Short version:
Yes, ECU tuning should be made after every change made, that will affect engine rpm behavior. Almost every tune will affect it. Although it is rarely dangerous to engine when some minor changes have been done. Let's say catback, CAI installment and you can drive without ECU tuning. But when changing headers (with different primary lengths/different setup like 4-2-1 -> 4-1), intake (different measures than OEM) and
especially when changing cams, it is mandatory.
If not, it is more than possible to melt holes in your pistons or force your crankshaft to new measures through knock.
Let's put it this way. Mitsu ECU calculates Injector Pulse Widths (basically, how long injectors are open = how much fuel is being injected to the engine) by these values:
AFRMAP*AIRTEMP*BARO*MAFSCALING*MAFSMTHG*TEMPBAROAI RFLOWLKUP*INJSCALING*MAFSOURCEMAINxMAFMULTIPWARMUP
(* WARMUP*COOLANT* MAFMULTIPWARMUPxLOADBASEDMULT*O2FEE DBACK)
+ accel1 - accel 2 (still to be fully worked out)
+INJDEADTIMELOOKUP*INJLAGRES
As you notice MAF flow is one of the basic information used on. Stock ECU rom has a couple of maps that basically are MAF calibration measures where it translates MAF flow (Hz) to LOAD. When these are made to the current setup, you can put, lets say AFR 12.5 on your Fuel Map and it'll be just that from the pipe. But when you have changed your engine's behavior on some rpm's, ECU wont know about it and still continues to make the assumption that you are running stock setup. As you know, Engine runs on closed loop on high LOAD (you give it much throttle) and after ~3000-4000rpm.
Of course if, let's say for example you have put a bit worse fuel in the tank -> affects all rpm -> ECU notices change through lambda values -> compensates it by Long Term Fuel Trim (LTFT, which it will use on higher rpm's also). Mitsu ECU has 3 different LTFT's. Idle, Mid and High. High. ECU makes the decision of what LTFT to use by LOAD. But ECU only reads lambda on stoichiometric mixtures which are basically when you are cruising. And if you make changes which will result in gains on high rpm you will basically make the engine run on stock IPW (fuel injected) which will result in lean mixture on those rpm's! Lean mixture of course results in higher temperatures on the combustion chamber -> possible hot spot's will start to glow -> possible pre-TDC ignition of mixture -> knock ->
POSSIBILITY OF REALLY MAJOR ENGINE DAMAGE!
Did I make my point impossible to understand
But about re-tune. If done properly (which I won't think any local dyno guy will) it will result in much better fuel economy and what is more important: more linear power curve through all rpm!
And like I said before. There is no need for standalone ECU becaus stock ECU is as programmable.
