I'll try to address Larry's questions ...
WRT what are CDI, what are Inductive...
Most newer ignitions are inductive. Some of the early MSDs were CDI-based,
but I believe the later ones are now inductive multiple spark, or at least
provide energies comparable to inductive systems. The early Magneti
Marelli's were CD's, as were the old Delta Mk 10's (or something like that).
Most everything else is inductive. Anything saying CDI on it is not
inductive.
Most of the breakerless ignition conversions (using either hall-effect,
magnetic, or optical sensors (like the Perlux Ignitors, or Hayes, or Accel)
are inductive storage. All a breakerless inductive storage unit is doing is
replacing the points with a transistor switch to turn the current on and off
to the ignition coil. The points used to do this with the cam and rubbing
block, and the condensor was there to minimize the arcing and erosion on the
contacts when you interrupted the circuit. The condensor is now inside the
electronic units. The other function they provide is computation of the
dwell (the amount of time the coil is ON and current is flowing). It takes
an appreciable amount of current and time to "saturate" the transformer
inside the coil. The longer you run the current, the more energy available
to make a spark. As the rpm's increase, the time avaialable to do this gets
quite short, as you still want to have a spark for at least 1 msec (on an 8
cyl engine at 7,000 rpm, for example). So, you need to turn the coil back on
as soon as possible to reach maximum current. At very high rpms, you don't
reach maximum, and thus the energy decreases with increasing rpm.
You're correct on the basic assumption that the ignition doesn't really add
any power to the engine. If you get the mixture lit properly, and it burns
completely, then you're making maximum power.
But its not always that clear cut. Over the entire rpm and load range, the
demands of the engine, and the fuel/air mixture flow patterns are very
complex. Many SAE papers have been written on the subject, and its still
hotly debated today. The latest technology is optical sensors that can
actually "look" at combustion taking place in the cylinder and adjust the
fuel injection and timing accordingly to manage the combustion process.
Usually, lean makes power, but as you know, when its too lean, you detonate,
and burn holes in important parts! So, keeping combustion at the right level
is critical. The ignition is part of what does this. It lights it
initially, and may relight it again, or several times before the process is
complete in that cylinder, and the piston has reached bottom. The flow of
buring plasma is a science in itself...so the simple ignition has quickly
elevated into some complex thermodynamic physicss issues!
It's safe to say a high energy spark has a very good chance of really
agressively lighting off your mixture, where a wimpy or short spark could
light it, and then have it go out again (what happens when you have a fouled
plug). This is usually noticeable. CD ignitions used to be popular on older
engines that had oiling problems. The CD could fire through the oil, but as
engines ran leaner and leaner, the CD's very short spark (about 1/10 the time
of an inductive one) was just not enough to keep the fires lit.
Personally, I run an old Homer Howard multiple spark inductive storage
igniton. Its breakerless with a hall effect trigger, and has proved very
effective. Homer's no longer with us, having passed away several years ago,
and no one took over his business that I'm aware of. The real benefit of
breakerless ignitions for racing, is that they eliminate constant adjustment.
Just set the distributor and forget it. For this, any of the aftermarket
inductive breakerless systems should be fine, as long as they prove reliable
in your particular applicaiton's environment.
One other point to mention is ignition timing. When you replace points and
condensors with breakerless ignitions, you have a high chance of changing
your advance curve.
First, the points rubbing on the cam used to load the mechanical advance
mechanisms. When you remove that mechanical load, the advance mechanisms may
not advance the same, and/or "bounce" at low (idle) rpms, causing problems.
If you use a magnetic reluctance type sensor (the coil and magnet type, NOT
hall effect) you also introduce a characteristic retard at higher rpms. This
is an electrical effect of the inductance of the sensor's coil. It cannot
provide a straight line advance curve like the points did, and will always
have less total advance than the mechanical mechanism offered. You can only
compensate for this by introducing greater initial advance, and/or completely
recalibrating the mechanical advance mechanisms for the particular sensor on
a distributor machine.
Regards,
Myles H. Kitchen
1965 Lotus Cortina Mk1 #128
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