I failed my emissions test due to high nox. Max/Measure 15mph HC - 108/48 CO - 0.71/0.02 NOX - 767/767 rpm @ 2261 25mph HC -- 83/24 CO - 0.58/0.02 NOX -706/876 rpm @ 1957 engine has less that a 100 miles on it, i had a shop build the long block for me and when i got it, it came with autolite 3923 plugs, and after doing alot of research, i found some good reasons and decided i would be better off using 3924 plugs that are 1 stage hotter, but i dont recall the reasons at this time. im sure if i were to go with the colder plugs 3923 that it would help with the nox but by how much, and enough to bring it down to an average level? 199 and 237 are the average for passing vehicles. im currently in the stage of tuning, and i have it all good and ready to put on the road with a 14/15:1 afr. so im not sure if the tune is the problem or the hotter plugs, so i need to know if i need to step to a colder plug or should i richen up the mixture.

Are you using an EGR?

yep everything on my car is smog legal, have all emissions equipment on it, brand new magnaflow cats.

after doing some reading i come to find out that every stage colder plug you run will drop the combustion chamber temps 70-100 degrees celcius, so i installed the autolite 3923 plugs (one step colder). also found out that every 10* timing is advanced the combustion chamber temps go up 70-100 degrees, and while the smog test my ignition was advanced to 40* or so. so im going to burn a "smog chip" with a max ignition advance of 25* and hopefully bring the combustion chamber temps down 200*C or so. but i still dont know how well that will bring the nox down. im even thinking of making the "smog chip" have a max advance of 15*. i remember once when i couldn't get my car to pass a few years ago it was high on nox, i dont remember by how much, but the tech ran the test with the timing advance connector unplugged to make a consistent 6-8* advance and it brought the nox down below the max levels. anyone have any input on this?

High NOx is usually caused by a bad EGR valve/solenoid, or being lean.

http://www.discountconverter.com/cat...erter-&News=98

Yes, the colder plugs should help, too.

got my retest today and passed

Max/Measure
15mph HC - 108/21 CO - 0.71/0.02 NOX - 767/240 rpm @ 2215
25mph HC -- 83/25 CO - 0.58/0.02 NOX -706/266 rpm @ 2003

i did a few datalogs and found out at rpm/load the egr was only open 70% for the 15mph test and 80% for the 25mph test. i change a few things and got it to stay open 100% at that rpm/load.

things i did to make it pass: egr to 100%, one step colder plugs, max ignition timing 25*, multiplied fuel tables by 1.13 although i think the o2 sensor pulled this fuel out anyway. the smog tech said that the colder plugs probably didn't help, i still think they did cause colder plugs = lower combustion chamber temps = lower nox. i think i could have got it to pass just by opening the egr 100% and not doing the other stuff, but i wanted to be on the safe side.

i've seen a few people ask this question..... so this goes to show you that you can get a 383 stroker with a mild cam to pass california smog.

I wonder if that little snake oil can they sell at the auto stores to pass emissions would have helped.