OK, this post is resulting from the many PM's I receive about these topics. I will include enough information here that will enable you to choose your general type of engine management and estimate the cost WITHOUT having to ask general or stupid (yes, they do exist ) questions. Once you know what you want then you can ask for specific advice.

Credits and Thanks: Myself (practical application, writing and more); DemonSVT (tuning advice and MAF transfer function interpretation +more); PA3LSVT (MAF/PCM knowledge +more); Bnoon (engine knowledge +more); +all other pioneers and people who know who they are that added to the mix

What happens when you change the engine size and need to adjust the idle and maximum throttle airflow values?

Bigger injectors, bigger MAF, larger block, so what's the story?

Lets use the example of a normal contour 2.5L with 17#/hour injectors that has been modified to accept a 3.0L block.
A hybrid 3.0, or a straight 3.0 swap will still require proper mods to the engine management.
The stock maf is generally fine for most hybrid 3L applications since it will be difficult to max out its flow potential with only 0.5L increase in engine displacement.
The fuel injectors on the other hand are too small and the engine will run lean at high rpms. 19# injectors are more than sufficient for any 3L swap we've seen up to about 225-235 wheel horsepower!
However, lets say you're starting with 17# injectors and need to run bigger so you might as well go with the 24# injectors from the 3L; and stock MAF is ok for now.
The first thing you find is that the PCM doesn't know you have 24# injectors so it still runs the same injector pulse-width as it did with the 17's; only problem is the 24's inject 41% more fuel at the same pulsewidth than the 17's!!!
Therefore, the pcm must know that you are using 24# injectors so it can adjust the pulse-width. The PCM cannot be controlled by tuning the MAF for larger injectors if there is a really LARGE change. 17's-24's is over 41% larger!!!
I would say that MAF trickery will only work well for injector changes of around 25% or less.
Let me define MAF trickery. This is when you use a MAF "calibrated" for a different injector size than the car originally came with; this is also using an SAFC, eManage or mafterburner to scale the MAF voltage before the PCM reads it and in effect does the same thing as the MAF alone but is more flexible; this is also the same as using a MAF optimizer which is the least effective next to just swapping the MAF itself.
The reason that you scale the MAF voltage down is to make the pcm think less air is coming in so it lowers the pulse-width in order to drop the fuel now that your injector is 41% larger. However, when you do this you loose some of the sampling resolution that was designed into the stock MAF sensor.
Think of it this way: The normal MAF voltage range is about 0.5 volts to 4.9 volts. 0.5v is about the lowest flow the maf will see and 4.9 is the maximum flow that the maf can handle. On a stock 95 tour, that 4.9 volts may correspond to 280 CFM or so. On a Cobra mustang that 4.9 volts might be 500 CFM of flow.
No matter what MAF you use, this example of the contour PCM will interpret 4.9 volts as 280 CFM. If you are running injectors half again as big, you need to drop the voltage down to about 3.5 volts to compensate. Well, this works fine to get the fuel right at Max throttle and rpm, but the computer only "sees" (for example) 200 CFM of flow at 3.5 volts we'll say. With less airflow being seen, the calculations are all going to be off in other areas and it now forces the pcm outside is "learnable" range. Consequently the car will not run right.

Now look at the bottom end:
If 0.5 is usually the lowest voltage the pcm will see, then the average idle voltage should be a little higher, say 0.6 volts. A 40% reduction from 0.6 volts is about 0.42 volts; this is 0.08 volts lower than the lowest voltage the pcm expects to see from the MAF.
Therefore it doesn't know what to do with a 0.42-0.50 volt signal and will treat it all pretty much the same. Hence the piss-poor idle quality and off-idle throttle response of the more highly modified 3L projects.
Now this is still simplified because the 3L will draw in more air over the 2.5L but the example still applies.

REMEMBER: The MAF is just a sensor that operates off of a 5 volt reference source from the pcm. That means it is unlikely that it will operate from a full 0 volts to a maximum of 5 volts. The operating range must be within the 0-5 volt range. My MAF used to put out 0.08 volts just with the key on and NO airflow going through it. You may as well approximate 0.1 volts minimum just to turn the electronics on, and figure aobut 4.9 volts at the top of the scale too.

The solution is to use a CHIP or Tweecer, or standalone engine management system. The stock maf is usually adequate for most swaps and the MAF curve won't need to be altered, just the injector sizing.
IF you are going to have an extremely large change in airflow and your maximum airflow requirements have exceeded the stock MAF range (i.e. 3L turbo) then you are going to need a MAF that is sized for larger flow. Now it becomes even more impossible to use MAF trickery ( BEEN there Done that!!!) so you need a chip yet again to go along with the bigger MAF. This time you use the chip to input a new injector size and a new airflow function.
The MAF transfer function is a data table that corresponds MAF voltage output to Airflow Volume.
1 volt might be 50 CFM and 4.5 volts might be 300 CFM.
If you need a biffer MAF, then you need access to this data. PRO-M supplies this data with their MAF's and is invaluable in programming the chip into the right "ballpark" the FIRST TIME. If you have this data then you minimize the risk of damage and the number of re-burns you must pay for. If you want a MAF from another car, then make sure the computer from the car that the maf is coming from is "Read" by the chip maker so they can copy the MAF transfer function from it and put it into your chip.

Last:
Using MAF trickery on top of a chip. It doesn't make sense to use it if you are tuned right except under certain cirumstances. With the eManage and the SAFC you can make small changes in fuel without doing any real harm to the calibration in order to fine tune without paying for a reburn. I run the eManage at that same time as my chip so I can fine tune AND use the datalogging to store data on my hardrive. Then I can reference it for better understanding of how my engine is performing.
In the long run it will save money if you can make small changes with the A/F computers so you don't have to constantly get your chips re-burned; assuming you have been tuned into the "ballpark" somewhat close.

~costs:
chip -- $300-$700 depending on source and tuning
Tweecer -- $400-$500 (correct me if I am wrong)
eManage -- $300 - $1000 (depending upon add-ons and options; full featured, highly flexible)
SAFC -- $300 (nice display w/good basic features...less expandability)
custom MAF -- $200 - $400 new
used MAF's -- $25 - $125
Injectors -- $5 - $300+ for a set depending upon size and new/used

Tom

Originally posted by warmonger:

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19# injectors are more than sufficient for any 3L swap we've seen up to about 225-235 wheel horsepower!
However, lets say you're starting with 17# injectors and need to run bigger so you might as well go with the 24# injectors from the 3L;

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Why even go to 24 then?

There's been just one 3L that got to the power range that might need the extra fuel (DavidZ's) - and he was still using his stock 19's with no extra A/F management. Seems you could avoid cost, compleexity and tuning headacche by just sizing for what you need to begin with? IIRC people running 19# out of the box had much lesss trouble getting there 3L's to run right.