Pipes: 3" throughout or 3" down to 2.5"... and more

[Woodnutz78]

Hey guys - I'm getting way ahead of myself on this, as exhaust upgrades are still probably at least a year away for me, but I'd like to get your opinions in advance. I've currently got the 2.5" axle-back SZ setup, as it was from the previous owner, but as I've looked at potential changes to the upstream components it seems foolish to not go for the 3".

SZ has 3" to 2.5" taper mid-pipes that would clearly work, and while I understand there's some flow loss in the step-down, I wonder if it would even be noticeable with the restriction at the turbo outlet mitigated with the 3" DPs. Sure, the obvious solution would be to just get the 3" axle backs... well, that's $700 that would be nice to avoid, especially since I've got some nice parts already in place.

For any who are curious, here's my high-level plan:

Phase I: Get it running "right" - misc deletes and maintenance: Spoiler replace, clutch bleed del, carbon can del, AIV del, EGR del (with engine in ), TB coolant lines del, antenna & rear wiper del (on the fence on those), etc., 120k, clutch. Clean the F out of the engine bay. Between now & through winter 2019.

Phase II: Flow - All exhaust, from the DPs down, including what I asked about above. Intercoolers and maybe the Goose Honk Killer (if not sooner). Pop charger is in, but who doesn't do the intake first... oh, wait... my Z's previous owner didn't (which was fine with me). Later 2019.

Phase III: Balls - Injectors, boost jets & tune. Winter 2020.

Phase IV: Bigger balls - TBD.

As always, gladly welcome your input & advice.

[Ace2cool]

If you're not planning on upgrading intake piping, I honestly don't see the need to go a full 3" out the back. Obviously you are bottlenecking the system a bit, but it's not terrible if you're planning on stock turbos and stock piping up front with a single MAF.

[Woodnutz78]

I wasn’t sure if a dual intake was really needed for stock turbos... at least not at stock boost. Wasn’t planning on it, but plans can change of course. Well, first things first anyway.

[DCaff300ZX]

I run upgraded bb turbos/DP's, custom tune and larger injectors, upgraded smics- pretty standard stuff, about 400-450 HP, all on stock exhaust except for the exhaust tips which are larger exits without any issues. Really as mentioned, larger exhaust isn't really needed until you reach the 600+ HP area where you need to maximize everything to get more.

[Woodnutz78]

Thanks for the feedback. As my very generalized plan shows, I'm not even considering upgraded turbos for quite some time. It's easy to say now, but I see myself being quite content with the power gains I get from going to 14-15 psi in Phase III.

Now, of course, if something were to happen to my poor little stock turbos at some point, it would be just plain wrong to not make an upgrade ...hence the TBD for Phase IV.

Once again - I greatly apprecaite the advice, gents!

[NolimitZ32]

Actually, the step-down benefits flow velocity. As the exhaust gasses cool down they contract so to maintain flow velocity a step-down is introduced where less volume in less cross-sectional area equals maintained velocity. Even with the stock turbos a 3" DP with a 3" to 2.5" TP and CB will benefit performance, how marginal or noticeable it will be I couldn't even begin to predict. That said, if you are going for massive power the 2.5" creates a choke point and is ill-advised, what that magic number is I have no idea. Knowing all this, I opted to go 3" all the way back (with my HKS 2530s) because I love hearing the turbos spinning when I'm idling.

[Woodnutz78]

That’s actually a good point - I’ll keep that in mind as things progress. I have a lot of cleaning up/prep work and maintenance to take care of before I get to do more than window shop. The car runs great as is, but I think a lot of that stuff will really help me get familiar with the car.

Plus, it eases the blow to the wallet!

[DCaff300ZX]

That’s actually a good point - I’ll keep that in mind as things progress. I have a lot of cleaning up/prep work and maintenance to take care of before I get to do more than window shop. The car runs great as is, but I think a lot of that stuff will really help me get familiar with the car.

Plus, it eases the blow to the wallet!

I also planned to stay OEM turbos if possible, but as mentioned a locked up ps turbo changed plans and going back to the old turbo tech not even a possibility for me...something every turbo guy needs to decide and have a solution in mind. Personally I started searching for used bb turbos as soon as I landed back home with the car, and was lucky enough to find the ones I got for a great price, with DP's and all nearly new.
I didn't know the exact info NoLimit gave, but yeah- in addition to some exhaust manifold porting the 3" DP's stepping to the 2.5 piping afterwards was given as a beneficial situation for my build given being well under 600 HP, and Doug did say 3"" was only necessary as NoLimit suggests, but also was not an issue if the exhaust note wasn't a problem. I tend to prefer a less obnoxious but still healthy exhaust note, which somehow easily Doug achieved with my cobbled setup.

