Ignition

[jadney@vwtype3.org (Melissa & Jim)]

>From: Toby Erkson <Toby_Erkson@ccm2.hf.intel.com>

>Just to set things straight:  I have a COMPUTERIZED ignition system, not a 
>capacitive discharge system (CD).  There is a *very* big difference between 
>the two!

>As for the energy output, to quote Dr. Jacobs, "It's not that simple."  A 
>big ol' blast of electricity isn't always the answer.  Too much spark will 
>cause cavitation, which is when the spark punches through the fuel mixture 
>so hard that it actually pushes it away from the spark and it doesn't 
>ignite.  Too much spark will also reduce the life of a spark plug.  Also, 
>ignition requirements vary not only on RPM but on the density and 
>temperature of the fuel mixture as well as spark plug conditions (like a 
>fouled plug).  It's not that simple.

Toby,

Of course it's not that simple.  I don't know who "Dr Jacobs" is but if it 
is important to you, I can drag in the name of my PhD friend who specializes 
in automotive control systems on the mech eng faculty here at the U of WI.  
I discussed the problem with him when this issue came up on the newsgroup 
and he admitted he couldn't think of any reason why the stock Kettering 
spark could be considered second rate.  If your "computerized" ignition 
system allows you to program in a complex-function multi-dimensional advance 
curve that can take into account lots of engine parameters (air temp, head 
temp, RPM, load, etc) then of course you can improve on the simple-function 
two-dimensional (RPM, load) stock distributor advance curve.

If that was your point then I agree with you, but I do not agree with the 
contention made by some that there are ways of producing "better" kinds of 
spark that, even if delivered at the same timing, will magically produce 
more power, improved mpg, and less emissions.

In this discussion, I believe that it is important to break the issue down 
into two parts: timing and spark.  I am in complete agreement that there are 
large improvements to be had through improved means of timing--and lets make 
it clear that any advance mechanism is part of the timing.  On the other 
hand, the improvements to be had from changes in spark are much less 
dramatic: reduced maintenance, reduced plug wear, greater reliability, 
improved system stability (doesn't need to be periodically re-adjusted.)

In the end, however, all ignition systems will find it necessary to deliver 
a spark and about the only factors that are under any control of the system 
are the rise time and the duration.  A CD system is able to control these 
nicely in such a way to reduce plug wear, but one should not expect any 
increased performance unless you are comparing it to a Kettering system that 
is not in good repair.

>The stock system, of course, does provide plenty of energy and it does what 
>it has to well -- for stock I'd use nothing but Bosch.  But that energy 
>isn't fully utilized.  With my system my performance times have increased 
>slightly and so has my MPG.  I'm still in the process of data collection so 
>I don't have all the figures yet, but my MPG went from 14 city 17 highway 
>(stock ignition system) to 16 city 21 highway (computerized ignition, stock 
>wires, plugs not indexed).  Obvious advantage:  For a car that drinks 
>nothing but 92 octane the computerized ignition has already begun paying 
>for itself.

If this is the kind of gas mileage that you are getting then you have much 
bigger problems to deal with first.  If you choose to live within the 
emission limits that were in effect when your car was new then you should be 
getting on the order of 20-22 mpg.  If you are willing to ignore the 
emissions regs then 25-28 is attainable.  Of course your mileage will be 
expected to suffer during competition.

I understand that you have invested a great deal of money in your ignition 
system, and I also understand that it is human nature to defend and to come 
to believe in that which we have purchased.  To do otherwise would be to 
admit that we made a poor choice.  But you are treading here on the thin ice 
of wishfull thinking.

Jim
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       Melissa Kepner                                    Jim Adney
       jadney@vwtype3.org              jadney@vwtype3.org
                             Laura Kepner-Adney
                             Madison, Wisconsin
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