OK, you fellows with the electrical engineering degress have it all over me for pure knowledge base - and I quote: ******************** 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 |****************************** 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