The Narrow-gap Spark Plug Trick...

[Steady Eddie]

Does narrowing the spark plug gap to 45-thousandths work OK with an OE (no tuning)
ECM/PCM??

As a trial, I narrowed-up the gap on a fresh
set of 41-993 AC Delco Platinums, and stuck them in. I was really, really shocked at how bad the original 41-932s were worn..many were
greater than 100-thou at the gap. This was at
about 60K miles..no codes set for mis-firing,
it started right up, nothing to point to bad
plugs or any issues.

I had found out about the ECM's in-ability to
"see" a high-RPM plug mis-fire, which occurs
most often "on-the-grade" at RPMs higher than peak torque. The ECM--OBD2 brain-box will not
throw a code or light up the SES light under
these conditions because it does not have the
"resolution" to see these rapid events. It is blind to them, and some have blamed the resulting mis-fires to valve spring float, it is not.

The narrow-er gap solves this problem. As a part of this test, I put on a new set of NAPA
Max Plug wires, the 8.1L set, as they are a tad
longer than the L21 Vortec wires and curve over the valve covers and dip-stick tube without touching.

Does it all work?? You bet!! Thanks, Mike....

[Steady Eddie]

Does narrowing the spark plug gap to 45-thousandths work OK with an OE (no tuning)
ECM/PCM??

As a trial, I narrowed-up the gap on a fresh
set of 41-993 AC Delco Platinums, and stuck them in. I was really, really shocked at how bad the original 41-932s were worn..many were
greater than 100-thou at the gap. This was at
about 60K miles..no codes set for mis-firing,
it started right up, nothing to point to bad
plugs or any issues.

I had found out about the ECM's in-ability to
"see" a high-RPM plug mis-fire, which occurs
most often "on-the-grade" at RPMs higher than peak torque. The ECM--OBD2 brain-box will not
throw a code or light up the SES light under
these conditions because it does not have the
"resolution" to see these rapid events. It is blind to them, and some have blamed the resulting mis-fires to valve spring float, it is not.

The narrow-er gap solves this problem. As a part of this test, I put on a new set of NAPA
Max Plug wires, the 8.1L set, as they are a tad
longer than the L21 Vortec wires and curve over the valve covers and dip-stick tube without touching.

Does it all work?? You bet!! Thanks, Mike....

[Tom N]

I would like to know how narrowing the spark gap affects emissions.

-Tom

[Steady Eddie]

Tom--

A good question..in my case, I have two things
going for me: #1--the P12 is a Class 6 MDT, and as such, the emmissions rules are a bit relaxed.
#2--In my state and area (county) there are
no annual smog checks.

That said, your engine is still going to run in
"closed loop" control via the O2 sensors, so
emmission-wise everthing should be OK.

[Oemtech]

If my memory serves me correctly... from my racing days and DIY work. A wider gap is more for leaner AFR and it takes more AMPS to produce the spark. So, if you have high resistance wires and a wide gap you get a poor spark. With low resistance wires and a narrow gap you get a better spark. This works to a point.... to much of anything usually turns out to BAD.

One last trick is to index your plugs.

[Steady Eddie]

Dale wrote:
"...One last trick is to index your plugs..."
==============================
I agree. But here's what AC Delco said when I asked them...and a big reason to stay with
OEM AC Delco Plugs...

The OE spark plugs have no gasket. They fit into the heads with a chamfered hole. The plugs
are supposedly indexed right out of the box.
*IF* you stick with the GM Service Manual's
14 ft. lbs. of torque spec, the twist required
into a "clean" head-spark-plug-hole will bring the electrodes right around to a near perfect
index in relationship to the combustion chamber
and the valves. I mean, they told me, the indexing is with-in degrees of being spot on.
Not so with any-old-brand of plug and not so in
engines that have a "crush" gasket under the
spark plugs. THOSE you index with different thicknesses of gaskets/washers....

[Oemtech]

Hummm.... Learn something everyday. I will check the next time I am installing a set of plugs.

[Full-Timers]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Steady Eddie:
Does it all work?? You bet!! Thanks, Mike.... </div></BLOCKQUOTE>Please define "Does it work?"� My stock setup works too.

[Full-Timers]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Steady Eddie:
Dale wrote:
"...One last trick is to index your plugs..."
==============================
I agree. But here's what AC Delco said when I asked them...and a big reason to stay with
OEM AC Delco Plugs...

The OE spark plugs have no gasket. They fit into the heads with a chamfered hole. The plugs
are supposedly indexed right out of the box.
*IF* you stick with the GM Service Manual's
14 ft. lbs. of torque spec, the twist required
into a "clean" head-spark-plug-hole will bring the electrodes right around to a near perfect
index in relationship to the combustion chamber
and the valves. I mean, they told me, the indexing is with-in degrees of being spot on.
Not so with any-old-brand of plug and not so in
engines that have a "crush" gasket under the
spark plugs. THOSE you index with different thicknesses of gaskets/washers.... </div></BLOCKQUOTE>Hum... Interesting. Could be true but I doubt they would go to all that trouble to index the plugs. I could be wrong. If I ever pull my plugs I will mark them first and see if the marks on all plugs end up in the same place.

[Steady Eddie]

As posted by Full-Timers:
"...Interesting. Could be true but I doubt they would go to all that trouble to index the plugs. I could be wrong. If I ever pull my plugs I will mark them first and see if the marks on all plugs end up in the same place..."
====================
Your comment is probably right. Moroso Performance makes a "kit" that consists of
soft copper indexing gaskets for the 14mm
tapered seat spark plugs. They are in 3 sizes:
.060,.080, and, .100 thick. I just did the plug
change and I should have felt-pen marked mine, (don't use a pencil!)
but I was remembering the AC Delco comments and
just tightened them in there with my torque wrench..click, click on each one....

[Ultra RV Products]

Dang Eddie!

Are you trying to get the award for the most post's in a week!!!

[Steady Eddie]

Mike wrote:
"...Dang Eddie!
Are you trying to get the award for the most post's in a week!!!.."
===================

Mike---

I'm about all spawned out..just one old retired
guy wrenching in his driveway, all alone, and
with minimal tools and money, trying to figure
out the ins and outs of the GM OBD2 and why this
huge Coach ran so poorly...now I believe it was
running all this time in some amount of spark retard...I'm the kind of guy who sits on the toilet and actually reads the Service Manuals...
my next post was going to be on the subject of
the PE (power enrichment) circuit and the timer
that lives in there as compared to the PE stuff
in the LT1 "hot rod" motor...but I think I'll
go take an enema now....or drink some prune juice, or something..

[Oemtech]

Steady,

That post gives a whole new meaning to a "morning sit down".

[TandW]

It has not been my experience that plugs are manufactured pre-indexed. Not any brand with single welded looped ground electrodes. Usually you can do an index of all plugs, without add-on washers if you have two sets to work with. I find it more accurate to file a 45 degree mark on the plug tip inline with the electrode gap. I also don't like the idea of putting "anything" on the porcelein insulator. Lastly, I overgap the plugs by .005". I check my plugs yearly, at least, to acertain firing conditions within the cylinders. This has been my method for years. Good luck!!