The car:
2009 CTS-V
LSX block (427ci, built, 9.2 compression, dynoed 610/550 on an engine dyno NA)
Mast 305 heads
2.9L Whipple
Injector Dynamics ID850s
This car has been through the ringers and is hopefully on the road to recovery. I'll set the stage of what happened yesterday.
This particular car used to have a 434 with a TVS 2300 blower at 15psi and only made 630whp. Why the numbers were so low was always a mystery. That 434 let go when a very thing part of the block let coolant rush into the oil pan. It was replaced with the 427 that's in it now, and was switched over to the 2.9 Whipple.
We street tuned it in the morning and got that down, then hit the dyno. We began making pulls, and started off pretty conservative at 11.3 AFR and only 12* of timing for 13psi. I slowly started adding timing in and took it to 11.5 (this is on 93 octane). Everything was fine, although the whole thing was still making low power. It made a best of 670whp with only 14* in it. Anyway, I took it to 16-17* and got a weird reading on the scanner, but also felt something similar to a misfire. It happened at 3600rpm and again at 6100rpm. It also lost power on that pull. Gunshy given the history of BS this owner has gone through, we aborted the dyno session because I had no idea what happened.
My main question is... Can anybody shed light on what would cause the narrowband on bank 1 to report a quick lean spike? I felt it (similar to a misfire) but cannot come up with an explanation of what happened.
We left the dyno, and driving down the street, the blower belt decided to shred a rib, which got bound up in the tensioner. We pulled that belt off and kept driving, got a new belt, and threw it on the car. We went out to test on the street to try and recreate the issue, and it happened again at part throttle, low boost (not wide open throttle). We hit the highway and tried WOT again, and it was fine then... pulled like a freight train until the tires washed out. Now it was also making 16psi instead of ~13.5. It never did that misfire feeling again, even on repeated WOT pulls.
Go figure, the damn NW102 started sucking shut at WOT. Threw on an LS3 throttle body, same problem. We ran out of time at that point. The owner is going to try an LS2 throttle body next, and a C5 102 as well.
Anyway, my biggest concern is what was happening with the picture I posted. I would think a misfire wouldn't show a lean blip on the narrowband, and am hoping somebody may be able to shed some light on what's going on.