I have a 2.5" catback with the cat still installed, and I intend to keep it that way. It's more important to me to be smog legal these days than make every last hp out of my setup, but I do like to make what I can. I am wondering how bad the stock downpipe is and if it makes sense to change it out if the cat is still there. I also don't know if any are made that bolt to the cat rather than delete it. I have only found ones that delete the cat so far. I know downpipes aren't legal in CA without a C.A.R.B. exemption, but they are very easy to swap out for stock if it's bolt in. Furthermore, if you don't get a bright polished downpipe, it will eventually look like old piping and pass for stock I would think.

So what I would like to know:
-Is it even worth messing with performance wise if the cat is still there.
-Are any aftermarket downpipes made that keep the cat in the system and where can I find them. Websites?
-Are there any C.A.R.B. legal one's that would make this all very simple. I'd just get that.
-If it helps, the other mods that might make it more worth it are a K&N intake (K&N filters are obviously legal), a tubular exhaust manifold with equal length primaries (it's C.A.R.B. exempt). The free mods that clear up the intake track like the bov dump tube removal, waste gate solenoid restriction removal, 1g bov installed etc. Just little things undetectable by smog inspectors.

Obviously my goal is to make modest power without compromising the smog legal aspect of the car. I'll buy anything that's C.A.R.B. legal because you cannot be harassed over it. But I'm also not afraid to add simple things like a downpipe because even if I couldn't pass it off as a stock looking piece, its easy to swap out. It's also something that wont cause an engine light or any negative effects. Sorry to be so long winded, but I know a bunch of guys will jump on to say "Just get one that delete's the cat. It'll make more power". Thats not the goal here.

Go big with your exhaust. Go to a good muffler shop and have it custom made. There are high flow cats available. You can make more power and still keep all the emissions parts.

Isnt your car old enough to not have to pass the smog tests? its an antique pretty much, would a 82' Chevette still have to pass smog? I would like to hope that there is some way to be able to run an older car out there, lets face it at some point emmisions items for any car will no longer be able to be bought new and its illegal to install used emmisions items on a car (federal law as I understand it), I doubt for instance that you can go to Mitsubishi and buy a brand new cat for a 1g (and maybe not even a 2g), so then what? can you put historic plates on it at some point? 20yrs? 25yrs? around here it used to be 20yrs and its now 25yrs.

Even a 1975 muscle car must pass smog. 74 and older is exempt. Yes you can buy aftermarket cats, but they must be C.A.R.B. (California Air Research Board) approved ones. I have a brand new cat on mine and passed smog already. So no, a 90's 4 cyl isn't even close to being felt bad for by the state . EGR, EVAP, and all systems must be in working order even back into the 70's. Its extreme here.

Here in Toronto Canada, all passenger vehicles 1988 and newer need to pass emissions, regardless of what motor is in there. Non obd2 vehicles have to pass a tail pipe sniffer test. Obd2 vehicles just need to have a non CEL ecu status.
Luckily here I have the option to run a high flow cat. Running 3" DP to 3" cat to 3" magnaflow.

The biggest exhaust improvement is the downpipe. A catback with a stock downpipe and cat probably doesn't even increase power on an otherwise stock car.

donniekak said:

The biggest exhaust improvement is the downpipe. A catback with a stock downpipe and cat probably doesn't even increase power on an otherwise stock car.

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A catback does on a turbo car, there are many dyno results to prove it. The muffler has a horrible restrictive design. I wider diameter pipe with a straight through muffler made a noticeable difference for me.

The reason upgrading the down pipe is such a good improvement is because aftermarket downpipes delete the cat. Much like the rest of the system past the cat, all you can really do is increase the pipe diameter. So the question is: If keeping the cat, is changing the downpipe even worth while? I dont see it being that big of an improvement. Trying to see if anyone has done it or knows it to be worth while.

I couldn't trap any faster than 113mph with a stock downpipe and cat no matter how must boost I ran. Running a straight 3" pipe catback did nothing. Going full straight 3" got the car to almost 120mph with no other changes.

With the stock cat and down pipe the car wouldn't make over low 20's boost. With the straight turboback I could hold 28-30psi with the 16g, and spool a few hundred rpms sooner.

This was a bone stock car with the only mods being an evo3 16g, walbro pump, 1,000cc injectors, act 2100, and e85 eprom tune. Stock ic, stock intake, stock air filter even.

Th biggest gains I've ever seen from just a catback was 4-5hp on a stock car. A downpipe and intake are good for around 20hp together.

The way an exhaust makes power on a stock car is mostly by increasing boost because of lower back pressure. The same wdc with less backpressure makes more boost. The main restriction is the stock downpipe and cat, removing a restriction after the cat does nothing. If you have a 1.75" pipe going into a 2" pipe, making the 2" pipe a 3" pipe does nothing.

