I just spent 2 1/2 days fabricating an entire exhaust system from scratch. I started with a box of mandrel bends and a piece of 4" pipe...

I used to run open downpipes on my twin turbo camaro with 7-8psi internal wastegates.

I built two 3" pipes (one from each cyl bank) into two 3" magnaflow catalytic converters, then into a 4" cat-back (that I made for $155) and no muffler.

Now I can't get more than 5psi of boost. I'm going to answer my original question and blame the cats (back pressure) for the low boost. I remember reading about someone else not getting good boost with a smaller exhaust than I have. I know the easy solution would be to cut off the cats, but being legal for the first time since 2002 feels pretty good. Plus its really quiet now, major sleeper!

Is this a serious back-pressure problem or do I just need to crank up the boost? Its amazing to find out that two "highflow" 3inch cats are that restrictive!

 

The new setup sounds like it should be pretty free-flowing. I only used a 3" downpipe on my single turbo with the w/g plumbed back into the downpipe (no cat), and had no problem getting 10psi. But, if the exhaust is the only thing you changed to cause the lower boost numbers, maybe you do have a restrictrion somewhere. Do you have any pics of it?

 

The ONLY change I made was the exhaust. the exhaust turned out better than I expected. the y in the pipe is actually pretty free flowing if you could see it from the inside. A cap I had on the heater hose near where the evaporator canister should be melted on my way home from work last night. I found out when I saw the puddle of coolant under my car this morning. That means either underhood temps are up or its an awful coincidence.

Edit:
Burns clean though, lots of water out the tailpipe and almost no smell. I should run to the emmisions station before I cut out the cats.
Also the coolant temp isn't noticably hotter.

 

This may sound kind of obvious, but did you check the intak pipe for leaks and the exhaust to turbo pipe for leaks? Could it be the WG got damaged? Have you tested the WG?

Is it simply that the turbo output pressure on the WG has changed enough that you need to change the spring to re-balance it.

5PSI seems like a large amount to lose by changing over two similar exhaust systems.

EDIT: Maybe the old exhaust had a large amout of back pressure and did a lot to keep the WG from opening so a spring was installed to make it open easier. Now that the back pressure is less, it is opening too early and therefore less boost is seen on the intake side.

 

I tightened all 15 couplers and searched for leaks, all i found was a tiny split in a vacuum plug on the TB (purge canister port).
Before, I just dumped the exhaust to the groung right next to the oilpan with two 2-3/4" downpipes. The wastegates are internal and were rated at 7-8psi. I saw 8 psi. I guess the output pressure has changed enough to need to recal' the wastegates. now I'm thinking i'll need to worry about the pressure before the turbos, since its obviously going to be much higher.

 

What size are those turbos? that's a lot of exhaust to be having issues with...

 

they're T3/T4's with .63AR turbines. I'm thinking that the cats just aren't as "high flow" as they're supposed to be. I'm going to try calling Magnaflow and see if I can get a refund.

 

The magnaflow cats are the old Carsound cats which are actually GM full size truck cats. They are actually some of the better flowing cats out there.

Honestly, I’m stumped, I’d believe that whatever your exhaust is behind those cats is the problem before I’d belive that those cats are it since they are actually quite free flowing, I have a new one on my truck and sell quite a few of them. For that matter, they should be much, MUCH less restrictive then what was behind anything that came from the factory with that turbine housing.

BTW, you’re local to me, do you spend any time at any of the local tracks or car shows? Usually I spend _a lot_ of time at capitol (enough that most people recognize me up there), but I haven’t had anything running yet this year to take so not this year at all.

 

Yeah, I don't get it either, as you can see from the pictures the one cat is used. I bought it new and put about 5000 miles on it and went 14.39 on 2.73s and an open rear, through that cat, a slipping 700r4, crap 225-60-16s, No turbos. then the cat sat for a year while I was at school. I cut off the old exhast, bought another cat and used them both for this project. If just one of the cats was a restriction, I imagine the other turbo would work harder to keep the boost up, but the cat looked clear to me.

You can see whats after the cats in the picture... If it looks bad to you guys, let me know! And the inside of that Y is totaly clear of blockages and overhanging pipe. I cut it with the portaband. you can see the built-up weld in the center because there was actually a 3/8" gap there (no overlap). All TIG welded.

After sustained boost I get a faint rotteneggs smell.

I live between Towson and Perry Hall, a mile north of 695. I make a trip or two to ceceil county. Also I've gone down to rt 70/ Northern Parkway/ Riesterstown rd/ Wabash Ave area. Not this year yet because I don't go anywhere without my Brother who is still in Atlanta for the next 4 weeks.

I'll takeoff the cats Tuesday and put in some straight 3" pipe in their place to either pin-point or eliminate the cats as the "problem". Wastegates tested ok on the air compressor. Maybe I should check my boost guage? It does feel considerably slower though.

 

I wouldnt say your new system isnt too restrictive. Its just not as free flowing as open downpipes, odviously. So i'd just turn up the boost. You would be lucky to get no loss of boost going from open downpipes to a full exhaust with cats. Just my $.02

 

I cut off the cats today and found that the honeycomb in the used one (~5000miles, underwarrenty) came loose and turned sidways blocking the exhaust. I put straight pipes in their place, and all is well now.