Pre-cat bypass vs. secondary cat bypass
 

In a previous thread, the topic of secondary bypass pipes came up. The Radium King (a forum member) suggested the following:
“i think, from a flow turbulence reduction point of view, that it is best to have the restriction as far down-stream as possible. in the case of the Boxster, this means deleting the pre-cats (the ones on the headers) and keeping the secondary cats (the ones on the mid pipes).

so, it would be less expensive, and provide more power, to get a set of those cheap Chinese headers on eBay (they come without pre-cats and are designed to connect direct to the mid-pipes) and keep your current mid pipes. this will require some post-cat o2 sensor kung-fu, however, as you have to either (a) extend the sensors to after the secondary cats (and perhaps add a bung) (b) get a ROW flash so that your car doesn't throw an emissions code, or (c) get one of the many o2 sensor simulators or 'foolers' such as sold by fvd (or fab your own).”

I vaguely recall reading something about bypassing the pre-cats in another discussion. I read about secondary cat bypass a lot. I wanted to bring up discussion in another thread to find out more information. Has anyone done this? Can anyone verify that one is any better than the other?

The stock headers are of atrocious design, and getting rid of them is certainly worth it. However, the only real good way of doing this is to find someone to flash your DME over to the Rest of World program (RoW) so that you don't have catalyst efficiency codes turning on your Check Engine Light (CEL).

Hypothetically, you could weld in new bungs into the mid-pipes, after the secondary (now only) cats and extend the wiring for the post cat O2 sensors, but this is far less ideal than just re-flashing the DME. If running North American software and you do not do this, you will trigger a CEL for cat monitoring.

On my 02 S I am running TurboWerx headers (which are catless) and a stock 03/04 exhaust (stolen off my 03 S with 3.8 swap) and RoW software and it's a great setup.

jake raby recomends sport cats on the mid-pipes and catless headers. i think you see cat bypass mid-pipes on boxsters for several reasons:

- most other vehicles have only one cat (located on the mid-pipe) so the catless mid-pipe is a standard mod.
- a mid-pipe is an easier swap than headers.
- until the cheap eBay headers came out several years ago, a set of catless mid-pipes was cheaper than a set of headers.

now, with the cheap ebay headers, the smart money is on the pre-cat delete and keeping the secondaries (or go to sport secondaries, or total cat delete). ps, i've done a lot of seaching and can't find anyone with much bad to say about the cheap headers, and at $89 ...

I am getting ready to have TurboWerx headers installed, but my indy shop guy says there is another way to go catless without having to reflash.

He has a PST2 but told me he had not ever done a reflash. Do you know the correct procedure on the PST2 to get the car reflashed to RoW?

You'll need to get your DME and Immobilizer codes from the dealer for your car. Then, using the PST, go into the programming section, select DME. You'll have to enter and confirm both codes, and then you can select whichever version of software which is correct for the car you have (i.e. 986 S) and select from the various programs, i.e. "986 S RoW."

If I call the dealer and give them my VIN, will they give me the codes?

This is a quote from a previous thread that I contributed to.


I've experimented with several different exhaust configurations before choosing the final system for my 3.2 S. I found the following set-up produced the best results in terms of performance and reliability (keeping my vehicle OBD II compliant).
Here's my ditty:

1. A less restrictive, high quality after -market cat back exhaust.
2. By-passing/deleting the secondary cats (they're like having an extra set of thumbs).
3. Headers and cats that stick with the basic principles of the oem design, however, they incorporate longer, equal length primaries that flow into a merge collector, along with lower cell - high flow cats (something on the order of HJS 200 cell).

The final result yielded the following: No need to relocate O2 sensors, make bungs, chase CEL's, reprogram the ECU, or worry about emissions ! Excellent results in terms of improved power and performance. And perhaps equally important - great sound ! All this begs the question; who makes headers and cats that follow this design principle ? In all of my research and experimentation I discovered 2 companies that manufacture such a system. M&M exhaust in Germany ($$$$$$$$$$$$) who's products are imported exclusively by FVD/Brombacher and FabSpeed.

To further elaborate on the topic, on the assumption that I decided to forgo the oem headers/pre-cats set-up, in addition to sourcing excellent, high quality headers (which was never a problem), in turn I needed to find find some high quality, less restrictive secondary cats (forget the oem ones - you may as well stick a banana in your tail pipe - it would be less restrictive). Therein lied the problem. Not only were there very few aftermarket manufacturers that offered these items, the ones that did were never the correct application. Generally, the cell count was too low. In which case, a good deal of back pressure was lost. This in turn resulted in a significant loss of low end to mid range torque. As most are aware, for those who primarily "street" their vehicle, low and mid end torque is essential.

