Wendell Allen

Maybe I just don't understand this ignition stuff yet.

1) I'm curious about the sparks duration. According to MSD's web site, the 6A last for 20 degrees of rotation. But if memory serves, on the regular points set up, it has about 30 or so of dwell. If this box can be triggered by points (or electronic ingition for that matter), at what point in that 30 degrees of dewll is the spark triggered, assuming a points set up?

2) Why don't I want the spark to last for a full 30 degrees as the original dwell? Wouldn't that promote more complete combustion?

3) From a cost savings point of view, is it a reasonable and more frugal idea to simply install a less expensive electronic conversion kit in my stock and still good distributor (as opposed to buying a new one) and add the box?

4) Can anyone explain to me the relationship of the spark intensity, for lack of a better term, and duration and the resulting flame travel (of the ignited fuel) over time? It would really be cool if you could explain it in terms of crankshaft rotation. I realize things like compression and octane will affect this, but these and all other things being equal, I'm curious about this relationship.

ScubaJKD

I would point you to the MSD website. http://www.msdignition.com. Hopefully lars will jump in and clear it all up for you much better than I can. One tip I would give you is that if you are going to spring for the MSD6a go ahead and jump up to the 6al and get the adjustable rev limiter. Cheap insurance. :seeya

thejaf

Tell me this isn't screwy. I've been ironing our timing in our 1972 with the help of Lars, who rebuilt my ditributor. Anyway, when I bought the car it had (and still has) and MSD 6A box. however, it was wired in parallel to the stock ignition :confused: :smash: WTF purpose would that serve?

In other words, the power supply for the MSD 6A was wired correctly to starter, however the red and orange wired went to the + side of the coil along with the 12 volts and ignition key wire, and the white and black wire along with the distributor went to the - side of the coil. The original factory ignition system did not go through the MSD at all. Did Bubba just not know what he was doing?

427Hotrod

1) The dwell time refers to the degrees that the points are closed. That is the time the coil is "saturating" or charging itself up.

Think about how the coil is wired. In simplest terms, you have 12 volts (or 9 volts or so in reality when running) going in the "+" terminal and flowing through the windings of the coil out the negative terminal to ground. The negative wire connects to ground through the points that are screwed to the plate inside the dist. When the points open the circuit is interrupted and the built up energy has to go somewhere. Remember, inside the coil is an iron core that all those windings are wrapped around. There is a tremendous magnetic field built up in that dude since current has been flowing around it. When the circuit opens and that field collapses the current goes out the coil top (through secondary windings that step up the current even more.)

You know the rest. It travels through the coil wire to the cap center terminal and through the rotor that just happens to be "timed" to be in line with a particular cylinder's plug wire at the correct point to get a spark delivered over a plug gap in a cylinder.

The condensor is primarily there to reduce arcing of the points when they open. Think of it as a little accumulator that absorbs some of the initial hit so the points don't end up welding themselves together.

That's a pretty basic description. It's actually amazing that the sucker works as well as it does!

2) So as you can see, the original spark was a one shot deal. It did not last anywhere near as long as the MSD.

3) The MSD is a large capacitor that is able to "charge up" in a fraction of the time a coil can. You have those big heavy feed wires to it. The points or whatever system you use, triggers it to release it's current. The advantage is that instead of 12 volts or less feeding the coil, you now have upwards of 450-500 volts sent to the coil that then steps it up again and you have some tremendous potential. Since the capacitors can recharge so quickly it has the ability to provide multiple sparks at lower rpm (under 3000 or so) After that there just isn't enough time to do it and it ends up being one big spark.

4) Notice I said potential. There is lots of hype in ignition systems. Basically an ignition system will only produce as much energy as is required to get across that plug gap. No more. Once you start modifying an engine for more power which comes from more cylinder pressure, the spark is harder to produce so a strong system can get it lit. A weak one won't be able to jump the gap.

Cylinder heads, compression, cams, intakes, carbs, headers are all designed to get more air into the cylinder to increase cylinder pressure. That's about the only way to increase power. Until you do much to it, stock systems are usually capable of handling things.

The MSD is a neat addition but don't expect miracles. I doubt you will feel any more power at WOT unless you had other problems. It will help starting and low speed driveability especially if you have pretty radical setup. On my Dominator equipped car it helps low speed and will keep plugs from fouling. I still trigger mine with a set of Accell points just because folks say it's crazy. They have been in there for almost 4 years and have never had to be reset. This motor made 700+ hp on the engine Dyno and 615 RWHP with points and an MSD6. It turned over 7500 rpm and never missed a beat and I'm still running the same plugs in it I used when I fired it up a year ago!

I do suggest doing some creative wiring with the MSD system to allow quick return to "stock". Mine died once and luckily I had enough wire to "McGyver" it to get home. It is now set up where I can be back to plain points with the shuffling of two wires. Just in case!

Hope all of this helps!


Jim

Wendell Allen

Great explaination, 427. I now realize how ignorant I was in regards to ignition, and how I remain so to a lesser extent! Your information really helped straighten me out.