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1981 931 - Need replacement catalytic converter

 
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canyonblaster  



Joined: 20 Aug 2012
Posts: 386
Location: Denver, CO, USA

PostPosted: Tue May 01, 2018 9:59 am    Post subject: 1981 931 - Need replacement catalytic converter Reply with quote

Cross posting from part wanted section.
Appreciate any advice!

http://www.924board.org/viewtopic.php?t=43696
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canyonblaster  



Joined: 20 Aug 2012
Posts: 386
Location: Denver, CO, USA

PostPosted: Mon May 07, 2018 5:56 am    Post subject: Reply with quote

ZERO luck but I did find a hollowed out one available @ Parts Heaven.
Anyone have insight as to how the car will perform & sound with a hollow cat?

Thx
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924RACR  



Joined: 29 Jul 2001
Posts: 8087
Location: Royal Oak, MI, USA

PostPosted: Mon May 07, 2018 11:09 pm    Post subject: Reply with quote

Should work well with a rodded-out cat... or check with Dan (ideola), I'd almost swear I gave him one years back, might still have it as a valid spare...
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1535
Location: Somerset, UK

PostPosted: Tue May 08, 2018 5:29 am    Post subject: Reply with quote

Why bother with a Cat? Straight through pipe fwiw!
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1980 931 - forged pistons, Piper cam, CGT turbo, 951 ducted FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife LSD
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3forall  



Joined: 29 May 2015
Posts: 17
Location: Canada

PostPosted: Tue May 08, 2018 7:21 am    Post subject: Reply with quote

I'd say replace it with a cheap resonator. Without the cat (straight) it gets too loud and the resonator helps the sound be a little less agricultural.
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canyonblaster  



Joined: 20 Aug 2012
Posts: 386
Location: Denver, CO, USA

PostPosted: Tue May 08, 2018 7:53 am    Post subject: Reply with quote

Thx guys. I don't want it too loud, I already have a rear muffler delete with a low restriction Ansa/Monza type twin tip. It actually sounds great currently (other than the rattle from this old cat).

I like the idea of less restriction but don't I need the replacement piece to have the connector for the bypass pipe the car has? I'm not very savvy about how to deal with this.

Thx!
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1535
Location: Somerset, UK

PostPosted: Tue May 08, 2018 6:09 pm    Post subject: Reply with quote

You are best off without any restriction/mid box/cat between the turbo and back box. Change the back box size to tune the sound you are happy with. The turbo naturally reduces the note. The back bock reduces further with limited back pressure/restriction for better spool up.
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1980 931 - forged pistons, Piper cam, CGT turbo, 951 ducted FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife LSD
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canyonblaster  



Joined: 20 Aug 2012
Posts: 386
Location: Denver, CO, USA

PostPosted: Thu May 10, 2018 2:05 pm    Post subject: Reply with quote

The car has to pass emissions every five years. Next test is in 2 years. Local emissions center mgr says it has to have a visible CAT. He also warned that although it only has a to pass an idle test (not a full roller test), w/o a CAT it may not pass....

I may have a line on a used OEM CAT with lower miles, and Parts Heaven in California has a hollowed out CAT for sale. Not sure what to do.
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canyonblaster  



Joined: 20 Aug 2012
Posts: 386
Location: Denver, CO, USA

PostPosted: Sun Jun 10, 2018 5:17 am    Post subject: Reply with quote

This is a bummer... In trying to chase down the annoying rattle noise coming from somewhere around the exhaust, my tech thought it was the CAT. Well, he removed my exhaust and gutted the cat, AND cut open the resonator just behind the CAT, removed the insulation and some kind of retaining ring that was broken anyway, and welded it all up and put it back on...

The rattle remains and is now louder! Ugh.
It my be anything. Possibly the turbo bearings?

THE WORST part is this, now the car won't spool up and build nearly as much boost. I have a gauge and it was making 10 psi @ 4-5K and now it struggles to get to 5-6 at ANY RPM.....

Is this from lack of back pressure? Now what am I going to do!?
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 1497
Location: Not Detriot - NMI

PostPosted: Sun Jun 10, 2018 5:25 am    Post subject: Reply with quote

Could be the pop off valve in the compressor housing rattling and now leaking boost by badly?
I suppose a broken WG (waste gate)spring could cause these symptoms too but I'm not sure if that ever happens..

Post a video of the noise and other symptoms.

I don't think you would hear turbo bearings until you hear the wheels hitting the housings, and you would be smoking like crazy..

Is the turbo not spinning up or just not building pressure?
If the pop off valve is leaking and rattling it would spool like crazy and make little boost..

But, if the WG is leaking and rattling the turbo wouldn't spool for crap but still give whatever boost you get from the little spooling it will do..


Take your air compressor and dial it down to like 10-15psi..
Take the WG signal line off your upper charge tube and actuate your WG by blowing air pressure in that line.. You should hear a really creeky springing noise and it will slam back shut when you release the pressure..

If it doesn't do anything try it up to about 30psi but I wouldn't go any higher than that..

Do you pull 1st gear to redline at WOT atleast once every time you drive the car as the manual suggests?
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canyonblaster  



Joined: 20 Aug 2012
Posts: 386
Location: Denver, CO, USA

PostPosted: Sun Jun 10, 2018 6:45 am    Post subject: Reply with quote

Thanks! I wish I could find one local tech who really understands this car....they all seem to chase their tail....
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1535
Location: Somerset, UK

PostPosted: Mon Jun 11, 2018 1:13 am    Post subject: Reply with quote

The turbo recirc valve and itís spring can rattle. Mines done this since itís rebuild on and off. Will only do it on idle as thereís no boost to seat it.

A boost leak will reduce boost. Possibly a slightly gapped recirc valve

I burst wastegate membrane will overboost not underboost. A wastegate stuck open may cause under boost but would also cause the turbo to overspin
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1980 931 - forged pistons, Piper cam, CGT turbo, 951 ducted FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife LSD
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252
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