Fordnatics List Archive

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Mail From: Bryan E Berndt <(email redacted)>


I have one more detail to give all of you who have helped me out
with my bad milage in my TC. Towards the end of the tank that measured
such an offending mpg figure, I put in new spark plugs and wires. Now
with the use of ascii art for the first time I will atempt to recreate
the condition of the old plugs, and maybe you can come up with the same
conclusion I did.


____
| | <--ground electrode
| _
|--| \\-- <--bad pos. electrode
| |
-------
| |
| |
If you can see the meaning in the picture,the positive electrode was worn
so bad that it had a real good slope to one side like someone chopped off
a corner. Also the gap was .055--.062 in. It is supposed to be
.032-.036 in. I would imagine that these were the original Motorcraft
plugs, and their time had definatly come. This certainly didnt help my
milage!

******************************************************************************
=-->GOOCH<--=
'88 T-bird T forTURBO!
* *Rockford Fosgate*Coustic*Sony*Jensen*
'93 Trek 9000 ->BOING!!
(email redacted)

******************************************************************************

This read-only message was archived from a public mail list.
Mail From: (email redacted) (Jay Fletcher)

Bryan Berndt wrote (and drew):
____
| | <--ground electrode
| _
|--| \\-- <--bad pos. electrode
| |
-------
| |
| |
If you can see the meaning in the picture,the positive electrode was worn
so bad that it had a real good slope to one side like someone chopped off
a corner. Also the gap was .055--.062 in. It is supposed to be
.032-.036 in. I would imagine that these were the original Motorcraft
plugs, and their time had definatly come. This certainly didnt help my
milage!

The thing that is really hurting you here is probably a "rounded" center
electrode, not the increased gap. Sparks like to start from places where
the electric field gradient is large. The field lines generally follow the
surface of the conductor. Therefore, where they bend around sharp corners
or points are likely places for sparks to originate.

When the electrode wears, it loses material from the places where sparks
most often originate, the edges of the center electrode. This rounds it
out and makes fewer nice juicy spots for sparks to start. The smooth
electrode, IN COMBINATION WITH a larger gap make spark generation less
likely (and misfire more likely). More misfires mean lower efficiency,
less power, and poor milage.

A larger gap by itself is a good thing (if your ignition is up to snuff,
as most modern systems are). A larger gap increases the probability that
a spark will occur in a part of the chamber where a readily combustible
part of the mixture exists. This reduces misfires, and therefore increases
efficiency. I know of one list member who runs a 0.080 gap in his 5.0
Mustang without any problems. He would be well advised to keep his
electrodes sharp however, either by replacement or filing.

As a side note, multiple spark systems also achieve higher efficiency
by increasing the probability of ignition, although they do it
temporaly, rather than spacialy. If you keep sparking long enough,
chances are that a combustible portion of the mixture will happen through
the gap and be ignited.

Copyright 1994. Jay W. Fletcher