Exhaust diameter, 2x45 or 2x50

[MWe]

As many people know the standard exhaust system for a 325i m20b25 is 2x45mm.
While this diameter is sufficiënt for a bone stock engine, producing around 175 hp there is a point where it would choke the engine.
Most of the time NA builds with up to around 200 hp will not consider larger diameters, i’ve seen 2.9 stroker builds with around 230 hp running 2x45mm.

I’m now at the point where I have a m20b27 stroker producing 201 hp. The exhaust fitted is a standard 3-piece system in a rough state, so I’m looking to upgrade this one in the near future.
I’m now researching what would be the best option, should I go 2x45 mm or 2x50 mm for this build.
So a full system with equal length headers, probably supersprint full system.
Future upgrades will be a standalone ecu, to het rid of the stock LMM, and CoP or wasted spark.
Cam is a schrick 272, so mild street upgrade, I’d like to keep the torque because it’s a street car.

2x45mm will keep the torque, maybe a bit more and will add around 10 hp I suspect.
2x50mm will probably drop the low end torque bit will add it later in the rev range, I suspect.

Does anyone have an idea what the best option will be, and at what point it is best to switch to a larger diameter exhaust, are we talking around 200 hp, or more in the range of 230 hp?

looking forward to reading your views and experiences.

[digger]

There is no magical point as it is a spectrum that depends on the quality of implementation such smoothness of transitions, bend type and radius and the component between the pipes such as muffler, cat, resonator etc which might be more important.

I would use 2x2" 2x50mm as there are no real downside except minor cost perhaps and probably more readily available parts. iirc Alpina went to 2x2" or the metric equivalent

[82eye]

are you running a header now? the stock manifolds choke it at the engine.

i run a long tube header, custom mid, and stromung catback. the catback is stock diameter. the mid reduces down from the header diameter to stock for the catback. i run catless with an x-pipe and o2 sensor at the cat position.

i run a 272 regrind cam, would probably go new billet if i did it again and would deck the head a little for higher compression. we just skimmed the head for resurface and used a stock head gasket.
other mods include a crank scraper, pan baffle, and lightweight flywheel.

car runs great and sounds glorious. it's not for everyone though, it's louder than it is fast. i don't street race, but a lot of guys won't challenge it just on the sound alone. super hilarious as a mini-van with a decent driver will walk on me still over distance lol

[MWe]

Currently running stock manifolds, with a 3-piece original exhaust system. O2 sensor placed at the front pipe of cilinders 1-3.
Looking to upgrade to a equal length header, not sure about the rest of the system yet.

Good to hear that your car runs great, I'm a bit curious about the sound because I'm considering an identical system with an x-pipe and only a muffler at the back.
You have a movie somewhere on YouTube of your setup?

[82eye]

Currently running stock manifolds, with a 3-piece original exhaust system. O2 sensor placed at the front pipe of cilinders 1-3.
Looking to upgrade to a equal length header, not sure about the rest of the system yet.

Good to hear that your car runs great, I'm a bit curious about the sound because I'm considering an identical system with an x-pipe and only a muffler at the back.
You have a movie somewhere on YouTube of your setup?

grab a decent catback and start there. there is more fun / gains there for the dollar, and it's a bolt in on stock.

i have the ie longtube header and don't really make enough power to warrant it. it's hard to get a header that adds much, and the ones that are out there are pricey. i'd get the catback and hunt for a deal / used header.

never bothered with soundclips but that does have me thinking.