Engine internal modifications that affect SMOG

[Lefty_lifestyle44]

Hello everyone, I wanted to pose this question so I could obtain some more knowledge on the smog process and what mods can cause you to fail (and hopefully this can become a guide to others like myself who don't really understand how it all works). Most all of us know the external stipulations, things like air intakes, cat deletes, etc. But, as the title states, I'm interested in how changing up the internals of an engine can affect the outcome of your smog test and why they do so. My particular application is an M20 2.7i I plan to build someday. Thanks!

[bmwman91]

Honestly, if you run a functional catalytic converter + O2 sensor and don't have some sort of bogus ECU tune, you should be fine. Hell, even a crap tune would probably not cause you to fail SMOG as long as the O2 sensor and cat are present and functional.

I have a 2.1L M42 with zero stock internals and 11.5:1 compression, and I have never had a single issue passing the tail pipe sniffer. I got close to failing on NOx once, but that's because I cooked the cat by running rich during tuning, so I shelled out $250 to have a replacement Magnaflow cat welded in and it's been blowing clean for the 6 years since then.

The visual can be a pain, and I have talked to SMOG shop owners about it. The state has been increasingly coming down on them for ignoring the visual requirements, so many are no longer willing to take the risk. However, let's just say that I have never been failed on it.

[dasmanschaft012]

Anything between the MAF and the cat cannot be modified unless the mod has an EO number and the EO sticker is present.
A simple chip tune will probably not cause a fail, as tunes mainly correct AFR at higher RPM.
A cone filter on stock MAF is ok. A muffler is ok.
Cats must be OEM or aftermarket cats must be 50 state legal (EO number) to pass visual.
Carbon canister and purge must be present and connected (you should get a CEL without evap solenoid)
CEL must show up at key to ON, and then turn off at IGN/RUN to pass.
Long tubes, any aftermarket header fail visual. (IE's shorty headers theoretically might pass, but probably not visual. I dont have personal experience with anyone using these.)
Non-BARd motor swap will fail.
Large injectors without a tune probably will fail rich.
O2 sensor must be present and functioning
TPS must be in adjustment or you wont idle right.
MAF must be working right or your AFR will be off.
Often, one or more of these sensors are not working correctly, but they're not broken enough to show a CEL.

[bradmer]

Hello everyone, I wanted to pose this question so I could obtain some more knowledge on the smog process and what mods can cause you to fail (and hopefully this can become a guide to others like myself who don't really understand how it all works). Most all of us know the external stipulations, things like air intakes, cat deletes, etc. But, as the title states, I'm interested in how changing up the internals of an engine can affect the outcome of your smog test and why they do so. My particular application is an M20 2.7i I plan to build someday. Thanks!

This thread while 24v related may contain pertinent information to your quest.
Good luck

https://www.r3vlimited.com/board/for...ne-swap-thread