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View Full Version : Overhauling exhaust, few questions...



MrBluGruv
01-27-2010, 09:10 AM
Lately I've been kinda concerned about passing inspection as more and more push is being made to check emissions, and since I've got pretty much no emissions restriction hardware any more I feel I need to at least get back to passably legal. I figure while I'm down there hacking stuff up, I may as well do multiple things at once, but I have a few questions before I do:

1. It's been my understanding in the past that the front O2 sensors are present to measure AFR and such to control engine mixture, and the second sets are the ones designed to sniff and make sure the catalysts are doing their job, is this correct? And if so, following that logic I guess, would that mean to be legal I really only need to worry about having cats between the 1st and 2nd sets at least? when I got the first cats removed the guy was swearing a storm because of the difficult angle of those cats and the fact that my manifold flange bolts that connected them were slightly rusted, so I'd like to avoid that hassle if I can. Or if that wouldn't be enough, should I consider investing in high-flow cats to replace where all 4 stock ones were?

2. High-flow cats. From all I've seen they are essentially straight-through design yes? I want to avoid getting rid of the tonal quality and brashness that came from removing all my cats if I can, as the stock cats may as well be mufflers in and of themselves.

3. Electric cutouts. This is a much more subjective question, for anyone that's running cutouts on your MM or have run them on any of their cars, is there a particular brand that stands out to you? where would you recommend placement in the exhaust line? I do realize this isn't really a performance adder, and admittedly I'm doing this purely for the sound and volume, and with that I'm kind of curious for those that have them what kind of volume should I expect, and again where should i put them for greatest impact on volume?


Thanks guys, I'm pretty sure that's all I was curious about. I tried to collect my thoughts into one so I wouldn't spam post the board with stupid questions. :P

Stranger in the Black Sedan
01-27-2010, 09:15 AM
Any cat is going to muffle. The high flow cats usually have shorter bricks and/or modified cell spacing in the brick so there is less restriction, but you are still passing exhaust through a ceramic or metallic brick and it is going to attenuate noise.

MrBluGruv
01-27-2010, 11:11 AM
True, but my deal is I don't want to disrupt the transient "pop" and cackle of my exhaust for the exhaust note overlapping. I'm pretty sure it would reduce the amplitude of my exhaust, but not the crackling popping overlapping sound? At least that's what I'm aiming for. If it would, I'll just toss my old pair of cats back on and put the cutouts before them for when I want loud, that'd probably be a smarter move anyway wouldn't it?

Stranger in the Black Sedan
01-27-2010, 11:57 AM
Cats are cats, I'm sure its gonna reduce that sound you want (why do you want your car to sound like a pickup truck w/ glasspacks on it anyway? just kidding, sort of). A single pair of cats will be louder than your 4 original cats, if that's what you're getting at, but its still not going to sound like straight pipes no matter what "high flow" lingo is on the box of the cats

MrBluGruv
01-27-2010, 08:04 PM
Even if you aren't joking, I really appreciate the input, hell I prefer it. If I remember correctly, you've been a proponent for catted exhausts and not over-bearingly loud, so I know you won't be giving me a fluff opinion on the subject.

At this point I'm thinking what I may do is install the two cats I still have and put cutouts before them, this way when I'm running legal it's not too loud so I don't attract an outrageous amount of attention, but then can flip the switch and get absurd volume if I'm going to a meet or by chance the strip.

That being said though, can I get any confirmation on the duty of each pair of O2 sensors? I'd really like to do this once and get it right the first time.

MrBluGruv
01-28-2010, 10:59 AM
Could anybody clarify the purpose of each pair of O2 sensors for me?

FordNut
01-28-2010, 11:57 AM
The front ones are what the computer uses to adjust AFR. The rear ones are to verify the cats are working.

MrBluGruv
01-28-2010, 08:57 PM
Awesome, thank you very much sir. :)