So...... a few MS questions.

[ComicArtist]

I know MegaSquirt is a topic that's beaten to death, but I've read all the sticky's multiple times, searched, and there's a couple more questions I've got before I dig into it myself.

 

First, I just wanted to fully grasp just exactly HOW MegaSquirt works. If I know how it works, it'll be easier to put together because I'll know WHY I'm putting it together.

 

And after all the reading and studying, I can't quite grasp how the Spark aspect of MSnS works. Being driven by a mechanical distributor, how does the MS board control the spark? It seems that unless the L28ET is running with coil packs or EDIS, there's no way to control the spark. Perhaps I'm just overlooking it, but I can't seem to see it.

 

Second, pretty much all the writeups I've seen are for '83 L28ET's. I'm running an '81 motor, which has the different CAS and distributor. Should that make a difference in how I wire the board?

 

Thanks for taking the time guys. I just want to understand it fully before I build.

[mobythevan]

The sticky does have a section on using the 81 CAS and Dizzy because Tony D. was using that setup. If you can't find I can locate it again.

 

Remember that the 81-83 cars had the factory computer controlling the spark timing through the distributor, megasquirt does the same thing. It is easier to understand if you look at the rotor arm of an old distributor versus a computer controlled car like the 83 distributor. You will see that the end of the rotor arm is much wider allowing the computer to advance and retard timing, but still keeping the rotor close to the post. So a wide range of timing can be used by the computer. If you use an old style dizzy with MS that does not have the wide rotor arm and you weld it so there is no mechanical timing control, just ECU timing control, then you could end up being far away from the post when you try to send spark. For large advance number you could have problems with the spark not jumping to the post and get misfires I suppose.

 

The two key items between a dizzy for computer control versus mechanical control is no mechanical advance and wider rotor arm on a computer controlled dizzy.

[ComicArtist]

I gotcha. So does MS work WITH the stock ECU or does it completely eliminate the stock ECU?

[mobythevan]

It eliminates the stock ecu.

[DCZ]

On a related topic, I have been reading up on controlling spark with Megasquirt using the 82-83 optical dizzy and I haven't found the answer to my question:

 

When first converting to MS, do you leave the distributor alone so that you have a static "mechanical" timing of 23BDTC like you would with the stock ECCS? That way the numbers in the timing map in MS would just be advance beyond the 23 degrees.

 

Or do you rotate the distributor so that your static timing is 0 degrees and the MS advance numbers reflect the total timing advance?

 

Maybe this becomes obvious once you start playing with the actual system, but so far I have only been reading as much documentation as I can find.

[mario_82_ZXT]

I leave the dizzy in the center of its adjustment and then change the trigger angle in the megasquirt so that both the stock timing marks and the megasquirt's timing lines up.

 

It's in the install sticky IIRC.

[ComicArtist]

If it eliminates the entire ECU, what happens to the lower wiring harness? The one that operates the headlights, interior gauges, turn signals, etc. In all the diagrams and writeups I've read I haven't seen anything regarding them.

[z-ya]

If it eliminates the entire ECU, what happens to the lower wiring harness? The one that operates the headlights, interior gauges, turn signals, etc. In all the diagrams and writeups I've read I haven't seen anything regarding them.

 

Nothing happens to that harness. Keep it if you want lights etc (oh, and power to the coil too..).

 

The Megasquirt should be wired completely separate than the stock harness. For factory EFI cars, you need to remove the engine harness and stock ecu. For early Zs with carbs, there is no engine harness to remove (note engine harness, not engine room harness).

 

 

 

Pete

[ComicArtist]

Hm I'm starting to get it. I'd like to see someones setup in person, but I doubt that will happen.

[cygnusx1]

There is not much to see. Mount the MS box in the car somewhere, and run about 30-35 wires from the MS box to the engine bay (or buy the pre-made bundle). Make the wires long enough to route anywhere around the engine bay, to the furthest point (cooling fan area?). Make sure you throw in a few pairs of sheilded wires running with the bundle (will be used for the dizzy). Also run a nice heavy pair of wires, I prefer shielded, from the MS area to the fuel pump area.

 

Variations to the wiring result from the way you get power to the MS and the fuel pump. ie. where you mount the relays, manual switches, or the MS pre-made relay board.

 

When MS gets power, it is designed to turn on/off/control a fuel pump relay with one of its output wires.

 

The rest is cutting, soldering, crimping, and tidying up.

 

 

 

 

Hope this 10,000 foot picture helped out.