924Board.org

epsylon · Posted: Tue Jun 10, 2003 2:31 am Post subject: exhaust comaprison

are the exhaust systems the same from the cat back on the 924 and 931?

i replaced my original 924 exhaust system and i can't remember what it looked like to compare it to my 931.
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'82 931
'78 928
Howell Henderson
South Padre Island

gohim · Joined: 02 Nov 2002 Posts: 4459 Location: Rialto, CA

No, the exhuset system for the 931 and 924NA are completely different.

They bolt together is in different places, the 924 has a cat, muffler, and resonator, and the 931 only has the cat and resonator, no muffler, since the turbo acts as a resopnator.

The 931 exhaust is also about a 1/4" larger diameter than the 924NA. The aftermarket 931 and 951 exhaust are interchangable, and the diameter is about the same as the stock exhaust for the most part. This would indicate to me that the 931 exhaust tubing diameter is more than adequate for the 931, since if this was too small, the 2.5L engined 951 would not have come with the same size.

epsylon · Posted: Tue Jun 10, 2003 11:43 pm Post subject:

ah ok. modifying the exhaust on my 924 made a noticeable difference. obviously the turbo is a restriction on the 931 so i shouldn't expect the same results as on the 924. what i am looking to do is run a test pipe instead of the cat and replace my ratty looking final muffler, leaving the diameter of the system as is. i have seen aftermarket test pipes for the 931 but i have not seen any 931 final mufflers.

so what you are saying is the aftermarket 951 final mufflers are direct bolt on? no custom bends or brackets required. what do most people run on the 931?
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'82 931
'78 928
Howell Henderson
South Padre Island

CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS

Umm, just as a "data point", my 931 has a resonator and a final muffler. It is a Eurospec so that may be the difference, a resonator in place of the cat converter. Perhaps there is confusion on terminology, what I am calling a resonator is a cylindrical tube about 4" in diameter and maybe 18" or 24" long, located behind where the wastegate dumps back into the main exhaust flow. The final muffler (as I am calling it) is oval in cross-section located just before the tail pipe. It's about 4" tall, maybe 8" wide and 18-24" long.

I'm going to go out on a limb here and suggest that Gohim meant to say that the cat acts as a resonator, not that the turbo does, and that what he is calling a "resonator" is what I am calling a "final muffler".
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'79 Eurospec 931

epsylon · Posted: Wed Jun 11, 2003 5:00 am Post subject:

my 931 has a cat and no resonator. our definitions of final muffler are the same and that is what i am looking to replace. i can't find a source for these for a 931. which was why i was asking if it was the same final muffler as the 924. he suggested that the 951 final muffler is the same. any thoughts?
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'82 931
'78 928
Howell Henderson
South Padre Island

78 924 N/a · Joined: 20 Mar 2003 Posts: 769 Location: Pacific N.W.

http://www.tweeks.com/ ,has one listed separately for 931

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1978 924 n/a

CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS

Looking at the PET CD, the muffler on the 931 and the 924 NA is a different part number. But, ... the muffler comes with several feet of exhaust pipe that runs forward, and that may be the difference (bends, diameter, length, etc). The 931 muffler (and length of exhaust pipe) mates with a 4-bolt flange to the rest of the exhaust pipes running forward.

It looks as if the 924S, 944 and 951 all have a 3-bolt flange attaching the rear section of pipes/muffler. Someone who has actual hands-on will have to verify this though. Once again, the actual silencer may be the same, or might work, but with the attached length of exhaust pipe they don't appear to be interchangeable.
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'79 Eurospec 931

924 turbo · Posted: Wed Jun 11, 2003 8:50 am Post subject:

The 951 uses a 4-bolt flange, just like the 931. I wouldn't bet my life that they're interchangeable, but I've seen the 951 cat-back section and it, at least, looks identical to the 931 cat-back in shape. I did notice that the "flattened" section was less restrictive on the 951. Otherwise, they appeared to be the same diameter, as well. I didn't get a chance to do any measurements, though.
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Jon Furst

gohim · Joined: 02 Nov 2002 Posts: 4459 Location: Rialto, CA

The TWEEKS catalog shows the same part number for the final muffler for both the 931 and 951, #400-720.