[_k_b_k_]

Hi everyone!

Just took ownership of my 19 Z4 M40i. This is my first BMW, first roadster, first 6 cyl and also first modern car too. Loving it so far, but I found the noise it makes to be a bit muted even in sport plus. Also, I bought the car with the intention of getting it tuned (stg2), as I knew as nimble as it is, the stock ~330 hp is not gonna be enough

So yeah, I have an EU model, supposedly with the OPF fitted, although I haven't put the car on a lift yet, will confirm. I read in a couple places online, that the OPF does kill some of the exhaust note, is that true?

And my main question is...as I'm a bit confused on the various parts/components of the exhaust system...so for stg2(+) I'd need decat or sports catted downpipe, and so could I also delete the OPF (I know I'll need to address that in the ECU accordingly too)? Is there anything else between the DP and the OPF section, or is that how it comes together (DP -> mid section with OPF -> axleback section)? And also, can I fit the replacement pipes I've found (Manhart for example) without cutting and welding, or is that absolutely necessary (would like to be able to restore oem state for MOT if possible)

Any help is greatly appreciated. Cheers!

[t0pless]

Euro models were fitted with OPF. This is a reason for less power output and less exhaust sound. Whether you’re doing a custom tune or OTS, you’ll be able to deselect this if you decide to delete OPF.

I’m not familiar with OPF since I’m in the US. Surely this affects emissions readiness, but this is beyond my expertise.

The performance gap between catless and catted are virtually zero with a stock turbo. As you start to get to the 600+whp, the catted downpipe will start to become a bottleneck. At that point, you’re already doing port injection/DI injector upgrades, big hybrid turbo/full frame, and a built transmission or at least torque reduction tuning.

Big reason for me to go catted was to be emissions compliant (GESI is an EPA certification that’s recognized in all states minus California), and also you won’t have the smell of fuel.

DP—> resonator—> axelback. OPF would be somewhere after the DP I believe.

All axleback exhausts will require cutting since there is no flange. An exhaust for these cars are also not a bottleneck until well over 600HP, so modification is strictly for sound. Nearly all aftermarket axle backs will come with exhaust clamps. Installation is best on a lift than stands as the oem exhaust is heavy as hell. Yes, your OPF does affect your exhaust noise.

[_k_b_k_]

Oh, thank you so much for all that info.

I've no plans to go beyond Stage2(+) on 93oct, mostly because sourcing ethanol or some other octane booster is a pita here.

I'm gonna go with a catted dp mostly to avoid smell, and yeah like you said, it's not gonna be a bottleneck for the power levels I have planned.

And too bad about having to cut the exhaust, although I guess I could just put together a full catback system (wanna keep the OEM one untouched).

If you don't mind another question, I've also been reading that a TU version of the HPFP is needed for Stage2+ which adds just a little bit more power...so considering that the Z4 already has the TU version of the B58, does that mean it also has this fuel pump?

[t0pless]

TU pump is called “TU” as in technical update. It’s just BMW speak for 2nd gen. So yes, the z4 already has the TU fuel pump.

You won’t be fuel limited for your power goals. You’ll be octane limited. If you decide to move to meth or ethanol, you’ll then be turbo limited.

I highly encourage you to data log your car anytime you modify it. Learn how to read logs, and what parameters you should be looking at.

Unless if you plan on returning to stock in the future, save yourself the headache and just scrap the exhaust. These exhausts only fit a z4 m40i, so good luck trying to ever sell it if that was your plan.

Oh, and depending on your driving, upgrade your intercooler.

[_k_b_k_]

Thank you so much, again, that was a lot of helpful information.