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**Notes**
I thought this might be useful to have in an easy-to-find place. This was originally posted in another thread, but I always had a hard time finding it and have re-written it several times in the past. I would also like to add a PID closed loop boost control one as well (whenever I find it).
EDIGREG made a very nice fuel tuning post on quattroworld. It goes into a lot of detail on getting started with tuning fuel, specifically in VEMS and may be just what you're looking for. http://forums.quattroworld.com/s4s6/msgs/112142.phtml This post here on the project pad is more geared toward the general concept of tuning fuel.
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Alright then, let's get started. You will be sent a base tune with your ECU based on what your setup is like (big turbo, small turbo, injector size, etc.). If you do not have a base tune, we can go into that more later. You can always ask here too! Someone is bound to have something that will work for you. The timing map will be pretty good and you should not mess with this unless you really know what you are doing, and even then you should preferably tune timing on a dyno. What you are left with then, is tuning fuel. You can start tuning fuel without really knowing much. You just have to know a few basic rules.
Since you are pretty new to this whole thing, I'll start with the basics. For gasoline, the ratio at which the fuel burns completely is 14.7:1 (14.7 parts air to 1 part fuel). More air than that (above 14.7) means the car is running lean(er) and less air than that (and thus more fuel in relative terms) means the car is running rich(er). You will adjust the AFR (or lambda... I'll expand on that later) in VEMS by pulling up the VE (volumetric efficiency... its stupid I know, just think of it as the fuel table) table in the tuning drop-down menu. The VE table is set up with manifold pressure on the y-axis (in kPa rather than psi.. get used to it!) and RPM on the x-axis. You will see various values in each cell which correspond to how much fuel the injectors will deliver to the cylinders in that particular load cell. If you want the car to run richer, you simply increase the value in that cell. If you want it to run leaner, you just decrease the value. That is pretty easy while just cruising or idling where you can hold the engine at a specific RPM and MAP pressure, but it takes some practice to track the path of the engine across the map at wide open throttle and then make adjustments to the map accordingly. So now that we have the basics out of the way, lets move on to how much fuel you really need in any given situation.
As a rule of thumb, you should tune idle, cruising, and anything light load at 14.7:1 air to fuel ratio since that is the ratio at which the fuel burns the most efficiently. It'd be great if we could keep the mixture that lean under all conditions, but that just isn't the case for various reasons (mostly to do with heat).
Now... before we go much further, lets switch over to talking about lambda rather than air to fuel ratio because that is what you are going to see while tuning with almost any standalone engine management systems. The conversion to lambda is xxx AFR/14.7 = lambda. So if your current AFR is 14.7, then lambda is 1 (14.7/14.7). If your current AFR is 10, then lambda is 0.68.
So as I was saying before, tune your light load, cruising, and idle to run at 14.7:1 AFR or 1 lambda. Just increase or decrease the value for the cell (on the VE map) in which the car is currently operating, and watch to see how your lambda reading changes. It should go down as you increase the fuel you are providing the engine (less air and more fuel = richer... richer = lower lambda value). Now, as you increase manifold pressure, you will have to fatten (make richer) the mixture up. Use the following as rules of thumb:
-Anything below 120 kPa, keep lambda around 1
-Between 120 and 200 kPa (15 psi), shoot for 0.9 lambda
-between 200 kPa and 250 kPa (22 psi), shoot for 0.85 lambda
-above 250 kPa, start to fatten up to 0.8, continuing to as rich as maybe 0.78 or even richer as you approach 300 kPa (29 psi) and above
That should keep you in a pretty safe place, yet still make decent power. Others, feel free to comment on my numbers there... it's been a while since I have tuned anything unfortunately :( I guess it has been less than a year, but it feels like forever!
So basically, you are going to go out and make a bunch of wide open throttle pulls, watch your wideband 02 sensor, and make adjustments accordingly. If you start going too lean, let out of the throttle and add more fuel immediately. Don't keep beating on it while it is lean
There is a lot more to learn than just this... you will have to know how to increase boost, set limits on boost and fuel, etc., etc... it might seem a bit daunting at first, but don't worry! It will come with time. There is a lot of of great help available on this forum, so ask away and I'm sure there will be people willing to remote tune for you or talk you through some of the steps involved in starting to tune for yourself, recording logs, reading those logs, etc.
Good luck, and I hope you have fun with it!
-Chris
