I notice lots of basic ignition questions, so why don't we build a "sticky" of basic info? I'll start, and you guys keep me honest and add your own info: Like Jim Lore (flynbuick) has said, it only takes about 12,000V to light off the mixture, but most systems provide for more, since more capacity "overkill" doesn't hurt and can only help in less-than-perfect conditions (too rich, too lean, high rpm, poor wires, etc...). That's why there are all the high-voltage coils, high-dollar modules, caps, and wires and stuff out there. (90% of the vehicles don't need it 90% of the time, but like lots of stuff, we like overkill :Brow: ) Basically, there are two types of ignitions; Inductive and Capacitative (Capacitive??). Anyway, ALL inductive ignition (points, HEI, pertronix, etc) systems make a spark by applying a low voltage (12V or less) current to the coil primary windings and then interrupting the current. Applying the current creates (induces, hence the term inductive) a magnetic field inside the coil, and interrupting the current causes the magnetic field to collapse, which creates momentary a high-voltage current which travels the plug wires and arcs across the plug gap - "zap". The longer the coil can be charged with the current (longer dwell time), the stronger the magnetic field, and the stronger and/or longer the spark - "ZAAAAAAP". But it takes time to build and collapse the magnetic field, which is why at higher rpm there's less time to charge the coil, less room for errors in switching it on/off, so you want more system capacity ("overkill") . All a Points, or HEI, or Pertronix, or whatever system does is: start and stop the current going through the coil. Better systems are basically more precise/consistent in how/when they do their switching, which can allow for a more efficient coil, which can give you more of that wonderful capacity/overkill. But if you don't need all that extra capacity/overkill, you're chasing a diminishing return. Points (which use 12V for starting and <12V for running but still start degrading from day one) are the least precise/consistent method, and the GM HEI is probably the most precise/consistent. There's lots of advertising $$$ spent trying to convince you otherwise, but there's more engineering, R&D, and thought in an honest-to-goodness GM HEI module than in everything else combined, Ford and Chrysler included (imho). Only 'problem' with HEI is the epoxy-filled in-cap coil which can overheat and drop out at high rpm (4500+), which again, 90% of the cars don't do 90% of the time.
Capacitive Discharge (CD) Ignitions I think of CD as being the opposite of inductive ignitions - that is, instead of cutting on/off the current flowing through the coil, they use the coil like a transformer, sending high voltage (400-580V) to the coil, which steps up the voltage and sends it to the sparkplug They work by using one or more capaciters (small fast storage cells) to store electrical energy, and when the ignition is triggered the capacitor is discharged (CD) and the 400+V is sent to the coil. Capacitive Ignitions can be triggered by points, an HEI magnetic trigger, other magnetic or optical or crankwheel pickups, etc. There are two types of CD ignitions, single-spark and multiple-spark (Multiple Spark Discharge = MSD). CD ignition sparks tend to be relatively short, so most CD systems are multiple-spark - that is, they fire the coil and plug several times rapidly, instead of the single relatively long spark produced by Inductive Ignitions. Not many single-spark CD ignitions out there anymore.
And just to put a touch on the sparkplug wire/ performance myth. E=RI (cosine theta) When we deal with direct current circuits then theta is 1 because the curent and votage are in phase. However in auto the dischage in question is a transient current so it is most likely to be something less than one depending on the design of the wire. But for simpliciy let's use 1 for the cosine of theta and further assume a one amp current flow at the plug. Well, if you big dollar wires are 1000 ohms a foot less resistance than stock for a 3 foot wire that is a gain of 3000 volts across the plug for in a CDI system that may be operating at 40-50K or a stock system that may be at 25k. In other words a stock wire may have a 3000 volt drop (loss) relative to your big dollar wires. For a plug that fires at 12K potential across the electode, so what! It really would make no differnce in performance if the snap across the secondary of the coil was 100K volts or if you drove the primary of the coil with more current because that winding has lower resistance relative to stock as the result is the same. Once you have adequate voltage available he duration of the pulse and the frequency of the pulse during the correct postion of the rotating assembly is where you may get some performce gain. That is where the control of MSD box and the like come into play. However, old myths --they do die hard because everyone wants to believe they did in fact buy some cheap bolt on performance. And the marketing departments play into this human trait with the names like "Flamethrower" and "High Energy". Get them to show you an independently performed double blind dyno study supporting their increased performance claims and they will melt away just like Richard Petty and STP did. (But STP did make a lot of sales before the FTC shut them down for like of support for their claims)
Jim, and anybody else who wants to chime in; Got a couple of questions I'm not sure how to ask, but here goes... * The 12K Volts you mention it takes to fire a plug (I've seen the same number elsewhere), is that firing a "normal" mixture? What's normal? * What are the other main issues affecting ignition requirements? My understanding is that the fuel mix and temp/pressure are the two biggies...with lean mixtures and high pressures causing the most problems. * When the mixture goes lean or rich, how many volts are needed then, and what happens at the extremes? * As I understand it, Inductive ignitions give a longer zzzzzaaaaaaapppppp spark, while CD ignitions give a series of shorter but more powerful ZAP-ZAP-ZAP-ZAP sparks, is that generally the case? Which is "better" for a daily driver, a street machine, a race car?
