Downpipe advice

Turbo2g · Mar 6, 2018

drcolt said:
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.

I did that before in another car. Found out that’s a sure way to murder your cat. Cats work by becoming hot. Opening a cut out before it makes what still goes to the cat become unburnt gases on the element and it becomes clogged or otherwise useless as they burn off on the elsement insufficiency and no longer sufficiently reduce emissions. Causing you to fail an emissions test before knowing it’s bad.

motomattx · Mar 6, 2018

So what do people out there do when they drive to the drag strip and go drag racing on a built street car? there has to be a workaround, that crap wouldnt wash with me for one second, I would have a PO box in another state and register my car in another state and drive it everyday over there with no cat just to punish them.

donniekak · Mar 6, 2018

You can always run a straight pipe through a catalytic converter case.

I built a 3000gt for a guy in Arizona that passed the sniffer test with no cat. It just had a cat heat shield wrapped around the downpipe to pass visual.

It was a brand new engine running e85 as well.

Turbo2g · Mar 6, 2018

The way to get around it is to have a straight pipe and cat pipe that can be easily swapped in. Mine is welded in unfortunately. So I'd have to make flanges. Just the way the car was when I got it. Cars can still be quick with their cats in. Just a hindrance we all deal with. This gst had a bad cat when I got it. Wasn't plugged or anything. Had to weld another one in. A straight pipe or gutted cat would never pass sniffer here. Passed with flying colors after the cat swap. Now that a good cat is in I'd just rather leave it alone. I'm just trying to make decent power still.

Boomdeeze · Mar 6, 2018

Just do what I did and add one of these:

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Well I read the thread after I posted haha Looks like it was mentioned already.

Turbo2g · Mar 7, 2018

What do you guys think of this plan. The Apexi N1 down pipe is 2.5" inlet and outlet, which is my catback's diameter and will match up the the cat nicely. And if you look at it, it isn't a bright polished chrome look. It looks like it will relatively quickly become a normal looking pipe with some use. I will measure where to cut the cat delete part and bolt up the pipe to the turbo and weld at the other end to the cat. I don't think it will be detectable because I'm not going for a stainless look.. And I do think it will make a difference in power the more I think about it. Cat's are a restriction, however they have been proven to flow good up to a certain amount. So I feel the downpipe will still help make the entire system flow better. Remember, I'm not trying to break records here, just maximize my power in a car that's still 100% legal in the eyes of the law. I don't think anyone will ever think twice about the pipe with my plan to make it look stock. I'll even use old heat shield on it. I have old pieces all around the shop.

DJRace · Mar 7, 2018

Turbo2g said:
What do you guys think of this plan. The Apexi N1 down pipe is 2.5" inlet and outlet, which is my catback's diameter and will match up the the cat nicely. And if you look at it, it isn't a bright polished chrome look. It looks like it will relatively quickly become a normal looking pipe with some use. I will measure where to cut the cat delete part and bolt up the pipe to the turbo and weld at the other end to the cat. I don't think it will be detectable because I'm not going for a stainless look.. And I do think it will make a difference in power the more I think about it. Cat's are a restriction, however they have been proven to flow good up to a certain amount. So I feel the downpipe will still help make the entire system flow better. Remember, I'm not trying to break records here, just maximize my power in a car that's still 100% legal in the eyes of the law. I don't think anyone will ever think twice about the pipe with my plan to make it look stock. I'll even use old heat shield on it. I have old pieces all around the shop.

You're right; it wont make a difference in your power. Archer Fabrication's better flowing catalytic converter idea is the best solution. You could incorporate one after you get your dp in if you don't/can't get one at the same time. You will just give yourself more work by not doing both at the same time of course.

gofer · Mar 7, 2018

Here's what I did in SoCal living there for 8yrs with my DSM.

Purchase a turbo back, includes DP all the way to the muffler with a rally cat on there (looks like a catalytic converter to the CHP) which is essentially a straight pipe. --> 2.5" Rally Cat
It will pass a quick CHP visual but won't trick the REF or any emissions shop, definitely not CARB legal.

Store your stock downpipe/catalytic converter/catback exhaust and install it prior to going in and getting it smogged.

