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I figured with the addition of ecodiesel support for HP Tuners and the interest in modifying exiting tunes, I figured would start a post specifically for things I have learned by poking around in the ECM flash data. I don't have an HP Tuner device, so I'm not sure what can/can't be modified via there software, but some of this may be possible to be changed. Most of what I'll show will be from WinOLS, unless I can figure out a straightforward way to show it via hex edits.

If the information posted here doesn't make any sense to you it probably can't be explained via a forum. I say this because messing around with the ECM data can/will damage your truck or potentially brick your ECM. I primarily wanted to create a place for the "tweakers" to exchange information.

Personally, I am not an expert and have very little tuning experience (i.e. none outside of EcoD's), BUT any information I post here is reliable and I will clearly state if it has been tested on my vehicles. So, first up:

Axle Ratio Differences in Flash:

I finally got a hold of a 3.55 and 3.92 stock tune that were close enough that I could start figuring out the differences. I have successfully identified an axle ratio value within the FLASH data:


"Key Value" should be a unique value to help you find the location. For this it is: "00 01 3277 00 3277" (16 bit, HiLo, Decimal). I have tested this search and it has worked in 5 different files including Ram and Jeep. (Jeep's axle ratio was set to "00345" as expected.)

Is this the only difference? WinOLS shows 322 differences between the two files. A large number of them are related to checksum values. I have found a few other bytes with potential differences and I'll update this post if I can make more sense of the additional differences.

Next question is how do we test this? My truck has a 3.55 axle ratio and I have tested 3.92 firmware in it, but have not been able to tell a difference. But I have also not tried anything "scientific". Someone has mentioned RPM differences when using cruise control. Maybe monitor RPM's via AlfaOBD or other monitoring tool and see if there is any difference in RPM for a set cruise point?
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