Tuning For Emissions
 

Im trying to get my 95 miata to pass emissions, i have a cat and "evap", but no egr, but arizona doesn't check for egr so im not worried about the equipment inspection. when i first tried to pass emissions, i failed with 4x the amount allowed for HCs and COs, but passed on NOX. The second time i leaned out my idle and set the car to fun at a fixed 10 degree timing and made it so acceleration enrichment only kicked in at 50% throttle, and i still failed but was only 30% over on HCs and 2x on COs. I tried a 3rd time and set acceleration enrichment to kick in at 75% throttle and took some fuel from everywhere as i still passed on NOX so i knew i had some room to lean it out, I still failed on HCs but only by about 15% and I passed on COs, but I failed NOX at 15% over. My mods are an MS3pro, long tube headers, COP conversion, Skunk2 throttle body, a custom intake to let me delete the MAF sensor and run an iat sensor but still use the stock airbox. Other than putting some fuel back in to get NOX back down, what can i do to get HCs to go down even further and COs to not go back up, can i just mess with timing, or will i have to get kinda creative, with some none tuning solutions. Ive also attached the maps ive used for my fist and last attempt

HC and CO mean incomplete combustion. NOX means your cylinder is too hot. From those 2 things and your readings, I can only think that you would need to add fuel so that you're not creating NOX, and then add timing. The fuel needs more time to burn before heading out. After that, make sure your CAT is hot - don't turn the car off while you're waiting for them to get it on the machine.

i have a good guess how much fuel to add, to get NOX to be within the limit, but im clueless on timing, would 11 or 12 degrees be enough, or should i advance it to 14?

Normally I'd say your cat is junk, but it seems like your tune is junk. You can find base miata tunes all over this site that will get you in the ballpark for N/A miata ignition timing.
If you're really only running fixed 10deg of IG, then your combustion stability is likely very poor (because it's super retarded at low load), and it's also using more fuel (meaning higher emissions) to make the required torque to drive the mode (because it's super retarded and IG vs torque is a 2nd or 3rd order equation.)

This is from the NB1 MK Turbo base map:
https://cimg2.ibsrv.net/gimg/www.mia...047e374614.jpg

Im not particularly set on what the timing is, im just not good at tuning so I set it at a fixed timing as simple and quick solution, but its clear that im going to need to make a full spark map. If i were to use the base spark map would i be good to leave as is or should i modify it a bit such as removing or adding timing?

Yes. Start with a base map, and then adjust timing blindly. :beer:

and overall would retarding or advancing timing help?

Typically you want to target 10-12 around idle for emissions, 12-20 for most other applications. I’m not in front of a computer, but what’s your target afr at idle? Are you using EGO? When’s the last time you syncd timing?

I developed my smog tune with a gas analyzer. My test in CA on an early obd1 car is just a tail pipe at idle and 3krpm, (no dyno/nox) which is less difficult. In your case you need everything to be very stable.

Ultimately the afr needed to be 14.8-15:1 and the ignition at 9 deg/idle and 20deg at 3krpm. If they do not check the timing I would drop it down to 7-8. Lower timing will lower the nox and co. The lower timing will increase the egt which will make the cat more efficient. The lower timing can raise the hc, but can be offset with the afr target.

Ideally you need to tune on a dyno with a gas analyzer. One little trick that has worked for me is a little bit of methanol which runs cooler. There is a product called heet which is an easy source of methanol. It comes in two varieties, iso heet in the red bottle which isopropyl based and the yellow bottle which is methanol. 2 bottles per 5 gal has helped a little. Good luck.

I just looked at your tunes. You cant just use fixed timing unless the test is only at idle. To put together a tune to pass a dyno smog test that is testing HC, CO and NoX you need a very good cat and be very good at tuning.

You need to pay someone with a dyno and gas analyzer to do this for you. Your tunes are not even scaled correctly to maximize the resolution of the tables. Below are some rough tables. I'm leaving on vacation shortly so I'm limited on time, but maybe you might get an idea from the pics below. Still after developing a good strategy for your tables you still have to tune the VE table to be close to the AFR table. You don't want the EGO correction working excessively. A stable idle is paramount. It needs to be good without idle controls and great with.



https://cimg5.ibsrv.net/gimg/www.mia...ecadbb27b4.jpg
https://cimg6.ibsrv.net/gimg/www.mia...0144959945.jpg
https://cimg7.ibsrv.net/gimg/www.mia...c8b1c17bbd.jpg