Time for an update, and this will be long! Even though I kind of hate doing it (mainly due to not having a lift, the correct tools, or an assortment of mandrel bends) I finally started building the front half of the exhaust a couple weeks ago. If you'll remember, the Becker exhaust has an insane design flaw in that the initial run of the system, from the downpipe flanges to about where the mid-car exhaust mounts sit, is all 2" piping. It then miraculously jumps up to 2.5" for the rest of the system.
It's a horrible design and not mentioned in any of their marketing material. For reference, here's the complete Becker system installed on my car, but using Varex mufflers:
I had a couple goals while building something to replace it. Of course, the main goal being to move up to 2.5" throughout the system. The secondary goal was to have some sort of a merge, to clean up the sound a bit. The final goal was to reintroduce a cat into the system, so it doesn't smell so bad. I started off by putting together a basic design in my head and tabulating a parts list to create it.
First up was the cat. I knew I didn't want a restrictive stock cat, but being in CA, it's actually really hard to source a high-flow non-California compliant cat. The big sites simply won't ship to you, even if you tell them it's for offroad use. Amazon, Ebay, Summit, Jegs... they're all a no-go. This had actually deterred me for many months, as I'd also wanted a high flow cat for my e30 and had started my search there. Thankfully I eventually found a smaller shop that agreed to send me a 3" race cat for my e30 and a 3.5" race cat for the Z4.
I did the install on the E30 first and used the race cat to replace a very large straight through Magnaflow muffler I had sitting in the middle of the system. It went from sounding like this:
To this:
Just like the Z4, I also have a Varex internal bypass muffler on the E30. I just can't get enough of these damn things! While I awaited the cats, I started to collect the other parts for the system. I decided on 304 stainless to match the rest of the Becker piping and knew that I'd need 2 y-pipes; one to form a merge to run to the cat, and the other to split again to feed each of the mufflers. Once again I hit a snag, in that there are no reasonable dual 2.5" to single 3.5" y-pipes. Don't get me wrong, there are a few on the market, but they are terribly designed. Every one of them goes from dual 2.5" to single ~2.75" and then expands up to 3.5". Who wants a 2.75" restriction in the middle of their exhaust!?
Not good. There is actually one Flowmaster y-pipe that fit the bill decently, but it was out of stock until mid-February, wasn't stainless steel, and the dual section was splayed instead of parallel.
So in the end, I decided to make my own. I'd done it before, so surely I could do it again. Right? I hope. with that decision made, I ordered the piping required to fab it all together.
Once it arrived, I put the car up on jack stands to pull the front half of the exhaust. It was immediately clear that the exhaust leaked all over the place. The slip-on joints in the middle and rear as well as the downpipe flange. The 2" flange on this system is so small that it's bolt holes partially expose the opening of the 2.5" downpipe flange! It's crazy stuff. I pulled the front piping off and mounted up the new flanges on the downpipes. Alas, my first issue; these generic flanges were simply too big. I'd seen N54 specific flanges on the web, but really didn't want to pay $80 for them. I decided I could use my new chop saw to cut them down to size and aimed my focus at fabricating the y-pipes. I actually had a couple big mistakes that caused me to start over on these, but instead of going over them I'll just focus on what worked.
These y-pipes started their lives as 2.5" 45* bends and a 3.5" section of pipe. I started with cutting up the bends to fit together and run parallel to each other on exit. I measured the center of the lower half of the bend, and used that mark as the starting point to run tape line parallel to the upper half of the bend. That would be my cut line on the chop saw.
Then I mirrored this on the other bend, and made the cuts.
With them cut out, I got back under the car and checked to see if the distance between the pipes fit the downpipe outlets, and magically they did! So I tacked them together.
Next I took the 3.5" pipe and squeezed it in my vice until it was an oval with a 2.5" orifice, to match the 2.5" piping.
Then I measured how wide it was and measured that width out on the newly tacked together piping.
Back to the chop saw and she was cut out. And a test fit of the pipes shows what this will look like.
Next I removed the midsection of the exhaust to build it's y-pipe. The piping in this section is even closer together than the downpipes, so the y-pipe would need to be a bit different. First I cut the mid-pipe so the inlets were even, as the system was staggered from the manufacturer.
This time, instead of using the midpoint of the lower half of 45* bends as the starting point for the parallel cut, I used a point about 1/8 of the way across. By pure luck/magic, this worked first try as well! Phew!
Then I took a length of 3.5" pipe and tacked it to a 3.5" vband and once again squeezed the other end in my vice until it was down to 2.5".
Measured the long diameter of the newly formed 3.5" pipe and cut that width off the 2.5" V, to create the second y-pipe.
I tacked everything in place and reinstalled it back under the car to make sure it all fit and looked right.
Once I confirmed it would work, I also tacked the rear slip-on joints in place, and removed the entire rear section for finish welding.
With that out of the way, it was back to the front section, which would surely be more of a PITA. I'd tried cutting the flanges down to size, but they were so thick it was basically not worth continuing. I also needed two new 2.5" bends, as I hadn't accounted for a slight bend right at the downpipes. So I bit the bullet and ordered the N54 specific flanges and accompanying bends from Yonaka. Of course it took like 1.5 weeks to arrive, but yesterday it finally did!
To be continued...