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Why are spark plugs gapped at .040? now

 
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Old 12 Sep 2009, 01:19 pm
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Question Why are spark plugs gapped at .040? now

Just curious, because all the service manuals state .047-.052 guidelines. I also have the original mopar plug wires.Im gonna be changing both wires and plugs under your guys recommendations. Also does anyone know if the gearshifter should be lit up at night or if mine has a burned out bulb.
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Old 12 Sep 2009, 02:17 pm
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Default Re: Why are spark plugs gapped at .040? now

Chrysler found that they were having hesitation & missing problems when set at .050, they sent out a TSB to all dealers telling them to set the gap at .040.Also does anyone know if the gear shifter should be lit up at night or if mine has a burned out bulb.

Yes, it should be lit.
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Old 12 Sep 2009, 10:16 pm
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Default Re: Why are spark plugs gapped at .040? now

I would definately gap them at 40. The first time i put nitrous on one of my cars,it would cut out bad past 4500 rpm's because the gap was too much and basically putting out the flame. As soon as i re-gapped them to 35 thousands it ran awesome.This is just to show how important plug gap can be.
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Old 14 Sep 2009, 10:46 am
Mal Mal is offline
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Default Re: Why are spark plugs gapped at .040? now

It takes X voltage to jump (spark across) a Y size gap in air. It takes even more voltage to jump the same sized gap in compressed fuel/air mixture. This is the reason that forced induction gaps are normally smaller and also the reason a miss due to plugs normally shows up under fairly heavy acceleration at low RPM with a cold engine. Those are the conditions where the resistance of the compressed fuel air mixture is the highest.

I have always been interested in what gap will start a miss and though I have never had the opportunity in a PT; I heard a report here of .073 or so. My Miata with .040 recommended started to miss at .053 and a Saturn I drove for a while with .040 recommended started to miss at .055. None of that means squat, just interesting. I do know that a tremendous number of plugs and wires are replaced when all that is needed is to regap the plugs. Even wires obviously sparking to ground can be caused by excessive plug gap.

In the PT a .050 gap might well provide a better spark but when the gap wears the slightest above that, a miss is likely. No doubt Chrysler simply overestimated the voltage provided by the ignition system they are using in the PT and had to back off the gap originally recommended.

Last edited by Mal; 14 Sep 2009 at 10:59 am.
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Old 14 Sep 2009, 02:21 pm
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Default Re: Why are spark plugs gapped at .040? now

Quote:
Originally Posted by Mal View Post
It takes X voltage to jump (spark across) a Y size gap in air. It takes even more voltage to jump the same sized gap in compressed fuel/air mixture. This is the reason that forced induction gaps are normally smaller and also the reason a miss due to plugs normally shows up under fairly heavy acceleration at low RPM with a cold engine. Those are the conditions where the resistance of the compressed fuel air mixture is the highest.

I have always been interested in what gap will start a miss and though I have never had the opportunity in a PT; I heard a report here of .073 or so. My Miata with .040 recommended started to miss at .053 and a Saturn I drove for a while with .040 recommended started to miss at .055. None of that means squat, just interesting. I do know that a tremendous number of plugs and wires are replaced when all that is needed is to regap the plugs. Even wires obviously sparking to ground can be caused by excessive plug gap.

In the PT a .050 gap might well provide a better spark but when the gap wears the slightest above that, a miss is likely. No doubt Chrysler simply overestimated the voltage provided by the ignition system they are using in the PT and had to back off the gap originally recommended.
The Miata comes from the factory with a set of plug wires that are only slightly better than the PTC , that is the first thing that comes off of a stock class Miata, neither are much better than a wet piece of string inside a rubber hose...The reason that the plugs are gapped .040 is that the "made by the lowest bidder ignition systems " are a far cry from a Knock Your Ass Down System.......040 is about all the gap that it will fire under all conditions...
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Old 14 Sep 2009, 04:16 pm
Mal Mal is offline
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Default Re: Why are spark plugs gapped at .040? now

Yeah, most everybody goes to the NKG Blues sooner or later. Interestingly there is an optimum voltage for ignition systems. It's not the "more is better" we often assume. Too much and plug wear goes way up and paths are ionized in places you wouldn't believe. In the old days; there was a resistor in the "run" position of the ign system to keep the voltage down when the engine was running. It was removed from the circuit when the ign was in the "start" position so that the lowered voltage from the starter drain would not reduce spark voltage. I don't know enough about the modern ECU controlled ignitions but I would guess the ECU does the same...
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Old 14 Sep 2009, 04:25 pm
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Default Re: Why are spark plugs gapped at .040? now

Quote:
Originally Posted by Mal View Post
Yeah, most everybody goes to the NKG Blues sooner or later. Interestingly there is an optimum voltage for ignition systems. It's not the "more is better" we often assume. Too much and plug wear goes way up and paths are ionized in places you wouldn't believe. In the old days; there was a resistor in the "run" position of the ign system to keep the voltage down when the engine was running. It was removed from the circuit when the ign was in the "start" position so that the lowered voltage from the starter drain would not reduce spark voltage. I don't know enough about the modern ECU controlled ignitions but I would guess the ECU does the same...
I keep a set of "race" plugs gapped at 0.35, but run normally 0.40. I run about a 1/4th tank of race fuel mixed with 93 octane when at the track, and was watching my logs and saw increased knock, hesitation and misfire at anything over 0.35. So hooked up the laptop to the DSP and ran logs driving to/from work, 0.50 was horrible, 0.35 seemed like it was running rich, and 0.40 was perfect.

I get to the drag strip, I pop out my 'normal' plugs and drop in the 0.35 plugs, do my pulls, let it cool down and swap back to 0.40 before I drive home.
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Old 14 Sep 2009, 05:09 pm
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Default Re: Why are spark plugs gapped at .040? now

I had mine gapped at .035 before I installed my hard pipes from AMX and then the cars boost increased significantly, so the .035 was causing the car to miss and hesitate at 5500 rpms. I re gapped them at .030 and the car runs like a champ now.

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Old 14 Sep 2009, 05:54 pm
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Default Re: Why are spark plugs gapped at .040? now

Whenver I get this POS clutch replaced, I'm going to do a few pulls with mine at 0.30 too. Sounds like I might have a touch of knock at 0.35 since I did my hardened up-pipe. Feels like a subtle hesitation in third at about 1/2 power.
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Old 14 Sep 2009, 11:58 pm
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Default Re: Why are spark plugs gapped at .040? now

On any engine check your plugs and remove any microscopic dingleberrys ,they will stay hot and preignite just like a plug with a too hot heat range.On a Normally Aspirated engine I cut back the ground electrode so it comes out about halfway over the center electrode.This exposes the flame kernel and reduces the need to index the plugs EXCEPT in super critical racing applications.I havn't tried on any high boost gasoline engines but I use cut back electrodes on my blown alcohol drag boat.. When playing with plugs remember sharp edges are good ---dingleberrys are bad...
The best ground electrode configuration I've found for my tow truck is a Hawk Bill shape ,,good for a average mpg increase from 10 to 11 = 10 % on the same west coast trip in the same weather with the same racecars on the trailer...Was really worth the trouble and seemed to increase the torque at the same time. Mod is good for around 10 k miles plug life but these are easy to change...
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Last edited by edsidders; 15 Sep 2009 at 12:12 am.
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