Ford Tuning - Auto Trans Gear Ratio using SCT PRP
Ford Tuning - Auto Trans Gear Ratio
The vehicle speed in the shift functions is not always the actual vehicle speed that the transmission will shift at, at that throttle position. Here is why.
First there is a parameter called N/V, say N over V (this actually shows up on the Dynojet software right before you click OK, to a graph with all the values on it). This is the engine speed (N) over the vehicle speed (V) of the vehicle in direct drive ratio of the trans (typically 3rd gear in a 4 speed automatic trans). A typical 3.27 axle ratio Mustang has an N/V of 44.5, meaning that in 3rd gear (with an auto) for every 44.5 increase in engine RPM, you get a 1 MPH increase in speed.
In addition, there is a value called TIRE_REV, that is the number of revolutions that a tire makes in one mile. You cannot calculate this based on the tire size/aspect ratio since when the vehicle is loaded the tire has some "squish" to it, thus increasing its rev per mile since it's now shorter.
Now, the transmission shift schedule is setup for a certain N/V, this is called BASE_N/V_OF_VEHICLE. There is another value on some older cars called NV_PRODUCT. This value should be the BASE_N/V_OF_VEHICLE multiplied by MULTIPLIER_NV. These values are used to help calculate the correct N/V from the speedometer gear. On vehicles that don't have a speedometer gear, these last two values will no longer be available.
All of the speeds in the shift and lock schedules are based on this BASE_N/V_OF_VEHICLE. So, if you change the axle ratio what happens?
Assuming there is range in learning, and this will be covered later, it adjusts the shift schedules if you give it the right information. On, the cars where the PCM sends the vehicle speed info out to the dash cluster this means ensuring that the axle ratio and tire rev size values are correct. On cars with vehicle speed sensors, this means changing the speedometer gear so that the speedometer reads correctly. This is important since the PCM always assumes that the vehicle speed is correct whether it's calculating it from tire size and axle ratio, or getting it from the speedometer gear.
So, let's say you put a 3.73 axle in a Mustang that was setup for a 3.27. The MPH in the shift and lock functions is calculated by the BASE_N/V_OF_VEHICLE divided by the current calculated N/V of the vehicle. The PCM is smart enough to calculate a new N/V based on the driveshaft speed (which it knows since it has a sensor to measure this) and then divides this by what it thinks the vehicle speed is, based on info from the vehicle speed sensor or the programmed tire size and axle ratio. The PCM now treats this at the real N/V of the vehicle.
Here is an example. Let's say you change the axle in your Mustang from a 3.27 to a 3.73. At 60 MPH, with the 3.27 axle, the driveshaft is spinning 44.5 (this is the N/V of a production Mustang with a 3.27 axle) times 60 or 2670 RPM. Now, at 60 MPH the driveshaft is now spinning 3030 RPM. So it calculates a new N/V of 3030 divided by 60 to get 50.5. So, it would take the BASE_N/V_OF_VEHICLE, 44.5, and divide by 50.5 to get a ratio of 0.88. So, it now takes all the speed values in the shift and lock functions and multiplies them by 0.88. So, if you had a shift setup to be at 50 MPH, with this axle change, it would be at 50 * 0.88 or 44 MPH actual vehicle speed, but the trans shift schedule still says that shift occurs at 50 MPH.
Now there is a maximum and minimum amount that is allowed to be learned for this correction. In most cases, the best thing to do is to just allow up to 25% adjustment either way, so it is recommended to make MIN_LEARNED_N/V to 0.75 and MAX_LEARNED_N/V to 1.25. In some newer applications, rather than a ratio, it has a min and max clip on the actual learned N/V itself, rather than the multiplier. The values are called the same; they will just appear as an actual N/V rather than a multiplier.
The gear ratios for an automatic transmission can also be changed. If you switch the gear set in your 5.0L Mustang to a 4R70W, you will need to change these values to make it work correctly. The gear ratio values for an automatic are 1ST_GEAR_RATIO, etc.