OK, you fellows with the electrical engineering degress have it all over me for
pure knowledge base - and I quote:
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From: dardal@xxxxxxxxxxxxxxx
Date: Fri, 26 Mar 2004 08:37:11 -0600
Subject: Re: IML: Jacobs Ignition
Reply-To: mailing-list@xxxxxxxxxxxxxxxx
Quoting W Bell <cbody67tx@xxxxxxxxxxxxx>:
> What he DID talk about (which I suspect dealt with his current products) was
> about misfire in engines as a matter of course. That every cylinder did not
> fire off completely or at all during every revolution. He mentioned that if
> the "computer" sensed that a "fire" did not take place, it would immediately
> "strike" that cylinder again. He also talked about doing some research for
> the auto manufacturers in that area too. That might be the secret to why you
> saw the gains you did?
How can the computer "sense" that the engine missed? And even more unlikely,
can the computer be fast enough to charge the coil and fire again within the
cycle? One possibility of the improved mileage on these truck cases with the
Jacobs ignition is that the new ignition had better suited advance curves for
these small cam engines. Or even more likely, the previous point igntition had
problematic vacum or centrifugal advance units.
>
> The misfires he talked about might be more correctly termed "marginal fires"
> than full misfires as such, possibly.
It turns out that there is no marginal fire. The mixture either ignites or it
doesn't. There can be a cycle to cycle variation caused by variations in the
ignition process (which is one of the reasons and engine may run rough even
though there is no missfire) but by about 60 degrees past TDC, pretty much all
the fuel is consumed (unless there is a missfire, in which case all the fuel
will remain unburnt). The roughness is caused by variations in the pressure
vs. crankangle due to the differences in the way the flame propagates and the
rate of combustion. This typically happens at lower rpm's, as at higher rpm
the turbulence ensures quick flame propagation, which is less sensitive to the
ignition process.
D^2
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When I first looked into the Jacobs sustem, I ws "sold" more by the quality of
the components than by any understanding of the internal working - the fact
that it did improve life with the vehicles I put it in was enough for me. BOTH
these trucks had stock Chrysler Corp electronic ignition distributors,
therefore it was a super easy install - just a few wires to tie in and a good
place to mount the controlling "computer" module. There is a sensor wire from
distributor side into the brain box, and a feedback/controlling (my assumption)
wire back to distibutor, so it's genuinely possible that there is a
speed/interval sensing capability inherent in the system.
I always figured the old dump truck had enough wear in all timing components
to cause it's idle roughness and lack of economy (if any dump truck could be
acused of being economical!), it was a surprise to me that the ignition changes
made as much difference as they did. The '77 pickup was in fine condition,
though, and other than it's affinity for high octane, I had no problems with it
- would love to know why spark improvement would result in less demand for the
hi-test gas.
I had been seriously considering swapping out Lucille's stock points
distributor for an electronic one, until reading this latest thread. I never
have been fond of setting up a set of points! If you don't get it JUST RIGHT,
the whole thing suffers. I have never had problems with the stock Chrysler
e-ignitions, something I can't say for other makers.
I did do a Pertronix conversion on a 1962 Buick Electra, back in about 1975 or
76, which ran trouble free and really improved the car's performance and
economy. Hadn't heard of them failing before - would be very interested to know
if the failures were on old installations?
Kate Justet Triplett
Kate's Custom Gunleather
Monroe, Washington
Proud mother of a soldier
and owner of "Lucille" 1968 Crown
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