Anyone have experience in this regard? Looking at a 85561 MSD for some dyno. My thinking is to segregate the spark advance duties off of the ECM for the moment and just control fueling via EFI. Of course, I need RPM input and what-not for everything. So how would that be done? I'll compare the results of those tests on the dyno to results using an MSD 8366 and some ECM-controlled advance patterns.

Why?.

Does the 85561 still use a reluctor type trigger?.
If so the only difference is going to be the effects of the differences in air gap.

A remote mount coil HEI will do everything as well as an MSD, IMO.

If your distributor has shot bushings, then the MSD might be a lil, better, but so would just a fresh stock distributor.

IMO, CDI, DIS, or CNP are much better answers.

Better yet, DIS, with 4 CDIs so that you have multi-spark all the way to redline.

Well, I've got both units on hand (both MSD units). The MSD folks also concur that there's no difference. I was just wanting to do a little comparo of mech -vs- ECM controlled advance. This goes back to the carb -vs- EFI thing ya know.

To keep the ECM happy enough to fire the injectors need to provide a DRP. This would be a square ware, 0-5v. It can be either the controlled timing or the reference timing, wouldn't matter. Although the injector timing would shift to be relative to the DRP. Look at REF+ and REF- into the ECM, that is the minimum expected signal.

The ECM will instantly throw a code 42 and lock out it's timing control. But that wouldn't matter. The distributor is controlling timing.

RBob.

Crap. This means I'd need some auxiliary circuitry to run the non EFI distributor. Ah well, so be it. Will just use the 8366 then. Was worth consideration at least.

craig ive been using a probillet msd distributor for sevral reasons in the sister buildup of the 427 were doing here. which by the way dynoed out very impressively. once we make some tb mods ill get the new tune right and get it going here. as for the pro billet your stuck like i was becuase of the polymer gear only existing in the .500 shatf diameter. so youve got to run a msd pro billet becuase its the only shaft out there that can hold the gear. unless you were to make a bushing to intall on the distributor shaft to bush the .487 shaft to .50

Craig, I took the magnetic output from a MSD 8570, (same as the other MSD's) and hooked it to the ESC module mounted on a heatsink to the firewall and it worked perfect for over two years.

Couldn't you do the same and just zero out the timing tables and let the mechanical advance work like you want it to?

Just an idea...

I'd like to give it a whirl, yes...

Just make sure you connect the two wires properly to P and N of the module (positive and negative, duh). If you scope the reluctor signal, you'll see it start from 0, rise, drop quick to below 0 and rise up to 0. That is the correct way. The incorrect way will dip, jump, and decay to 0 (exact opposite or correct). The GM Module begins the REF hi pulse to the ECU when the voltage rises just a tad above 0, and stops the pulse just as it crosses 0 in the sharp fall. The ECU looks for the falling edge of the REF signal. Remember to ground the ignition module body, also.

BTW, there will be spark if it's corrected incorrectly, but the timing will change with RPM (in addition to mechanical advance), and in general just suck.

Ah well, this was a wash. Just put the mech advance unit aside, and ran the EFI controlled unit. Ran great, and MUCH more flexibility. Actually was able to gain a bit by changing how fast the timing came in from like 3500-5500. Really helped the characteristics. I think with several iterations on fueling and timing, along with WB and EGT data, all bets are off in terms of how good you can get the tune dialed in. Pretty incredible.