Pre-cat bypass vs. secondary cat bypass

[jwade]

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

[Johnny Danger]

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.

[backman_z]

Anybody have any isea how many "cells" are in the OEM primary and secondary cats?

[Johnny Danger]

Originally Posted by backman_z

Anybody have any isea how many "cells" are in the OEM primary and secondary cats?

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.

[harryrcb]

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

[Johnny Danger]

Originally Posted by harryrcb

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

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.

[jmatta]

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.

[harryrcb]

Originally Posted by Johnny Danger

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.

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.