3" exhaust upgrade woes - catback flange donut is only 2.5".

wild man · 11-15-2010, 02:33 PM

I'm working on my exhaust upgrades right now. But I am confronted with a situation that needs to be addressed. It involves the flange joint that couples the rear (cat-back) section of the exhaust system to the center section. The flange uses a donut type seal that is only 2.5" ID. I have previously upgraded the catback section to 3", but I used the flange from the pipe it replaced. This wasn't a problem because the pipe feeding it was only 2.5". But now, the center section is going to be 3" also. I could use the flange that is on the stock center section, but that would introduce a restriction down to 2.5" where the pipes meet, while the pipes themselves are 3". Would this be a tolerable situation, or am I going to get robbed on power?

What are my options, for eliminating this problem? The only thing I can think of, is to go to a exhaust shop and buy a 3" set of flanges.
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gray 89 951S - 3.5" MAF, 7006/8 (K27) @ 20psi, 55# injectors, 3bar FPR, 3" intake pipe, ported outlet pipe, 2.75" DP, 3" nocat mid&back pipe, open WG pipe, dual 2.5" vortex, late cam, Q'd up= custom chips, stg2 KEP PP, 930 disc, 5/33bar BBV, tall 5th gear; red 87 924S -

Pete R · Pete R's Garage

The exhaust will only flow as much as the 2.5. Doesn't matter what's on either side if at some point it still has to go through a 2.5 opening. ie. nascar restrictor plates. Now, I don't know how much flow your car NEEDS so it may or may not be noticable... but you will essentially only have a 2.5 inch exhaust

ksudar · 11-15-2010, 08:17 PM

easiest solution would be to buy a 3" exhaust band clamp, which will slip over each section of pipe and butt them together.

wild man · 11-16-2010, 03:09 AM

I can't seem to vizusalize what one of those looks like. I don't think I ever saw an exhaust system that had one. Wonder why that is?

If using the stock flange would make it act like a complete 2.5" system, then I am wasting my time making the whole thing 3" (except for the DP section, which is 2.75") . I might as well have just left the whole thing alone. But unfortunately I'm far beyond that point now, and I'm not ready to accept that it was all for nothing. So it looks like I HAVE to do something about this situation. My preferred method of upgrading is to use "modified" stock parts, for reasons of saving money, maintaining the stock look, and retaining 100% interchangeability with stock parts. But it is looking like by going 3", the cat pipe and the back pipes are going to have to loose that interchangeability with stock. What to the aftermarketers use at this pipe junction?

nize · nize's Garage

exhaust band clamp:
Amazon.com: Walker Exhaust 33230 Hardware-Clamp-Band-Stainless: Automotive

wild man · 11-16-2010, 05:28 PM

I guess that explains it about as well as it can be (pic worth 1K words?). That's actually a pretty nifty product there, especially for someone in a situation such as the one I am facing. It's straight up cut-n-clamp. No fuss, no muss. Seems just a wee bit ghetto for a porsche though, but it's not like anyone is going to see it. I will consider it, but 1st I am going to break out the plasma, and attempt to cut the stock flanges out to 3" and use them, if they are still usable when I am done sizzling away at them. I haven't had the unit all that long, and don't claim to be very good with it (yet). But since the doenut will then be out of the picture, I'd have to get the mating angle of the flanges perfect, and use (make) a gasket. And then I'd have to hope for enough room to able to get a wrench onto the bolt heads (and nuts). Wow, that just keeps looking like a longer and longer shot. But at least now there is a plan B.

Bonestock951S · 11-21-2010, 09:28 PM

You could use a 3" flange and a copper gasket.

wild man · 03:23 AM

That's probably what my plan A should be, since I'm figuring that cutting out the stockers to 3" with a plasma torch probably isn't going to pan out. I think NAPA can order me a set of flanges where one side has a built-in doe-nut that fits into the other side (doesn't need a gasket). Looks like I need to get over there and order them.

EDIT: I'm still wondering how the aftermarketers address this situation. I would think that they would go with 3" flanges. But that would prevent a 3" aftermarket rear (cat-back) pipe from being able to be used with the stock cat(mid)-pipe, correct?

billy mild · 11-22-2010, 06:12 AM

3" Cat Back DIY (lots of pictures now) - Rennlist Discussion Forums

Not sure if we are supposed to link stuff like that but I just did.

wild man · 11-22-2010, 06:37 AM

Quote:

Originally Posted by billy mild

3" Cat Back DIY (lots of pictures now) - Rennlist Discussion Forums

Not sure if we are supposed to link stuff like that but I just did.

I believe the rule is not to link to anything outside, that can also be bought from this site.

He used the old flange from the ansa pipe, which is EXACTLY what I had done previously for the back section of pipe. But since I am now going 3" on the mid pipe that it connects to, I can't tolerate the constriction that would exist between them using the 2.5" flanges.

I'd like to maybe change mufflers from the one I am using now due to it being somewhat too loud. That magnaflow is nice, but I would only use it if it has the same internal layout as the ansa or stock muffler does (you should be able to look straight through it). Does anyone know if this is the case? If so what is the magnaflow part#?

billy mild · 11-22-2010, 07:09 AM

All magnaflow mufflers are straight through design. On my old 924s I had a custom 2.5" exhaust made. it had a high flow cat and a Race Magnaflow Muffler. It was loud. The street ones are supposed to be a little quieter, but I don't know. So pick whatever magnaflow will fit your application.

nize · nize's Garage

search and ye shall find;
944 turbo exhaust

Bonestock951S · 11-22-2010, 10:17 AM

You can use a 3" flanged cat-back with a stock test pipe. Ran it for a while on my car. The bolt pattern is the same. I used a copper gasket. Just get yourself two 3" flanges, weld them to each pipe and seal with a copper gasket. Easy to disassemble and you don't have to buy donuts.

wild man · 03:10 PM

Well, after buying a band clamp at NAPA today, I decided to try the plasma cutter on the stock cat pipe flange, and it actually worked out pretty decently. So I stuck with stock (modified, but still fully compatible) - that always seems to make me happy. If you ditch the doe-nut, and cut like 1/8" off of the inside opening of the stock flanges, you will get about a 3" opening, or real close to it. That means you have to use a gasket, so the flanges need to be perfectly aligned with each other. Looks like it is going to work out OK though. I think I will look into a magnaflow in about 6 months or so.

It looks like I have matched the aftermarketers, using my modified stock parts. Apparently their 3" flanges also are bolt-pattern-compatible with stock, so I think I went the right way with this.