Ferrari Mailing List: Re: Spark plug reading?

[Rick Lindsay (rolindsay]

Hey Peter,
   Everyone and his brother will have opinions on this
issue but I'll add mine.

   First of all, a plug-cut should only be done (read:
only be interpreted) from plugs that have not been
subjected to recent idle.  That is, the technique
says: Go for a spirited drive and at the last second
of that drive, cut the engine WITHOUT allowing it to
idle - not even rolling into the garage.  The carbon
build-up comes from idleing and that can preclude an
honest read of the plug.  This site has a good
write-up on the idea:

http://www.centuryperformance.com/spark.asp

   Don't forget to go to page 2 where there are more
pictures.

   Your picture shows an insulator that is about the
right color, perhaps a little too cool (read: it isn't
staying hot enough to burn off deposits).  The ground
electrode color isn't too bad either.  Still, I would
expect the whole thing to be a dusty tan color if
everything was balanced properly.  The black carbon
around the base of the plug is not too much of an
issue with me.  It says that you may be running a
little rich.  Still, a little rich 11:1 to 12:1 is a
more powerful charge mixture than the perfect 14.7:1
ratio.  We must also remember that a leaner mixture
burns hotter.  Conversly, rich mixtures burn cooler. 
Therefore, air-fuel ratio or 'mixture' must be matched
to the plug heat range.  
   Then there's compression ratio.  That too changes
burn rates.  Higher pressure charge is harder to
ignite therefore, a stronger ignition is required. 
Ferrari ignitions are arc welders so if you're running
a stock ignition, you're probebly strong enough
(assuming it is in good health).

   Quick question: Do all of the plugs look similar or
is one bank different; or one plug different?

   Okay, that's just some stuff to ponder.  From this
and someone else's post with the 'right' answer,
you'll get the engine dialed in.

regards,

rick
'79 308GTB
--- Peter Pless <ferrarilist [at] pless.com.au> wrote:

> Does anyone know what this means?
> 
> http://www.pless.com.au/308plug.jpg
> 
> It's from my newly rebuilt 308GT4 engine. 10:1
> compression running on
> unleaded fuel.
> 
> Programmable ignition gives it lots of advance.
> 
> When the plug was pulled, I'd come back from quite a
> long drive, but the
> last bit of it consisted of traffic and plenty  of
> idle speed stuff.
> 
> What's with the carbon on it already? Engine's only
> done 600 miles....and
> I've been nursing it.
> 
> TIA
> 
> Peter
> 
>
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