[SpencerC_6MT]

Hey yall, I've been having issues with my car after a downpipe install in December and I'm hoping to get some other opinions on it. For context, the car also has MHD Stage 2 installed, and the issue only gets slightly better when E40 is put in the tank instead of 93. Plugs and coils replaced ~20k miles ago, stock gap NGK's and Turner coils. With an e-mix in the tank, I've tried both the 93 and E40 maps, which react similarly.

~2 months and 2000 miles ago, I replaced the stock downpipe on my M240i with a 200cell catted downpipe, and started having issues. At first I had an obvious leak from the v-band, amended by installing a new gasket (bought lightly used, seller provided a brand-new gasket) and really tightening the v-band, but my car continues to have timing pull issues. I have logs, which I can upload later, but the TL;DR is that everything you'd check for seems perfectly healthy. AFRs are consistent and healthy (12.5-12.1:1 near redline on 93 map, 12.8-12.5:1 on E30 map), fuel rail pressure hardly ever drops below 3000 psi, etc.

On a typical pull before the downpipe, I'd usually see ~2.5-1.5* of pull on cylinder 1 with minimal to none on other cyls. Post downpipe, I usually see similar characteristics, 3.0 is common on cyl1 with most others less than 1.0. But on an "off" pull, 6.0 cyl1 with around 3.0 on other cyls is pretty expected, which happens roughly one in five full-throttle events. Very occasionally it gets real bad, the worst I've ever seen being -12.5 cyl1, in which case the engine fully cuts power if I don't catch it first, but with no clear explanation. Even after these horrible ones, I can often just upshift and floor it again with no timing cut.

The strangest part, I've come to notice, is that gradually leaning into the throttle seems to make the problem more likely. Quickly going from 10-100% throttle will usually give me clean power till redline, but leaning into it over the course of a second will cause strong timing cut which slowly gets less severe over the rev range. Even weirder is that the timing cut appears before I reach full throttle, while the turbo is still spooling. For example, when I'm in my parking structure, I'll often go from 0-50% throttle and back while in 2nd just to hear the turbo noise - oftentimes I'll look down to see the datalogger caught 4.5* of pull in this case, even though boost doesn't build past 5psi and the AFR is seemingly healthy.

My best theory is that the higher-flow downpipe is causing the turbo to spool sooner than the DME expects for a given throttle position, thus causing the car to run lean in transient conditions, and the car continues pulling timing as a precaution. Checking the "aftermarket downpipe" option in MHD, however, didn't seem to make any difference, and I've also read that the option doesn't affect the tune - only prevents catalytic efficiency codes from triggering the CEL. However, if MHD wasn't set up properly to work with an aftermarket downpipe, I'm certain we would've heard about it by now.

My current plan is to replace the v-band and gasket when I get the chance, but I wanted some opinions in the meantime. Appreciate anything yall can contribute.

[n55david]

Originally Posted by SpencerC_6MT

Hey yall, I've been having issues with my car after a downpipe install in December and I'm hoping to get some other opinions on it. For context, the car also has MHD Stage 2 installed, and the issue only gets slightly better when E40 is put in the tank instead of 93. Plugs and coils replaced ~20k miles ago, stock gap NGK's and Turner coils. With an e-mix in the tank, I've tried both the 93 and E40 maps, which react similarly.

~2 months and 2000 miles ago, I replaced the stock downpipe on my M240i with a 200cell catted downpipe, and started having issues. At first I had an obvious leak from the v-band, amended by installing a new gasket (bought lightly used, seller provided a brand-new gasket) and really tightening the v-band, but my car continues to have timing pull issues. I have logs, which I can upload later, but the TL;DR is that everything you'd check for seems perfectly healthy. AFRs are consistent and healthy (12.5-12.1:1 near redline on 93 map, 12.8-12.5:1 on E30 map), fuel rail pressure hardly ever drops below 3000 psi, etc.

On a typical pull before the downpipe, I'd usually see ~2.5-1.5* of pull on cylinder 1 with minimal to none on other cyls. Post downpipe, I usually see similar characteristics, 3.0 is common on cyl1 with most others less than 1.0. But on an "off" pull, 6.0 cyl1 with around 3.0 on other cyls is pretty expected, which happens roughly one in five full-throttle events. Very occasionally it gets real bad, the worst I've ever seen being -12.5 cyl1, in which case the engine fully cuts power if I don't catch it first, but with no clear [...]

Turbo spool isnt the issue..

First two thinga id check is..

1. Check for anything loose that could rattle and fix it. Could cause the dme to think one noise is something else and pull timing.. 🤔

2. Pull the downpipe.and check that the substrate isnt broken inside and moving around..

Basically there is nothing a downpipe would do by itself if everything is in good working order and installed correctly that would cause that... and aince this started immediately after the dp swap...