Sorry to all for dragging out this thread as I get the digest version and
only get to check it every other day or so. :)
WHAT: Not to single Bob out but he did bring up a common misconception.
My advice--If you want better performance put in a sport coil and leave
the plugs exactly the same.
WHY: I was once taught the rule-of-thumb and knew that I would never
have to remember it unless I was designing my own engine and electrical
system from scratch. What the spark gap rule-of-thumb really tells you
is the average energy needed to comfortably jump a average gap.
The main reason for replacing a standard coil for a 'Sport' version is
for better performance. If you increase the gap along with the voltage
then the spark remains at the same strength and temperature--you are just
throwing your money away. Because the spark from the coil is created by
the collapsing field in the primary of the coil, the field has to be
reestablished before it can be collapsed again. At low RPMs both the
standard and sport coils perform virtually the same. The difference is
not measurable. The real difference comes from running at higher RPMs.
While a standard coil has a harder time making the needed voltage for a
quality spark--the sport coil has more than enough and some to spare.
This helps to keep correct timing, less misses and constant power at
higher RPMs.
Note: Most manufactures make long reach and extended gap plugs. These
are usually for heavy duty applications where oil fouling is the norm.
These plug often operate with a gap from .060 to .080 or more utilizing a
standard coil. Putting one of these plugs into an application that does
not call for it is asking for trouble and is NEVER advised.
>Date: Tue, 08 Jul 1997 07:24:06 EDT
>From: mgbob@juno.com (ROBERT G. HOWARD)
>Subject: Re: Reading Spark Plugs
>
>Hi Rick,
> Addressing the Sports Coil idea, at the GOF Newton sports coils were
>mentioned. In the discussion, it was stated that a rule-of-thumb for
>spark plug gap is .001" per M volts. Since the standard Lucas coil
>delivers about 22-25M volts, depending on incoming voltage, the spark
>plugs should be gapped at .025. The Lucas sports coil delivers about
>40M volts, so the plugs should be gapped at .040 to utilise the extra
>power. This was new to me, but it was then explained that Champion and
>NGK make plugs called "large gap" or something similar, that are
>especially configured for the extra gap and extra voltage. One might
>wish to check clearance between plug and piston if compression has been
>increased.
>Bob
John Knight '73 MGB (under restoration)
"If uniqueness makes us who we are, why be like everyone else?"
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