A problem I've noticed with the brakes of my roadsters is not so much braking power but heat dispersion. I recently upgraded the brakes on the Z3 after get them hot to the point of not being able to stop while trying to keep up with IMZ4N (it's always his fault if I have to upgrade something 
). The Z3 came with 286mm SOLID front rotors and was able to upgrade them to 300mm VENTED for $160 (plus new rotors, but that's more of a maintenance item).
Bigger brakes run cooler because they require less friction to generate the same torque. If you're thinking about upgrading your brakes to increase braking power, first make sure you're ABS doesn't kick in already; if it does, then your problem is more so with tires and suspensions, than brakes (or at least, brakes are not the place to start from)...
I've being doing some research about doing something similar on the Z4 and I thought I would share some thoughts since I read of people asking about it:
1) The 2.5i (as well as the Euro version 2.0i and 2.2i) have 286mm vented in the front and 280mm solid in the rear
2) Both 3.0i (and the Euro 2.5si) have 300mm vented in the front and 294mm vented in the rear
3) The 3.0si has 325mm vented in the front and 294mm vented in the rear
4) The ///M have their own things, so it might not be as easy to put ///M brakes in a non-///M car (the different offset and the ///M tradition makes me think it require some suspension parts to be changed as well... The ///M experts might be able to help more)
5) 300mm is the most you can fit with 16" rims; 325mm require 17" rims
6) All Z4 share the same master cylinder (although the p/n changed they are interchangeable): this simplifies the swap a lot.
7) The Z4 share the same brakes as the E46, making a lot easier to find parts at the junkyard
I would recommend you grab the whole caliper and carrier assembly by removing the brake fluid line and the two bolts connecting the carrier to the king pin/suspension. This doesn't apply only if you're planing on upgrading from 286mm to 300mm front: if so, all you need is the carrier (calipers, sliders and clip are the same). Remember that if you have 16" front rims, 300mm is the biggest you can go without having to change your rims.
Upgrade combinations
300mm vented front brakes
This is a good upgrade for the 2.0i, 2.2i and 2.5i. It will allow you to keep the stock 16" rims and you'll keep a good front-rear brake bias. The cheapest way to go...
All you need is the carrier and can be taken off the following cars:
E46 318d Touring
E46 318i Sedan
E46 320 All (Gasoline & Diesel)
E46 323i Convertible & Touring
E46 325i All
E46 328i All
Z4 3.0i Roadster
Z4 2.5si Roadster
Z3 3.0i (Roadster & Coupe)
325mm vented front brakes
This is a great upgrade for all Z4: you'll have the same front brakes as the 3.0si so I'll recommend upgrading the rear to 294mm. You will need 17" to clear the calipers.
You'll need the whole front caliper assembly from one of the following car:
E46 330i All
Z4 3.0si (Roadster & Coupe)
If you wanna be a perfectionist: the protection plate behind the 325mm rotor is bigger than what the 300mm or 286mm have (these two setups have the same cover). The rotors should fit with no particular issue as the cover does not interfere with the rotor. The problem with replacing this covers is that the wheel hub has be removed (you need a 46mm socket and an impact wrench) make it impractical if you are sourcing the parts from a junkyard. My friend Ed has been able to fit 325mm on a 323i Touring (originally equipped with 286mm as some Z4s) and had no issue. You can see some pix of his setup here.
294mm vented rear brakes
Another good upgrade for the 2.0i, 2.2i and 2.5i. It will allow you to keep the stock 16" rims and you'll have the same front-rear brake bias as the 3.0i.
If you go with 325mm in the front, I'll suggest you upgrade the rear as well to keep the bias within BMW specs.
You need the whole caliper-carrier assembly from one of the following cars:
E46 318d Touring
E46 318i Sedan
E46 320 All (Gasoline & Diesel)
E46 323i Convertible & Touring
E46 325i All
E46 328i All
Z4 3.0i Roadster
Z4 3.0si Roadster & Coupe
Z4 2.5si Roadster
320mm vented rear brakes
If you find a E46 330i, you might also want to consider a full brake setup off this car: it would give you 325mm in the front and 320mm in the rear and it would be a bolt on swap...
Alternatively, 320mm rear vented rotors can be sourced off of an X3 (any model, old or new). As noted below, the X3 might not be able to provide you a full front and rear swap: the front calipers are the same as the E46 330i, but the caliper carrier has a different p/n.
