Results 1 to 5 of 5

Thread: M156 leanest safe AFR for stock primary cats?

  1. #1
    Tuner in Training
    Join Date
    Nov 2016
    Posts
    38

    M156 leanest safe AFR for stock primary cats?

    I was going to delete the primary cats and make a new front section of the exhaust but hard-to-find flanges and effort to weld the bungs, bends and mounts is making me have second thoughts.

    What do you guys thing the leanest safe AFR at redline is that's safe not to damage the stock primary cats that are very close to the exhaust manifolds?

    I would normally run it at about 0.87 lambda or 12.8:1 if only the secondary cats are in place but this is probably a bit hot for the primary cats.

  2. #2
    Tuning Addict 5FDP's Avatar
    Join Date
    May 2012
    Location
    Rogers, MN
    Posts
    9,051
    I'm not a Mercedes guy but GM's can run 13.0 on the newest cars without a single problem.

    It's really not a worry anyway, that fuel ratio isn't going make things too hot. It's other factors like ignition timing or unburnt fuel entering the cat that you'd have to worry about. If the explosion stays in the cylinder where it should, the exhaust gases afterwards shouldn't be a worry at all.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

  3. #3
    Tuner in Training
    Join Date
    Nov 2016
    Posts
    38
    Quote Originally Posted by 5FDP View Post
    I'm not a Mercedes guy but GM's can run 13.0 on the newest cars without a single problem.

    It's really not a worry anyway, that fuel ratio isn't going make things too hot. It's other factors like ignition timing or unburnt fuel entering the cat that you'd have to worry about. If the explosion stays in the cylinder where it should, the exhaust gases afterwards shouldn't be a worry at all.
    Interesting. It's just that most factory calibrations these days seem to want to run richer at high RPM with lots of mentions of catalyst temperature. I guess even if the cat substrate stays intact, we still wouldn't know if the leading edge of the coating was burnt off, reducing the effectiveness of the cat because the cat check with secondary O2s isn't very fussy.

  4. #4
    Tuning Addict 5FDP's Avatar
    Join Date
    May 2012
    Location
    Rogers, MN
    Posts
    9,051
    Richer fueling is safer and they always tend to be richest at peak torque. Catalyst life is surely something they factor too, they want it to last a long time and they want anyone anywhere in the world to be able to drive it without risk of hurting something.

    Going from something like a 12.0 to a 13.0 for sure won't cause it to melt down.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

  5. #5
    Tuner in Training
    Join Date
    Jun 2019
    Posts
    28
    catalytic converters can handle any amount lean value. when you run lean EGT's go down because there isn't any fuel to burn. what damages catalytic converters is excessive fuel and ignition of excess fuel within the cat.