Its Friday afternoon and all of my coworkers are already sick of me for the
week. I can't go home yet so I might as well torture those poor souls on
the MG mailing list. As if ownign an LBC wasn't enough!
To continue the combustion discussion. According to basic theory, anything
that ignites the fuel air mixture at the right time should be sufficient.
So we should all steal the inductive ignition out of our Lawnboys and graft
them onto our LBC's since they are fairly reliable and obviously good
enough. There is no need to spend 300.00 for the latest multi-spark
capacitive discharge computer controlled combustion sensing never needs
ironing electronic ignition and bottle washer.
So why do so many racers buy these devices other than the obvoius benifits
of prestige and sponsership money? The answer is not complicated but it is
rather long winded, which, given the low level of activity in the office
this afternoon, suits me just fine.
Combustion and flame travel are statistical phenomenon. Notice that, when
your car knocks, it usually doesn't knock constantly. The fuel air mixture
is not homogenous- it has lean and rich areas that are harder to ignite and
burn at differant speeds. This causes the ignition delay period- the time
between the spark and the start of the flame front- to vary. Since, to
prevent detonation, you have to tune for the worst case (shortest) delay,
most of your combustion processes will start later than optimal.
A "hotter" ignition can sustain a larger spark across a wider gap over a
longer period of time. This means that the spark will contact more of the
fuel air mixture which is moving pretty rapidly past the spark plug. By
the law of averages, the more of the mixture that is exposed to the spark,
the more similar the mixture will be from combustion cycle to combustin
cycle. This will reduce the variability in ignition delay and allow a
person to tune the ignition to ignite the "average" mixture closer to the
optimal time.
A less common component to the advantage of "hotter" ignitions is the
elimination of non combustion cycles. This is particularly important at
low engine speed where fuel mixing and atomization is less complete. So
better ignition could make your car idle more smoothly.
Another reason that race cars use ignitions that put out more power than my
car is that, as the fuel air mixture gets more dense, it take more voltage
to cause the spark to jump the plug gap. Also, in race engines, they run a
lot of excess fuel which can aggarvate the non combustion issue. The real
benifit of multi spark ignitions, in my opinion, is at idle and in real
rich or lean mixtures, where a single spark may find an ignitable portion
of the fuel air mixture.
How much does this help? I believe that the highest claimed horsepower
increase for an ignition system was in the area of 2% to 3% This would be
about 1 horsepower at the wheels for my MGA. For the 175.00 they ask for a
typical low end ignition system, this is not a good return on investment in
my book. They do look cool, however- all those brightly colored boxes and
cool decals.
I would recomend that, if you must spend money on performance, you would be
better off renting dyno time and doing a really good tune up. If your LBC
is functioning properly, it won't benifit that much from a fancy ignition.
If it does benifit, it is because you either have one highly modified
machine or it is masking some other fault in tuning.
Concerning comments on my last post: I appreciate the encouragement
although others that read this may not. I am not sure whether older
gasoline would have higher octane than newer gasoline. It depends whether
the volatile components lost were high octane or low octane. I do know
that engine oil is very low octane. Concerning dual plugs on a Japanese
engine: the Japanese are very agressive in improving combustion chamber
design and have always (contrary to popular belief that they are copiers)
been willing to take more risks from a technical standpoint.
Regards:
Bill Eastman
61 MGA (in storage for the winter, which probably explains why I am writing
these boring posts!)
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