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I'm adding cats to my 5.3L swapped C3 vette. It's a mild 5.3 with Summit stage 1 HL cam, LS6 dorman intake 1 3/4" Fbody long tube headers. Car was so loud with dual 3" with xpipe full car exhaust I swapped in turbo mufflers to help calm it down. This is a drive/cruise car only and the exhaust smell is also not great. I got two 3" Flowmaster Cat Converter High Flow Stainless Metallic 200 Cell 2000130 Cataylitic converters to go onto the C3 vette. Looks like best placement is right off the 3" slip fit long tube headers. Im in the process of fabbing and adapting the dual 3" high flow cats into the current exhaust right off the headers. The cats are weld on which means they have like a 1" slip fit connection on both sides of each. I want to use a accuseal band clamp on the headers to make removal easy and then Vbands connecting to the Xpipe inlets. I don't think I can run band clamps without extending the 1" nipple to like at least 1.5" which I believe would be the minimum for a band clamp. Another option would be to weld on a .5-1" section of tubing to the end of the cat to make it long enough for a real accuseal clamp and cut some slots in for clamping down. I don't really want to do that as I then have to worry about inside weld slipping over the header. Not pulling the headers to weld a Vband on them, not a ton of room around header to the trans pan.
I don't use the wide band clamps (3" wide) anymore. I use the narrow band clamps (1" wide) like they use on newer vehicles. Assuming the exhaust tube going into cat goes inside the cat I would cut some slits in the cat so the band will clamp down.
Just a note in regards to building an exhaust. Back in the day I worked a shop that was one of the first to get a exhaust tubing bending machine. So the training from the supplier was as you build the exhaust to prevent leaks was this. On the upstream side of the muffler or cat the exhaust pipe should fit inside. On the down stream side of the muffler or cat the exhaust pipe should be expanded to fit over the down stream of the muffler or cat. Then at all joints a bit of muffler cement.
I don't use the wide band clamps (3" wide) anymore. I use the narrow band clamps (1" wide) like they use on newer vehicles. Assuming the exhaust tube going into cat goes inside the cat I would cut some slits in the cat so the band will clamp down.
Just a note in regards to building an exhaust. Back in the day I worked a shop that was one of the first to get a exhaust tubing bending machine. So the training from the supplier was as you build the exhaust to prevent leaks was this. On the upstream side of the muffler or cat the exhaust pipe should fit inside. On the down stream side of the muffler or cat the exhaust pipe should be expanded to fit over the down stream of the muffler or cat. Then at all joints a bit of muffler cement.
How deep would the slits have to be? I only have 1" to play with and I believe the narrow band clamps are 1.25". If I cut too far back I could create a leak point. I just dont like the idea of 3/4" or half the band clamp not doing anything as it will be hanging over the edge of the slip fit joint so like 1/16" of a gap and no clamping. I have muffler sealant but that makes it really had to get stuff apart lol. I like the wide 3" clamps bc you don't need slits in the joints for compression which makes sealing a bit easier.
With my current 80 Camaro LS project I make the slits about 3/4". The 1" band clamps will clamp down just fine. Muffler cement will come apart just fine if disassembly is required.
I ended up welding on 1.5” of 3” ID 304 I bought off Amazon (basically a 3” ID coupler) so I got two pieces from one coupler. Welded that on the one end so I have more clamp/slip for the header collector and clamps. I can always slit the welded sections for a 3” narrow band clamp. I finished it up all last night, all 304ss. The welds aren’t perfect but they aren’t bad. On the last 2 pics I had to make a slight bend as it wasn’t perfectly straight so I took out a mini pizza slice and bend the tubing to give me what I needed. Didn’t warp the vbands so I call it a win and should be leak free and hopefully smell less.
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Flowmaster high flow Cat weld on to clamp on? 
