Questions about headers and bypass

[Stunin_Stud]

So Im thinking of getting an exhaust and thought while Im at it get headers. Been doing alot of research and Im kinda confused about the cats. I hear that the headers are attached to the cats and if you remove them the CEL comes on. So If I change the headers would i get CEL ? and would it smell funny? I live in California so I need to be able to pass smog so Im not sure if changing the headers would affect it or not.

Also I have been seeing videos and hearing people talk about the secondary Bypass, such as the one fabspeed sells. Can someone explain exactly what that is and why people get it? does it just by pass the secondary cats? do you keep stock headers get a bypass and exhaust to prevent CEL or what.


lastly what type of headers would you recomend? the ones on ebay all seem to be the same and rebranded with some selling for more. Theres also an NHP one with cats attached to it, which is $1K. Ive heard they are all the same so what do you guys think.

sorry for sounding ignorant on this. Im just trying to be sure before I know which direction to go with my next project.

Thanks

[23109VC]

I don't have headers, and I'm no expert, but I've read about it a lot here...

this is what I BELIEVE to be true...

on YOUR car, a 2001 model, you have cats IN your headers and secondary cats in the next set of pipes that are in between the headers and muffler.

you can remove the secondary cats and likely still pass a CA sniffer test. techincally you would flunk the visual, asyou modified the exhaust system and removed cats, but the reality is probably NO smog tech knows that boxsters have TWO sets of cats, and if they see the first set, they are happy and pass you.

if you removed the front and secondary set,, you would fail.

i dont' know what would happen if you took of the fronts, but left hte secondaries.

the 97-99 cars had only cats in the headers...and there is sometimes confusion because what works on a 97-99 is not what is true for 2000 and on.

I believe the most common setup on a car like YOURS is to retain teh stock headers/cats and just put on a muffler and then maybe add the secondary cat bypass pipes.

another unforutnate thing is that nothign on these cars is cheap, and any reputable company seling this stuff wants an arm and a leg for it. for example, the NHP muffler and their bypass pipes are about $1200. it probably sounds cool and likely yuou'll get no ral power gain out of it. $1200 is a lot to sound cool.

my next mod will likely be a pulley...as you can get that for about $300 and you will actually get gains you can feel...

[ChrisZang]

Originally Posted by 23109VC

i dont' know what would happen if you took of the fronts, but left hte secondaries.

The biggest problem you would now face is that you have a set of O2 sensors BEFORE the primary cats (the ones integrated in the headers) and another set BEHIND the primary cats.

So here's what happens:
The after markets headers have usually only one set of holes for the O2 sensors so your second set would dangle in the air. To make things worse the DME will be very confused as both sets of O2 sensors now read the same values (and the DME usually expects the air before it goes through the cats to be dirtier than after). So if you just leave t like this you will have a check engine light and probably poor performance. But there are ways around this, do some search on the forums.

But long story short: You do not run into these issues if you leave the original headers on and replace the secondary cats with a bypass pipe, and this is also mechanically less challenging (there's a bunch of guys who had serious trouble with the studs with which the headers are attached to the motor)

[Stunin_Stud]

So technically I can play it safe keep the stock headers and get the bypass for 500 bucks and a muffler and get no CEL and still get a nice sound. how much of a difference in sound or power would headers make? I feel like the headers are all the same and the ones that are not are way to expensive and the power claims seem exaggerated.

[root_werks]

This is interesting reading.

My 97' pulled a CEL for failing cat (P0430). I was going to purchase both, but didn't realize they have 4 cats? I'd have to stick my head under there again, but I think I've seen only the 2 cats with an O2 before and after each cat.

Am I wrong here? I would think cat bypass pipes would throw CEL's all over.

[landrovered]

My 3.6 is catless and at present the after cat O2 sensors are clamped underneath the car. Surprisingly it does not throw codes as often as you would think. I hate having the CEL lit so I have found a $10 fix for this. Fortunately a spark plug and an O2 sensor have the same threads. I went to Autozone and bought a $4 set of anti-foul spark plug adapters. I then went to the hardware store and bought a set of brass caps to fit on the end where the hole is. The end of the O2 sensor is a tad bigger than the bottom of the anti-foul adapter because it tapers where the second set of threads is on the bottom. A few seconds with a drill on a drill press to enlarge the area at the bottom of the inside so the O2 sensor will fit inside the adapter. Screw the unit onto the end of the O2 sensor, clear the codes and Bob's your uncle. No more CELs.

[schoir]

Originally Posted by root_werks

This is interesting reading.

My 97' pulled a CEL for failing cat (P0430). I was going to purchase both, but didn't realize they have 4 cats? I'd have to stick my head under there again, but I think I've seen only the 2 cats with an O2 before and after each cat.

Am I wrong here? I would think cat bypass pipes would throw CEL's all over.

Your '97 only has 2 cats total. One on the left and one on the right.

The cats are located in between the headers (a/k/a exhaust manifold if it's stock) and the rear muffler.

The '97 has a total of 4 O2 sensors, and they are located on the cat pipe, just before and just after the cat.

Here is a photo of the right side stock header (exhaust manifold) on a '97:





Here is a photo of a Billy Boat header installed on the right side:





Here is a photo before the muffler installation, showing the headers installed with the stock cats for a '97. The red arrows point to the O2 sensors at the rear of the cats. The front O2 sensors are not visible here, but are at the front of the cats:





Do not take the caution about the exhaust manifold bolt heads snapping off when trying to remove the stock exhaust manifold from the head lightly:

http://www.ppbb.com/phorum/read.php?19,1552140,1552140#msg-1552140

Regards, Maurice.