The part you're missing here and that is still unanswered is if a downpipe and stock cat will be an improvement. Your improvement came from removing the cat. Everyone knows an aftermarket downpipe that deletes the cat will be an improvement. You aren't telling me anything I don't know. I live in California. Not Arizona where there are no rules. The cat has to be on the car and it has to be the C.A.R.B. approved one. There is no high flow cat that's legal for use here.

What I'm trying to figure out is if increasing the diameter of the downpipe makes any sense if it leads to the cat anyway. And if it does, if anyone knows where to get one that doesn't delete the cat.

Turbo2G ... you ask an interesting question. I can't say definitely a larger downpipe would help or not with the presence of a cat, I guess you'd really have a to get it on a dyno before and after to know for sure ... but that said, we all know turbos don't like restriction, so the more you open it up, even with a cat still there, I would think you would see some gains, just not as aggresively as you would minus that cat - which you clearly already know. It's also always been my understanding that taking away restriction closer to the turbo is better, and I'd say that cat is a good 2 1/2 feet away from the turbo (pipe distance), so my thinking is going to larger dia has got be better than the OEM restrictive setup.

Your other question about a DP. Years ago you could find any number of companies making DP's for these cars - the well is drying up unfortunately. My guess is you're not got going to find a larger dia bolt-on from turbo to cat. But, here's what I did about 17 years ago. When I bought my first Eclipse I was still dailying the car here in PA and I had to have a cat to pass emissions just like you. So, I bought a full turbo back exhaust system (I think it was an Apexi at the time). I took the downpipe, which eliminated the cat, and just measured back the distance of the cat and marked it. I cut it, and then took it to a muffler shop along with my cat and had them weld on a flange that matched up with my cat. As a matter of fact, I still the section I cut off out in my garage, I saved it in case I ever wanted to use it for something . So, I essentially had a performance exhaust with a cat. Later on, I bought a straight pipe same size as my cat off eBay and swapped the two when I had to go for inspection. I don't daily the car anymore, so those days are long gone and I'm under 5000 miles a year so I don't need emissions anymore. either.

Like you, sorry for the winded response, but hopefully that gives you some things to consider that actually relate to your question.

@Turbo2g So what I would like to know:
-Is it even worth messing with performance wise if the cat is still there. No
-Are any aftermarket downpipes made that keep the cat in the system and where can I find them. Websites? Yes I believe RRE makes it, but I would contact them to be sure
-Are there any C.A.R.B. legal one's that would make this all very simple. There was a CARB legal exhaust systems for our car but I don't think they survived with there stamps after all these year (with alot of luck my car came with the greddy carb legal exhaust I keep around just in case with the stamp) but as for CARB legal down pipe. No

These are short answers.

I do know what the OP means about how difficult it is to be smog legal and make power, after a few years of stressing and abiding by keeping my car as legal as possible I realize, whether your car is legal or not, if the cop wants to write the ticket they will write the ticket.

I'm in the same boat as you since I live in CA. I also have my smog due in May which I'm worried about . Unfortunately, it's impossible to have a 100% legal exhaust that performs to your expectations since everything that's required is a major restriction.

If you'd like to find a CA CARB legal cat you can find your options here (I believe the largest CA CARB legal cat for my 1g had 2.5" piping) : https://ssl.arb.ca.gov/AftermarketParts/catalysts

I plan on keeping my stock exhaust setup intact for when I have to get it smogged and then swapping on a full 3" setup. I may use a resonator or two and a high-flow cat with the 3" to keep cops away. While it may be a restrictive 3" setup, it'll still be a major improvement to the stock piping for the street! A flanged straight pipe will be in the garage which will eliminate my cat and resonator/s for when I'm trying to attract the LAPD...

If you're worried about a shiny exhaust you can either wrap it or paint it with high-temp paint (barbecue paint is cheap black satin high temp paint). Black does tend to hide things well but a 3" pipe is not going to ever look stock on our cars

Good luck with whatever you decide!

Believe me, I've been there done that. My 505hp vette is a smog nightmare every year. That one I drag race and went all out on. In this state v8's are considered gross pouters and subjected to a smog test every year. My GST is meant to be my street car, fun as possible, but legal. I need a car that I can't have a situation come up where I'm not going to have a car. I just mean legal, so it can't be taken away, or ordered to an inspector, or ticketed on parts, etc. I drive pretty mild these days, and it still gets looks from the cops, but I never give them reason to get me. I know I'll be pulled over eventually just fishing for a problem. I've heard the "license plate light is out" trick a couple times, then they look around more.

