Hello all,
So I have an Audi A4 1.8t (180hp version) with AJL engine and M3.8.2 ECU.

Rather than get an unknown "performance chip" and solder it in, instead I have bought a Golf IV ECU (06a906018bb) and a couple of "stage" bin files for it for review. (Already found some things I didn't like.. like the KFLUL tables both being maxed out at 12.5 load haha)

Mostly I have got it mapped out, at least in the important areas, but I have a query about what to do with the KHFM. Also some interesting information that may help actually answer the question.

So the AGU has 249.1 cc @ 3 bar injectors, and the AJL has 242.5cc @4 bar.

I have checked the parts fiche and both engines have identical MAFS, and the MLHFM tables are identical.

I also have the M3.8.2 AJL ECU bin/ map pack for comparison.

The interesting part is the difference in KHFM. So, the AGU ECU has a KHFM of 46800, and the AJL ECU has a KHFM of 32895.


I've taken a look at some other m3.x  ECU's as well (BMW's and the VR6) and they tend towards having KHFM's in the 25k range.

I've also read a *lot* about this over the last week and I have two conflicting views.

(Mostly summarised in this thread, conveniently
http://nefariousmotorsports.com/forum/index.php?topic=5745.0)

One school of thought is that it is an injector constant (certainly it appears to be a linear fuel modifier?) and the German name seems to imply this.
The other is that it is representative of the engines operating range as far as vacuum. This makes sense from the point of view of the NA cars having lower values - even the 540i (which you would expect to have higher load) has a much lower value than the 1.8t's. This also makes more sense in the context of the KHFM values affecting all maps with load as an axis.

On the other hand, the AJL engine runs higher stock boost than the AGU and so one would expect a higher operating resolution.
Both cars have near as makes no difference the same injectors. So all things considered it makes no sense why the golf ECU has the highest KHFM value; and the difference between them is nearly 30%.
(I also checked the A3 1.8t 557b ecu - same KHFM as the A4, and the same fuel gauge constant - as far as I can tell though it has the the same injectors as the Golf?)

And so practical question : I've adjusted the Golf map to match the AJL injectors in terms of TVUB and TEMIN. Should I also adjust the KHFM value, or just adjust fgat0?

Theoretical questions, does the Golf ECU have a higher KHFM because it was tuned to a higher expected range? (I guess it is a newer car?)
What's the practical benefit of a higher/ lower KHFM?



(as an aside, I do intend to modify the car further and ultimately do an ME7.5 swap - just baby steps. I'd also like to do things like upgrade fuel system, probably 550cc's and a bigger fuel pump, exhaust, etc, before doing this so understanding KHFM properly seems like an important interim step. )

Of course I will do this, but it's a matter of time and money

Yes, I took into account that the longitudinal engine has a 4 bar fpr (in effect the injectors are the same size). The A3 and A4 have 3 and 4 bar FPR's respectively and yet both have the same lower KHFM value.

So I think then that KHFM is not the injector constant per se, but the *injection* constant, ie, its purpose is to scale the load values for other maps.

I'd guess that the way the OEM does it is picks a load scale and then adjusts the maps to suit the injectors. FGAT0 is then the actual injector modifier (they set it to 1 as the maps are defined for stock injectors) and can be scaled for different injector sizes.

So, since the Golf map is set up to operate within this load range, adjusting KHFM is a no. I'll have to take a look at some maps with load as an axis to see what else I can see.

Thoughts?

There is some wot enrichment and advance offset maps too, but they are zeroed in all turbo versions I saw