TonyG

It doesn't.

The point was that most people think that they can chop out the cat and put in a section of straight pipe, save a bunch of money, and get the same performance of a true test pipe.

....they can't

The reason.... which you can see from the photos.... is because of the inner section of pipe that has a substantially reduced cross sectional area which you can't see from the outside.

I have documented this multiple times over the years with pics, albeit with a factory section of pipe which hasn't collapsed to the degree this pipe has.

No matter..., the point is that you should get a real test pipe instead of modifying the factory system.

TonyG

LuigiVampa

No one is giving a good explanation as to what is going on here. What causes the pipe to collapse? I am not an engineer but it seems to me it has to get so hot that it melts. Is that what is happening?

Also, why is chopping out the cat and replacing with a straight pipe different then buying something like a Bursch test pipe. Are you saying the straight pipe has too large of a diameter or too small?

Why would a straight test pipe cause a collapse?

I woke up this morning ready to chop out my cat and replace it with a straight piece of pipe.

Alan 91 C2

Tbe inner steel pipe gets cherry red (soft) and combined with the stress created during the bending process collapses.

I put the welded pipe back in the car. THIS NOT YOUR FATHER'S PORSCHE. Before best boost 1.6, now almost 2. Performance is WOW!

mclarenno9

I'm guessing it's usually caused by constricted thermal growth of the inner pipe. Once the pipe buckles it's game over.

Because you are not replacing the section which collapses, which is before the cat. You will get the benefit of no cat, but you could still have the collapsed piping upstream which is more of a detriment to performance than the cat was by itself.

LuigiVampa

Thanks for all the answers. I'm upset that my Sunday morning project is dead but would be more upset to have done all the work only to have everything screwed up.

One last question - where does the double walled pipe start and stop?

Alan said he cut out that section so I am wondering what that amount of work would look like.

TonyG

The double wall starts at the top flange all the way to the cat (the entire section of pipe), although it's usually the part of the pipe that runs horizontal that's the problem, although I've seen it collapsed up higher too.

There's no way to really fix it "properly" as it's a pipe within a pipe. People have tried... but unless you go all the way up to the flange, cut it off, then weld to that, you're going to have a cobbled up fix.

It's cheaper and better to simply buy a Bursch test pipe or any other aftermarket (more expensive) 2.5" test pipe.

TonyG

Alan 91 C2

The full answer for the collapse does include the flattened tube heated cherry red (thermal expansion) with no room for the expanded pipe to go, so it folds in. The outer pipe constains the inner pipe expansion.

The big clue to the collapse is the low peak boost Pressure for the turbo S.

I will resize the welded pipe picture and post.

LuigiVampa

Well, I guess I am not cutting out my cat. I'm probably going to go with a Lindsey 3 inch exhaust now.

Crazy Eddie

Guys
I hear where you all are coming from, but I have a stock gutted cat pipe,
that ran just the same as the Bursch test pipe did....
The issue is, if the pipe is collapsed vs one that isn't...
I tested my gutted cat, by rolling a tennis ball through it, ( when I had it off ) and it rolled right through ... Personally, I like the sound of the Bursch test pipe a little better, because it does this gurgle thing when I let off the gas vs the gutted cat pipe which muffles it a tad more ...
Also, the test pipe a great deal lighter ...
just sayin'

Van

I believe the root cause of the pipe heating up to the point of collapse is really a clogged or inefficient cat - this prevents the the hot exhaust from flowing through quickly enough. And, because of the double-wall, insulated design, the inner pipe can't dissipate the heat fast enough.

As stated, it gets cherry red hot, tries to expand, but can't because of the outside pipe, so the material ends up collapsing inward.

Crazy Eddie

I would put my money on that explanation

odurandina

so, we drive from Mazatlan to Austin needing 2 ski's after the long summer on Lake Travis is over... and the tourist season on the Mexican riviera set to begin... one guy will sell us 3 Yamaha 800s for $4,500, and this other guy down the street will part with 2 Yamaha 700s for $6,500... and to make the choice even more of a no-brainer, the 700s are 3 years older and look really tired....

so, that's a pair of (5 year-old) Yamaha 700s for $3,250 each,

vs three (2 year-old) Yamaha 800s costing $1,500 each.

so, which deal is better ? or let me put it another way. which sounds better ? 400 hours of service for an engine that you can rebuld 4 or 5 times and have her back in the water the next day (every time).... or 7 hours of service for an engine that you might as well throw away ?

that's right. the 800s are that bad... and the engines just as good as useless at attempting rebuilds.

Alan 91 C2

I am attaching my Frankenstein exhaust pipe (lot's of cuts and welding)

The pipe and cat bypass work amazing.

TonyG

It won't when you start making more power.

TonyG

Crazy Eddie

Why do you say that ?
Because of the cavity where the cat use to be ( creates a disturbance of the flow ) ? I was able to do an A vs B and from the seat of the pants it was hard to tell the difference. But I am probably only in the 300- 350 max HP range with the Vitesse kit I have, and the boost I am running...
Regards
Ed