resistance in the ignition cap

Mac · Post by **Mac** » Tue Dec 19, 2006 5:34 pm

I was looking at an old distributor with my friend last night, trying to figure out how i could do a swap to a solid state igniton system like AEM's CDI coilpack ignition module (http://www.aempower.com/ViewCategory.aspx?CategoryID=79).

he was looking at the cap and thinking about what a tough job the spark has to do going from the ignition coil to the spark plug, so we decided to check the resistance in the cap, turned out to be 20,000ohms just from the coil to the rotor and thats over a distance of about 2". we were both pretty shocked as 3000ohms for a foot (from our understanding) is generally what maximum resistance should be.

I'm thinking this aftermarket cap (which is fairly new BTW) is probably a piece of garbage judging by the amount of resistance and i'm going to try using an OEM one instead. it would also explain why the new engine and the old engine would tend to "skip a beat" when idling as they both use aftermarket caps.

takza · Post by **takza** » Tue Dec 19, 2006 5:42 pm

I use the cheapo caps...but I've heard people talking of using only the brass contact kind.

brianp · Post by **brianp** » Tue Dec 19, 2006 7:48 pm

hmm, well in the testing procedure for the cap and wires in the manual, it says together they should be 25 kOhms. It doesn't say what it would be by itself.

keith · Post by **keith** » Tue Dec 19, 2006 8:02 pm

20k ohms sounds a bit high, but it should be compared to a new OEM cap before making a judgment. The coil needs to see a certain amount of resistance for maximum transfer of power. Lower resistance is not necessarily better. A low resistance can result in the coil discharging too quickly resulting in a low duration spark and little power transfer. Too much resistance can cause a weak spark. Again low transfer of power. There is an ideal resistance for every size coil that results in the best duration and intensity of spark for maximum transfer of power. Mixing aftermarket parts designed for different systems can result in poor performance. An example would be using MSD wires with the stock system, not good. Those wires were designed for the MSD system. Go all the way or don't go at all.

Typrus · Post by **Typrus** » Wed Dec 20, 2006 7:13 pm

Well, one thought- go little resistance in the wires and cap and use a good plug gap. The resistance to jump the bridge would be increased by increased distance allowing for better coil buildup, and when it finally made the "tilt point" more of the coils energy would be put into the combustion chamber versus turning into heat due to excess resistance. I could be full of it, but to me, that sounds like a sound approach.
I'm not saying you can just go gap the crap out of your plugs and expect a good result. It'd take pulling, regapping, and reinstalling the plugs several times to find the proper setting for peak power. Then record the setting so you can maintain it for wear.

Back me up or smash me down here keith. It seems good to me, but you seem to have a hell of a lot more experience.

I think the IIA is wonderful for stock applications if its working right, but for a lot more fuel and air or for higher RPM's, an aftermarket full system might be best. I've been examining MSD's "6" series and am liking the digital one. A dual-condition rev limiter is one of my favorite features. If you could somehow link it to the clutch detector switch, you could electronically limit yourself from creating too much driveline shock. Say, a well-balanced engine pushing 130HP at 6300RPM. Have it so that with the clutch in, its limited to maybe 2500 or so RPM so that you can't really do too much driveline shock. Once the clutch is out, rev-limiter switches to, say, 7k. The advantage to this is not only that you save your clutch and tranny, but that in neutral, with the clutch out, you can still show off.

Just some thoughts.

keith · Post by **keith** » Thu Dec 21, 2006 7:04 pm

Increasing the gap is not the answer. Gap resistance is not a constant. The gap has a high resistance as the corona for the spark builds up, but once ionization occurs, the resistance drops to about ziltch, resulting in low energy and shortened duration.

A lot of engineering goes into these systems, stock or aftermarket. I'm not trying to discourage you from going with an aftermarket system like an MSD, just don't mix parts of one with the other, that doesn't work. If you go MSD, go all the way and get one of their complete systems, otherwise stay with OEM all the way.

Typrus · Post by **Typrus** » Thu Dec 21, 2006 9:46 pm

I had plans to install the 6 series with their coil. It wires into the current dist just for the triggering event. I'm not entirely sure on all of the hookup, but they meld it into the system somehow.

Mac · Post by **Mac** » Thu Dec 21, 2006 11:14 pm

what about resistor spark plugs?

Brass contacts is probably whats meant to go in there, not this shitty black stuff neihoff loves.