MSD and other CDI ignition half-truths

Circlotron · Post by **Circlotron** » Fri Jan 03, 2014 2:05 pm

Most everyone has played with an MSD ignition at one stage or another, so it seems. Was messing around with an old MSD6 the other week and decided to look at the secondary arc current. Made a test setup with about 1700 volts worth of zener diodes to simulate the voltage across a plug during coil discharge. One interesting thing is the amount of time there is spark vs no spark while it is multi-sparking at low speeds. The left part of the pic shows their version of events with the black trace on for about 40% of the time and off for 60%. Back in the real world the spark is on for 12.5% and off for 87.5%. Spark is 0.12mS every 0.96ms. Quite a difference from what they would have you believe.

I did the test first with a 60:1 E-core coil then a MSD #8253 coil with almost identical results.

Circlotron · Post by **Circlotron** » Fri Jan 03, 2014 2:18 pm

Next step was to measure the energy output. They quote 100 millijoules but this is only what comes out of the box. The figure out of the coil secondary is quite another matter. I've lost the screenshot for the test but with the advertised 100mJ coming out of the box only 35mJ comes out of the coil secondary!!! This is with an MSD #8253 coil. The losses are incredible. Even worse is MSD6 with Bosch metal HEI coil = 25mJ, MSD6 with old Bosch 9-220-061-400 points coil = 23mJ. Compared to that, a Holden blue motor HEI puts out about 75mJ and an ICE 10 amp system 158mJ. Here's a pic of the ICE in action.

immortality · Post by **immortality** » Fri Jan 03, 2014 3:31 pm

Maybe there is something wrong with your MSD box? If the MSD wasn't even close to what they advertise they would have been out of business a long time ago.

Circlotron · Post by **Circlotron** » Fri Jan 03, 2014 3:50 pm

I bought my first MSD6 in April 1981. Have fiddled with and repaired them many times since then. This one is as normal as you like. Not limited to MSDs, but CDI ignitions in general. Here's an MW CDI putting out 113mJ but only 51mJ out of the coil. Same 60:1 coil as MSD test. 0.15mS spark duration vs 0.12mS for MSD. A good inductive HEI has 1.5-2.5mS duration but not as much intensity.

immortality · Post by **immortality** » Fri Jan 03, 2014 4:27 pm

Fairly sure though that MSD does state that different coils will effect the output?
Seems like the failing is more the coil than the CDI box driving it.

I thought it was fairly well accepted that for low rpm stuff an inductive coil is best but the CDI systems shine at higher rpm where an inductive set up can't achieve full coil saturation?

edit: Fairly sure MSD also state that the number of sparks also change depending on rpm, more rpm, less sparks until you reach ( I think it's about) 3000 rpm when it becomes just one spark (what rpm did you simulate?).

immortality · Post by **immortality** » Fri Jan 03, 2014 4:34 pm

for curiosity sake, what coil would you recommend as a replacement for a 304 running the standard ignition coil/module set up?

Mine is till running the original coil and it's starting to break down when it gets really hot. Planning on re-locating the coil but may as well upgrade at the same time.

Circlotron · Post by **Circlotron** » Fri Jan 03, 2014 5:16 pm

My car had an ICE 85:1 coil on it when I got it about 3 years ago. Using OEM ignition box still. They don't advertise this coil anymore, only 60:1 versions but they might still have one lying around. Tricky to find a good place to locate it though; see pic. Failing that, the metal can ones used on 6 and V8 blue motors is not bad.

Post by **The1** » Fri Jan 03, 2014 5:25 pm

interesting.

I have only been able to play with one system myself on a V6 Supercharged 18psi @ LIM, the MSD system was a seperate coil box feeding from the DFI, swapping between it and factory system do WOT runs i couldn't get any further timing out of it or increase the plug gap over factory system.

I have one of VL400's box's here now with LS2 coils to test just have not had time yet.
http://pcmhacking.net/forums/viewtopic.php?f=11&t=2913

Circlotron · Post by **Circlotron** » Fri Jan 03, 2014 5:32 pm

immortality wrote:I thought it was fairly well accepted that for low rpm stuff an inductive coil is best but the CDI systems shine at higher rpm where an inductive set up can't achieve full coil saturation?

With an inductive system there is not enough time to fully charge the coil if you start from zero every cycle. The flipside of that is that there is also not enough time to fully discharge the coil either. A good system finds the right ratio between charge time and discharge time for any rpm so the coil doesn't discharge too far, to make sure the coil just hits full charge at the moment a spark is required. Most, including 304 module, do not. Ideally it would also track accelleration to make sure the turn-on point is far enough ahead in time despite the shortening time between trigger pulses during accelleration.

immortality · Post by **immortality** » Fri Jan 03, 2014 5:37 pm

The VS doesn't have aircon receiver behind the guard like yours so I was thinking of putting in there. Also means the standard loom won't need any modification, just need a longer lead from the coil to the distributor.

I don't know if the ICE stuff is available here in NZ. I do have an MSD coil floating around here from another car. It's the MSD - SS coil 70:1 turns, primary .355 Ohms, secondary 4.4K Ohms

MSD and other CDI ignition half-truths

MSD and other CDI ignition half-truths

Re: MSD ignition half-truths

Re: MSD ignition half-truths

Re: MSD ignition half-truths

Re: MSD and other CDI ignition half-truths

Re: MSD and other CDI ignition half-truths

Re: MSD and other CDI ignition half-truths

Re: MSD and other CDI ignition half-truths

Re: MSD and other CDI ignition half-truths

Re: MSD and other CDI ignition half-truths