I'm new to this forum, but I've been around on Zpost, Bimmerforums, and Bimmerfest. A lot of the Euro laws and Z4 models are slightly different, so I apologize in advance for my lack of exposure to them. Below is for the M54B30 engine found in the 2003-2005 U.S. model Z4. If you have the N52 engine, you may as well forget power mods (I can explain why if need be).
First, getting any real NA power is going to be very difficult with the 3.0i. Torque x rpm / 5252 = horsepower. This means that you can get more power by either increasing the torque or raising the rpm... make a bigger explosion or make more explosions happen in a shorter period of time. The M54 suffers from severe crankshaft harmonics that cause the oil pump nut to come off (or the shaft to shear) with repeated runs above 6k rpm. When this happens, you lose all oil pressure (and your engine). The only proven fix is a $3k BMW Motorsport oil system. Then, you're still limited by the hydraulic lifters. Hydraulic lifters use oil pressure to open and close the valves, and there's a slight ramp/delay as they open and close. A solid lifter conversion (like what the M's use) will run you $$, and you'll need a tune. You'll want some hotter NA cams ($1400 + 7 hours labor) to take advantage of the raised redline, and possibly lose torque down low where the car is now less efficient... not worth it to try to raise the redline in the end.
If you're not increasing the redline, you need to increase the torque to get more power. So let's look at increasing torque. A NA engine of a certain displacement will only make a certain amount of torque. This means you need to increase the displacement of the engine, either through increasing the stroke or bore. The engine already has a long stroke, so you can't really increase the stroke further without causing a lot of side forces on the piston (prematurely wearing the rings), increasing crankshaft inertia, and having a piston speed that's off the charts... basically, an engine that won't last more than a few races. And, a stroker kit involves a new crank, which is extremely expensive and labor-intensive.
You can't simply bore the engine, because the M54's aluminum block has steel sleeves. You'll need new sleeves, custom pistons and tune; even then you're weakening the block. You can only bore it out so far. The labor would be astronomical.
Exhaust - A catless exhaust is illegal in the U.S., and I'd assume it's probably illegal where you are too. If the Euro Z4's are like the U.S. Z4's, the cats are in the headers. Swapping them out for some high quality aftermarket headers (I personally have ceramic coated Supersprint headers) will yield quite a bit of power (15-20 hp). But, a full Supersprint exhaust is very expensive... and again, may not be legal where you are.
Tunes - I used to have the Conforti Shark Injector on my 3.0i before I supercharged the car. I loved it, but mainly for the throttle remap (made it more intuitive rather than more sensitive) and raised redline to 7k rpm (more time in each gear). You'll get a dyno-proven 5-10 whp out of it. You won't get much more than that out of a mail order tune.
Intakes - Aftermarket air intakes add very little power. They're mainly for sound. I used to have a "CAI" on my 3.0i, and I took it off because I noticed no performance change whatsoever.
The last thing you can do on the NA side is raise the compression. This means new pistons and possibly requires the use of high octane race gas at all times. All the heat in the cylinder's volume of air is now compressed into a smaller space, so the fuel is injected into a hotter medium. In order to avoid having it blow up prematurely, you'll have to run high octane petrol.
If I was to do NA mods, I would opt for exhaust, tune, and cams. All in all, you can get maybe 30-35 hp. You'll have to really wring out your engine to see that power.
Is it all bad news? What's there to do? Get an M? No. Forced induction! The M54 has a 10.2:1 compression ratio and it shares its forged crank with the E36 M3 (which have seen over 500 whp in certain applications). In a sense, the 3.0 liter M54 engine is a bit overbuilt. The car is begging for a low-boost supercharger or turbo setup. I run a VF Engineering supercharger kit at 6 psi, mainly because it was the right price at the right time. ESS has the best option IMHO (twin screw setup), because max boost is available from ~2800 to redline. A centrifugal supercharger makes max boost AT redline, and ONLY at redline, since the speed at which it spins is based on engine speed. Twin screw superchargers aren't quite as efficient as the rpm climbs, but they are a good match for the M54 engine. ESS also offer several stages of upgrades, because you WILL want more power.
A 330i stage 2 Technique Tuning turbo kit custom adapted to your Z4 would be even better than the ESS kit. TT uses a thicker headgasket to lower compression, so you can make 450 bhp with a bolt on turbo kit. It's less noticeable too, although I have heard of people sneaking the ESS kit through inspection.
A little history about myself - I've owned a Z4 3.0i for the past 6 years, and a Z4M for the last 3 years. My non-M has about $12k in modifications, mostly used performance parts; new they would be more than double the price. I've researched anything and everything with regards to making it faster. As it sits, it's about as quick as a stock M, but it's a totally different car. You can't turn a non-M into an M... you can put down similar numbers, but the feel is totally different. That's a whole discussion in and of itself.
My advice - only buy an M if you want an M. The M will always be better on the track and have more performance potential. The M feels different, and is a more difficult daily drive. I've said before that it's always talking to you, and sometimes yelling
. By adding a supercharger to your current car, it will retain all of the things you like - the feel won't change, the handling will remain the same, and the braking won't be altered. The car remains completely and totally YOUR car. The only things that will change are the sound (subjectively MUCH cooler) and the acceleration (WOW!).
