Originally Posted by sc2dave

Hey 69427, do you still have your distributor machine? If you do, are you willing to recurve my dist for a price?

Originally Posted by turtlevette

I've been wanting to try an MSD digital HEI module. As I understand it has a microprocessor or custom controller to trigger the charging and discharge of the coil, so dwell can be dynamic thru the rpm range, I think.

The other big thing is that it has a rev limiter. This would be really helpful for my trans am 4 speed that doesn't like over rev.

Can you give a mini lecture on how this module works?

Originally Posted by sc2dave

Why not just get an MSD 6 box? It'll give you multiple sparks up until around 3,000 rpm and a longer, hotter spark after that. You dont even need a module with it and the box should have a rev limiter too ,depending on what box.

Originally Posted by turtlevette

The trans am is almost completely stock and is worth more if I kept it that way. The MSD HEI modules are a stealthy mod.

Originally Posted by turtlevette

As I understand it has a microprocessor or custom controller to trigger the charging and discharge of the coil, so dwell can be dynamic thru the rpm range, I think.

Originally Posted by 69427

Okay, I changed my mind. There's a topic I want to discuss, and I think I'll let everyone digest the last (dry) post for a while, and do this one now.

A forum member asked me about coil overheating issues, and we discussed a few things that I think may be of interest to the collective.

Recall that I mentioned that the DC resistance in a coil is pretty much a byproduct of the wire size and length. What happens in a points ignition system if you mix and match coils?

Let's assume the alternator voltage is 13.2 volts, the dwell is 30* (out of 45*) and the original ballast resistor (assume it's a removable ceramic resistor rather than a ballast wire for this discussion) is 1.5 ohms, and coil primary winding has a similar 1.5 ohms.

At idle or low speed the coil primary current ramp up time is small enough to ignore for this discussion. So, the calculations:
The primary current is 13.2v/(1.5+1.5=3 ohms)= 4.4 amps. The ballast and coil resistive wattage numbers (I^2 x R) come out to (4.4^2)(1.5)=29 watts (instantaneous) on each component (because they're the same resistance). The dwell is 67% duty cycle (30/45) making the average power on each device 19.5 watts while the engine is sitting there idling.
Let's say the coil accidentally gets damaged, and is replaced by a generic parts store coil. You put your ohmeter on the primary terminals and find that the reading is 3 ohms. You know that the increased resistance in the system will lower the primary current (and energy), so you decide to bypass/shunt the ballast resistor to get back to a total of three ohms in the primary circuit. You hit the starter, and the engine fires right up. Success! Okay, lets see if anything actually changed in the circuit. The current (4.4 amps) is the same as before, and the system resistance (3 ohms) is the same, but now it's all in the coil, as the ballast is shorted out. Let's do the math. The resistive wattage is still (I^2)(R) so (4.4^2)(3)= 58 watts instantaneous, or 58 x (30/45) = 39 watts average. The careful observer will notice that the average power (dissipated in the coil) went from 19.5 watts to 39 watts (doubled!). Can the coil survive this continuous heating? I don't know. It depends on whether the coil can conduct away this heat, which is difficult to do when it is bolted to a surface that is 160- 200 degrees. Regardless, it's not a formula for reliability.
Third combination: Assume the parts store coil is one (1) ohm. We can do two things. Run it as is, knowing that the current will be higher [13.2/(1.5 + 1)]= 5.28 amps (hopefully the manufacturer has built in some safety margin, but good luck getting him to tell you what that margin actually is). The wattage in the coil will be 27.9 watts instantaneous, and 18.7 watts average. That's slightly lower than the original 19.5 watt baseline, so that should help offset the higher current concern a touch. (The corresponding wattage in the ballast resistor will be higher than the baseline though, and the reader can do the math if interested.) Second option is replace the 1.5 ohm ballast with a 2 ohm model to restore the current level to the original 4.4 amps if points life is a concern. The coil's average wattage in this situation is 13 watts. Directionally correct for increased component reliability.

So, two things: While most of the time we can swap coils around and not have the ignition fail on us while we're driving down the road, we are impacting the durability of the part (for better or worse) when the parts specs are changed. And secondly, with this new coil, did we increase the spark energy, or did we lose ground after spending our hard earned money? Unless we can actually get a direct answer from the manufacturer on what the inductance value is (or an honest answer of the calculated energy if you tell him what current level you're running) we won't know. You have to admit none of us would buy a cam or a set of tires if the manufacturer wouldn't publish the technical specs of the part they're selling.

More to follow.

Originally Posted by billyride

I have an LT-1 with the stock solid state ignition. With the vacuum advance connected the timing becomes erratic at higher RPMs. I'd like to put the pointstype distributor in but with a Pertronix module and coil. Is this easy meaning will the standard distributor drop right in? Is it unwise? if so, why?

Originally Posted by tommd

69427, I am not a Corvette person, but I do enjoy older vehicles and I just recently came across this ignition system thread. I found it very interesting and informative, and it certainly appears you have a very good understanding of the topic. I have a few questions and clarifications I would like to ask, if you are still checking this thread.