I've come across differing information. In the past, I've been told that the primary cats are 400 cell and the secondaries are 600. However, I tend to think that they're 400 cell all around.

I have played around with my exhaust as well, started with my 97 2.5, put ebay headers on. really felt the difference then I bought a 04 S so immediately put on the ebay headers. felt absolutely no difference, a slightly better tone but that was it,
Next I flashed the DME and added cold air intake with some great results. in an effort to make it better I eliminated the mid cats and added a Borla muffler, sounds great but definately lost that low and mid range torque.
since I put then on together I dont know if it was the elimination of the mid cats or the Borla muffler? WHAT DO YOU THINK?
BTW I just removed my headers to have them ceramic coated and noticed that Borla welded a venturi shaped piece of metal in the intake side of the muffler that really reduced the size of the pipe, I'm wondering if that has anything to do with the loss?
ANY opinions will be greatly appreciated :D

If I am understanding things correctly, its seems as though you have gone completely catless. If that is the case, then you have eliminated virtually all of the back pressure from your exhaust system. That would explain why you have lost so much low end to mid range torque. I would suggest putting the oem headers/pre-cats back on, and eliminate the secondary cats. By doing so, you will regain some essential back pressure, at the same time open up the flow of exhaust further down stream and shed some unnecessary weight.

I have replaced the secondary cats with Fabspeed bypass pipes and installed a modified "PSE" type OEM muffler on my '02 S. This not only gives me the sound I was searching for, but feels stronger to me, as well; though I do not have dyno data to back this up. I really question how much, if any, there is to gain removing the factory manifolds/pre-cats and replacing them with headers on a street car?

Seems like more trouble than it's worth for a few HP (at most).


Edit* After reading the post again, if Jake recommends replacing the manifolds, there certainly must be merit, but I think I'll pass...not worth the hassle.

yup completely catless. I did get a chance to take it on the road today with those venturis removed from the muffler and there is a definite improvement . I will take your advice and try putting in the original primary cat and see what happens, I wont be able to get to it till next week, will let you know what happened.

Hello Harryrcb,

What happened ?

The only bad thing
 

Is said to be the installation and the probability of having to deal with a broken bolt. I've read several stories of woe.

My 2000 S showed a definate torque increase from 1000rpm to redline with Dansk sport cats & 2.5L Top speed f1 headers & stock muffler. Much easier to start off from stopped & zero resonance noise. Now that the 3.6 is broken in it needs bigger pipes & collectors so I am installing ceramic coated secondary bypass pipes & 3.2 headers with stock muffler. Proper pipe size has to be matched to engine displacement for low rpm torque gains & bigger is not always better.

The Dansk 200 cell sport cats & Top speed headers are for sale $800.
pics are to big for 986forum so pm me if interested in buying & I can email them.

"The Dansk 200 cell sport cats & Top speed headers are for sale $800."

BYprodriver I sent you a PM.

Hi Guys,

curious to know where you picked up the sport cats from? Any links?

I recently replaced my Australian stock headers with some Top Speed units. Our stock headers look like they would be a neat upgrade for you American based boxsters. PM me if your intersted in buying them.

http://sphotos.ak.fbcdn.net/hphotos-..._7795380_n.jpg

Sam

http://www.********************************************.com/detail.php?PartNumber=R905897784&serial_number=109 2424

This is the company I got mine from but the picture on their site is of a different application. They actually look like the Fabspeed pic.

Sorry about not getting back to you. putting the secondary cats back did NOTHING. so after some thinking about what's going on and remeasuring everything, I realized the headers were 2" pipe at the flange and the mid pipes were 1 3/4" so I made new pipes with 2" dia and replaced the 1 3/4 " catless mid pipes. I was thrilled, got all my low end power back , increased my mid range torque considerably which showed up at the track went from 2:38 to 2:33 at Sebring. The muffler inlet pipes are still at 1 3/4" so I'm wondering what would happen if I replace those with 2" . This will be a little more difficult unless I make up a whole new muffler system.
The back pressure theory is not incorrect, I know if you go with too large a diameter pipe you loose all back pressure and lots of power, but these stock pipes are far from too large. I know some race guy who use 2 1/2" exhaust pipes with no muffler and gain hp and torque.
Around christmas time I put my stock s muffler back on because the Borla is realy loud and I went to visit my mother so I wanted it toned down, well the extra back pressure definitely reduce the power and it sounded strange, more like stock although throatier but not good.
anyway I'm working on another project now since I daubt I can get anymore hp and torque out of this engine. It was at 310 up from 256 before the pipe change but have not had a chance to run it on the dyno. This engine might be for sale soon if interested pm me. :)