Hi again,
Soooo IDFs sound great and the performance is better than a single carb but I'm a little tired of chasing the flat spot around and me and the garage smelling like gas all the time.
So, I've decided to just go full ITBs. To keep the car down time to a minimum the plan is to substitute a Microsquirt unit in for the Megajolt Lite Jr. This way I could re-use some of the wiring from the MJLJR and the mounting location, get the Microsquirt working just for ignition first and then swap out the IDFs for ITB's.
Last time I messed with this stuff was when I installed Megajolt v1.0 in my now gone '79 124 Spider. That was a crazy experience. It all worked but the tuning software was really rough, I had to solder resistors in all kinds of weird places, one of the capacitors exploded while I was driving on and on.
I have to say Microsquirt is a dream compared to that! I bought the unit with the extended wiring harness. All the wires have clearly printed text on them, it uses a modern connector and the unit is so small! Amazing!
I also picked up another weatherpack bulkhead connector where I'll run all the non-ignition related wires through and wired that all up.
Even with IDF/ITBs there is enough MAP signal to use for ignition timing. MegaJolt has the MAP sensor on-board so I would need to do something about that. From what I've read most people still use the external GM 1 bar map sensor which is this rather large box that's mounted remotely with a vacuum hose. I wanted something a little more discrete so tracked down GM part number 55573248. This unit is used on some LS engines and is the only unit I could find that attaches directly to the manifold with a BOLT and a tab, all the others of this style use only tabs. So I pulled off my vacuum log and drill a few mounting holes for it:
If anyone else is using this style MAP it does not use the same range as the external style so you have to adjust the settings in Microsquirt. Also pay special attention to the pigtail, its a little different from other units.
From what I could find the correct MAP range values are 128.14, -0.31. Using this works great for me and is accurate when compared to what the internet says the baro is for NYC, cause the internet is always right
Microsquirt also supports an expanded ignition map when compared to the MJLJR. I want to take advantage of this and improve the current setup. I had seen Digiplex specs and curves in other posts but they were all for shared plenum setups. Turns out the Ritmo Abarth 130 TC has both dual carbs and a MED408 digiplex ignition! It took me awhile but I managed to find a Czech or Polish set of spec sheets for the Digiplex 401, 402, 406, 408 and 411.
Here is the 408, you have to use the line numbers in the top right to understand the mm/hg and then its advance on the Y axis and RPM on the X.
Last hurdle is that the way Megsquirt and Digiplex view vacuum are different. From what I can tell one measures the vacuum in the manifold and the other the difference to outside, they also use two different units so after a bunch of converting I was able to translate the digiplex map point by point into the Megasquirt map.