[Woodnutz78]

I'll have to start my research and work on an emergency plan, so to speak. I would want a nice, reliable power bump of course, but still maintain good driveability - minimal lag, etc. Essentially, the most I can safely get without the need to upgrade engine internals. DCaff, that's awesome you found a great deal on yours. That would be the ideal situation, even if they had to sit on the shelf for a while.

I think my Z sounds great now with just the SZ dual tips - it's a smooth "hum" when cruising. Previously was a throaty rumble, but that was with just 5 cylinders Of course, I have no idea what it sounds like outside the car, but it's definitely not obnoxious. No idea what'll happen when parts start getting replaced, let alone boost turned up. I had an Integra GS-R back in the day and after repair from an accident, I ditched the resonator can in the mid pipe... it was pretty bad. Actually made me cringe a little when I hit the exhaust side VTEC at 4400/4500rpm.

I've just got to continue the research and keep my eyes open... same as I did in buying the car.

[Ace2cool]

Knowing all this, I opted to go 3" all the way back (with my HKS 2530s) because I love hearing the turbos spinning when I'm idling.

[NolimitZ32]

Just for you Aaron, I actually ran mine with no cat-back at all and plan to do so again once the current car is back on the road. Getting a couple 90 downturns fabbed with flanges to bolt up straight to the TPs, good thing most of the police in Houston disregard the 85db rule

[Ace2cool]

I literally did exactly that for like 5 years on this car. hahaha

[NolimitZ32]

It's heaven, and drowns out all the weird squeaks and squeals. hahaha

[Woodnutz78]

As tempting as y'all are making this sound, I'm sure all my neighbors wouldn't exactly be thrilled when I go out for my midnight Taco Bell run on the weekends. Not that I care or anything

Nolimit, I had Katy cops back in the day. Maybe they've changed since that place has grown into it's own little metropolis now, but they sure didn't let much of anything slide back then - 20 years ago.

[NolimitZ32]

Seeing as Houston and the surrounding areas have become the Mecca for street racing (TX2k) the police around here have generally eased up on stuff like loud exhausts, hell there are 1320 productions videos of guys doing 150+ being let off with a warning. There are still a few municipalities that will give you a hard time but most of those are the small uber-rich areas like The Villages, and some unincorporated parts of Pearland. For the most part the police force is young enough that they grew up in Houston with TX2k going on so they mostly just want people to not drive 120 in rush hour traffic.

[Woodnutz78]

Yea, I’ve seen the videos - pretty wild stuff. It’s the polar opposite here just north of the MD border in the middle of nowhere, PA. Fortunately, there’s a similar indifference... mostly since we’re in farm country. We don’t even have emissions testing at all in this county. There’s not much of a street racing scene in the region - take that for good or bad. Shoot, I’ve only seen one Z32 in person in my 6 years living here, and that was just this week. Now, muscle cars and big ol’ diesel trucks are a different story. At any rate... still gotta watch for the small town cops... both of them.

[NolimitZ32]

DUDE!!!! I just went snopes on your Sig quote. OMFG I AM DIEING LAUGHING!!!!

[Woodnutz78]

Yea, I had to read it two or three times to make sure I wasn't hallucinating. I just saw it yesterday and I just had to share. Surely something that was lost in translation...

I've lately been sifting through the FSM somewhat at random - I'm sure it makes me a bit of a nerd that I feel like I could read that things for days, but oh well. I'll have to see what other nuggets are in there.

[DCaff300ZX]

Yea, I had to read it two or three times to make sure I wasn't hallucinating. I just saw it yesterday and I just had to share. Surely something that was lost in translation...

I've lately been sifting through the FSM somewhat at random - I'm sure it makes me a bit of a nerd that I feel like I could read that things for days, but oh well. I'll have to see what other nuggets are in there.

You're not alone, when my NA started to become a problem searches there went so well that like you I started to look at EVERYTHING. That and any and all tech posts and such anywhere else, which led to a lot of good info on many of the issues a Z32 faces. This allowed me to determine whether or not I wanted to take it all on with a TT, and which I eventually did when the NA died.
I think the fsm is one of the best, most helpful things to come along for Z32 ownership, as a kid finding this kind of detailed info was VERY difficult and tech knowledge even harder to find without help from veteran mechanics and specialists who weren't always so willing to share the knowledge that made their paychecks.

[Woodnutz78]

DUDE!!!! I just went snopes on your Sig quote. OMFG I AM DIEING LAUGHING!!!!

To add on to this gem... after the initial head-shaking - take note that it states "It is not recommended to", as opposed to "Do not". I would say that you can't make this stuff up, but apparently you can. Funny thing is, in my line of work, we review manuals and have to try and think of some of the stupid things poeple might do. Foreseeable misuse.