Speeduino is a great starting point.
I found out for my engine type and what I was trying to do
the speeduino board wasn't going to get me to where I nneeded to go, but............
It may be hard to calculate but at high rpm trying to get the fi and spark maybe impossible to do with a single mega
If I look into a stock ecu from a 6 cyl I believe there are at least two processors, so a mega and uno(?)
I read alot of pages and alot was true, but one speeduino for all the engines ever made, give me a break!
It might be better for the beginner engine electrical people, like I've seen and advice is given on this page
to start with a single cyl, lawn mower type engine, w/ homemade sensor and use a igniter and mega with carb.
I bought a few and after a few false starts and hiccups i decide the spd board was the wrong way to go
what I needed was to test commponents,
use a uno with one output, learn to program it, with the arduino ref, which is the start and complimentart
I found the smart cop causes so much noise it will stop the board.
the mega has six interupts and the uno two,
they are right next to each other, using these causes alot of interference between the two
old ide cable, some had grounds runnin between data lines to prevent cross talk,
separating interupts if you use them w/ a output low ground or no use may be more beneficial.
hope this helps