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Old 05-02-2018, 09:10 AM   #1
89_740Turbo
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Default 16T Turbo with 3inch downpipe to 2.5 inch exhaust?



Hey everyone,

Back again for another question. Mind you, I've already tried researching this online but only came up with Subaru forum stuff. I wanted to ask here.

I am buying a 16T turbo, high performance fuel injectors, manifold, all the appropriate attachments and a 3 inch downpipe that will reach the catalytic converter. So, essentially, I will be creating a full turboback exhaust, but really just a catback since I really only need piping from the cat to the muffler.

Anyway, my question is - Is it logical/feasible/performance restrictive/stupid/an OK thing to do/a stupid thing to do, to have a 3inch downpipe attach to a 2.5 inch catback exhaust?

If it's not worth it, that's totally fine. I found a 3.00" diameter universal fi mandrel bent tubing kit online I'd buy and just fabricate/turn into an exhaust. I was just trying to save myself a little time and money IF possible by buying a prefabricated 2.5 exhaust online.

I've read 2 very different answers about this. One being negative that the 3inch to 2.5inch will be restrictive and will give increased HP but not a lot. The other being positive that the 3inch to 2.5inch theoretically would create higher thrust. Who knows. Anyone have anything to say on all of this?
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Old 05-02-2018, 09:45 AM   #2
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You will not cause any issues putting a 3" downpipe down to a 2.5" cat back. It will flow more than stock and work just fine. If and when you want more power just remove the 2.5" pipe and go to 3" for more flow.
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Old 05-02-2018, 10:13 AM   #3
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You will not cause any issues putting a 3" downpipe down to a 2.5" cat back. It will flow more than stock and work just fine. If and when you want more power just remove the 2.5" pipe and go to 3" for more flow.
Hey man. Glad to see you and thanks for the response. So, I think I'll just go ahead and use the 2.5 for now. Will there be a significant power increase from a 2.5inch to a 3 inch?
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Old 05-02-2018, 10:20 AM   #4
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I would say it'll be noticeable. I would expect the majority of the gain will come from removing the transition honestly.
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Old 05-02-2018, 10:57 AM   #5
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I would say it'll be noticeable. I would expect the majority of the gain will come from removing the transition honestly.

Hmm. If it's that noticeable then I might just wait and have a 3inch built instead of going with a 2.5 inch. No point in wasting time on a 2.5 if the gain is that noticeable.

thanks!
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Old 05-02-2018, 12:20 PM   #6
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You will not cause any issues putting a 3" downpipe down to a 2.5" cat back. It will flow more than stock and work just fine. If and when you want more power just remove the 2.5" pipe and go to 3" for more flow.
Lmao well would you look at what just popped up on bringatrailer.com! hahahahaha

https://bringatrailer.com/listing/1984-volvo-240-9/

240 Volvo with the same engine as mine, 16T turbo, 3 inch downpipe to 2.5 inch exhaust. Well time for me to read up!
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Old 05-02-2018, 12:36 PM   #7
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Keep in mind that in stock form, the S/V70 T5 cars that originally used the 16T turbo had 60mm exhaust piping, so a 3" DP + 2.5" cat-back OR full 3" system will work fine and be a step up from the factory setup in terms of flow.
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Old 05-02-2018, 09:01 PM   #8
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Tbh the difference between a well made 2.5" and a 3" is pretty negligible especially on a turbo like that. The 16t already spools up quick.

Will it spool up quicker with a 3" exhaust... Yes
Will you notice it... I vote not

FWIW my old shop teacher built a BMW with an ls1 and a borg warner turbo the size of a melon... He's using a 3" exhaust making over 1000 hp.
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Old 05-02-2018, 09:54 PM   #9
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Yes but just because he is using 3" does not mean he is leaving a lot of HP on the table. If he stepped up to a 3.5 or 4" he would gain a huge amount of power.
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Old 05-02-2018, 11:31 PM   #10
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Really depends on how you use the car. If you really use lots of boost regularly, go for the 3”. If you drive like 90% of us do,it will be negligible. 3” is harder to package neatly under the car too unless lots of time is spent bending it.
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Old 05-03-2018, 12:06 AM   #11
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Even having my 3" pressed as close as possible to the stock tranny mount, I still risk scraping as well
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Old 05-03-2018, 12:10 AM   #12
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Also if you're throwing a new downpipe in there, put a vclamp on it before the cat or around there to make pulling out the downpipe worlds easier.
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Old 05-03-2018, 01:16 PM   #13
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Originally Posted by LeedomtoFreedom View Post
Also if you're throwing a new downpipe in there, put a vclamp on it before the cat or around there to make pulling out the downpipe worlds easier.
I'm doing this as I weld up the new downpipe. I have a 15G conical on there now, I have a 19T with angled flange waiting and I have an HE341 if I get super bored.

V bands at the right spot and I have a usable downpipe that I just change the upper section on.

At least in my mind.
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Old 05-03-2018, 04:14 PM   #14
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Go for the three inch system. I used to use a 2.5" and it was fine but when I went to a 3" it was much more fun. Plus you already have the down pipe. You could buy the catalytic system from JT tuning to have a pretty much bolt in system.
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Old 05-03-2018, 04:31 PM   #15
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Built a 3 inch for my 240 because I figured why not since I'm already making a new exhaust. Barely noticeable, but I am only running a 15g with the conical flange.

Room for future improvement.
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Old 05-03-2018, 04:40 PM   #16
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I have 3" from the downpipe through the cat on my C70 which then transitions to 2.5 under the back seat. I'm running a 19T at 22 psi through it, makes plenty of power.
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Old 05-05-2018, 09:11 PM   #17
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Alright thanks for the answers guys.

