by Tuner » Tue Apr 15, 2008 1:26 am
Some small diameter 8 cylinder TPI distributors have a module that was also used in the Delco Voyager marine ignitions that has a built in advance curve that makes some sense for a hot rod motor. It’s the “limp-home” mode for if the ECU fails and can’t supply the timing control function. Unfortunately, it also has a 6000 RPM rev limiter in it, an actual rev-limiter, not a weakness in design, or whatever. There are some other modules than the marine unit that don’t have a rev limiter and do have a good advance curve and an interesting retard function when triggered with the jumper wire used to set the timing on the marine applications. The wire is a GM part that you plug into the module and connect to 12V to cause the advance function to zero out so you can set the initial without the timing wandering around with engine speed changes while you’re trying to time it (like un-plugging the tan wire on the ECU applications). The timing jumper is only to be connected and disconnected when the engine is running in this case, on ECM applications the tan wire is only disconnected and connected when the engine is off, they don’t go to the same pin on the module. I’m sorry, I don’t know what the applications were, I was just playing with a pile of junkyard TPI distributors with my distributor machine and it was a decade ago. I do remember the OE GM modules were the only ones that had a reasonable advance curve in them. The aftermarket units I checked (including some ‘brand name’ hotties) were just a retard for starting and a sudden switch to more advance at 2500 or so and then retarding after that – junk. I think chances are any one you might pick up could have been manufactured in any one of many off-shore locations and be different in these respects. I think the GM ones will have the characteristics of the original specs for their part number.
All electronic ignitions have the retard with RPM characteristic except for those which trigger off the preceding cylinder and then compensate in calculating the timing like a 7531 MSD or a programmable EFI. All electronics have a “slew rate”, a fixed amount of time it takes for the input signal (the trigger) in to be switched through the circuit in the amplifier and become output (the spark). The time required for the system to fire is constant, for example the time of 1 degree at 1000 RPM, and the crank turns 8 times further (gets closer to TDC) at 8000 RPM in the same amount of time. The timing light is doing this too. All electronic components have their own fixed slew rate characteristic. “Rise time” is the familiar term used to describe the voltage change in a coil circuit, such as when comparing CD to induction systems. Strangely, some of the “high-output” modules have more retard than the GM module, they may not use a “zero cross” type detector circuit.
The genuine GM module (D1906) has nearly the same retard characteristic as most common CD boxes. If you run either one with a locked advance you will have very nearly the same amount of retard over the RPM range.
You guys who never had the pleasure of using a points ignition to operate racing engines are at a real disadvantage for the not having had the experience. The CD boxes make it so a sloppy carb tuneup that’s way too rich can still be ignited and the engine run more or less OK in spite of spark plugs fouled so bad they wouldn’t run at all with a points induction ignition. In the good old daze you had to get the carb a lot closer than people get away with now because a CD will ‘bandage’ it.
Right from the get-go the HEI has always been capable of at least 5 times the energy delivered to the plug gap of points and twice that of common CD systems. The first couple of years there was trouble with module failures because of high fly-back voltage caused by open plug wires (that also killed the IC in the alternator’s internal voltage regulator - there’s some trivia for you – the fly-back pulse is as much as 400V in the coil primary and so all the wiring in the car connected to the + post of the battery). The energy of a healthy system has always been about the same with the stock parts. Granted, the CD systems have the fast rise time that fires fouled plugs but they don’t deliver as much total energy to the gap as an HEI. The CD does deliver it in a form that’s more beneficial for a sloppy carb tuneup. The HEI is better for firing lean economy and emissions mixtures because of the long arc duration.
The articles in Rot Hod magazines were (and still are) written to please the advertisers who pay the magazine staff’s wages and supply them with “benefits”. (Would you have it any other way?) The bad rap HEI’s got (only 4500 RPM, etc.) was because “unbiased” dyno testing was done with the “manufacturer’s recommended plug gap” which was at least .060” for most GM cars and .090” on some Oldsmobile applications. If you run the HEI with gaps that make sense for the type of ignition it is (induction) and feed it properly (volts and amps) it will easily turn more RPM than most 8 cylinder pushrod race engines. No one thinks it’s unusual to use .018” or .020” with a magneto, even a MSD Pro Mag 44, why should it be unusual to use a .025” gap with the HEI? We had to run points (induction) systems that way to make a high compression - high RPM engine run well and didn’t think it was unusual until the CD revolution in the 70’s and the “my gap is bigger than your gap” wars started. It didn’t take long for anyone who was doing much maintenance on the new systems to figure out the wide gaps just cause more rapid failure of the insulation of the HV parts of the system, wires, caps, coils, etc. In their tech literature in recent years even the MSD folks recommend .025” to .032” for over 13:1 and less than that for boosted and nitrous engines. (Welcome back to the local universe is my spin on that.)
It doesn’t matter how or where you lock the mechanical advance on a HEI, because the rotor is attached to the rotating pole piece (trigger wheel) the rotor phasing doesn’t change.
