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Old 08-19-2019, 12:08 PM   #1
tiger.1000
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Default Gap between rotor arm tip and distributor cap contacts

What effect does the gap between the rotor arm and the distributor cap have on the performance of the car ? Is there an ideal setting? Does the size of the gap affect either ignition timing or starting from hot or cold?


Many thanks.
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Old 08-19-2019, 01:26 PM   #2
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Default Re: Gap between rotor arm tip and distributor cap contacts

I can tell you that it makes a difference. I have a 1967 Corvette with the 350 HP 327 ci engine. After a full restoration and engine rebuild, the engine would not reach the yellow zone on the tachometer (5600 RPM). After a lot of research, I discovered that AC/Delco had shortened the tip in the rotor .060" in 1973 to reduce emissions. This rotor became the de-facto replacement for all GM distributors from that period. A search found a firm that was making replacement rotors with the original tip length. After installing one of these rotors, the engine would rev to the top of the red zone (6000) with no problem.

I don't know how this translates to early Fords, but it made a big difference in my Corvette.

Last edited by tubman; 08-20-2019 at 06:40 AM. Reason: Drove the car yesterday and actually looked at the tachometer
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Old 08-19-2019, 02:48 PM   #3
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Default Re: Gap between rotor arm tip and distributor cap contacts

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I can tell you that it makes a difference. I have a 1967 Corvette with the 350 HP 327 ci engine. After a full restoration and engine rebuild, the engine would not reach the yellow zone on the tachometer (5800 RPM). After a lot of research, I discovered that AC/Delco had shortened the tip in the rotor .060" in 1973 to reduce emissions. This rotor became the de-facto replacement for all GM distributors from that period. A search found a firm that was making replacement rotors with the original tip length. After installing one of these rotors, the engine would rev to the red line (6000) with no problem.

I don't know how this translates to early Fords, but it made a big difference in my Corvette.
In actuality the "E" rotors required a different coil , and used very little extra voltage but the rotor index was off a bit at high rpm. A bad idea from GM but got them past the noise police etc.....
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Old 08-19-2019, 02:50 PM   #4
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Default Re: Gap between rotor arm tip and distributor cap contacts

Heres the deal a wider gap does in fact use more voltage to jump the gap. However its like winning the lottery for 1 million dollars and the local gov wants 5,000 , who cares there is enough to have tons to spare
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Old 08-19-2019, 05:49 PM   #5
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Default Re: Gap between rotor arm tip and distributor cap contacts

Bubba,

Does an early Ford coil setup produce enough voltage to overcome the problem?
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Old 08-20-2019, 04:25 AM   #6
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Default Re: Gap between rotor arm tip and distributor cap contacts

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Bubba,

Does an early Ford coil setup produce enough voltage to overcome the problem?
Sure, its not the early ford coil as much as it is the six volt system. The coil has a pretty good reserve voltage range when used with a good ground and condensor. They wont shoot a 1 inch gap but will do 1/2 etc......
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Old 08-20-2019, 06:34 AM   #7
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Default Re: Gap between rotor arm tip and distributor cap contacts

An electrical engineer can give a better explanation but here is a rough idea. A coil is a building and collapsing magnetic field and how it collapses has an effect. The plug fires in a compression and is the end point for the circuit so that gap is a different effect from the one on your distributor rotor. You can actually pull a plug wire and watch and inch long spark fire a plug. Amazingly this is a fix to get a fouled plug to fire. Go figure. I believe the Model a had a spec for this as you can separate the cap and easily measure the gap.
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Old 08-20-2019, 07:01 PM   #8
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Default Re: Gap between rotor arm tip and distributor cap contacts

Ford called out for .015" - .018" gap between the rotor and cap terminals. I don't know why they decided that was the specification their assemblies should be built to. However, if that gap didn't matter I am quite certain Ford would not have specified such a tight tolerance.
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Old 08-21-2019, 04:33 AM   #9
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Default Re: Gap between rotor arm tip and distributor cap contacts

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Ford called out for .015" - .018" gap between the rotor and cap terminals. I don't know why they decided that was the specification their assemblies should be built to. However, if that gap didn't matter I am quite certain Ford would not have specified such a tight tolerance.
Its not that the gap doesnt matter ( it does) as the system is designed to give the ignition a usefull life .
In reality all systems should be "blueprinted" to have max spark energy , especially in a higher performance example with increased compression.
My post was meant to say a larger gap wont make the system fail as there is a reserve available......i get them when they have smoked all the reserve for years and failed .......
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Old 08-21-2019, 10:47 PM   #10
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Default Re: Gap between rotor arm tip and distributor cap contacts

I have a radio installation manual for 30-35, for early auto radio. There it mentions rotor gap and fixing it by peining the tip and filing it to a .004-.005 gap

A larger rotor gap helps with firing fouled plugs, forces the spark to jump the gaps if there's enough voltage, a fouled plug can bleed the voltage off down the insulator without jumping the gap---- one of the ways to start engine with fouled plugs it to pull out the coil wire to create a spark gap
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