Next question: Can I put on a 16T turbo with the stock ECU? I've read people have had good luck just leaving the car stock after adding the turbo. I read its reccomended to get an O2 sensor. Is it mandatory or recommended. What other mods will i need after the 16 turbo addon?
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Old 05-05-2018, 09:25 PM   #18
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Originally Posted by 89_740Turbo View Post
Alright thanks for the answers guys.

Next question: Can I put on a 16T turbo with the stock ECU? I've read people have had good luck just leaving the car stock after adding the turbo. I read its reccomended to get an O2 sensor. Is it mandatory or recommended. What other mods will i need after the 16 turbo addon?
Short answer: yes.

ECU doesn't care what turbo you have, however bigger turbos can make more boost / move more air than the stock ECU was programmed to handle. That being said, you can swap out your 13C for the 16T and run moderate boost no problem. It is recommended that you install a wideband O2 sensor which will allow you to monitor the a/f ratio in boost and provide feedback when increasing the boost. You don't need any other engine mods to use the 16T however you will need to figure out a wastegate bracket once you reclock the turbo for rwd use. It can be as simple or complex as you want. I went simple and used a piece of 1/4" steel I had laying around.
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Old 05-05-2018, 09:32 PM   #19
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Originally Posted by oldschoolvolvo View Post
Short answer: yes.

ECU doesn't care what turbo you have, however bigger turbos can make more boost / move more air than the stock ECU was programmed to handle. That being said, you can swap out your 13C for the 16T and run moderate boost no problem. It is recommended that you install a wideband O2 sensor which will allow you to monitor the a/f ratio in boost and provide feedback when increasing the boost. You don't need any other engine mods to use the 16T however you will need to figure out a wastegate bracket once you reclock the turbo for rwd use. It can be as simple or complex as you want. I went simple and used a piece of 1/4" steel I had laying around.

Thanks for the reply. The guy selling me the 16T has the setup specifically modded for the 740 which is what I have, and that includes the Watergate bracket.

So brass tax: I can add the 16T, downpipe, flange and all turbo mods to the car, including the bigger fuel injectors, without having to change anything else? The reason I ask is if it ends up being that I have to mod a bunch of other stuff after adding the 16T, then I'll stick with my TD05-12B turbo and just add a downpipe to that. I'm not looking to be a speed demon and pull 12 second 1/4 mile times Haha I'm entertaining the 16t because hes offering to sell it at such a good price, BUT I'll still be highly satisfied with me TD05-12B turbo speed with a downpipe, upgraded exhaust and cam.

If any of that makes sense.

Basically, not looking to be a racer. If 16t upgrade requires too much, I'll be sticking with the stock turbo.
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Old 05-05-2018, 09:34 PM   #20
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Originally Posted by oldschoolvolvo View Post
Short answer: yes.

ECU doesn't care what turbo you have, however bigger turbos can make more boost / move more air than the stock ECU was programmed to handle. That being said, you can swap out your 13C for the 16T and run moderate boost no problem. It is recommended that you install a wideband O2 sensor which will allow you to monitor the a/f ratio in boost and provide feedback when increasing the boost. You don't need any other engine mods to use the 16T however you will need to figure out a wastegate bracket once you reclock the turbo for rwd use. It can be as simple or complex as you want. I went simple and used a piece of 1/4" steel I had laying around.
Also, not looking to add a 16t and put a lot boost. If I were to add it, it'd be higher boost than normal but not top end.
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Old 05-05-2018, 11:15 PM   #21
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Originally Posted by 89_740Turbo View Post
Also, not looking to add a 16t and put a lot boost. If I were to add it, it'd be higher boost than normal but not top end.
You can bolt on a 16t and run 10psi with no other mods. Maybe more, but as mentioned above, get a wideband if you want more boost.
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Old 05-05-2018, 11:43 PM   #22
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One thing to consider is that you will need the oil/water lines for the 16t turbo. The 12b uses different lines. I have the oil/water lines if you need them.
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Old 05-06-2018, 11:32 AM   #23
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Originally Posted by soclosenotnear View Post
You can bolt on a 16t and run 10psi with no other mods. Maybe more, but as mentioned above, get a wideband if you want more boost.

Okay thanks.

Should the exhaust be installed first before the turbo or can the 16t be installed with the stock exhaust for the time being? Can the exhaust be installed on a 13t turbo or will it create backpressure issues?
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Old 05-06-2018, 11:37 AM   #24
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The issue you have when you install a different turbo is that you at the very least need a down pipe that will mate up to your turbo. You can run an open down pipe until you take your car to an exhaust shop.
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Old 05-06-2018, 11:38 AM   #25
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Originally Posted by soclosenotnear View Post
You can bolt on a 16t and run 10psi with no other mods. Maybe more, but as mentioned above, get a wideband if you want more boost.
I found this online - https://www.matthewsvolvosite.com/fo...ic.php?t=51868

and in it, it says

"It’s not uncommon at all for folks to upgrade their downpipe to achieve greater flow while leaving the remainder of the exhaust in stock form to keep tailpipe volume more reasonable. Stock resonators and mufflers do impede flow at higher horsepower levels but not nearly as much as the catalytic convertor in the stock downpipe . This is in part due to the exhaust gasses cooling considerably by the time they reach the resonator and muffler such that the gasses are more dense, take up less space, and therefore create less back pressure as they exit the tailpipe. "

Thoughts on this application: 16T, 3" downpipe ---> temp stock exhaust @ 10psi and eventual 2.5" exhaust?
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