The figure is just a ball park. I think an accepted range is 10-20K volts but that is affected by cylinder pressures which vary from engine to engine, the composition of the gas in the cylinder, and to some extent the shape of the electrodes of the plug. It is unclear to me how much the range of rich to lean mixtues affect the voltage potential necesaary to ionize the cylinder mixture which makes the plug fire --meaning arc. Somebody has studied this and no doubt this information is published somewhere. By way of illustration the dielectric properties of dry air at sea level pressure is about 3 million volts per meter. (Roughly that translates to 2286 volts over a 30/1000 inch stock plug gap before the air is ionized under those conditions) In other words if you took two plates and spaced them one meter apart under those conditions you would have to raise the potential between the plates to 3 million volts before the air between the plates would ionize and an arc would jump between the plates. Now once the air is ionized and the arc is established you can stretch the distance betwen the electrodes. Just think about the old Jacob's ladder devices on the early Frankenstein movies. They start the arc low in the apex ( or v) of two diverging wires and as the arc goes up the Jacob's ladder it stretches as the width between the wires increases.
Cool info! btw, what were those Jacob's ladders for, besides really cool effects in scary movies? And where can I get one for my living room? :Brow:
James, all else being equal, it is harder to fire a lean mixture than an optimal mixture. The GM HEI was developed, at least in part, to fire larger plug gaps of .060-.080", to fire leaner mixtures. It is harder to fire a spark through higher cylinder pressures. That is why plug gap is reduced in very high compression, and supercharged/turbocharged applications. You also have consider voltage as well as current. A common analogy compares electricity flowing through a wire with water flowing through a pipe. Current is the amount of charge flowing past a point in the wire per unit time, like the amount of water flowing through a point in a pipe per unit time. Voltage is the amount of push, or the difference in potential from one end of the wire to the other, or in the case of water through a pipe, the pressure. One of the limitations of points systems is the limited amount of current they can deal with, without seriously shortening their life. That is the reason for running them at reduced voltage(with a ballast resistor), and using a condenser(to absorb excess charge as the points open to prevent arcing) The GM HEI module can deal with higher current, and output is higher. The MSD CD ignitions use big capacitors to store more current and push it into the coil at much higher voltages(450-535 Volts). The resultant sparks are much hotter. MSD quotes spark energy output in milijoules. The 6A ignition generates 105-115 mJ per spark, and there are multiple sparks generated per cylinder firing. The mJ figure is a measure of how much heat is produced across the plug gap to initiate combustion.
Thanks for chiming in, Larry! :TU: I've seen the milijoules in MSD advertisements, but don't have much to compare to. Any idea how many milijoules different ignitions (points & HEI in particular) have?
Energy ( in millijoules) is given by W = 1/2*L*i^2 . The GM HEI has L = .008 H and current limited to 5.5 amps. So, W = 1/2*.008*5.5*5.5 = .121 J or 121 mJ.
Thanks! Just did some veddy interesting reading on joules, watts, etc. 1 joule = 1 watt per second. In the equation W = 1/2*L*i^2, I take it that W=work (joules), i=current=amps. What is L, and the H - length of time? Any idea what the numbers are for a stock-type points setup? Also, we're assuming that the more joules the merrier, but what's more important, the time or the current?
Don't know what the numbers are for points. I believe a points system cannot handle more than 2 amps. The GM HEI can handle up to 5.5 amps and with certain modle/coil combinations, up to 8 amps. The current makes a fatter, hotter spark. This is a good article: http://www.chevyhiperformance.com/techarticles/67498/
Interesting article - If L is the same, and you go from 5.5 to 8 amps, that's going from 121 to 256 mJ - Yow! Wonder what Ignitionman would have to say about this...
Some things to consider when choosing ignition systems include the quality of the fuel we must use. Since it is a combination of chemicals, each with its characteristic vapor points and burn temperatures, it becomes beneficial for the capacitive ignition to offer several light off points, allowing complete burns as a result. I have, on every occasion, experienced an improvement in driveability and smoothness when using an aftermarket ignition. Some of the reasons include the energy necessary to light off the mixture in high compression environments, diluted environments when using high overlap cams, and especially when having to use pump gas. There are several other reasons to use an improved spark, but these scratch the surface. Ray
Side question: Is there a significant benefit to using a magnetic pickup (GM HEI or Ford Duraspark distributors) over points to trigger an MSD capacitive discharge ignition box? (at rpm levels where points bounce isn't an issue.)
I think the magnetic pickup is going to be a more accurate trigger than points, especially at higher RPM's. Still, if I was building my ignition system on a budget, I would buy a good CD box and trigger it with points. Then save for Daves magnetic trigger conversion.
As for supercharged/turbocharged applications,the spark plug gap is reduced,but also the ignition system has a higher voltage to prevent the spark from being snuffed out. As for the question about points vs. magnetic systems: No question about it,magnetic systems are so much more accurate and reliable than point type systems. I know some of you like the old points type systems,but they cannot take the abuse and high rpms that today's modern magnetic systems deliver. One noticeable difference with magnetic systems is the engine starts quicker and the throttle response is much better. You also get better economy and efficiency.
Thanks for the input on points-triggers. You see, I've got this old Ranchero with a 200 six (high rpm? hah!) and an extra MSD 6A box just sitting there...
Of course it is! :bglasses: Since you live in Huntsville, I hope its Alabama, and you are a rocket scientist for NASA... :beer :TU: Are you? ou:
