Nasty VR noise with MSD 6AL and pro-billet distributor

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zackbass
MS/Extra Newbie
Posts: 3
Joined: Thu Aug 10, 2006 12:28 pm
Location: Rockaway, NJ

Nasty VR noise with MSD 6AL and pro-billet distributor

Post by zackbass »

I’m working on putting a megasquirt II (v3) system on my racing Flathead for fuel and ignition and have a few concerns with the VR signal I’m getting. The engine is a bit one the extreme side, 303ci running with about 12lb of boost from a 4-71 blower, roller cam, vacuum pump, pretty much the works. It's quite fragile and dangerously close to the edge between world class performance and grenade. I’ve got a huge amount of money and time invested in the engine and I’m quite concerned about its health with an unproven ignition system. A misplaced spark could be absolutely catastrophic in my case.

The ignition system is a MSD 6AL with a pro-billet distributor and a huge Mallory ProMaster coil. The 6AL, coil, and the MS Relay Board are all in pretty close quarters. Basically it’s about as electrically noisy a system as can be imagined. Previously I’ve used an MSD vari-curve box in the same configuration (the relay board is actually going in its place) to tune my timing curve and it has worked perfectly which now appears to be an absolute miracle since the VR wires used to run ALONG the coil wire for about three feet (they go to the same place, why don’t we zip tie them together!). I’ve now got the VR wires taking a separate route that is much saner.

I’m getting everything wired up now and testing all the parts along the way but I’m having some concerns about the noise in my VR signal. It takes about 15 turns on the hysterisis trimmer to get something that doesn’t cause the ECU to go absolutely nuts and ~25 turns to eliminate all the false triggering. At this point I’m able to get a good signal down to about 100RPM. I’d like to be able to pick up lower but I’m afraid of the false positives creeping back in and seriously screwing up the ECU.

So here’s my Big Set of Questions:
1. Do I stand to gain signal cleanliness by changing my VR wires over to shielded twisted pair? Should I?
2. How common is it for an ECU to freak out over a bad signal and hurt the engine? In testing I’ve gotten lots of bad failure modes including RPM runaway, nasty spikes, and cutouts with my hysterisis trimmer just out of tune.
3. How was the old MSD box able to use the signal? If I run the VR wires in the same configuration as the Vari-Curve used there’s no chance of the MS ever being able to pick up something that looks even remotely like an RPM signal. I’d probably have trouble picking out the VR signal on my scope!

Here are some pics of the setup as of now: http://s5.photobucket.com/albums/y156/zackbass/EFI/
You gotta find first gear in your giant robot car.
Mr Gadgets
Experienced MS/Extra'er
Posts: 187
Joined: Tue Sep 21, 2004 10:15 am
Location: Michigan

Post by Mr Gadgets »

Zackbass,
I am new at this myself... I am running a 6AL and had it setup for VR from my MSD dist. I had noise problems from my injectors, it seems, so I removed the ignition control from the MSII to eliminate any problems with it.
In the process there is a thread about "noise on map signal" that you should review. I added a 10kufd cap across the input to the 6AL and was told that would help. Well I found noise from the PWM circuit, and did some filtering there.. I have not put my 6AL back on the MS, but the point is, maybe your noise is more than just the VR signal.

It is interesting reading, and in the boating community, where I come from.. they say that the cap on the input to the 6AL keeps the box from dieing.. Take alook, see if any of it applies to your situation..

Good luck with it!! Sounds like a great project..
Dick
zackbass
MS/Extra Newbie
Posts: 3
Joined: Thu Aug 10, 2006 12:28 pm
Location: Rockaway, NJ

Post by zackbass »

Thanks for info. I’m pretty sure the noise is due to the 6AL/coil because all my sensor signals are absolutely perfect until I bring the high voltage ignition into the system by hooking up the white signal wire on the 6AL.

I get an insignificant amount of noise in my ECT and IAT from the ignition but nothing at all in MAP. The MAP signal is pretty ugly from my extreme cam but it doesn’t seem to be causing any problems.

I started the car for the first time yesterday and it lit right up as soon as I hit the starter. A little bit more work with the hysterisis trimmer and minor wire routing and it’s working very nicely now. I got the low end of the ignition and VE tables dialed in and it’s running quite smoothly.

Sometimes I have trouble picking up the tach signal while cranking due to the hysterisis trimmer. I can’t seem to find any pattern to when it decides not to work since it happens so rarely, but hope to fix the situation soon.
You gotta find first gear in your giant robot car.
scowman
Helpful MS/Extra'er
Posts: 57
Joined: Tue Jul 11, 2006 9:45 pm

Post by scowman »

I have a very similar problems with my coil in cap HEI. I have not found a solution yet. I'm trying to isolate the coil with a special cap from MSD/Summit.

Hopefully moving the coil away from the VR will solve my problem.

I have tried shielded wire, resistor plugs, independent grounds, GM 8PIN moudules, etc., but as yours, my problem starts when I plug the coil in.
MS-II V-3 Megatune 2.36 Relay Board
1974 Jeep J10
Stock AMC 360 with smog removed
GM TBI with 55# injectors
Duraspark with HEI External Coil and 8 Pin Module
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