I feel bad for you guys that live in CA. I've read threads like this before in the past ... I don't how anyone does anything to their cars out there. I'd have a hard time moving there if I ever had the opportunity just b/c of this stuff. Is it the whole entire state? Or, just certain counties?

The downpipe won'make much of a difference unless you get a more freely flowing o2 and Downpipe. Also your turbine is restrictive itself. Your biggest power killers right now are the TB elbow and intercooler. I just checked you DSM profile and you should hook that recirculation tubing back up, worry about your exhaust when you have a bigger turbo, and get a decent intercooler and 2" piping all around. Maybe a boost controller after you can cool down that air. Then look at tuning solutions. Your ability to go fast isn't really hurt by CARB at the level of perfomance the car is at, you just have to work around it.

DJRace said:

I just checked you DSM profile and you should hook that recirculation tubing back up

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Not sure what you're talking about here.. I don't have any tubing disconnected.. Did you see something in my engine pic or something?

As far as the other stuff, that's another California issue. I can't upgrade the turbo, do aftermarket piping and fmic stuff. It will fail visual and be unregisterable. I was thinking about a downpipe as an easily hidden mod. However it doesn't sound like it's worth my time or money to mess with. The cat can't go away, so there's no sense messing with it. The only parts that can be changed are those that have a C.A.R.B approval. There aren't many. K&N has it, some manifolds etc have it. After cat exhaust mods are legal.

Will they actually know if you have a non stock turbo? are they that good at Dsm's that they know what a 25 year old car should look like stock? and will they really take your car if its modified? and do the cops actually crawl under your car to see if the exhaust is different?

Very plain ic piping to a water to air ic in the stock location would be fine. I agree with the motomattx; an OEM 16g would be fine. There are a lot of OEM parts that are upgrades; the EvoIII hot parts for example. Most fit with little to no modification. Porting will always pass visual inspection.

I was referencing this when I mentioned your recirculation tube.

Turbo2g said:

The free mods that clear up the intake track like the bov dump tube removal, waste gate solenoid restriction removal, 1g bov installed etc.

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motomattx said:

Isnt your car old enough to not have to pass the smog tests? its an antique pretty much, would a 82' Chevette still have to pass smog? I would like to hope that there is some way to be able to run an older car out there, lets face it at some point emmisions items for any car will no longer be able to be bought new and its illegal to install used emmisions items on a car (federal law as I understand it), I doubt for instance that you can go to Mitsubishi and buy a brand new cat for a 1g (and maybe not even a 2g), so then what? can you put historic plates on it at some point? 20yrs? 25yrs? around here it used to be 20yrs and its now 25yrs.

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You would buy a new cat and weld it in I’d think. You just won’t get an oem cat.

DJRace said:

Very plain ic piping to a water to air ic in the stock location would be fine. I agree with the motomattx; an OEM 16g would be fine. There are a lot of OEM parts that are upgrades; the EvoIII hot parts for example. Most fit with little to no modification. Porting will always pass visual inspection.

I was referencing this when I mentioned your recirculation tube.

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Oh I get it now, I guess I'm saying it wrong. I didn't remove it, I did the cut of the tube that goes way into the inlet to the turbo. It's all hooked up like stock though.

motomattx said:

Will they actually know if you have a non stock turbo? are they that good at Dsm's that they know what a 25 year old car should look like stock? and will they really take your car if its modified? and do the cops actually crawl under your car to see if the exhaust is different?

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If you get caught racing, or doing something reckless like that, the car can be taken. That's much more likely to happen if the car is modified. If there is any doubt in the legality of the car, you get sen't to a referee. Until you have a statement from the referee that the car is legal, and it passes a smog test, it is revoked from use. It's all an expensive, time consuming process that you want to avoid. All of these test cost money, every document has fees involved. There is likely a ticket involved from the beginning of the incident. It's time at the DMV and money. Best just to stay legal.

Not worth touching the exhaust too much then keep it legal. Look for gains elsewhere.

Just a brain fart. How about leave your exhaust as is, from cat forward. Get a new larger DP installed with an electric bypass plate before the cat. I had that on a the DSM I bought as a donor car. The laser had stock exhaust with the exception of a 2.5" DP with the BYPASS valve before the cat. Is there a way you can go that route without getting your peePee slapped?
Press a button and you're wide open exhaust, when you're done having fun, close it back up to a quiet sounding car.

Thats a great mod then. Haha

Turbo2g said:

Oh I get it now, I guess I'm saying it wrong. I didn't remove it, I did the cut of the tube that goes way into the inlet to the turbo. It's all hooked up like stock though.

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Drcolt, you can get pulled over for being too loud though.