jaybnve said:
I'm used to working on the older iron myself (60's Ford musclecars), and in those vehicles, equipped with carburetors, altitude is critical to the tune. Typically you will jet the carburetors down at higher altitudes, to compensate for thinner air at higher altitudes (you will get less oxygen at WOT, so you need less fuel to burn with it).
For the GT, and with any modern EFI vehicle, I can't imagine that there is a big difference in how the vehicle is tuned for various altitudes. First, the GT is equipped with a MAP sensor, so it knows the absolute pressure in the manifold. Variations in air pressure caused by altitude differences can therefore be compensated for automatically by the ECU. Further, there are two oxygen sensors in the exhaust that monitor the air fuel ratio. The ECU is designed to monitor the A/F ratio with the oxygen sensors, and add fuel to the mixture if it goes lean, or take fuel away if it goes rich. I guess I don't understand how a change in altitude could affect the tune, given the engine controls of an EFI engine.
Having said that, it is certainly clear that the engine will make more power at lower altitudes, simply because there is more oxygen available to burn with the denser air. Of course, the supercharger tends to smear this difference a little, as compared to a naturally aspirated vehicle, but the supercharger is not electronically controlled; it applies a fixed amount of airflow based on the engine speed and pulley sizes. So, at any given engine operating point, the engine should be making more power at sea level than it does at higher altitudes. Of course, that is one of the correction factors applied to dyno results.
Short answer - I personally don't believe that the altitude you are at will affect your tune parameters; sounds to me like it is just rumor, left over from the days of carburetors. If I'm wrong about this, I'd love to hear a good, technical explanation of why ....
Chris, that is an excellent reply, sir! If you would not mind, I would like to expound on this subject somewhat.
First off, the front O2 sensors (HEGO - heated exhaust gas oxygen sensor) help to dictate fueling during CL (Closed Loop). CL operation generally happens when the vehicle is at least 120 degrees or higher, depending on the calibration, and the time delay for OL (Open Loop) to CL transition has expired. The pcm always attempts to obtain a stoich value for fueling. Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. Gas stoichiometry applies when the gases produced are assumed to be ideal, and the temperature, pressure, and volume of the gases are all known. Thus the general stoich ratio of gasoline and air is 14.7:1 (Ford uses 14.64:1). This will produce the best emissions and drivability.
At WOT, or beyond a specific TP (throttle percentage), the vehicle will go into OL, and run off of, you guessed it, the OLFT (Open Loop Fuel Table). On a Ford, this table is generally comprised of 10 columns on the X-axis, and 8 rows on the Y-axis. The X-axis is expressed in RPM, and the Y-axis is either in TP (Counts), or Load (aka VE; more on that in a moment). Within the cells that make up these columns and rows, there are stoich values that the pcm will use to dictate the fueling of the vehicle. Here's an example I just whipped together to show is the OL fueling of a Ford GT:
(These values below are only "Commanded", not "Actual". For Commanded to become Actual, the MAF transfer must be properly calibrated; more on that in a moment.)
Amongst many other sensors, there is also the MAF, which is the sensor at/near the airbox on most vehicles. This sensor measures the amount of incoming air volume. The MAF is the most critical sensor on the vehicle, and it should always be properly calibrated/tuned throughout the entire curve. Within the calibration, there are 30 points to a Ford MAF transfer. Each of these points account for the actual voltage output of the MAF transfer, expressed either in volts, or A/D counts, which represents voltage digitally. Furthermore, the output of the MAF transfer function is used to calculate volumetric efficiency, expressed in Load % (0 to 200%). Without VE being correct, the pcm will not use the correct spark and fuel values given from their respected tables, thus possibly resulting in an extremely dangerous situation.
With a MAF equipped vehicle, so long as the MAF is not pegged (more on that in a moment), and the MAF transfer is properly calibrated, the pcm is always able to properly adjust the fueling and the spark for a given amount of measured air charge. This is how manufacturers are able to make 1 tune for the same car so that the vehicle can operate 100% properly within all conditions. In the case of custom tuning, a MAF controlled vehicle properly tuned at sea level, say Mobile, AL as an example, can drive to Denver, or even higher elevation, and the pcm will properly adjust for the change in density. Futhermore, most all air meters have temperature compensation, both built-in the meter and in the pcm calibration. If you cool the engine area off enough for ACT1, or inferred ambient, to reduce, it will modify the MAF transfer function.
In regards to a "pegged MAF", this is nothing more than the electronics becoming saturated. Basically, the 5v (1023 counts) MAF limit of the pcm is exceeded, therefore the pcm does not use anymore data from the MAF, and in turn will use a default Load (not as in "weight", but rather as in incoming air volume) table to determine VE. Of course this only happens when the MAF is beyond the 5v limit. Since the pcm isn't able to actually measure true air Load, and instead uses default data to gather this from, generally spark will go up and fuel will lean out = BOOM! Ford inputs in a 4.785v limit (980 counts) from the factory, but this can be increased to 5v (1023 counts) to provide a slight bit of more room.
With all that being said, I've only touched base on some of the main areas, but obviously there is so much more to the inner workings of a pcm, as well as tuning, than this. So many smaller things make up one large picture. This is why I feel that ALL individuals with performance vehicles should educate themselves within this industry. It not only helps ensure that you protect yourself, but it actually promotes better tuners! Here are some questions that you can utilize when finding a potential tuner:
1: When you tune the fueling of a vehicle, specifically say for the OL (Open Loop) portion of the tuning, do you datalog the measured output of the MAF versus AFR (air/fuel ratio), in order to calculate difference ratios, from the commanded AFR versus the measured AFR, to apply to the individual airflow values of the MAF transfer, so that your commanded AFR now becomes actual AFR? Or do you instead just tune the AFR strictly through the OL fuel table, from which commanded AFR can never be actual AFR, and calculated Load by the pcm will not be correct?
2: What about for closed loop tuning? Do you map out the MAF transfer via a datalog of the measured output of the MAF versus the STFT (short term fuel trim) values, in order to calculate and apply difference ratios to the individual airflow values of the MAF transfer, so that the STFT values are attempting to achieve within +/- 5% of stoichiometric? Or do you instead not even calibrate these areas of the MAF transfer?
MDT said:
Can the dealer tell that you have changed your tune? And deny your warranty? Or can they tell even if you reflash the orginal? That's my main concern until the warranty expires at least. It's a 30k engine plus install. The only reason I would consider it is that they say the engine runs cooler and/because the fans turn on quicker.
MDT, I made a post here concerning just that:
http://www.fordgtforum.com/forums/showthread.php?p=38211#post38211
I hope all this helps someone!
Kind Regards...
