Possibly throwing myself on the fire here, but I want to outlay my idea that makes sense to me and I haven't really heard anyone else doing, or atleast talking about.  I think about it a lot as I read peoples threads and sometimes it scares me because I realize that their entire tune might rest on a given sensor reading a certain way at that given point in time. Literally hundreds if not thousands of variables come into play to possibly change that fine tune.  The fine tune that is based of a possibly sketchy base tune.  Because the base tune was done on bad sensor information.

Here goes how I currently do it, this is a by product of my single turbo car and the confidence it gave me in running these systems off as little as possible.  This all assumes you are starting with something like the popular Mbox file for the 2.7t and a 2.7t based engine, or CH on a 1.8t and so on.  VW/Audi/Bosch all did a very good job of essentially putting base tunes in the cars, the maps that the car runs on when things like a MAF or all 02 sensors are removed.  Timing adjusters, egts, all sorts of things can be removed or disabled even for just a short time to get the car running right at a more basic level.  You have to adjust these "base" maps at the most basic levels for whatever your changes have been.  If you havent modded much of anything and are just turning up boost, then you would essentially be changing nothing.  If you have a 3.0L and are running a 770s on e85 and 1200cc injectors with a 90mm maf you might be changing a lot.  You need to know that you have corrected things at a base level for your fuel and air flow.  You need to know that those adjustments you made are spot on and not just spot on when your sensor feed back keeps it that way.

What happens when your car cannot run right down a given sensor?  In my honest opinion then that means your car is not safe.  If your fueling is so wonky that you must have maf readings that then compensate you might not realize this if all you ever did was tune the entire system.  I think some of you would be surprised with your DIY tunes if you tried to run speed density.  Or unplugged your 02s.

I see it preached a lot to try and have 02 corrections near zero.  Well I agree, but how do you even know that the narrow bands are correct?  In my experience with personal cars and dozens of customer cars logged with back up wideband data for various reasons the stock values are garbage.  I know we all shoot for minimal trims but I know that most have them all over the place.  Thats scary to think about when fuel differences and atmosphere differences come into play.  I do not even feel that depending on STFTs is safe.

To conclude what I am saying, my advice is to lock you car in with true wideband data and limited me7 internal feedback.  For sure feel confident in running with no 02s, no maf, no egts to know where your fueling really is.  If you see weird timing jumps, eliminate the timing changeovers either in maps or physically.  When you get it all good, then give those sensors back to the car it only makes it better.  Also if your MAF is off, you know for sure, because if it is calibrated properly your fueling should not change drastically, it will be slightly leaner on MAF yet they should follow each other.

I reckon that's very good common sense advice, i.e. applying a level of due diligence to your primary sensors and ensuring the base tune is sound before fine tuning. TT owners are lucky enough to have wideband HEGOs anyway and my LTFT has been hovering around 1% ever since I had my pro tune so that's all good news. I clean my MAF and IAT sensors regularly (every few months) and check for DTCs on the engine management module every week. If anything is suspect, I will get a new sensor ordered up. In three years, only the coolant temperature sensor has failed so I don't accept that Bosch sensors are 'trash'.

IIRC, the Vagcheck guys used to issue similar cautions about tuning with the car in a potentially unfit state and they have a good reputation for diagnostic thoroughness and pre-tuning diligence checks.

Doug

Luck of the draw. On MKIV's these sensors go bad left and right. 1 out of 4 customers complain about bad part throttle response and turns out its just a bad MAF. I certainly do NOT trust these sensors.

Yes, they do seem to be. Luckily, I've not had any problems. Owners of older TT wanting a tune were always advised to upgrade their coil packs. Some guys even carried spares in the car!

Doug

I second a few thoughts:

1. get fueling right with minimal use of sensors
2. use feedback (the sensors not included above) and make sure things work just as well
3. incremental adjustments always, nothing radical

And I have a question. Is modifying existing maps recommended, or creating "big maps" ? What I means is, when a map with a 2D domain is to be modified, say RPM and actua.l load being the domain's axes, should we simply scale the load axis to bring the maximum higher, or create an extension to the map beyond that load?

Using big maps may require the maps to be moved around a bit in order to fit in the .bin file, sort of like playin Tetris in the memory