First, let’s go over the basic overview of how ECU controls an engine. In the simplest term, an ECU reads from the sensors (MAP/MAF, TPS, Crank, CAM, CLT, IAT, etc), do some calculations, then output fuel and spark. Injection of fuel does not need to be precise while it is critical for the spark timing to be very precise.
ECU might also turn on fan, shut a/c compressor off, control the idler, turn fuel pump on, boost control, alternator control, and so on, but we are only focusing on fuel right now.
The fuel amount is calculated based on a static but critical pump efficiency or volume efficiency tables on a MAP (speed density) based calibration. What’s pump / volume efficiency? It simply means how well your engine can breath/pump air at a given RPM, see ShelGame's post below for detail explanation on the difference of the two. MPtuner uses pump efficiency table while Megasquirt uses volume efficiency table.
Let’s compare some pump efficiency of the stock calibrations and my custom calibration in MPtuner.
You can find MP tuner here
http://www.turbo-mopar.com/forums/sh...l=1#post593977
Calibration files here.
http://www.turbo-mopar.com/forums/fo...ost&order=desc
Navy blue is from 89 2.5L MTX/ATX
Red is from a 89 2.2L ATX Chrysler TC by Maserati
Pink purple is from a 87 2.2L MTX/ATX
Green is a 89 2.2L TII MTX
Light blue is my 2.2L with a ported cylinder head and big valves and other modifications.
You can easily see how using 89 2.2 TII calibration on a 89 2.5 will make the engine run rich at higher RPM, even though both 2.2TII and 2.5TI uses the same injectors.
You can also see how using my calibration on a stock 89 2.2 TII will make it run rich.
Every ones uses different parts to modify their engine, which resulting in every modified engine has a different pump / volume efficiency. Tuners often have to make an educated decision (best guess) of the pump / volume efficiency since most tune are done remotely. Sometimes the tuner gets lucky, other times the calibration is way off. Personally, I learn my lesson and I refuse to tune an engine if I can't touch the engine physically.
Do you need to re-tune after changing one part?
Here is another example.
Navy blue is my ported head with big valves on my 2.2L.
Light blue is a stock 782 head with back-cut valves on the same 2.2L block, same turbo, same injectors, same everything as Navy Blue.
Light green is a stock 89 2.2 TII for comparison purpose.
One part can make a major difference in the Pump /Volume Efficiency!
My advice for tuning?
At minimal, buy a wideband and prepare to work with your tuner to keep trial and error until you get a tune that works for your engine.
The better options would be buy or make a socketed ECU, buy Moates Ostrich, buy wideband, learn to tune your own car, then tune your own car.
You would also want to check your tune every time you changed a part and re-turn if needed.
As always, I'm not prefect, if I miss something or wrong, please kindly point it out and I will edit this post.
Edit after Shelgame's explanation regarding to pump \ volume efficiency.
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