In summary, 320mm vented rear rotors can be sourced from the following cars:
E46 330i All
E83 X3 All
320mm can be used only with 325mm front as you don't want to have your rear rotors bigger than the front. Although many variables go into calculating brake bias, the disk size ratio of a 325mm front - 320mm rear conversion is similar to what the 3.0i has so it should be fine. However such a brake setup was never chosen by BMW as a setup for the any Roadster.
Another note regarding 320mm rear brakes. In this case too they have a different protection plate (as for the 325mm front rotors setup), and in this case it gets in the way of the bigger rotor. Once again, removing the plate is not as practical as you have to remove the parking brake assembly and likely the wheel hub. To fix the issue I would personally just bent the plate of the way... Sometimes I agree with the saying that "is something doesn't fit, your hammer ain't big enough"...
Notes and Credits
PS1: the E46 323i
I did some more research about the E46 323i and it appears to be somewhat in between when it comes to front brakes: some body style have 286mm while others have 300mm. Also it appears the same body style could have different configuration between Euro version and US version... I would recommend you bring some measuring tape when going shopping through junkyard just to make sure you get the correct one.
PS2: possible sources
IMHO, buying the calipers new from BMW is not cost efficient, as you might as well go with some fancy non-floating & multi-pistons brake kits. Here's a couple of hints of where to source the parts you need:
1) of course you can check with your local junkyard and car dismantlers: where I got mine the guy actually removed them, in some junkyard you remove the item yourself and then negotiate the price
2) eBay is a good source for the: the US has usually has a lot of calipers off of E46 so that's a source...
3) There are some dismantlers that specialize on BMW and will ship you the product. The two I can recommend because I have personally used are:
- Autobahn in El Cajon here in San Diego
- Bavarian Auto Recycling up in the Bay Area. The nice thing about them is that calipers come tested (the o-rings are replaced if leaking), cleaned, and come with a warranty (not sure of how long)... Of course they are more expensive than what you could get elsewhere.
Just complement any of these setup with some Brembo Slotted or Zimmerman cross drilled rotors (I got mine from Ed @ Bimmerbum). I won't recommend buying rotors on eBay as some sellers get plain rotors and drill them in house.
PS3: sourcing the front brakes from an X3
The X3 has the same front calipers as the 330i (325mm vented); however the carrier has a different p/n so it might not be a perfect bolt on.
Credits
cj10jeeper: comments on several things
mamasuon: comments regarding the dust plates on the 330i and Z4 3.0si being different
). The Z3 came with 286mm SOLID front rotors and was able to upgrade them to 300mm VENTED for $160 (plus new rotors, but that's more of a maintenance item).Bigger brakes run cooler because they require less friction to generate the same torque. If you're thinking about upgrading your brakes to increase braking power, first make sure you're ABS doesn't kick in already; if it does, then your problem is more so with tires and suspensions, than brakes (or at least, brakes are not the place to start from)...
I've being doing some research about doing something similar on the Z4 and I thought I would share some thoughts since I read of people asking about it:
1) The 2.5i (as well as the Euro version 2.0i and 2.2i) have 286mm vented in the front and 280mm solid in the rear
2) Both 3.0i (and the Euro 2.5si) have 300mm vented in the front and 294mm vented in the rear
3) The 3.0si has 325mm vented in the front and 294mm vented in the rear
4) The ///M have their own things, so it might not be as easy to put ///M brakes in a non-///M car (the different offset and the ///M tradition makes me think it require some suspension parts to be changed as well... The ///M experts might be able to help more)
5) 300mm is the most you can fit with 16" rims; 325mm require 17" rims
6) All Z4 share the same master cylinder (although the p/n changed they are interchangeable): this simplifies the swap a lot.
7) The Z4 share the same brakes as the E46, making a lot easier to find parts at the junkyard
I would recommend you grab the whole caliper and carrier assembly by removing the brake fluid line and the two bolts connecting the carrier to the king pin/suspension. This doesn't apply only if you're planing on upgrading from 286mm to 300mm front: if so, all you need is the carrier (calipers, sliders and clip are the same). Remember that if you have 16" front rims, 300mm is the biggest you can go without having to change your rims.
Upgrade combinations
300mm vented front brakes
This is a good upgrade for the 2.0i, 2.2i and 2.5i. It will allow you to keep the stock 16" rims and you'll keep a good front-rear brake bias. The cheapest way to go...
