chaoscreature

I was really curious about the innards of the Porsche catalytic converters and was also looking to build another exhaust fixture to make some exhaust systems.
A fellow Rennlister was kind enough to send me his cat for me to gut and weld back together. I was really hoping to do more but things just didn't work out so I basically just hollowed out the system and welded the covers back on.
The most intersting thing that's truly noteworthy here is that the 1996+ exhaust is true "dual exhaust" because they welded a plate into the center of the X-pipe. The 1995 Exhaust mixes in the middle.
The stock metallic cores used by Porsche are pretty high quality. They seem to be about a 300cpsi design, really nice stuff. Considering this was 15 years ago this is really top of the line catalyst design.

Below are some pics documenting the brick removal, the bricks themselves and ultimately the slightly scarred but re-assembled cat.
Weight savings was probably around 10 lbs here. I really should have weighed the thing before/after.

The first picture shows the cat before cutting.
Pic 2 is halfway cut through. There are 2 layers of steel in there
Pic 3 shows the two exposed catalysts.
Pic 4 And the cats are now gone
Pic 5 shows the plate installed to seperate the right/left ports.

luial

Any other benefits aside from weight savings?
Is there a noticeable difference in throttle response? (assumed from clogged or aged cat)

chaoscreature

Pic 6 is a close up of the catalyst
Pic 7 shows the daimeter of the catalyst is about 4"
Pic 8 is another shot of the catalyst. The shorter piece measures 3.5" long if you are curious.
Pic 9 is the whoel shebang welded back up, minus the innards.

If I had built a welding fixture the best way to go about this would be to cut the whole converter section out completely and replace it with a 4" tube. That would also open the opportunity to use short absorption mufflers in place of the catalysts with better flow and a small amount of sound reduction.

chaoscreature

luial,
If your cats are clogged for sure you will notice a difference.
In this case, the cat that i gutted does not belong to me, nor does it fit my car so i will not have any feedback for a little while on the results.
I replaced my own cats with 100cpsi cats and before that 200cpsi cats and there is no difference in throttle responcse. More than the cats I feel that the choice of tubing size and configuration is more important. I used 2.50" exhaust with an X-pipe first and there was a noticable drop in low rpm torque whereas my current setup feels good everywhere (2.25" tubes, no crossover, 100cpsi cats)

Ed Hughes

Those look very nice Peter.

NP993

Interesting thread, thanks.

Cupcar

Thanks for the set of facts and close up pictures Peter.

Fabspeed has a video on You Tube that shows the gutting of both the 1995 and 1996 systems here that shows some more:
Questions:

1) Do the 100 cell cats work to attain CA test limits?
2) Where did you source the Cats?
3) Did you use Burns stainless flat to oval transitions at the Bishoff flanges?
4) Did you cut the Bischoff flange plates yourself or source them and are they stainless?

Bill Verburg

Thanks for that, Joe's exposition was very complete and informative right up to the point where he started talking about mixing banks, he then departed into the typical ill-informed spiel typical of hawkers.

He is correct in stating that a connection linking the left and right banks of 90* V w/ 90* crank V8s is a very beneficial feature, but he is incorrect in stating that this is a hp or torque advantage in Porsche flat 6 or Ferrari V8s w/ 180* cranks.

This is because of the fundamental difference in firing order.

American V8 all use 90* cranks w/ 90* V blocks and are called odd firing engines
Porsche flat 6 and Ferrari V8s use 120* or 180* cranks and are even fire engines
here is the difference is firing patterns B -Boom, S-silence
90 deg crank (1999 IRL, NASCAR, drag racing v8s, etc:
Left__collector: B-S-S-B-S-B-B-S
Right_collector: S-B-B-S-B-S-S-B
180 deg crank (Porsche flat 6 2000 IRL, CART, former F1 V8s, etc:
Left__collector: B-S-B-S-B-S-B-S
Right_collector: S-B-S-B-S-B-S-B

the consecutive cylinders firing into the same collector on a bank is what gives odd fire engines their distinctive rumble and also temporarily overloads the collector, temporarily overloading one side then the other, this does not happen w/ even fire engines where there is always alternate side firing.

The alternate overloading of each side means that an alternate gas path is quite useful in relieving the overload and any sort of connection anywhere in the system will suffice, these H connections are totally insensitive to location or configuration

alliteratively the exhaust on an odd fire can be arranged so that 1 Left cylinder fires into the right collector and 1 right cylinder fires into the left collector, this is what Ford did in their GT40 w/ it's bundle of snakes exhaust and is also used by some Nascar teams some of the time. Porsche also experimented w/ these sorts of exhaust in their 910s some of which were odd fire and some even fire

There can be advantages to blending the pipes in carefully crafted ways to enhance the flow of one cylinder by using the flow and/or acoustics of another cylinder, but the blending has to be carefully done paying close attention to pipe lengths widths and transitions, *** has done any outstanding job w/ these sorts of exhaust.

Cupcar

I should have warned about the snake oil sales pitch from the guy in the video! He does show the inside of the cat nicely.

In doing some research about this, I found this catalog http://catalogue.jpgroup.dk/pdf/jpgroup2/porscheuk.pdf which is interesting. The 993 pages we are interested in are 54 and 55. I didn't know that Dansk made the FVD stuff. Note they make 3 mufflers 2 that are TUV certified for noise and 1 that is not. They make a Cat setup very similar to what Peter made himself.

Ed Hughes

Thank you for that explanation Bill. That is the most lucid detail of why an X pipe works for some motors, that I've seen.

Ed Hughes

He lost me when he showed he's not good at math. The measurements written on and spoken by him on the size of the stock cat: 10" is not the same as 155mm.

Falcondrivr

Bill, I've read these threads before and never really got it until now. Excellent explanation. Thanks!

Bill Verburg

The first time I ran across a crossover used on an even fire engine was 1970 where Triumph used a crossover just after the exhaust left the heads, the purpose was noise abatement and it was very successful at that, If you put a '69 Bonne next to a '70 there was much less noise from the '70

chaoscreature

Questions:

[I]1) Do the 100 cell cats work to attain CA test limits?[I]
If the engine is running cleanly and is well maintained they will pass the sniff test, but would definitely fail the visual. I have heard that a properly running warmed up engine with no cats can pass the test.

[I]2) Where did you source the Cats?[I]
The company I work for makes 100/200 and 300 cell cats. For some reason or another we do not sell athe 100 cell cats here in the United States, only to Australia. I can't even get more for myself now...

[I]3) Did you use Burns stainless flat to oval transitions at the Bishoff flanges?[I]
No, I hammered the transition peice into shape on an anvil.

4) Did you cut the Bischoff flange plates yourself or source them and are they stainless?
The flanges are made from A36 mild steel plate and were cut on a laser.

Bill V. was the knowledge base I used to make this exhaust setup work. I have 0 dyno graphs to prove anything but I have made about 7 different iterations of exhaust for this car and this is hands down my favorite.

Fabspeed makes wonderful high quality exhaust parts, but I am not a fan of the design layout they used for the bypasses shown in the video. It's a little more difficult but you can twist the exhaust tubes around so that they pass eachother like I did instead of using ultra tight 180 bends.
I forgot the actual numbers but a 180 degree tight radius bend has as much restriction as adding 10 to 15 feet of straight tubing to your exhaust. Our cars inherently have another 180 degree turnaround in the muffler as well.
Again, I have no actual factual data to present here. Just observations and experience from my own designs.

Ed Hughes

As I contemplate this more, is the boom/silence relationship why I've seen some old British inline 4's with split exhaust manifolds: 2 and 2 each going to a pipe, so two pipes leave the engine bay?