[DeTomaso] WARNING: .050 may be the wrong kind of Bomb

Mon May 25 10:59:41 EDT 2009

You want the plug to be hot enough to keep it clean, and not hotter.  Too
cold, and it fouls.  Too hot, and it acts like a glow plug for pre-ignition.

An article I read years ago said that Chevy experimented with plug gaps as
wide as 0.080.  It said they got better performance the wider the gap.  The
negative was the wider the gap, the shorter the plug life.  My understanding
was the wider gaps knocked larger divots in the electrode each time they
fired.  They concluded that 0.035-ish was a reasonable tradeoff between
performance and plug life.

The same article said that there was a tradeoff between good performance and
plug life with respect to the hardness of the plug - i.e. the materials used
for the electrodes.  My understanding was that it wasn't the hardness that
made the plugs work well, but rather the conductivity of the metals - i.e.
great conductors were typically softer and thus wore faster.

It said that:

1.  Ford plugs were very good conductors (great performance) while also
being very hard (great wear).  It was unusual at the time for a magazine to
rate a Ford product higher than Chevy.  The article was written when
everything was pro-Chevy.
2.  AC/Delco plugs were almost as good conductors and were also almost as
hard.
3.  Champion's had the best performance, but were very soft, and were not
recommended for street driven cars.

I don't remember what they said about Bosch.  It was before the split fire
craze.

FYI - It only takes a little bit of oil sucked into the cylinder to cause
pre-ignition.  I had removed the bolt between #7 and #8 to attach a throttle
return spring.  When I replaced the bolt, I didn't get it torqued down
enough.  I kept hearing pinging when I didn't expect it.  Naturally, I
reduced the advance, ran better gas, and so on.  Later, I pulled the intake
and noticed a trail of oil mist in the #7 intake port.  I made sure I
tightened the intake this time, and the pinging went away.  Sometime later,
I pulled the head and noticed little dings all over the #7 piston.

-----Original Message-----
From: detomaso-bounces at realbig.com [mailto:detomaso-bounces at realbig.com] On
Behalf Of pantdino at aol.com
Sent: Sunday, May 24, 2009 2:35 PM
To: detomaso at realbig.com
Subject: Re: [DeTomaso] WARNING: .050 may be the wrong kind of Bomb

My understanding is that a too-hot plug will get superheated at the 
tip, which will possibly cause the ceramic to fail.  A very hot tip 
could then also serve as a source for detonation.  The shock wave from 
that will damage the metal of the plug.

But why do some plugs look fried and damaged from detonation and others 
look good or fouled?
My understanding is that a lean mixture will cause detonation and 
piston-top damage and will make a hole in the piston if it continues 
long enough.

So it sounds to me like your intake manifold is not delivering equal 
mixtures to the cylinders.
The fried plugs didn't look that way after mixed driving because when 
you accelerate the accel pump dumps in some more gas to richen the 
mixture.  But after 400 miles of 3500rpm on the carb cruise circuit the 
lean conditions took their toll.

But Jack knows 1000 times more than I do.

Jim Oddie

-----Original Message-----
From: Julian Kift <julian_kift at hotmail.com>
To: dfcex at excaliburre.com; rimov at charter.net
Cc: De Tomaso List <detomaso at realbig.com>
Sent: Sun, 24 May 2009 12:02 pm
Subject: Re: [DeTomaso] WARNING: .050 may be the wrong kind of Bomb

The plug electrode damage may be self destruction due to 
heat/detonation. Too
hot a plug in some cases will create detonation, especially if you are 
on the
verge of being there anyway. FWIW I run the Autolite AR
-32 plug, which 
as I
undestand it is one of the colder plugs Autolite manufacture.

Julian
> From: dfcex at pacbell.net
> To: rimov at charter.net
> Date: Sun, 24 May 2009 08:14:11 -0700
> CC: detomaso at realbig.com
> Subject: Re: [DeTomaso] WARNING: .050 may be the wrong kind of Bomb
>
> Paul, very high dome pistons.
> This is why I had to go with large cc heads, to keep the compression 
around
> 10.5:1.
> The valves are brand new Ferera's, not cheap.
> But I'm not sure the damage to the plugs was caused by contact. On #6 
the
> tip is still intact, yet the electrode is eroded aprox. 1/4" below / 
inside
> the threaded case. On #5 the tip is 1/2 intact, and the electrode is 
about
> 1/3" down. I don't see how the electrode could have been impacted 
while
> being "blocked" by the tip. The tips don't look bent, either.
> If the bits and pieces are already gone, I'll proceed to check the
> compression and leak-down.
> Assuming it checks out I'll add a couple more rings and go with .045, 
then
> take it to the Dyno Shop to check the advance, etc. I was thinking 
about
> having it dynod anyway.
>
> Dan
> Dan F. Courtney
>
> La Jolla, CA
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