Re: oilheads-digest V1 #47

[BAnder6125@xxxxxxx]

In a message dated 1/4/04 8:34:28 AM, bill@xxxxxxxxx writes:


> I tend to think the holed piston is more likely to be caused by the longer
> Autolite spark plugs, which I would imagine place the spark closer to the
> piston crown.  Yes?  No?
>
Saab actually made an engine that used a little button on the piston crown as
a ground electrode to a special plug that had only a center electrode!
Holed pistons usually are caused by pre-ignition which can be caused by a
plug running too hot. Advanced timing and an air cooled engine would
exacerbate
that condition. Viewing the center electrode under magnification and seeing it
look like it has been peened with a hammer is a sign to go to a cooler plug
right now! Autolite plugs IMO are rather junky and there are better
alternatives.
A secret for smooth running with a lean burning engine is to get the plug
spark to contact as much of the tumbling incoming fuel mixture as possible.
The
autolites place the spark better than the two electrode OEM plug and that
helps.
Dyno tests show that HP increases with plug gap size up to the point where
the ignition system can't reliably ionize the gap. Two ways to get a wider gap
with the 1100's decidedly weak ignition system: 1) I use Bosch 4417 plugs.
They
use a surface discharge center electrode insulator and the ignition system
'sees' the actual 50 thousandths gap as much smaller. Place a set in and
you'll
get at least a 10% increase in idle speed; then try a 5th gear roll on up to ?
with your friends.   2) Many modern engines are using iridium tipped plugs.
Wire electrodes concentrate the charge better and will ionize larger gaps, and
iridium electrodes can be made significantly smaller in diameter than
platinum. NGK and Nippondenso make them, but I don't have a clue if any fit
the 1100;
I'd guess you could run a 40 thousandth gap with them on stock ignition if you
could fit them.
My Toyota MR 2 Spyder uses iridium plugs with coil on plug ignition, DOHC
with bucket and shim valve adjustment and the 1st check  of valve clearance is
at
60K mi. Don't understand why BMW can't be a little more up to date.
Get your Google going and you will find lots of info on spark plugs. Be very
careful and always err on the side of a cooler plug!
Bruce   '94 R1100RS

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