Putting Flowmasters on. Take off the cats or no? They're going bad anyway.

[PaladinMan187]

Ok. I know this looks like a novel so if you dont feel like reading, hit the back button now. So I'm selling a car this week and plan to take a couple hundred to put a Flowmaster and SS tips on the truck. 2002 Silverado RCLB w/8.1 6spd ZF, 2WD. As I was starting the truck on Monday I heard what sounded like a metric a$$ load of marbles rattling under the truck. I tracked it down last night when I was doing an oil change to the rear cat for the passenger side bank. Yes the truck has true dual exhaust until it hits the muffler under the bed and then goes to one pipe. I had read that the 8.1 engines do not like it when you take off the cats. Can anyone verify this? I had planned to leave the cats so I wouldn't have to worry about stink testing when I got the new exhaust but now that one of them is rattling I might have the whole exhaust removed just before the cats and save it in case I fail the lovely state emissions test we have here in Maryland. My plan is to just have them cut the two pipes so I can get a slip collar to clamp them down if I ever need to reinstall the stock pipes. If these motors don't NEED cats then I'll take them off and have a slightly better flowing exhaust. If the 8.1 NEEDS a cat to run correctly I'll just have to bite the bullet and have them replace one when I get the Flowmaster put on. Any help would be appreciated. And no, there are no sensors behind the cats so I do not have to worry about kicking codes for cat inefficiency.

[Low97tahoE]

off with the cats!.....IMO

[99chevy7.4L]

From 96 on the Cats were hi-flow anyway. Taking them off or keeping them will see no gain either way. However I don't know if the Computer will like it or not hard to say. Ditch the flowmaster idea if your looking for a gain. Go with magnaflow or Borla if you want a performance gain. If you like the sound then stick with the flowmaster. If it were me look at a magnaflow 12599. Its a dual 3 inch in and dual 3 inch out with a true X-flow design. It voids the need for a x-pipe. You will notice a gain in torque at least I did.

[smiles383]

If you remove the cats you need to get the second O2 sensor tuned out or the check engine light will be on . I would reccomend getting a tune anyways .

[PaladinMan187]

If you remove the cats you need to get the second O2 sensor tuned out or the check engine light will be on . I would reccomend getting a tune anyways .

Both you guys didn't see my note at the end of my rant saying that there is NO SENSOR AFTER the cat so there is no possibility of the computer kicking a code for catalyst inefficiency. I had read somewhere that these motors enjoy a specific amount of backpressure and the cats were in place to take care of that issue. I'm not worried about the computer figuring out that they are gone, I'm curious if the motor will run poorly without the right flow rate set by the cats. YET these motors are dual purpose in that they are truck/marine motors and the boats that have the 8.1 have either under water exhaust or through hull exits. Therefore I don't think I have anything to worry about. And yes, I'm only getting Flowmasters so I can have the Big Block sound that tweaks the throb of flowmasters.

[smiles383]

most OBD2 vehicles have the downstream O2 sensors . A quick search at Rockauto.com shows 4 sensors so ??? Unless your truck is non emission or export only I would have guessed it would have had 4 O2 sensors . As for your question about the cats I don't believe there are any on a Mercruiser with the "Silent Choice " exhaust which is open exhaust when you flip the switch .

[PaladinMan187]

My truck is a very rare item. It was a military order that was shipped to Germany as a personal vehicle for a member of the US military. I didn't think this would have any bearing on my emissions equipment but it may. There are bungs for O2 sensors but none there. Hmmmmmm.

[jake760]

Your 8.1 will run perfectly fine without cats. Engines do not need backpressure, they need exhaust velocity which is accomplished by using the right size pipe. People think engines need backpressure after they install large free flowing pipes and loose power, when actually it is the loss of velocity that has killed the scavenging effect that helps power.