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Well tuned engine?; Explain to the noobie?
Topic Started: Jul 18 2009, 07:49 AM (1,933 Views)
Bad Bent
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Facetious Educated Donkey

I'll say that the object is to light the fuel/air mixture. Will a longer gap do a better job, maybe but it does take more voltage to jump the wider gap. Be the first on your block to experiment and report the results to us? But there are so many other factors... :shake
Our ignition system is designed to jump a gap of .039-.043 (1985-93) and .041 (94-00) for 1.0s. A hotter plug might make more of a difference but more heat is not good for the engine. Advancing the timing 3-5 degrees BTDC than the factory 6-5 BTDC for a longer more complete burn seems the best way to improve gas mileage and control carbon build up. IMO B-)
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starscream5000
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Got 70 MPG?

Bad Bent
Aug 12 2009, 06:09 PM

Our ignition system is designed to jump a gap of .039-.043 (1985-93) and .041 (94-00) for 1.0s.
That's odd then, my sticker on the '95 just says to gap at .039", doesn't say any other gap at all...

I'll have to try out .041" on the next spark plug change and report my findings...
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mwebb
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FOG

i would not increase the gap beyond what is specified .
yo can see 3 spark waveforms in the last post in the "EGR" thread , one with a bad wire one in open air and one in a "normal" firing cylinder

Burn time is a function of spark plug gap -
larger gap or higher combustion pressure increase KV demand for the initial spark to jump across the gap .
burn time is longer with less / lower KV on the firing line , so spark burns longer
longer is better .

there is also the problem with excessive KV caused by worn or excessive spark plug gap looking for an easier path to ground , which may not pass through the spark plug gap and is a common cause of secondary spark system failure.

anyway the engineers who designed the system think their specified gap is the best compromise , until i see a compelling reason to disagree with that, my gap is set to .039"
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Dallas


the recomended ngk plugs (the bprXey-11's, where the X is a number. probably 6 I think for stock) are gapped at 0.045" out of the box, and I usually throw them in as is on cars I service. not one has reported an ignition problem. if it cant jump an additional 6thou over stock, you may need a new coil.

too big of a gap and it will require more power out of the coil to spark it (potentially reducing the coils overall life) it will also create a hotter spark, keeping the plug itself more clean (assuming the heat range on your plug is correct) and unless you are misfiring, you WILL NOT see any performance/mpg improvements by readjusting your gap (unless it was way wrong from the beginning/the old ones were bad)

to little of a gap and it wont be strong enough to ignite the mixture and misfire. your coils charge time is controlled by the stock ecu, and from my understanding doesnt have control to vary dwell time based on engine rpm (msd boxes do, and work the coil harder in the lower rpms for a stronger spark) so its at a set amount, so when you demand more power out of it (larger gap) you arent really getting more power out of it at all. bigger gap + stronger coil might net you a little something, but it works the coil harder to discharge that power over a wider gap.

that said a stock coil should have no problems driving a 50thou gap, not that I would recommend it, but it may be a good test to see if your coil is up to snuff. that said, I run an aftermarket coil and cranked up the dwell time and run the biggest gap 23psi will allow me to run (which is under 25thou)
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Dallas


the engineers also designed it back in 1989
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Woodie
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starscream5000
Aug 12 2009, 09:55 AM
My plugs are currently gapped at .039" per the sticker under the hood. Does the car get better fuel economy with a larger gap or a smaller gap?
Unlikely you could show any repeatable results from testing either way, unless you're talking about .005 to .070 gap extremes. Used to be that you could tune engine performance by changing the gap, one would work better at high speeds and one would work better for starting. The fine print here is that ignition systems sucked and you were lucky if you could get 12,000 volts at the plug trying to light a mixture that was vaguely created by a carburetor to be somewhere near optimum. Now the shittiest car around provides a 40,000 volt spark and a computer controlled mixture much better matched to conditions.

Get some regular NGKs or Nippondensos, look at them for half a second to see if the gap is somewhere near 1 mm, put them in with anti-seize and a torque wrench, then change them at 30K miles as the book recommends.
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