YERFDOG · Mar 7, 2018

As stated above, our best bet is to store the stock downpipe/cat and throw it back on for SMOG day. I recently wanted to upgrade my downpipe but still decided to keep a CAT on it. I didn't want to spend a lot on the CAT because any SMOG legal cat for our cars is still only 2.25-2.5". (I wanted a true 3" exhaust). I bought the Megan Racing V2 downpipe which is 3" and bought a 49 state legal cat that is 3" and high flow ($45). It still acts as a cat so I have no check engine ect.. and runs clean. I then brought it to an exhaust shop to have it fitted to my existing catback (Thermal R&D). BUT the most difficult part was buying the CAT online because it "cannot be sold or shipped to CA". So I had to ship it to a friend in another state then he shipped it to me. Talk about B.S....how can we buy and ship a cat-deleting downpipe but not an actual working CAT?!?! But anyway, just thought I'd share.

Turbo2g · Mar 8, 2018

I have come to the realization that I may have a high flow cat installed. Its a magnaflow California complaint cat, however the more I research it, it seems they only make high flow design cats. Anyone know if that's true? I'm looking around and all I see is "high flow" mentioned with any cat products magnaflow produces. So maybe the downpipe will be more beneficial than I thought. I'm gonna keep looking into it, but as of right now I think it's a high flow cat. Unless anyone knows something I don't.

Turbo2g · Mar 8, 2018

From manganflows website:

"Most MagnaFlow catalytic converters are higher than OE in flow capacity. The 400 CPI cell density of most MagnaFlow catalytic converters will out flow an OE for the same application while providing the necessary functionality."

Key word "most" I know. But it seems like it's likely a higher flowing cat.

ERJ-145 driver · Mar 9, 2018

Ill be moving to Cali soon and this will also be my problem now! Besides the CAT will the EGR delete and OBD-II cause the inspections to fail? I don't mind installing cats, but will they fail a car of other mods under the hood? thanks

gofer · Mar 9, 2018

Any modifications to the car that aren't C.A.R.B. compliant will cause it to fail, even the air intake must have a C.A.R.B. compliant sticker if it's aftermarket. If you get sent to the REF by the CHP a catalytic converter that isn't OE from the same model year DSM will fail. From what I know, there's no C.A.R.B. legal high flow cat from Magnaflow.

If you're fortunate enough to have a relative that lives in an area around the country that doesn't require emissions your best bet is asking them if you can register the car at that address and plate it out of state.

Turbo2g · Mar 9, 2018

ERJ-145 driver said:
Ill be moving to Cali soon and this will also be my problem now! Besides the CAT will the EGR delete and OBD-II cause the inspections to fail? I don't mind installing cats, but will they fail a car of other mods under the hood? thanks

If you did like a lot of people and deleted EGR and things like EVAP system, have no cat, and have aftermarket parts bolted onto the engine in various places, I'd recommend selling the car where you live and starting fresh here if you still want a DSM. If you don't do the work yourself, it will cost thousands to have those systems diagnosed, parts located, and reinstalled. Most people remove those system because they could't figure out the issues they were having with them in the first place. Having an fmic is begging for cops to get you, Any intake or exhaust piping not stock (except from the cat back) will be immediate failure uppon inspection. You will have to pass the 5 gas sniffer after you get your CARB compliant cat in, so the EGR, EVAP, O2 systems, and all sytems wil have to be working, no engine light, if obd2 readiness monitors set. There's a lot you have to look forward too.

ERJ-145 driver · Mar 9, 2018

Well that really sucks! If worse comes to worse I’ll register it as an Off-Highway Vehicle (race car). At least that way I can track the car at least. I’ll have to look more into this requirement. Thanks guys

Silkybuttah · Apr 5, 2018

Have an exhaust shop make you a cat delete pipe that you can swap out with the cat

doubleclutch · Apr 5, 2018

While you are in there, try to get a less restrictive o2 housing. Either port your 2g housing or get an EvoIII housing and port that. I think there may be one or two cast iron housing in the aftermarket that would look stock. The 2g housing is really restrictive.

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