AdrianE

I'm running 4 BUR9EQP's which have resistors built in, and I logged the "trigger at last home" which does give a consistent 2. All of the cables from the ECU and MSD CDI boxes that route from the cabin to the engine bay pass through the same hole in the bulkhead. Even so, in the cabin, I tried to keep the 'CDI to coil' wires away from other wires.

The cable from Hall sensor to ECU is shielded (and grounded) cable. I could try something similar for the ECU ignition output wires, which trigger the CDI boxes. If these wires pick up noise they could cause the CDI boxes to output to the coils at the wrong time (or not at all), couldn't they?

So apart from the spark plug leads and coils, where does most of the noise come from? The actual CDI box or the 'CDI to coil' leads?
Also do high power CDI boxes like the MSD Digital 7+ (190mJ / 535V) generate much noise directly in the 12V battery supply they're connected to, because they draw fairly large ammounts of current (eg. 7A) very quickly (eg. <1ms)?

AdrianE

Apparently they do. I found this on MSD's forum http://www.msdignition.com/forum/showthread.php?t=9747

On their website MSD recommend using the P/N 8830 noise filter in all 7 and 8 series ignition installations. Maybe it's not necessary for Digital 6 or DIS-2 ignitions, but if you use 2 or 3 of them surely this has got to have some effect on the battery supply? Has anyone else with a moderate-to-high power CDI measured battery voltage with a 'scope when the engine's under load?

I've attached a couple of pics showing how badly mine's been affected. Note that RFI noise from the coils / plug leads could also be a major factor here.

13B-RX3

iTrader: (2)

Are you sure the ignition is causing that and not a bad alternator?

AdrianE

Its not obvious to me if I need a new alternator / voltage regulator, but then again I haven't seen much to compare it with. Maybe the regulator can only do so much when you have power hungry CDI's!

Even so, I calculated that the 'time' between the spikes and it does match up with engine rpm ... where each spike is caused by firing leading or trailing. Leading causes all of the largest spikes as I would expect from the MSD Digital-7+ and MSD coils, because they consume the most electrical current / power.

I've wired in the MSD 8830 noise filter (capacitor) to the Digital-7's power supply wires, and at idle rpm's noticed that the battery supply to the ECU is a bit smoother than before and the spikes are smaller. But worryingly, I'm also losing 0.5V! It's averaging around 13.4V now instead of 13.9V. I've yet to measure it under load at high rpm, which is where the spikes are really causing trouble!

I could add a filter to the ECU supply itself, but I want to minimise the RFI coming from the Leading coils / spark plug wires first.

AdrianE

I think you could be right about the alternator, I've been reading up on alternator problems and these spikes are probably symptoms of blown doides in the alternator. Also the drop in battery voltage when the engine is running could be due to one of the alternator stator coils failing.

So I replaced the alternator with a supposedly good s/h one, and checked the voltage to the ECU again. It was hovering around 14V at idle but had +/- 1 V ripples (ie. much larger than before), and the spikes were still there! I switched the headlights on and drove up the road keeping the revs down then checked the voltage again. Now it had dropped to 12V with +/- 1 V ripples! Damn it, turns out my original alternator was better than the one I replaced it with...

I don't want to risk buying a new or recon'd alternator and then have this one damaged too by RFI or whatever else is causing these problems. But if I did I guess I could try disconnecting the Digital-7 completely and then just using the DIS-2 to fire the just the leading plugs using the standard trailing coils.

Apart from the Digital-7 firing the MSD DIS coils what else could be cooking the alternator?

13B-RX3

iTrader: (2)

Where are you pulling power for the MSDs from?

AdrianE

They're both wired straight to the battery terminals, and the battery is in the trunk and earthed to the chassis there. So the MSDs power leads are quite long but the cable guage is thick enough to pull a lot of amps.

13B-RX3

iTrader: (2)

You might try running a large capacitor close to the MSDs (1 farad+). You can find them at car audio shops. They are used to correct the exact same problem you are describing (fluctuating voltage from a high load).

AdrianE

I'm already running the MSD 26000uF capacitor on the Digital-7's supply, but I mounted it closer to the battery for the moment. It didn't filter out those voltage spikes.

So the next thing I'm going try is using just the DIS-2 to fire just the leading plugs via the standard trailing coils. I'll just set the ignition split to zero and see if this has any effect on reducing the voltage spikes. If it doesn't then I'll know that its not the Digital-7 (and MSD coils) that are causing the problem.

C. Ludwig

iTrader: (27)

How much boost are you planning to run and on what fuel? What power level do you expect? Good inductive coils may do the job and be a LOT less of a headache.

AdrianE

I'm planning on running as much boost as I can manage reliably on UK 95 RON pump fuel with water injection. I have a new HKS T04Z waiting to to be fitted on a large SP engine. But I intend to swap this for an HKS T51 KAI or GT42R which is when I will go half bridge. But for now I just have a GT35R running at 12psi which I'm using to debug this ignition system.

Had I known how much trouble these CDI boxes were going to give me, I probably would have gone for LS1 coils with built in ignitors instead. I didn't realise high power MSD CDI's and the DIS coils could generate so much RFI in the battery supply. Its funny but neither the boxes nor the coils are getting hot even after 1/2hr!

Having come so far down this road, I don't want to just sell 'everything' I bought and start over again. MSD told me the DIS-2 should work ok with their DIS coils, but I may just have to ditch the Digital-7 and go with something less powerful instead. I suspect the Digital-7 is what's causing the problem, even when feeding the standard FD coil (which eventually gave in and caused misfiring) and probably worse with 2 DIS coils in parallel!

13B-RX3

iTrader: (2)

Wait a minute, you are running two coils off one channel of the digital7? I can't see the draw from the MSDs causing your problem solely. I am running a DIS-4 and two digital6s with a smallish alternator and my ignition never skipped a beat at 30 PSI running methanol. I am sure the 7 draws a lot of current but i cant see it pulling more than two 6s. What gauge wire did you use to extend the leads for the MSD? I too run my batt in the trunk but rather than extend the wires i tied them in to the wire for the starter (2/O not 2 gauge). What i did when i was having ignition issues with my car was i took a piece of plate steel and drilled and taped it to accept 4 spark plugs, than i hooked the leads to the plugs and grounded the plate. I had an extra CAS and spun it with a cordless drill while watching the tach on the laptop. At around 6000rpm i would loose the trailing plugs and it would only fire on the leadings. After changing a few settings i had nice purple spark on all the plugs all the way to 13,000rpm. Never had any problems since.