Hello All!
I have a 2012 L96 6.0L engine I rebuilt and built for more power, I had the block bored 0.030" oversized. I installed flat top pistons with valve reliefs -1.2cc. The factory 823 heads were never milled and I used a cometic 0.051" compressed thickness head gasket. I installed a Texas Speed L92 stage 2 truck cam. 220/232 0.600 lift 112 LSA. Long tube headers. Factory truck intake. Factory harness and factory E38 ECM Black bear tuned. Using EFI and data logging custom tuned.
I run 93 octane 10% ethanol. Under load and low rpm the engine stumbles just enough to feel a slight bump in my butt. it never stumbles at higher rpms. I replaced the plug wires and plugs and it still did it. I replaced all the coils and it still does it. I then installed an Ethanol content sensor inline in my fuel line to read the ethanol content so I can run E85 and the E38 is now tuned to correct fueling on the go according to the Ethanol content. Running E85 the stumble has gone away almost completely but every once in a while I feel a slight stumble. I replaced the oxygen sensors, mass air flow sensor and MAP sensor and it had no effect on it. Being that it stumbles always running 93 octane and is almost completely diminished running E85 I have come to believe its octane related as E85 is somewhere around 100-105 octane depending on percentage of Ethanol to gasoline. The L92 6.2 has flat tops and has 10.5:1 CR with 823 heads. My L96 with flat top heads shouldn't be more than 10.5:1 CR
Last week it did a thing at startup. It backfired through the intake at startup then cleared up and ran fine. It has never done that before or since.
What gives?

 

That's definitely not to high of compression for pump gas. You have to start getting over 12:1 to even test that. You have something else going on but you can mark that one off atleast

 

10.8:1 here... 93 is fine with my tune, but 91 is an issue...Unless I tune it for 91.

I would say it's your tune...maybe pull some timing and see how it goes first ?

 

My cam is a little lumpy but I was thinking maybe the stumble at 1500 rpms loaded was just my cam making it stumble. I sent data logs to black bear and they said everything looks fine

 

01 SS has 11.5:1 and is tuned for 93 but I still add VP Unleaded Octanium per fill up for an extra 6 points. Just a little added insurance to keep detonation in check and the car is more responisive which puts a on my face. There's a couple of fuel stations around here that sells 90 octane ethanol free and I'll be trying some of that fuel with the VP additive soon.

 

99% chance you need a good tune. The other 1% chance involves a psychotic break and the car not even existing.

 

I agree with others about the tune….but the backfiring through the intake is mechanical. I’d do a compression AND a leak down test to confirm that everything upstairs…and downstairs is playing well. If they check out, I’d then send my injectors off to be flowed and cleaned…dirty or sticky injectors can cause a stumble.