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Any custom exhausts available for 931?
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bureau13  



Joined: 07 Sep 2017
Posts: 81
Location: South Florida

PostPosted: Thu Oct 11, 2018 4:12 am    Post subject: Any custom exhausts available for 931? Reply with quote

So is everyone running the stock exhaust with cat? And if not, what did you replace it with?

I had a couple turbo RX-7s in the past, and I know one of the things people always did was reduce exhaust backpressure, usually be removing cats, etc. I don't see any ready-made products available for this, although that may simply be due to the fact that these are rare cars, and rather old.

Back when I had an exhaust leak, the one good thing about it was all the popping on decel...maybe I'm immature but I want that back ha ha...without the otherwise terrible exhaust leak sound of course...

Obviously quicker spool-up would be welcome as well...
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)
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CÚdric  



Joined: 27 Aug 2004
Posts: 1766
Location: Sweden

PostPosted: Thu Oct 11, 2018 5:11 am    Post subject: Reply with quote

The stock one is very good on the row version, and lasts forever, in combination with small production numbers makes for a tiny after market. Why not modify it instead? Decat and maybe a smaller muffler?
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1603
Location: Somerset, UK

PostPosted: Thu Oct 11, 2018 5:52 am    Post subject: Reply with quote

My first attempt on my RoW 931 was removing the front box and using a sports rear box by Dansk. Nice sound, less back pressure.

Front + Mid section - Zircotec ceramic coated at the front on the original steel, stainless mid section


Stock rear section v's Dansk sports


My second attempt is using a custom tapered exhaust commissioned by BassGT briefly on his race car and now on mine. It expands to 3.5'' off the turbo and then tapers back to 2'' at the rear. Massively helps with fast spool with no back pressure but the taper allows for maximum gas speed as it cools towards the rear. Rear box is not much bigger than a bean can. Sounds real nice.

Need a better image of the current exhaust!


Huge front section against the J pipe. Briefly ran a screamer pipe on the WG. Not a nice noise on WOT


Briefly ran no rear box. Again, not a great noise, so small bean can installed. Note the heat discolouration of the Dansk in this pic versus the original pic at the top!

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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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bureau13  



Joined: 07 Sep 2017
Posts: 81
Location: South Florida

PostPosted: Thu Oct 11, 2018 6:45 am    Post subject: Reply with quote

So looking at various parts diagrams, etc, the ROW has a silencer of some sort where the US version has the cat? My problem, and maybe I'm reading this wrong, is that in the US, the cat is where the wastegate pipe connects back into the exhaust, so for me to try to remove that, I'd need a part made to replace it, or I'd have to get something custom-made, which I don't really have the capability to do. Maybe I need an old ROW exhaust, but that's probably not very common here in the US...
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1603
Location: Somerset, UK

PostPosted: Thu Oct 11, 2018 6:50 am    Post subject: Reply with quote

You mentioned 'custom' in the title. Custom doesn't come off the shelf. Just employ a metal guy or exhaust specialist?
_________________
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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924RACR  



Joined: 29 Jul 2001
Posts: 8118
Location: Royal Oak, MI, USA

PostPosted: Thu Oct 11, 2018 10:56 pm    Post subject: Reply with quote

Excellent detail, thanks Carrera!! I aim to do something similar myself to mine someday...

Question for you regarding the wastegate - you said you ran a "screamer pipe" on it, what do you mean? I'm curious, have heard that separately dumping the WG is preferable for performance, but being as this is a street car I definitely don't want to suffer with a crappy sound...

So far, sounds like deleting the cat/center for the 3.5" pipe tapered to 2", with rear resonator, is the preferred way. Definitely would love to see pics!

More details - you mention expanding to 3.5" off the turbo, would that be immediately at the turbo outlet flange or after the two bends? If I'm reading correctly, looks like an immediate bump to 3.5"... but do you need a transition piece from the flange, or just build the flange at 3.5" Sounds like the former...

Thanks...
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bureau13  



Joined: 07 Sep 2017
Posts: 81
Location: South Florida

PostPosted: Fri Oct 12, 2018 1:58 am    Post subject: Reply with quote

You're right, I did. Probably poor word choice, but it does appear that's the only way to do it now.

Carrera RSR wrote:
You mentioned 'custom' in the title. Custom doesn't come off the shelf. Just employ a metal guy or exhaust specialist?

_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1603
Location: Somerset, UK

PostPosted: Fri Oct 12, 2018 5:30 am    Post subject: Reply with quote

924RACR wrote:
Excellent detail, thanks Carrera!! I aim to do something similar myself to mine someday...

Question for you regarding the wastegate - you said you ran a "screamer pipe" on it, what do you mean? I'm curious, have heard that separately dumping the WG is preferable for performance, but being as this is a street car I definitely don't want to suffer with a crappy sound...

So far, sounds like deleting the cat/center for the 3.5" pipe tapered to 2", with rear resonator, is the preferred way. Definitely would love to see pics!

More details - you mention expanding to 3.5" off the turbo, would that be immediately at the turbo outlet flange or after the two bends? If I'm reading correctly, looks like an immediate bump to 3.5"... but do you need a transition piece from the flange, or just build the flange at 3.5" Sounds like the former...

Thanks...


