leaner mixture produces more nox and will kill your cats faster. your cat needs a certain amount of co in it to work effectively. nox + uv = smog

save your cats and just go catless till its smog time. i never understood the point of getting a hi flo cat. a cop thats going to cite you for catless downpipe is going to know the difference between a 3inch pipe and the stock one.

 

I agree with this scenario(after being pulled over). However, a cop sitting in traffic behind you will smell a non-catted exhaust much more easily than a hi-flo catted dp, which makes you more likely to be pulled over in the first place.

 

I was recently at an exhaust shop and chatting with the owner, he told me cops are cracking down hard on catless cars now. It's no longer a fix-it ticket or smog ref. They take your cars to the smasher now, which has happened recently to two of his customers.

From his experiences with his customers, he said at least if you have a cat, they tend to be more lenient and give you a ref and fix-it. To me, it's not worth the 20-30hp I'm losing by having a high-flow. Also, catless cars sound different and smell, a HUGE tell for any cop nearby, especially behind you.

Do as you guys will, my high-flow has 110k miles on it, and I passed smog with some 0s on my emissions.

Also, if you want to be legal, smog shops can take a legal cat replacement part, cut the piping, and put 3" piping on it and weld it in your exhaust and it's completely legal. Cats today are much better flowing, so legal replacements are basically high-flows.

Edit: Forgot to add, while leaning out a mixture can create addition NOx, since the factory tune is very rich (except for on a few select years), I believe leaning it out shouldn't increase the NOx significantly as long as it's a good tune.

 

Wait people are getting their cars crushed for having no cats? What cars were they? I've never heard about this before

 

Didn't mean to derail this thread too much, but one was a catless mustang the other was a catless corvette. Shop owner said both looked very flashy too. Never saw the cars in person.


To the OP, any new updates on your car?

 

You might want to swap out the CAI with a stock one, unless it has a carb sticker. Even though i passed smog with my mods, i failed in visuals.

 

Hey guys, thanks again for the great info! So here's what I did:

1. Swap out Cobb high flow catted DP with stock DP (2 cats)
2. Putt putt over to the smog check station on first gear
3. Had to wait for another car to finish up first, so I left the car on
4. PASS WITH FLYING COLORS!!
IDLE: HC - 17; CO - 0%
2500 RPM: HC - 5; CO - 0%

Talk about a HUGE difference. Now... to hypothetically put it all back ;-)

Edit: Oh... you guys should have seen the guys face at the smog check station... like, "What the deuce did this guy do?" Bwwaahahaha

 

Ditto. I want to know how its even legally possible for them to do it. Worst they can do is tow + ticket.


@OP - Good job those are some healthy numbers. Now go gut your hi flo catted pipe!