benefit to running 3.5" downpipe?
 

I see a lot of 3.5" downpipe setups that reduce down to 3" catbacks. I'm wondering if it even makes a difference

It can help with spool, gives less back pressure at first, then flowing like normal after it spooled faster

On my TII I tried a couple aftermarket exhaust configurations with the same motor and turbo (stock hybrid w/ 60mm external WG) configuration.

First 3.5" turbo back single with one muffler- JIC Spec 90.
Very loud at all rpms with in the low frequency (boomy) with a bad resonance right around 70mph cruise rpm on the freeway.
Full boost @ 3,400rpm in 4th.

Switched to Racing Beat Rev TII 3" turbo back with pre-silencer to dual mufflers.
Fairly quiet at idle/cruise, really quiet under load until WG opened- Psssh noise like air trying to escape my exhaust tips.
Full boost @ 4,000rpm and much less power tooling around off idle.

3.5" downpipe and mid pipe section to Racing Beat Rev TII 3" "catback section".
Fairly quiet compared to JIC Spec 90 at idle- much less boomy, but more exhaust noise than full RB system by far. Pretty loud and raspy sounding at full load even before the WG opened.
Full boost @ 3,400rpm in 4th- no noticeable performance difference over full 3.5" exhaust at any rpm.

Back to full 3.5" turbo back with a Vibrant Streetpower muffler instead of original cannister style.
Surprisingly quiet at idle compared to original JIC Spec 90 cannister style muffler, but still louder than with RB cat back and very boomy. Very loud under full load, but boomy not raspy until WG opened. Surprisingly quiet at cruise with no booming resonances.
Full boost @ 3,400rpm- no noticeable performance difference over 3.5" downpipe/midpipe and 3" "catback" section. No noticeable difference on dyno either.

I preferred the deep tone of the single rear section though over the chainsaw rasp of the Racing Beat duals, others maybe didn't prefer the overall louder exhaust.

I watched some out of car videos of both single and dual rear section, but all you can tell is idle/free rev sound or open wastegate sound because its at the drags.

Well- might as well post.

3.5" downpipe/midpipe to RB 3" dual-

3.5" turbo back single with Vibrant street series muffler. This vid is an aftermarket turbo with slightly smaller exhaust side and dual 44mm WG- but sounded really close to same as stock hybrid with bigger P-trim exhaust and single 60mm WG.

Thanks for the info! 600 rpm is a really good difference. I have to measure my RB exhaust. It's a real old one that came with the car. I don't even know if where it splits is 60 mm piping. It almost looks smaller, but I never measured it. In the future I will look into fabing a 3.5" dp and mid section.

I was reading on the BW efr iwg turbos, and I saw people saying a full 3.5" exhaust were causing boost spikes.

Ok, I measured my piping where it splits and it is 60mm. I'm used to seeing 2.5" 16 ga, so I almost thought it almost looked closer to 2.25".
Also the piping on my exhaust is thicker than 16ga and is mild steel.

as usual Blue TII, thank you for sharing your experiences and insights. this has always been one of my back burner questions (you know, something that i figured didn't make any sense asking until i got there) and it's great to see and hear the differences.

The further away from the engine, the cooler the exhaust gases. Cooler exhaust gases have less volume for the same mass.

An ideal exhaust system would be a gently tapering cone.

Through experiments (on an N/A car) I found power increases going up to a 2.5" exhaust system at the header. I also found no power decreases when putting a 1.75" ID restrictor in the exhaust tip. The only decrease was exhaust drone on the highway.

Are you saying that 5" tips on small exhausts arent faster??

Only if our bumpers had a better contour for a 5" exhaust. Lol jk,

Thanks for the awesome info. People! If I ever upgrade my turbo, I will be going 3.5" to at least my rear subframe area. And thanks to WANKfactor, 5" tips, or quad dtm style mufflers

Funny story: Drove out to Tulsa with a friend in his GC, which had Super Drager or something exhaust. (Not parts store crap) He put an oil filter in the exhaust. Literally, a used ol filter that he cut the top off of, put in the exhaust like a silencer, and held in with a couple screws. He didn't even gut it first, said it smoked for DAYS when he first put it in :)

No difference in power. But the car was annoyingly quiet, it was driving me crazy that all you could hear was a little wind whistle, so I took it out for the drive home :)

I guess this isn't an uncommon thing to do in the Subaru world. I'd search for oil filter silencer but the ATF has gotten wise to the "cleaning kits" for threading oil filters onto the end of your pistol for use as an untaxed/unregistered (therefore unlegal) silencer and I don't know how deep they care about it. (Which is another rant, because IMO silencers should be compulsory instead of discouraged... hearing loss is a bitch)

the noise comes from the exhaust pulses, if you drew a graph of exhaust pressure, you'd get a sine wave, and the height and steepness of the wave = noise. the turbo works because it chops up the wave a little, and then the small tips work, because its a little restrictive and it turns the wave into a smoother flow.

the other way would be a dual pass muffler, where the waves can cancel each other, but its complex.

its also why a v8 is quiet, even though the cylinders are about the same size as a chamber in a 13B, there are more of them, and there is much less going on in each one, so the peaks are shorter, and its easier to bunch them together

On my original 3.5" turbo back JIC Spec 90 exhaust it came with a "tip silencer" which I basically had to run all the time because otherwise it was instant arrest loud.

Funny thing, like others are saying above- even with this tip silencer with one tiny 1.75" opening the power felt about the same.

The RB exhaust doesn't really need the pre silencer. It's plenty quiet without it, and at will reduce your backpressure.

Yes ^^

It is hard to believe the 3" straight through presilencer is causing any backpressure.

It might be the three cheated radius bend cut/welds in the downpipe to presilencer section of the RB exhaust?

I saw those cheater bends too.