All you need is the carrier and can be taken off the following cars:
E46 318d Touring
E46 318i Sedan
E46 320 All (Gasoline & Diesel)
E46 323i Convertible & Touring
E46 325i All
E46 328i All
Z4 3.0i Roadster
Z4 2.5si Roadster
Z3 3.0i (Roadster & Coupe)
325mm vented front brakes
This is a great upgrade for all Z4: you'll have the same front brakes as the 3.0si so I'll recommend upgrading the rear to 294mm. You will need 17" to clear the calipers.
You'll need the whole front caliper assembly from one of the following car:
E46 330i All
Z4 3.0si (Roadster & Coupe)
If you wanna be a perfectionist: the protection plate behind the 325mm rotor is bigger than what the 300mm or 286mm have (these two setups have the same cover). The rotors should fit with no particular issue as the cover does not interfere with the rotor. The problem with replacing this covers is that the wheel hub has be removed (you need a 46mm socket and an impact wrench) make it impractical if you are sourcing the parts from a junkyard. My friend Ed has been able to fit 325mm on a 323i Touring (originally equipped with 286mm as some Z4s) and had no issue. You can see some pix of his setup here.
294mm vented rear brakes
Another good upgrade for the 2.0i, 2.2i and 2.5i. It will allow you to keep the stock 16" rims and you'll have the same front-rear brake bias as the 3.0i.
If you go with 325mm in the front, I'll suggest you upgrade the rear as well to keep the bias within BMW specs.
You need the whole caliper-carrier assembly from one of the following cars:
E46 318d Touring
E46 318i Sedan
E46 320 All (Gasoline & Diesel)
E46 323i Convertible & Touring
E46 325i All
E46 328i All
Z4 3.0i Roadster
Z4 3.0si Roadster & Coupe
Z4 2.5si Roadster
320mm vented rear brakes
If you find a E46 330i, you might also want to consider a full brake setup off this car: it would give you 325mm in the front and 320mm in the rear and it would be a bolt on swap...
Alternatively, 320mm rear vented rotors can be sourced off of an X3 (any model, old or new). As noted below, the X3 might not be able to provide you a full front and rear swap: the front calipers are the same as the E46 330i, but the caliper carrier has a different p/n.
In summary, 320mm vented rear rotors can be sourced from the following cars:
E46 330i All
E83 X3 All
320mm can be used only with 325mm front as you don't want to have your rear rotors bigger than the front. Although many variables go into calculating brake bias, the disk size ratio of a 325mm front - 320mm rear conversion is similar to what the 3.0i has so it should be fine. However such a brake setup was never chosen by BMW as a setup for the any Roadster.
Another note regarding 320mm rear brakes. In this case too they have a different protection plate (as for the 325mm front rotors setup), and in this case it gets in the way of the bigger rotor. Once again, removing the plate is not as practical as you have to remove the parking brake assembly and likely the wheel hub. To fix the issue I would personally just bent the plate of the way... Sometimes I agree with the saying that "is something doesn't fit, your hammer ain't big enough"...
Notes and Credits
PS1: the E46 323i
I did some more research about the E46 323i and it appears to be somewhat in between when it comes to front brakes: some body style have 286mm while others have 300mm. Also it appears the same body style could have different configuration between Euro version and US version... I would recommend you bring some measuring tape when going shopping through junkyard just to make sure you get the correct one.
PS2: possible sources
IMHO, buying the calipers new from BMW is not cost efficient, as you might as well go with some fancy non-floating & multi-pistons brake kits. Here's a couple of hints of where to source the parts you need:
1) of course you can check with your local junkyard and car dismantlers: where I got mine the guy actually removed them, in some junkyard you remove the item yourself and then negotiate the price
2) eBay is a good source for the: the US has usually has a lot of calipers off of E46 so that's a source...
3) There are some dismantlers that specialize on BMW and will ship you the product. The two I can recommend because I have personally used are:
- Autobahn in El Cajon here in San Diego
- Bavarian Auto Recycling up in the Bay Area. The nice thing about them is that calipers come tested (the o-rings are replaced if leaking), cleaned, and come with a warranty (not sure of how long)... Of course they are more expensive than what you could get elsewhere.
Just complement any of these setup with some Brembo Slotted or Zimmerman cross drilled rotors (I got mine from Ed @ Bimmerbum). I won't recommend buying rotors on eBay as some sellers get plain rotors and drill them in house.
PS3: sourcing the front brakes from an X3
The X3 has the same front calipers as the 330i (325mm vented); however the carrier has a different p/n so it might not be a perfect bolt on.
Credits
cj10jeeper: comments on several things
mamasuon: comments regarding the dust plates on the 330i and Z4 3.0si being different