My first comment/clarification:

In your first post (many years ago), item 4, you discuss the condenser, and although the total thread is long and I may have missed it, I do not remember seeing much more discussed on it. You state the purpose of the condenser is to provide a temporary current path to prevent point arcing. Yes. That is what I typically read in most sources and I DO agree with that. But… I believe that purpose is misleading because it does not go far enough. My understanding is that the condenser is needed to control/dampen the collapsing of the coil’s magnetic field Yes, but only in a points/mechanical switching application. (in an oscillating manner) I'm unclear if you're stating that an oscillation occurs in a points system (it does), or that it is desired (it's not). to provide a proper and strong spark. As “evidence” of this, is one has a faulty condenser, the symptom seen is seldom arcing/burnt points, instead, the symptom seen is poor ignition, poor engine running. Very possible. A faulty/leaky/resistive capacitor can excessively slow down the magnetic field collapse rate, reducing the energy and voltage levels available to the plugs, causing light to significant misfire. The user typically will not tolerate the poor running to ever reach the burnt points symptom.

So I am asking for clarification, am I correct in understanding that for proper understanding of the condenser’s primary function in the ignition system, one should see its purpose as one to control coil collapse Yes, thus provide strong spark It allows the designed spark energy/voltage to occur, although it does not factor into the actual energy stored/delivered into the system. and the prevention of arcing Yes, while important and a by product of providing proper coil collapsing, is somewhat secondary? I'm unclear what you mean on this last part.

Thanks,

Tom

Originally Posted by tommd

69427, thank you for your response. First, yes, I am discussing only points/condenser systems, not electronic. I believe my "different understanding", may be that you are describing the condenser as a bandaid, I see it as an integral part of the resistive, inductive, capacitive circuit that was designed from the start to provide the proper spark. Yes, it's an integral part, but keep in mind what the whole purpose of that list of parts is: to take energy from the battery/alternator during the dwell period, and deliver it to the plug at the end of the dwell period. And during dwell, the coil has voltage/current through it, storing energy, while the condenser is shorted to ground, supplying no energy during the ignition process. The reason I mentioned electronic systems earlier was that a switching system that does not include a condenser is a more efficient system, delivering a higher percentage of the coil storage energy to the spark plugs. But either way, let me move on to my next question:

It seems that now-a-days, since the OEM points systems are long history, it seems we look at all condensers as equal, where as back in the day, each OEM specified a particular condenser for a particular coil. ranging somewhere between 0.2 to 0.35 or so mico farads. It seems now, we just grab any condenser that physically fits. It seems to me, since the condenser is important to good spark, the proper size would be needed for good spark. Should we be more concerned about what condenser we are using for good spark, or do all condensers in this range provide good spark, but just different arc suppression? I do understand that too much capacitance allows point material transfer to the negative point and under capacitance to the positive tip, but how much is the spark strength/energy affected?
Thanks,
Tom

Originally Posted by tommd

69427, okay, thank you. So on the condenser size, you are basically saying that yes the size is somewhat important, but apparently not critical, and because of a lack of specs, we do the best we can! Yeah, that's about it. If I still ran points on any of my cars it might be amusing, as an EE, to wire up some ignition parts on my bench, and then see what the coil waveforms look like on the 'scope, and seeing what the waveform changes are when mixing and matching parts. But, with limited spare time to play with my car, I'm spending most of my efforts pulling weight out of the car, and tweaking the suspension geometry for a bit better lap times.



Next question- ballast resistor. You did address this and If I understood correctly, you emphasized that it is a current regulator for the coil, not a voltage regulator. Yeah, but I prefer the term "current limiter". Okay, I can follow that. I also believe I am correct that it is often bypassed during cranking to provide better spark while under cranking conditions. Yes and no. Usually you don't need a better spark, as warm weather or warm engine starting requirements are pretty easy. Where the starter shunt circuit really comes into play is during cold weather starts, where the oil is thick, the plug electrodes are cold, and the battery voltage is low due to all the current loading of the starter. The shunt circuit, bypassing the ballast resistance, allows the coil primary current (current is how the energy gets in the coil) to ramp up to "normal" levels despite the battery being at abnormal/low levels.

But the next part is what I have learned, but have only limited confirmation of (read most think I am nuts!) First, my understanding is ballast resistor designs may not all be the same, so this may not apply to all. But with at least some systems, I understand that the ballast resistor regulates current depending on rpms in an indirect manner. That is, at low speeds, when the frequency of pulses in the coil are low, the current is high based on the coil circuit reactance. At these lower speeds with high current, the resistor heats up more and its resistance goes up, lowering the current to the coil. At high rpms, because of the increase pulse frequency, the circuit reactance increases, thus lowering the circuit current, thus the ballast resistor current is also lower, which allows it to cool off, thus its resistance lowers, allowing more current to flow to the coil to help make up for what was lost due to the frequency increase. This helps to better match the current/coil characteristics over a wide range of rpms for better spark efficiency.

Do you follow what I am trying to explain? Is this something that you are familiar with?

Tom