6thgenva

Long story short I converted obd1-obd2 and got it tuned on nepTune. Well for a number of reasons I need a high flow cat. Well the cat I am looking at doesn't have a spot for an o2 sensor. I am aware that normal obd1 cars get all their afr from the o2 sensor further up. Since this car was originally obd2 I don't know if it still receives that info from the one further up or the one on the cat. Also the car was tuned with the stock cat on , how much will the high flow cat throw my tune off...(will it hurt me more than help me). I'd really appreciate some advice.

Fabrik8

The new cat will change tuning if there is a difference in exhaust flow, like a change in backpressure or velocity. A cat isn't supposed to present much of a flow restriction but that's really hard to actually do, and different cat designs and models won't be the same. So it may be worthwhile to get it retuned, but it's pretty difficult to know how much of a difference it will make.

The upstream O2 sensor is still the primary O2 sensor in OBD2 setups, and the downstream (rear) O2 sensor is usually used to determine whether or not the cat is working efficiently. That's why a lot of cars have a wideband O2 sensor before the cat and the other O2 sensor is often a narrowband.

6thgenva

Alright so don't bash me up here I'm just trying to get everything straight. So if I get the cat I don't need the backflow o2 sensor since the afr is determined by the upflow sensor. My problem lies with my increase in air flow in the exhaust , which would lower my back pressure (which is normally a good thing). However, since my car is tuned to the specs on it now I have a possibility of running lean since the car is tuned for its current set up.

So can I conclude that the ecu will not automatically adjust the afr from the upflow o2 sensor to match the reduction in back pressure ?

Fabrik8

I'm not bashing anyone, I'm just answering your questions.

The ECU will adjust the AFR, yes, but the change in backpressure may change more than just the AFR. The change in backpressure will affect the efficiency of the engine (for the better usually) so the tuning may need some slight tweaking. Talk to your tuner and ask their opinion; they've probably dealt with this situation before and have some insight on how much (or how little) tuning needs to be changed. Like I said, it's hard to tell how much the different cat will affect things, so it's also possible that nothing at all needs to be changed with the tuning. Ambiguous, I know.

6thgenva

I wasn't talking about you man. You have helped me out plenty of times. Thanks man....I was referring to others up here

Thanks man that helped alot, and Ill talk to him to see his insight on it