cliffyk

I have been retyping all of this quite a bit lately and I thought it needed to be in one place.

So called "calibrated" MAFs are not calibrated to just a specific injector capacity, but rather to a specific injector size AND a specific tune; usually a stock tune. They are a kludge, a left-over from the old days when EFI system tunes could not be changed at all, or were quite difficult to change--as such they have no place in the modern EFI world.

They work by having the output voltage reduced relative to the airflow, either by having the signal conditioning electronics in the sensor altered to report that less air is flowing than really is, or by placing a stock sensor in a larger diameter housing so that sampled flow represents a smaller portion of the total flow.

Both of these procedures result in the MAF "lying" to the PCM about how much air is being ingested by the engine; telling it that less air than is really flowing is coming in.

This is how they allow you to run larger injectors without changing the MAF curve or injector parameters in the tune.

For example, let's say you install 42lb/h injectors in a 2003 GT that cam with 21lb/h units, and install a MAF "calibrated" for the 42lb/h injectors--no changes to the tune are needed as the calibrated MAF lies to the PCM and tells it that 50% less air is flowing.

The PCM says OK and calculates an injector pulse that would create the target AFR based upon 21lb/h injectors (which it believes are still installed). This pulse will be 50% shorter than would be needed with 21lb/h injectors at the real airflow, however since the real injectors are 42lb/h the target AFR is achieved with no change to the tune.

But there is a fly in this ointment...

The PCM also uses incoming air flow, and how much the engine could consume at the current rpm¹, to calculate engine load. The calculated load is then used in nearly every other engine control operation performed by the PCM, in particular ignition timing and fueling.

But as the incoming air flow is incorrect (the "calibrated" MAF is lying to the PCM), the calculated load is also incorrect which is not good. Let's look at just a single example of how this incorrect load can really mess things up.

Here is the Spark Borderline Table from the stock tune for my '03 GT (I have added the 10° base timing to the table values to make things more clear).



This table defines the maximum spark advance to be allowed at various load and rpm combinations. If the PCM calculated advance is higher than the table value the table value will be used, if not the PCM's calculated value is used.

Note that at 40% load, at 5000 rpm, up to 42° advance will be permitted; however that at 80% load only 25° is allowed.

Now imagine that you are really running at 80% load, however your "calibrated" MAF is telling the PCM that 50% less air than is really flowing is coming in. The PCM says "Cool, 40% load, let me pump that timing up to oh, let's say 38°"

Guess what happens to your engine at 80% load and 38° advance? It ain't pretty...

Many other engine control calculations are load based (AFR based on ECT and load being one).

The bottom line is that the only reason, other than to mess with your tuner's mind, to buy a "calibrated" MAF is if you are not planning on tuning your engine properly (or at all) using a modern tuning system. They are legacy kludges that should all be gathered up and tossed into the dustbin of automotive history...


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¹ - This explanation is close enough for this discussion, however it is actually a bit more complex than this.

² - For reference, here's an old 9-point MAF data sheet I found on the web:

Hangwire

Hellll yeah cliffy. The other day I was briefly using search to try to find one of your posts about this topic. I #1 wanted the specific information on why these "calibrated MAF"s are bad and what kind of issues they cause and #2 and most importantly I want examples of calibrated MAFs. Is a "MAFia" a calibrated MAF? Call them all out! To the gallows with them!

cliffyk

To clarify a bit...

If you have the real transfer function data (preferably a 30-point map) for a "calibrated" MAF it can be used properly and successfully.

However if you do not have that real map and have to leave the tune set at the stock transfer curve, and injector parameters set at or about the stock values; then screw around with making guesstimates as to the injector slopes, breakpoint, and minimum flow rate--you are just asking for heartache trying to attain maximum power and drive-ability.

And if you get a MAF "calibrated" too far away from the stock injector values, like the example I used, a complete disaster because of the load issues discussed above can be the result.

I was recently asked to review a tune for a setup using a "calibrated for 60lb/h injectors" MAF--it made me ill to see a tune for actual 60lb/h injectors, that had to be defined in the tune as 19lb/h injectors in order to make the silly "calibrated" MAF happy.

In fact that was what set me off on making this posting...

Trackstar1319

So This topic is really confusing for me, but it has been lingering in my mind for a week. I am in the middle of a 5.4l 2v swap, and I had the stock 19# yellow top injectors. I figured since I was upgrading to the hardball'r intake and this motor I
would upgrade my injectors also. They say to use at minimum 24#, but recommend 30. Well I got a good deal on 39# injectors. Will I have any issues with these after the tune with my stock MAF?

uberstang1

Stickkkky.

wolverine8490

The question that I am always wonder is what if the stock MAF is pegged running F/I. Which MAF's would be recommended in that case? I ask this as I am almost certain mine is pegged and I am confused as to how to correct the issue.

cliffyk

No, however you or your tuner will have to set the low and high injector slopes as appropriate for the 39lb/h injectors--FWIW, here's the data sheet for the FRPP 39lb/h injectors (these have the USCAR connector--what brand are the 39's you have?)

The low and high slope values are shown in lb/s, just multiply them by 3600 to get the values in lb/h.

cliffyk

The 90mm Lightning MAF is a good choice for any application up to 550fwHP or so.

Here are some transfer curve comparisons:



You can see that the 90mm Lightning unit pegs just a bit North of the PCM's 3830lb/h limit.

Beyond that, for example if using the Bazooka 3000 (and running over 550fwHP), you need to rescale the transfer function, injector slopes, and standard air charge values in the tune so as to keep the load calculation accurate.

wolverine8490

Awesome, that is just what I needed to know. I was thinking thr 90mm lightning MAF, but some say they are only good to ~450rwhp. Since 550fwhp would be around the limitation of my build, I will stick with with the lightning MAF.

cliffyk

"They" may be talking rwHP. 550fwHP, applying a 15% driveline loss, is roughly equal to 467rwHP.

Note: I added the qualifier "(flywheel)" to the sidebar box in the MAF comparison chart above.

Engineers (and I am one of 'em), when selecting engine control components, automatically think in terms of flywheel horsepower.