How to: Flex Fuel on C5
Pro
Joined: May 2013
Posts: 740
Likes: 83
From: MN
Interesting, thanks again for this. I've been running v3 on my M6 car for a couple years now and it's been working great. I can't remember whether I started with one of your .hpt files or whether I pulled a stock '04 Z06 file from the tune repo as a starting point, though. Do you think there's any reason to mess with it?
Thread Starter
Melting Slicks
Joined: Nov 2013
Posts: 2,049
Likes: 316
From: USA
Probably not, there's quite a few diffs in idle spark and stuff like that you'd expect between a MT and AT car. The throttle mapping is quite a bit different too, which kind of surprised me (A4 is a lot more aggressive in normal mode... as in TC on and not in comp driving mode).
Worth a look/compare probably, but if you're happy with it, no reason to mess with it. If it was me, next time I changed something significant or was bored I'd probably start over from scratch with the stock M6 file.
Worth a look/compare probably, but if you're happy with it, no reason to mess with it. If it was me, next time I changed something significant or was bored I'd probably start over from scratch with the stock M6 file.
Racer
Joined: Apr 2014
Posts: 408
Likes: 491
From: Grand Forks ND
2024 C5 of the Year Finalist - Modified
2023 C5 of the Year Finalist - Modified
Great writeup on this thread. I'm thinking about taking my supercharged 02 down this rabbit hole. Hoping for a cheap/easy large power increase.
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1st Gear
Joined: Jul 2024
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aaronc7. I am finishing motor build on my 2002 C5, forged bottom end, twin 55mm precision turbos, and so on. Now I am at the point of figuring out E85 as the 91 octane varies so much here in Nebraska and I would like to get away from meth injection that I was running on this setup previously. I have read through this thread and it has some really good information. With it being a 2002 what would be your suggestion on going about E85 conversion ECU and programming wise?
Le Mans Master
Joined: Jun 2016
Posts: 5,457
Likes: 4,017
From: Cincinnati, OH
aaronc7. I am finishing motor build on my 2002 C5, forged bottom end, twin 55mm precision turbos, and so on. Now I am at the point of figuring out E85 as the 91 octane varies so much here in Nebraska and I would like to get away from meth injection that I was running on this setup previously. I have read through this thread and it has some really good information. With it being a 2002 what would be your suggestion on going about E85 conversion ECU and programming wise?
Safety Car
Joined: Jan 2012
Posts: 3,956
Likes: 520
From: Da U.P.
aaronc7. I am finishing motor build on my 2002 C5, forged bottom end, twin 55mm precision turbos, and so on. Now I am at the point of figuring out E85 as the 91 octane varies so much here in Nebraska and I would like to get away from meth injection that I was running on this setup previously. I have read through this thread and it has some really good information. With it being a 2002 what would be your suggestion on going about E85 conversion ECU and programming wise?
Thread Starter
Melting Slicks
Joined: Nov 2013
Posts: 2,049
Likes: 316
From: USA
This thread doesn't specifically address the 2002 to 2004 wiring changes, but I know that info is on the forums.
Once you get that aspect figured out, then you should be able to follow this as written.
All I did was look at 2003 and 2004 pin out to see what moved and where, and I moved the pins in the harness.
You'll likely want to do some 2002 vs 2004 file diffing and copy over things that make sense. One thing I can think of is the 2002 fuel system is slightly different so you I'd copy over the fuel level sensor scaling values to ensure maximum accuracy. Just one example.
Once you get that aspect figured out, then you should be able to follow this as written.
All I did was look at 2003 and 2004 pin out to see what moved and where, and I moved the pins in the harness.
You'll likely want to do some 2002 vs 2004 file diffing and copy over things that make sense. One thing I can think of is the 2002 fuel system is slightly different so you I'd copy over the fuel level sensor scaling values to ensure maximum accuracy. Just one example.
Last edited by aaronc7; Mar 12, 2026 at 10:02 PM.