Hi Vaughn

The front pipe comes off a stock sized flange with 2.5'' dia to match turbo but increases in size to 3.5'' quickly buy using sections to create the increase in bore size through the two initial bends. Once past the waste gate it tapers slowly to 2.5'' by the time it reaches the joint between front and rear section. Never took a good photo of the front section off the car!!

The screamer was a test as the exhaust initially did not have an entry pipe for the WG. So ran a screamer for a few days. The noise on boost was too raw and agricultural versus a nice aggressive race/rally sound. Regardless of performance, I decided to fabricate the WG into exhaust.

WG plumbed in

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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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924-76  



Joined: 05 Jun 2006
Posts: 1359
Location: Ontario, Canada

PostPosted: Sat Oct 13, 2018 3:15 pm    Post subject: Reply with quote

944 turbo (951) rear section (catback) is a direct bolt-on to the 931.
I simply empty the cat and replace the rear section with a used 3" 951 stainless pipe with a megaflow box.
I'm very happy with the result
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bureau13  



Joined: 07 Sep 2017
Posts: 81
Location: South Florida

PostPosted: Tue Oct 16, 2018 3:42 am    Post subject: Reply with quote

That's good to know, I didn't realize they were the same...

924-76 wrote:
944 turbo (951) rear section (catback) is a direct bolt-on to the 931.
I simply empty the cat and replace the rear section with a used 3" 951 stainless pipe with a megaflow box.
I'm very happy with the result

_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1603
Location: Somerset, UK

PostPosted: Wed Oct 17, 2018 6:51 am    Post subject: Reply with quote

If you want off the shelf 944 turbo option https://www.fabspeed.com/porsche-944-turbo-951-maxflo-exhaust-system/
_________________
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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Then www.924board.org/viewtopic.php?t=31252
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bureau13  



Joined: 07 Sep 2017
Posts: 81
Location: South Florida

PostPosted: Thu Oct 18, 2018 12:33 am    Post subject: Reply with quote

That does sound nice, but it seems pricey for a cat-back single exhaust. Also...+25/+29 hp/torque? Without touching the cat? Seems...questionable...

Carrera RSR wrote:
If you want off the shelf 944 turbo option https://www.fabspeed.com/porsche-944-turbo-951-maxflo-exhaust-system/

_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)
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Mike9311  



Joined: 14 Dec 2004
Posts: 678
Location: Chicago-ish

PostPosted: Thu Oct 25, 2018 12:00 am    Post subject: Reply with quote

924RACR wrote:


More details - you mention expanding to 3.5" off the turbo, would that be immediately at the turbo outlet flange or after the two bends? If I'm reading correctly, looks like an immediate bump to 3.5"... but do you need a transition piece from the flange, or just build the flange at 3.5" Sounds like the former...

Thanks...


You would need a cone to expand up to the 3.5". If you do a 3" it works without a cone. Working on an exhaust now for the '82. Currently planning a 3" down tube to 2.5" system with a 50 or 100 cell race cat and on from there.

Thanks RSR for the screamer pipe description. Always wondered how the wastegate dump would sound. Now I know "not good"

Still needs some tweaking to make it decent but this is a flange for a 3" pipe. It would fit into the recess. Otherwise you would need to have a cone off the flange in such a way to allow for mounting clearances.


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1980 931 ...off with the cobwebs
1982 931 Entwicklungsfahrzeug
1979 924 NA M471/M481 / ohne M650 Herausnehmbares Dach
0.5 1982 931 smashed parts car
1982 931 parts car?
1980 924 NA (R&D lightweight)
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924RACR  



Joined: 29 Jul 2001
Posts: 8118
Location: Royal Oak, MI, USA

PostPosted: Thu Oct 25, 2018 12:14 am    Post subject: Reply with quote

Very nice, Mike... how many of those flanges you planning on making?? I'd be in for one...

Was just under my Turbo last night, changing the oil, looking for an option... the only other thing I think makes sense to me would be to just put a v-and clamp on the straight section pre-cat after the O2 sensor, and run custom from there back. Not keen on cutting up my stock exhaust, but if that's the simplest option...
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'79 924 #77 ITB racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype
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Mike9311  



Joined: 14 Dec 2004
Posts: 678
Location: Chicago-ish

PostPosted: Thu Oct 25, 2018 12:21 am    Post subject: Reply with quote

924RACR wrote:
Very nice, Mike... how many of those flanges you planning on making?? I'd be in for one...

Was just under my Turbo last night, changing the oil, looking for an option... the only other thing I think makes sense to me would be to just put a v-and clamp on the straight section pre-cat after the O2 sensor, and run custom from there back. Not keen on cutting up my stock exhaust, but if that's the simplest option...


I'm not keen on cutting up a stock one either even though the one on the '82 now has been poorly repaired by the previous owner.

Once I have one made, I can make up more.

The plan is for a lightweight 0.035" wall pipe but I will make sure it fits any gauge 3" pipe just in case there are differences in the OD between brand/gauge.
_________________
1980 931 ...off with the cobwebs
1982 931 Entwicklungsfahrzeug
1979 924 NA M471/M481 / ohne M650 Herausnehmbares Dach
0.5 1982 931 smashed parts car
1982 931 parts car?
1980 924 NA (R&D lightweight)
1982 931 WB wana-be GTR race car
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