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?

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.

thelostartof said:

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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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?

I would say it'll be noticeable. I would expect the majority of the gain will come from removing the transition honestly.

EivlEvo said:

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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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!

thelostartof said:

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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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!

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.

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.

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.

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.

Even having my 3" pressed as close as possible to the stock tranny mount, I still risk scraping as well

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.

LeedomtoFreedom said:

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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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.

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.

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.

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.

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?

89_740Turbo said:

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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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.

oldschoolvolvo said:

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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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.

oldschoolvolvo said:

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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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.