Exhaust - Midpipe area

[Kabnine]

For those of you who have actually seen under your car, and under the plastic trays, is there enough room in there for a high flow cat stuffed in the midpipes? I know nobody worries about them, but I would like to at least make an effort to make it legalish after I remove the ones from the downpipes and replace the rest of the exhaust.

[froop]

Stock mids include cats already and the entire pipe is exposed, not hidden behind under trays.

I would think that if you were to remove the primary cats in the DPs you won't pass emissions anyway so there's not much of a case for midpipe cats. May as well weld in resonators that look like cats.

[Kabnine]

Originally Posted by froop

Stock mids include cats already and the entire pipe is exposed, not hidden behind under trays.

I would think that if you were to remove the primary cats in the DPs you won't pass emissions anyway so there's not much of a case for midpipe cats. May as well weld in resonators that look like cats.

In the past I have used Hi Flow cats that have passed emissions. But then the systems have always been sourced from Australian manufacturers who are familiar with out laws. I was just going to source the DP's from overseas, a muffler from overseas, and have the midpipes made up here with an appropriate cat, if it would fit under the shielding and in the 'tunnel'. I haven't really seen under the car as of yet.

[froop]

Hmmm I guess it's possible however both the N54 and N55 have their primary cats in the downpipes probably due to the heat. The N54 has secondary cats in the mids whereas the N55 has none.

Seeing as your N54 already has cats in the mids, would replacing them with another set of pipes with cats make much difference or did you want to go with wider midpipes?

As for whether they would fit if you were to go down that path, that should be a definite 100% seeing as we already have cats there in the mids anyway. If you pop your head underneath you should be able to see the mids and cats and part of the downpipes without even having to lift the car.

[Kabnine]

Originally Posted by froop

Hmmm I guess it's possible however both the N54 and N55 have their primary cats in the downpipes probably due to the heat. The N54 has secondary cats in the mids whereas the N55 has none.

Seeing as your N54 already has cats in the mids, would replacing them with another set of pipes with cats make much difference or did you want to go with wider midpipes?

As for whether they would fit if you were to go down that path, that should be a definite 100% seeing as we already have cats there in the mids anyway. If you pop your head underneath you should be able to see the mids and cats and part of the downpipes without even having to lift the car.

What I hoped, and to be honest I really need to look under it, was that I could combine the two midpipes earlier into a larger pipe with a single large high flow cat, flowing straight into the muffler section.

I guess I might just get the downpipes now and while they are under it installing them, they can let me know what sort of room etc I am looking at.

[froop]

Originally Posted by Kabnine

What I hoped, and to be honest I really need to look under it, was that I could combine the two midpipes earlier into a larger pipe with a single large high flow cat, flowing straight into the muffler section.

I guess I might just get the downpipes now and while they are under it installing them, they can let me know what sort of room etc I am looking at.

I don't see why you wouldn't be able to do that.

HPF (when they were still around) did a 3" catback exhaust which would have meant they merged the 2 DPs into a single 3" at the mid which went all the way to the rear outlet. They also resonated it somewhere but I'm not entirely sure where.

[Stuart@BMRAutowerkes]

I have a hunch an N54 would pass emissions with only secondary cats if the engine was already warmedup, I heard somewhere the primaries were only there for startup / cold idling.. but could be wrong.

I've seen stock N54 DPs with only 30,000kms or so on them and the cats have pretty much fallen apart inside already so doubt they do anything for emissions in that state.