355 Test Pipes- how to fabricate/install with pictures

I recently had a problem with a failed oe cat on a 2.7 355 with a Tubi race exhaust.
Now one can purchase oe cats for 6g us (10g cdn at the local f dealer , despite an only 8 percent exchange) or aftermarket cats from hyperflow,fabspeed or tubi.
I have decided to go with Hyperflow cats but in the meantime my car was out of commision.
I hollowed out the broken cats and reinstalled them on my car.....but what I found was a boominess that was not pleasant to the ear.
I'm certain there is also turbulence in the exhaust streem due to the hollowed out cats ...so this was not the solution.
You could buy test pipes but they cost around $1200.00 cdn ($1000 us) .
I decided to fabricate my own test pipes to see the plus/minus of this setup (possibly just save the new cats for etest time?) using the factory oe cats for donor parts.
It will take a few days but what I intend to do is take photos of the entire process with measurements for those that my wish to tackle this project.
This will apply equally to 2.7 or 5.2 cars with the only mod for the 5.2 being to weld an extra oxygen sensor bung on the rear of the test pipe.

The first part of this project is to take your stock cat and cut off the stainless intake and exhaust flanges, as well as the oxygen sensor and overheat sensor bungs.
I used a sawzall hacksaw to roughly cut the parts off being very carefull to NOT cut the flange pipes as they enter the cat shell.....just cut the cat shell.
Later with the sawzell I will trim around the( flange & bungs ) parts & use a desktop grinder to remove the balance of the old welds. The parts are then ready to be rewelded to the new test pipe.

The first photo is the cutting of the original cat, to save the intake and exhaust flanges as well as the oxygen and overheat bung.

Note: I decided to use the oe cat flanges as 1)they are the right size 2)are good quality stainless 3) you donot need a pipe bender or muffler shop and the entire fab can be in your home garage with the addition of a mig or flux wire welder, grinder and standard hand tools.
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Was unable to post as fchat was unavailable......anyways..

-first step is to remove the airbox
-remove heat shield
-disconnect bypass pipe vacuum line
-disconnect muffler (I left it sitting on the bumper).
-disconnect oxygen sensor and overheat sensor
-disconnect and remove old catalytic converter

Next you take your old cat and cut off the flanges as mentioned earlier. Make sure you cut outside the welds with your sawzell .....you want to roughly cut the shell only leaving the flanges, bungs ect intact. Later you can cut and grind off the excess material to make the parts ready for welding.
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Goto your local automotive supplier and pickup some pipe.....either stainless or automotive grade steel.
As I plan to use a high heat ceramic coating I went with steel.

The pipes will each need to be 10 1/2" long x 2 1/8" outside diameter.

The oe intake flange will need to have the end cut 1/2".
The oe exhaust flange will need to be kept its original length.

Start by inserting the intake flange all the way into the pipe . Insert the exhaust flange about 1/4" into the other end (with the cat attatchment bracket aleady on).
Do the same for both test pipes and install in the car .

Hang the muffler on its brackets- line it up where you want it to sit in the final install.

First line up the angle of the intake flange to the pipe and remove and put a temporary spot weld.
Re insert / verify ok/ then adjust the exhaust flange angle (goes off to the side and a little up)...till lines up perfectly (I used a marker to make 2 seperate marks when lining up the flanges- helps to align for the spot welds). Put a spot weld and reinsert- verify all ok.

Do the same for both sides /make any necessary adjustments/ verify ok and fully weld

After the flanges are welded, bolt in the test pipes and mark the spots for the oxygen sensor bungs and the overheat sensor bung.

Drill, weld and the fabrication is done
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At this point one has to paint the pipes with a high heat (2000 degrees) ceramic paint.
You can either bake the paint in an oven or on the car.
The process for the car is to run the car for 10"/shut down 10"- run car for 20"/ shut down for 20"-
take car for 30" drive and shut done (you are done)
The manifold paint is made by VHT and comes in other colours.

An hour after painting the pipes can be handled......next you install your oxygen sensors (with anti-seize).
Install the test pipes and your done.
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Thanks, it took about 5 hours at a leisurely pace.
It probably cost no more than $40.00 in materials.
If I used full stainless (for the polished look) it would have added another $20.00 to the bill (plus a little time to polish).
But my tubi was polished originally........look at it now.

 

I don't have any equipment to record it.....but I can say.....when that bypass valve opens it is LOUD....with no cats and a race tubi (and this was a run on the highway....not under a bridge).
It actually took me by surprise.
There is no 'boominess' whats so ever.....so that problem was corrected by the test pipe (over a hollowed out cat).
But I must admit it is too loud for me so I will taking the test pipes off and installing the hyperflow cats.

 

Thats an excellent idea.
I found the test pipes too loud with my race tubi.

Can you post photos (top view) of the installed 'mini muffler' test pipes?