So you've decided to swap the long legged terrible gear from the Lincoln Mark VIII and want to know your rough final RPM? Hopefully you opted-in for the correct Traction Lock and are not just throwing a ring gear on the open faced garbage in there currently.
Basically In short, you take the RPM tach reading in the 1:1 gear on the trans, with the original differential gears, and record your MPH for absolutely no reason at all. Depending on what gear you test with (3.08, 3.27 or whatever) and where you test, you could be going "fast" so watch out for the law.
Simply put, if you are at 2000 RPM at 45 MPH in 3rd with a 3.27 gear and you swap to a 4.10, you should be in the neighborhood of 2500-2600 after the swap.
So, lets say you do add a 4.10:1 and originally had a 3.27:1, your RPM after the gear swap will be the answer below.
This calculator ignores the MPH because you're in control of how fast you want to go. Just read the tach and be done. As your speed increases, so does the gap. 5000 RPM in 3rd with a 3.27 is going fast, so it will put you close to 6200 RPM at that same speed with a 4.10. See how the gap can spread? 5-600 RPM spread at 45, and 1200+ at the hauling ass speed.
Tire size is not needed with this calulator either. We think it's pretty dumb to make it that hard when trying to figure out a simple mathematical formula on how much more RPM you will run with a gear swap. Now if you change your tire height after you record the pre gear swap RPM, you're SOL using this calculator. You can always use google as your search.
Example of stock VS a 4.10 gear in our tuning software.
This is the MAX RPM at a given TP for when the shift will be COMMANDED, NOT completed.
|MPH|*|NV_Base|*|RATIO_IN_TRANS| = |RPM|
**Max recognized RPM is 5100 with stock gears due to it being 127.5 MPH
Commanding Shifts:
Ok so let's break down gear RPM in a scientific way.
The ECU is set for a base value of N/V (vehicle speed vs engine speed) of 40.
40 = 3.08 or 3.27 gears in the 93-98 ECU.
Estimated tire revs per mile with the 225/60/16 OEM tire is roughly 782 revs per mile with an unloaded tire.
The ECU OSS / Estimated tire revs per mile pulsewidth is 752 stock.
Therefore the pressures, shift schedules, lock up and unlock schedules are all following this. N/V*Multiplier=product. Which addresses all transmission shit. Now you figure 60mph * 40 * 2.839 (first gear ratio) to get a rough RPM shift to 2nd = 6800 ish.. but wait, there's more!! You have to accommodate for wot shift RPM command, timers, offsets and MBT.
A car with a 3.08 or 3.27 gear will command a 1-2 shift at wot at around 5200 RPM. While taking a brief time to complete a shift mechanically, this sets the actual 1-2 shift on the tach at 5800-6100. This is why all of you that manual shift your cars break them after too long untuned. Jmodding does absolutely nothing to the commanding shift, only speeds up the mechanical portion. You need to be able to address the timers, offsets and other things.
Now if you speed up the mechanical portion even more by adding a 4.10 (50 N/V or more) or higher number gear, your commanding shift just dropped lower. Bam, you then potentially bounce of the rev-limiter before the mechanical can catch up. Another reason these cars become donors.
Sum it up with this.
Stock cars will for sure break if beaten to hell as the tuning for the trans is lazy. Figure by changing tire/wheel size to a smaller diameter setup can act like a lower gear. Revs per mile is up in the 810 range on a 245/45/17 so math it out, and always choose your best route... and never skimp on doing research.
Revisting the N/V; Scenario: If you change the axle in your Mark VIII from a 3.08 to a 4.10.
At 60 MPH, with the 3.08 axle, the driveshaft is spinning 40 (N/V of a production Mark VIII with a 3.08 or 3.27 axle) * 60 = 2400 RPM. After the 4.10 gear swap, at 60 MPH the driveshaft is now spinning roughly 3194 RPM.
Now it calculates a new N/V of 3194 / 60 = 53.2. Finally the ECU figures a new N/V Base Value like so; 40 / 53.2 = 0.75.
So, it now takes all the speed values in the shift and lock functions and multiplies them by 0.75. Meaning, if you had a shift setup to be at 50 MPH, with this axle change, it would be at 50 * 0.75 or 37.5 MPH actual vehicle speed, but the trans shift schedule still says that shift occurs at 50 MPH.
Screen shots of how N/V will effect your tuning:
- Credits to GLB for narrowing this down some
Tuning Advice:
- Credits to Derive Systems
At WOT, the trans shifts based of one of two things, whichever one happens first. Either the vehicle speed that is in the shift schedule functions or the WOT engine speed scalars, TRANS_WOT_SHIFT_RPM_xy, where xy is the shift that that scalar controls, like 1-2. What this means, in most cases, is that this is where the shift is COMMANDED and not where it will occur. In some transmissions it can take up to one second to fill the on coming clutch. If the engine is accelerating at 1000 RPM per second (not unusual for 1st gear with a 3.73 axle ratio) that means that from the commanded of the shift to the actual shift point, the RPM will increase by 1000 rpm. So, if you set the TRANS_WOT_SHIFT_RPM_12 to 5000 rpm, the shift will occur around 6000 rpm. This is important to know when setting up WOT shift points.
Check out the RPM Calculator for more info..
