Narrow Band Tuning Risk Mitigation

[tschmitt416]

Background:
So my wideband failed (AEM sent a replacement, but it's gonna be a while) but I need to get this car working in the next week or so. It's a 2005 Jeep Wrangler 2.4 that I turbocharged and put a spring on the wastegate so it will only make like 5 psi of boost max. I have an SRT-4 MAP sensor and have rescaled VE, and spark tables and am ready to test. The car runs and drives fine when I go conservatively, but I'm not gonna try boost until I get some advice. The engine is the same block as the Neon SRT-4, but has a higher compression ratio (9.4) and normally takes 87 octane fuel. When it was stock, I could never get the thing to go open loop while driving. The tune file has PE trigger at 5V TPS, which never happens. So even in WOT, the Daimler Chrysler engineers seemed to think this thing was fine running stoich AFR.

Question:
I'm looking for opinions on the risk level of me tuning everything (including boost) using LTFT in closed loop at stoich (like 14.3 AFR because ethanol). Like I said, I will not go above ~20 psia (5 psi boost). I would tune the VE until I get zeros all around in LTFT, then go back in and set PE to the AFR I want under boost and lower the TPS threshold to enable PE to like 3ish V (open to suggestions). I am running 93 octane now. I extrapolated the spark timing tables to scale for boost, and would probably go in and just subtract like 3 degrees all around while tuning at stoich to avoid knock (add it back once I verify PE with wideband). Also, 93 octane has an auto-ignition temperature 150 degrees C more than 87. Will 5 psi really increase engine temps that much? Especially with a 3 degree timing retard?

I have read about people doing this and realize there are mixed opinions. I think I have mitigated risk by using high octane fuel and a timing retard, especially considering the thing never bothered to enrich at WOT anyway. I am an engineer, not a tuner and do not pretend to be an expert. I humbly ask for the opinions of those of you with more experience than me. Thank you.

[Evo8john]

It's a bad idea to run that lean N/A even, let alone with any positive manifold pressure. You need to get some more fuel in there or it's going to get hurt. If you must drive it, just err on the side of caution and over fuel it for now, but I would just stay out of boost all together until you get a wideband back in it.

[Hemituna]

Stoich @ 5lb boost wont ever hurt it unless of course you are running toooo much timing.

[tbrtuning]

Stoich @ 5lb boost wont ever hurt it unless of course you are running toooo much timing.

Agreed. It will lean out some from adding timing so keep that in mind if you tune it like that. 5 lbs of boost and stoich probably occurs a decent bit on OEM Hellcat tune even I would guess.

I would recommend wideband to know for sure what it's doing but you could do what you're saying to do a quick tune and get it close. You could add some timing and get an idea of what fueling is going to do when you're done before enabling PE also.

I think many people get far too hung up on a certain AFR but it really is a combination of AFR and timing. Even after you enable PE and add a little timing if it leans out a bit more than you expect it should be fine as long as you aren't getting knock.

[tschmitt416]

Thanks for the replies. I figured 5 psi was low enough that stoich wouldn't hurt anything, but it's good to hear from some people who have been doing this a while. I absolutely agree that wideband is the way to go, and I will verify my work once it arrives.