:
: I read the other day about how AMC 258cid
: inline Sixes went to a spark plug for '79
: with an extra-extended set of electrodes.
: The theory, which
: is still stated in various spark plug
: science/technology literature in relation to
: such plugs, is that this moves the spark
: further down into the
: combustion chamber, which reduces mixture
: variance in the immediate vicinity of the
: electrodes by getting the spark AWAY from
: the quenchout area (which coincidentally
: reduces HC emissions). It also has the
: effect of making the flame front hit the
: piston fractionally sooner, which is said to
: have the positive effects of advancing the
: timing slightly without the detonation
: problems.
:
: The spark plug that does all this is Champion
: RN13LYC, Autolite 925 or NGK ZGR5. This is
: right smack in the middle of the two heat
: ranges generally used in Slant-6s with early
: ('60-'74 style) heads. All mechanical
: aspects of these plugs (seat, hex, thread
: size and length, etc.) is identical to the
: standard SL6 plugs.
:
: Now, my one remaining SL6 car is in storage in
: another country, so I can't readily try
: these plugs in it yet, but I did carefully
: put a set in my 2.5 litre LeBaron this
: afternoon. As some of you may know, the
: K-car 2.2/2.5 is based heavily on the 225. I
: confirmed "no piston-to-spark plug
: interference" by removing all the
: plugs, disabling the ignition, and
: installing ONE of the special plugs until
: the gasket seated, then unscrewing it a few
: turns, cranking the engine, removing and
: inspecting the plug. I did this a few times,
: putting it in further each time, until the
: plug was all the way in. No problem. Given
: the huge negative deck height of the SL6, I
: cannot imagine there being any interference.
:
: So far, so good...cold starting and running
: seem better and the idle seems smoother,
: compared to the month-old, perfectly
: good/clean standard-length plugs I had in
: there before. I'll be keeping a close eye on
: mileage; this may be an easy way to pick up
: a little efficiency and driveability (and
: maybe shave a few tenths?)
:
: DS

: I read the other day about how AMC 258cid
: inline Sixes went to a spark plug for '79
: with an extra-extended set of electrodes.
: The theory, which
: is still stated in various spark plug
: science/technology literature in relation to
: such plugs, is that this moves the spark
: further down into the
: combustion chamber, which reduces mixture
: variance in the immediate vicinity of the
: electrodes by getting the spark AWAY from
: the quenchout area (which coincidentally
: reduces HC emissions). It also has the
: effect of making the flame front hit the
: piston fractionally sooner, which is said to
: have the positive effects of advancing the
: timing slightly without the detonation
: problems.
:
: The spark plug that does all this is Champion
: RN13LYC, Autolite 925 or NGK ZGR5. This is
: right smack in the middle of the two heat
: ranges generally used in Slant-6s with early
: ('60-'74 style) heads. All mechanical
: aspects of these plugs (seat, hex, thread
: size and length, etc.) is identical to the
: standard SL6 plugs.
:
: Now, my one remaining SL6 car is in storage in
: another country, so I can't readily try
: these plugs in it yet, but I did carefully
: put a set in my 2.5 litre LeBaron this
: afternoon. As some of you may know, the
: K-car 2.2/2.5 is based heavily on the 225. I
: confirmed "no piston-to-spark plug
: interference" by removing all the
: plugs, disabling the ignition, and
: installing ONE of the special plugs until
: the gasket seated, then unscrewing it a few
: turns, cranking the engine, removing and
: inspecting the plug. I did this a few times,
: putting it in further each time, until the
: plug was all the way in. No problem. Given
: the huge negative deck height of the SL6, I
: cannot imagine there being any interference.
:
: So far, so good...cold starting and running
: seem better and the idle seems smoother,
: compared to the month-old, perfectly
: good/clean standard-length plugs I had in
: there before. I'll be keeping a close eye on
: mileage; this may be an easy way to pick up
: a little efficiency and driveability (and
: maybe shave a few tenths?)
:
: DS

: Are these plugs for the later small plug heads,
: or the early heads with the spark plug
: tubes?

: Eric-
:
: This info was in my original post. They're for
: early-style heads. There may be an
: extended-electrode plug that would fit the
: later-style head; I have not looked.
:
: -DS

: Oh. Well, I have a '73 engine in my Satellite,
: so that will work for me! I always used
: Autolite 56's, so I dont know much about
: spark plug numbers. Are these plugs you
: mentioned a good heat range for a 50-50 mix
: of in town/freeway driving?

: I like Autolites, myself, too. They are no
: longer possible to obtain in Canada (that
: I've found in the Toronto area) so I had to
: settle for Champions.
:
: The Autolite 56 is a good plug. The
: harder-to-find 4256 is better (the 42xx
: series has a necked-down centre electrode,
: which provides more mixture access to the
: spark) and the special-order MP4256DPE
: (which is used on some Harley-Davidson
: motorcycles) has the necked-down centre
: electrode AND a cut-back side electrode,
: which still further improves mixture access
: to the spark. These are all variants of the
: basic #56 plug, they all have the same heat
: range and mechanical characteristics, just
: with different electrodes.
:
: The long-electrode plug (Autolite 925) has a
: heat range that is between number 55 and
: number 56, so should suit just fine. Because
: I can't get Autolites, I can't run a
: comparison between the MP4256DPE and the
: 925, but after 150 miles today, my 2.5 is
: running fine on the long-electrode plugs and
: seems to have a bit more pulling power. I'm
: keeping an eye on mileage.
:
: Most of the "gimmick" plugs
: ("Splitfire" etc.) are a waste of
: money, and I've had poor results with Bosch
: Platinums. Plugs of essentially standard
: construction from Autolite, NGK, Champion or
: AC are the ones I stick with, but as you can
: see, within that constraint there are LOTS
: of different variants to try out!
:
: DS

: I agree with you on the "gimmick"
: plugs Dan. I got some platnum tip plugs
: (Autolite's) and could honestly tell no
: difference. I just thought that maybe they
: would last longer. I am going to get an
: Accel Super Stock coil tomorrow, so I think
: I will get some of the plugs you recommend
: while I'm at it. As always, thanks for the
: advice Dan!
: Eric W