Platinum Plugs and Ignition Systems by Ray Paulk <raypaulk@kersur.net>
An ignition system is the entire package required to light off the fuel. The
most interdependent components are the plugs and the coil. They must be
considered as a system.
Compromises
The stock coil on most cars is a compromise between cost and performance.
They generally put out enough "juice" so the vehicle runs pretty good
with a warm engine running up to mid-range RPMs. As a secondary cost vs.
performance compromise, the factory usually installs steel-tip plugs with a
relatively narrow gap. The narrow gap compensates for the weak coil helping to
assure a spark at low coil outputs. With a cool engine or high RPMs, the coil
does not have the power to properly fire the steel tip plugs. Studies by Jacobs
Electronics have shown that stock engines generally mis-fire 3-5% of the time
when warm and up to 20% of the time when cold. These misfires both waste fuel
and potential power.
Basic Theory
When a spark plug ignites fuel, it starts a small fireball, known as a
"kernel", which propagates throughout the chamber. Studies have shown
that the ideal kernel size is .100 inches. With the same intensity of a spark,
the power output from an engine increases as the size of the kernel increases,
up to the .100". This translates into an ideal spark gap of .100".
Unfortunately, this would take a coil the size of the engine itself to provide
the required energy. Therefore, we must "play" with the gap vs. coil
power to obtain the best performance for the system. For a given coil, if we
start with a small gap, we only get a small spark. As we increase the gap, the
intensity of the spark also increases....up to a point where the coil is no
longer capable of generating enough voltage. For any combination of coil and
plug, there is an optimum gap setting for the best results.
Platinum and Other Specialty Plugs
The advent of Platinum plugs and other specialty plugs, is an evolution in
the search to find a plug which would fire more reliably than the normal steel
tip plugs at a wider gap with the lower voltage output of a stock coil. It was
discovered that by coating the electrode from which the spark emanates with
platinum, the spark would ignite at a lower voltage for the same gap. Therefore,
the gap can be wider than a steel tip plug for the same voltage. The wider gaps
mean more power and improved fuel economy without loosing the relative spark
reliability.
The compromise of the Platinum plugs is that they do fire at a lower voltage
which inherently means a less intense spark which does not ignite the fuel well.
Although the ignition kernel may be larger, the intensity is lower which makes
it propagate slower. You get less HP from the fuel consumed.
It is well known that steel tipped plugs produce the most intense spark.
Think of it as conductivity. The lower the conductivity, the lower the spark
voltage for a given gap. How conductive is steel vs. Platinum? Before Platinum,
I recall ads in Popular Mechanics about new and innovative copper core spark
plugs. These worked well for a short time but unfortunately the copper couldn't
withstand the spark and burned up fairly quickly. Coating steel with platinum
counters this effect. Ideally, you want a spark plug with low conductivity
combined with a coil large enough to fire it over a relatively wide gap. This
brings us to the "super coils".
Super Coils
One of the best improvements one can make to the performance of your engine
is to install an after market "super coil". You should get better fuel
economy, more power and a much cleaner running engine. On the down side,
stronger coils are bigger. The better ones, which really make a difference, no
longer fit in the stock location. But on a Fiero V6, look at the stock location.
Keep in mind that the worst enemy of a coil is heat! You want the coil in a
place where it can dissipate heat quickly. Setting right on top of the engine is
definitely not the best place. Actually, the best place in a Fiero engine
compartment is on the side near the air intake. I mounted mine on the left inner
fender wall just under the vacuum canister; it is cool and just a short jump to
the distributor.
The Fiero 2.8L V-6
The Fiero version of the 2.8L V6, with its HO heads, presents a unique twist
to the plug/coil balance. The HO heads "want" to ignite the fuel more
that the regular 2.8. Therefore, the "rules" which apply to normal
engines may not apply to the Fiero engine. The equation changes. For instance,
in a normal 2.8 a gap of .035 might be optimum for steel plugs and .045 might be
optimum for Platinum plugs. On the Fiero, these numbers might be .045 and .055
respectively. It may be such, if you happen to have a better stock coil, that
the Platinum plugs may never perform as good as the steel plugs. This is
unlikely, but possible.
My System, Experience and Opinion
I installed an entire Jacobs ignition system with an Ultra coil. I normally
run Autolite steel tip plugs but recently tried some RapidFires which are steel
tipped with a reduced area electrode. Since race car plugs generally have
electrodes filed to a point for better spark, the RapidFire tip seems like the
right approach. On the down side, the RapidFires have a preset gap of .045 with
distinct instructions not to reset. What I gained in the reduced tip I probably
lost in the narrow gap as I don't notice any change.
Initially, after installing the Jacobs system, I didn't see any improvement
in MPG. I could feel better acceleration, but it only clocked a gain of .04
seconds in 0-60 time runs. Over a course of 3 months, my time trials gradually
got shorter eventually knocking off more than 1/3 of a second. The idle improved
as well as my high RPM torque. Via discussions with Dr. Jacobs, I was advised
that my gradual time trial improvements were the result of the better ignition
burning away the deposits within the combustion chamber and exhaust system. The
engine was starting to run as designed. Recently I changed my O2 sensor and EGR
valve. Both were very clean with none of the normal soot. In my last state
inspection, the tester remarked that my car had the lowest emissions of any car
he has tested (and this is a place which has them lined up out the door.) Of
course he followed it up with "and this is a Fiero, they always fail!"
which may have influenced his first statement. My system gives me a better
performing, cleaner running engine. I haven't seen any improvement in fuel
economy, but that may be because the more power is more fun to play with. A good
ignition system can't compensate for a lead foot.
Bottom Line
Bottom line is that you must find out what plug works best with your coil. If
you're looking to upgrade your coil, I suggest Crane, MTD and Jacobs in that
order from better to best. In my opinion, the Accel coil is only a baby step
better than stock and not worth the expense. If you are going to make a change,
do something significant.
References
For further reading may I suggest "The Doctor's Step-By-Step Guide To
Optimizing Your Ignition" by Dr. Christopher Jacobs, Jacobs Electronics,
Midland, TX. Tel: (800) 627-8800.
The book put me to sleep several evenings, but it is thorough and gives one a
good understanding of ignition systems. I was fortunate enough to have several
long discussions with "the man" himself, Dr. Jacobs.
Ray Paulk
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