Cat Bypass Straight Pipe

[lucifer572]

‘86 Carrera Coupé w/Wong Chip

Doing a Cat Bypass. Removed cat and cut center of cat out at the maximum OD of the cat. Cat OD measurement where honeycomb material is measured at 5 2/8th inches. Intend to weld straight pipe in place of original.

Is there a recommended diameter for a straight pipe replacement? Should I stay with approx 5” or is smaller diameter recommended? Finding any exhaust tubing near 5” is so far impossible.

Any recommendations is appreciated.

[freeform911]

I think the bursch bypass is either 2.25 or 2.5"...don't need a 5" can...

[SCadaddle]

Any recommendations is appreciated.

Since you asked I'll give you my thoughts:

1) Go purchase a ready made "Cat Bypass" pipe. If what you have was previously a functioning good factory catalytic converter pipe, you could have sold it to someone that needed it to significantly fund the purchase of a ready made pipe.

2) Now that you have the ceramic guts out of it, spot weld it back together, then make sure the flanges line up between the crossover pipe and the muffler flange by bolting it back into your exhaust system. IF it fits, remove it and weld all around.

3) Realize if you wanted to simply "gut" the ceramic portion of that particular catalytic converter, you could have done it with a piece of rebar from inside the muffler flange end to break apart the ceramic "honeycomb" and dumping it into the garbage in no time.

4) Return to thought #3 above, and realize you've got 3 pieces of scrap metal on your hands, but you learned something!

[hcoles]

Plus - you possibly ruined a very valuable cat. Here in CA they are needed to pass smog.

[RarlyL8]

Most OEM catalytic converters are shot so maybe not such a loss.
The design of the bypass pipe is important to retain even flow and velocity. If the exhaust enters a large void velocity is lost which will result in lower torque. Our BP pipe has 2.25" inlets with a 2.5" tube diameter. The OE cat has a very restricted flattened lower tube, a well designed bypass pipe or premuffler will correct this and add significant power.

[famoroso]

Personally, I like the idea of a gutted cat / "stealth bypass."

I had Steve Wong dyno tune my '88 with a stealth bypass and a stock looking muffler that had one of the four stock baffles deleted. Sounded kinda angry, pulled STRONG, meanwhile... It looked bone stock.

[dw1]

When it comes to a cat substitute, I prefer the euro premuffler over the straight pipe.

[DNickel]

I just bought the Bursch from PP and 3 new gaskets. Easy Peasy.