facts...whether the results are good or not so good!!!
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