M3 OB2 cats

[hoveringuy]

I got a set the other day and since I don't want the Treehouse CABs I need to re-route slightly. I had a section of pipe that fit perfectly and that was relatively easy.

First, you can see that they really have double-wall pipes. The cats weigh maybe 15 lbs more than the e30 cats because of that.

What I thought was really interesting is that the pipe diameter on the M3 cats is actually slightly LESS than on the e30 (background) by one or two mm. I'm trusting that BMW know what it's doing here and it makes me wonder if the e30 cats are really constricting at all?

Did BMW do this for catalyst efficiency? The M54 has the cats basically bolted to the head so this seems like an intermediate step to that.

[Beej '86 325es]

So that's why they weigh so damn much. They were a pain to get installed by myself because they're so heavy. After I get my emissions done I'm switching to an AA track pipe, if not for the performance then for the weight loss.

[hoveringuy]

I'm not concerned about the weight so much but yeah, they're solid and very nicely made. The weight is down low and in the middle of the car. It will be worth it if they are as quiet as my e30 cats.

I want them to fit well and flow well enough for me to retard my exhaust cam without causing reversion. I haven't mounted them yet; the fit remains to be seen but I will post pics.

[Beej '86 325es]

Are you welding a bung for the O2 sensor before the cats? Or are you doing an OBDII swap?

I've just got a bung welded in the pipe for the fwd bank of cylinders, just forward of the cat. I've yet to run it with the sensor and verify this is a good location though.

[matt]

The idea behind the dual-wall pipes (and the idea behind going to tubular exhaust manifolds on OBDII cars, and the secondary air pump) relates 100% to emissions.

The lower thermal mass of the headers means they will take a shorter amount of time to heat up and help with catalyst light-off. The secondary air pump injects a bunch of air to help warm the cats up even faster. And the dual wall pipe is so that the exhaust loses as little energy (heat) as possible before it hits the cats.

[hoveringuy]

The idea behind the dual-wall pipes (and the idea behind going to tubular exhaust manifolds on OBDII cars, and the secondary air pump) relates 100% to emissions.

The lower thermal mass of the headers means they will take a shorter amount of time to heat up and help with catalyst light-off. The secondary air pump injects a bunch of air to help warm the cats up even faster. And the dual wall pipe is so that the exhaust loses as little energy (heat) as possible before it hits the cats.

True. I agree that it helps emissions by creating faster light off but I would think it also has benefits to flow by reducing exhaust gas cooling. The pipes near the engine have normal wall thickness and once they get into the double-wall area are very thin (for lower thermal mass).

I'm sure they are quieter, too.

Headers are also wrapped with insulation, but I think it's to keep temperatures up for turbo efficiency or to reduced under-hood temperatures.

I really don't think these will make a big difference over the e30 cats, but I think the rear muffler will.

I'm using a bung on the OBDII manifold for the OBD1 ECU. The other bung is for my wide band. My ECU is only getting 3 cylinders.

[dougsic]

A hack saw? You cut through those stainless pipes with a hack saw?

It's amazing what you've been able to accomplish - OBC correction device, M54 swap - and with so few tools.

I have a full shop and I couldn't even get my car to start. Embarassing.

[hoveringuy]

A hack saw? You cut through those stainless pipes with a hack saw?

It's amazing what you've been able to accomplish - OBC correction device, M54 swap - and with so few tools.

I have a full shop and I couldn't even get my car to start. Embarassing.

Doug,

The hacksaw was for something else... I cut through the pipe with my pocket knife and some dental floss. Ha ha!

I try to be resourceful with what I have but it can take a looong time.

i.e. I use a hose clamp as a guide to keep the cut square.

[hoveringuy]

One word:

Holy. Sh!t.

Ok, that's two words, but damn! I swapped over to the OBD2 center section and e36 muffler tonight and took it for a quick drive.

Immediately it felt slightly stronger up to around 3k rpm with a very nice sound to it. After that, it's completely uncorked and pulls like it has 20+ more hp.

Mind you, I had the stock e30 muffler up until now.

It actually fits better than the e30 cat section and is more tucked in. The muffler section is touching the the subframe slightly and will need to be adjusted. Exhaust tips will also need to be adjusted.

Other than that, I'm really pleased. Now I can start retarding the exhaust VANOS and it will really wail.

[hoveringuy]

Before and after pics of e30 cats and OBD2 cats. This is with stock CAB's; I re-routed to avoid the CAB otherwise pipes would have fit fine with the Treehouse CAB's. Some say that the pipes will also clear the sway bar without modification but I needed to remove 1/2" of pipe from the exhaust flange area to move them up a little.

I still need to fabricate the hangar for the mid pipe, I'm using a copper plumbing strap now.

I also had a chance to take some logs on my LM-2 wideband and I found that my A/F ratios went from UNACCEPTABLY LEAN at WOT and higher RPM's to ALMOST PERFECT! Yeah!

[mattdk318i]

I had the same experience with my full E36 exhaust. It pulls harder than most DOHC e30s with the same engine but aftermarket exhaust.

[hoveringuy]

I had the same experience with my full E36 exhaust. It pulls harder than most DOHC e30s with the same engine but aftermarket exhaust.

I may upgrade the e36 muffler. Can anyone comment to the difference between the e36 and e36 M3 muffler?

I will say "no" to ebay headers, but I am interested in M3 euro headers. Himizoli in Budapest is working to use the S54 hydroformed headers by welding on the e36 flanges. It sounds like a great idea.

[m_dudey]

is there a difference for the midpipes between a M and non-M e36? for OBD1 and OBD2?

[hoveringuy]

I know OBD2 midpipe is the same between the two and I think it's the samd for OBD1 but not 100% sure on that, I haven't checked. I know the muffler is different on M3 but I don't know how it's different.

It's touted as an uprade. Why??

[m_dudey]

great to know hoveringguy! now when I look for midpipes, it doesn't necessarily have to be from a M3.

the other thing I found out on realoem is that the mufflers for the 95 m3 and the 96-99 m3s are different. different part number at least. not sure what the difference is though.

you mean from a M muffler is an upgrade for a non-M? maybe just a throatier exhaust note.