I have a 99 SSEI series 2 engine. im from Indiana.
I need to replace my cat and i have to stay emission compliant. Id like a high flo bolt on type converter. i believe its 2.5 in and 2.5 out dia. I have seen different grades of converters but i am unsure on the grading system and how it relates to the emissions. I know i will need to weld one side but would prefer the other side is bolt on.
Any advice would be appreciated. Thank you!!!!

In order to go "highflow" bolt on, you would have to upgrade the current pipe from 2.5 to 3". zzperformance.com and 3800performance.com have the 3" highflow cats that bolt on. Otherwise, it wouldn't be highflow.

As far as emissions goes, as long as the rear O2 sensor sees the exhaust in a different state vs what the front O2 sensor sees, you will be fine.

Now, my question is going to be, what other mods do you currently have or plan on doing to the engine?

I am installing a true cold air intake now

In order to achieve maximum results from a high flow cat, one must also increase airflow through the engine. Usually a bigger cam, ported intake manifold, bigger valves and replace the restrictive exhaust manifolds with headers and a faster supercharger. Otherwise, the highflow cat is just a waste money really.

Thanks for your advice. The cat needs to replaced anyway so I just thought goin with a high flow bolt on like the from zzzz

What is involved with inspections in your area? Is the emissions test involving a sniffer, or something different?

The nice thing about these cars is that the cats are very straight forward, and an exhaust shop can drop a unit in blindfolded, but depending on the level of emission testing, you may need to look at a heavy metal cat, which can add up fairly quick. Sometimes the standard high flow units can be hit or miss as far as keeping emissions happy.

emissions here are just plugged into the cars computer to be sure all the senors have read and completed their cycles. no sniffers for a 99 here. any thoughts?

you can go a few ways, you can just put a stock style one in and not worry, or you can strait pipe it and get your computer tuned to not look at that. or you can get hpt and do it yourself, this is the most expensive but if you plan on modifying your car in increments it will be necessary to tune your car at each point to account for the mod, and the tuners are likely going to want a couple hundred each time.

You could get a O2 simulator. Its a little device that plugs in where the O2 sensor use to go and it makes the PCM think the O2 sensor is working.

Good input guys, thank you and I saw those simulators.
Now I'm running into a new ???
I can get a stainless down pipe with a hi flow cat from ZZP. In my area the stainless would be a good idea for the winters here and road salt. The rest of the exhaust is aluminized coated metal that is in great condition. I need to know about welding the stainless to the aluminized metal pipe,,,,,,,, I have read many different "theorys" on this. Any have experience doing this? Thank you