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Spark plug gap with high voltage coil

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MGTF1250Dave

MGTF1250Dave

Jedi Knight
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Aloha All,

I've read several times that when a higher output coil, like a Lucas sport coil replaces the standard Lucas coil the spark plug gap should be increased. The standard Lucas coil is 20KV and the Lucas sports coil is 40KV. What I don't recall ever reading is what the new gap should be or how much larger it should be opened. Any suggestions would be greatly appreciated.

I am also considering upgrading my ignition system with a Pertronix system to replace the current points and standard Lucas coil. I would retain the coil and points back up in the boot.
 
Minority opinion : Leave the gap alone !

Opening up the plug gap increases the electrical stress on the rotor and cap; and no one makes uprated versions of those. Rotor failures in particular are distressingly common these days.

In case it's not clear, those "voltage ratings" are only potential voltages. The actual system voltage is limited by the spark plug gap. Once the voltage is high enough to jump the gap (under compression), the voltage quickly drops to near zero.
 
TR3driver said:
In case it's not clear, those "voltage ratings" are only potential voltages. The actual system voltage is limited by the spark plug gap. Once the voltage is high enough to jump the gap (under compression), the voltage quickly drops to near zero.
Click to expand...

Excellent point! Good of you to remind us of the fundamentals.
 
I think stock gap is what, 25? I have the Lucas Sport coil and used to run it up to 35. Too much. I have since backed down to a very tight 30. Nice tannish-white plugs.
 
Didn't someone post just yesterday how a rotor failure spoiled their weekend ?
 
I am running a MSD 6AL, with MSD coil, pertronix iginition, 9 or 10 MM helical round spark plug wires and spark plug gap of .050 for 3 years now, and have not had a problem yet, Where is that wood dash

Hondo
 
TR6BILL said:
I think stock gap is what, 25? ...
Click to expand...

I bought a set of plugs for the TR3 this last weekend, and thought it was 0.025" also...until I looked in the Triumph Service Instruction Maunual. They said to set them at 0.032". Which is exactly what the Champion L82YC's were set at out of the box. :yesnod: Is this the right gap for these plugs??
 
martx-5 said:
I bought a set of plugs for the TR3 this last weekend, and thought it was 0.025" also...until I looked in the Triumph Service Instruction Maunual. They said to set them at 0.032". Which is exactly what the Champion L82YC's were set at out of the box. :yesnod: Is this the right gap for these plugs??
Click to expand...
Dunno. But the Driver's Instruction Manual says .025". I've always assumed that either the Service Manual was a misprint or Triumph revised the figure, but ???
 
This article by Dan Masters is about a good and concise of an explanation of the relatioship betweeen coil voltage and plug gap that I have seen.
But it doesn't tell the whole story. As pointed out high voltage to arc across a wide plug gap can have adverse effect on rotor, and cap.
The last long paragraph in this article address the relationship of coil/plug gaps.
https://www.vtr.org/maintain/ballast.shtml
 
TR3driver said:
Minority opinion : Leave the gap alone !

Opening up the plug gap increases the electrical stress on the rotor and cap; and no one makes uprated versions of those. Rotor failures in particular are distressingly common these days.

In case it's not clear, those "voltage ratings" are only potential voltages. The actual system voltage is limited by the spark plug gap. Once the voltage is high enough to jump the gap (under compression), the voltage quickly drops to near zero.
Click to expand...
I totally agree with Randall. For the reasons he gives. You will notice that late model ignitions have larger distributor caps & rotors (more insulation) for just this reason.
Minority #2.
D
 
Aloha Randall,

A '56 service bulletin Sports/17/B reduced the L10S spark plug gap from .032" to .025" to reduce the chance of misfiring under normal street driving conditions. In '57 the spark plugs were changed to Champion L87Y in service bulletin Sports/26/B. I didn't find any later service bulletins regarding spark plugs.

So I conclude from everyones advice there is no need to change the spark plug gap when converting to either a Lucas sport coil or a Pertronix system.
 
I opened up my plugs from 25 to 30 thou after installing the Lucas Sport Coil; no problems so far.....
 
From .025 to .035 here with the Pertronix and standard ballasted coil, here.
 
I'm at .025" with Pertronix ballasted coil and points. At .030" I had bad causing of low rpm bucking when the car was lugged down. Now it runs perfectly at the low rpms, so there they will stay.
 
I should add that my engine is stock except for the Pertronix, no egr and no carbon cannister and 100k+ miles on it.
Oh yeah, and it's a TR6, not a 3A.
 
Dave, something no one asked. What type of plug wires are you running?

Original gap of .025 was arrived at with wires that were carrying essentially no resistance. And coils that were rated no more than 25K volts.

If you have metal core wires or something that is equivalent, less than 1K ohms per foot, then you can open up to .032. If you are running radio suppression wires, or wires that have radio suppression caps(like MB and almost all German cars(Bosch manufacture) run, then go with .028

If I were in your shoes I'd start with .028 and see what happens. Hawaii Import Parts can get you good caps and rotors. At least the quality of the one I got for my 6's was good.
 
Aloha Ron,

I am using stock lucas coil, copper core bumble bee wire, non resistor plug connectors, Champion RL87YC spark plugs and stock distributor set up. I'm using resistor type plug because non resistor type are difficult to find. I don;t have a radio or stereo installed, the exhaust is my only sound system.
 
Hmmmm... I've been having occasional wierd problems similar to what Paul mentioned, at low rpm. Changing the rotor always fixes it. Since I'm getting tired of buying rotors ever few hundred miles, I'm going to try setting the gap from .030" (Crane XR700) to .025".

Tom
 
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