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Brakes upgrade 300mm to 340mm

3K views 11 replies 5 participants last post by  marco_polo 
#1 ·
Did anyone upgrade their 120i or below 300mm front brakes to the 340mm setup from a 125i/125d? It seems like a plug and play swap of the caliper and of course disc/pads combo. Don't really wanna go for the M brakes as it's an overkill and wanna keep my 17" alloys, but current brakes are quite bad tbh. Fun fact - based on realoem it looks like 120i F21 has 300mm front discs, but 120i F20 has 340mm.
 
#2 ·
#4 ·
OK so I guess I messed some things up from realoem - didn't see the 340mm discs were for the M brakes. Apparently 125i/125d brakes are still bigger, but they're 312mm rather than 340. So now I'm wondering if it's even that much of an upgrade from the OE 300mm or the M brakes are the only choice here?Considering Polish potholes 18" RFTs are too much, I know some people report that 17" rims fit on the M brakes, but it's a rather tight fit.
 
#6 ·
You could go one step up if you've got the £££££ and fit the Blue M sport brakes from an F30 335D. The front disc on them is 370mm with a slightly larger caliper. 370mm is the F20 M performance size. That's a nice upgrade but you would definitely need 18" rims as the clearance even on the 18 is minimal (but it looks great) The ride on 18" non run flats is fine if you ask me, i run Michelin and find it comfy

That's what i did on mine, cost me nothing but one thing worth noting is future costs when you need discs/pads etc.
 
#7 ·
340mm is f20 performance size mine came off a m140. Real OEM shows this. 370s are only listed for F30s and M cars. First thing to note is they sap some power as they are much heavier than the 312mm. If you want from 300 to 370 that is a massive weight difference and the rotating wheel is the worst place to put it. I could feel my car had lost some performance I fitted lighter wheels to get it back. I have non runflats its the first thing I get rid of when I change my car.
 
#8 ·
To the OP what is it your not happy with on the stock brakes out of interest ? maybe they just need a good service, fluid bleed and new pads as in good order they should be plenty for road use, quite often customers come to me complaining brakes are not good enough wanting stronger pads but they have never been serviced in years which is often all they need, brakes despite being the most safety critical aspect of a car are often never looked after correctly :(

If they are in good health and you just want more bite/stopping power then upgrading the pads/fluid can make a huge difference, just depends how far you want to go as if you get some proper high friction pads to give really strong stopping power then downside is you get more dust and possible squeal at low speeds so always a bit of a compromise, lot cheaper and easier than replacing all calipers and discs etc though.
 
#9 ·
cossied said:
340mm is f20 performance size mine came off a m140. Real OEM shows this. 370s are only listed for F30s and M cars. First thing to note is they sap some power as they are much heavier than the 312mm. If you want from 300 to 370 that is a massive weight difference and the rotating wheel is the worst place to put it. I could feel my car had lost some performance I fitted lighter wheels to get it back. I have non runflats its the first thing I get rid of when I change my car.

That's the front M Performance kit, listed under my own car as an example. If you click on the blue part numbers you see the kit is listed for the F20 up to the F36. If you look at part Number 10 - 370mm disc

The M140 comes with 340mm discs as standard with the blue calipers:


The M cars, like the M2 for example use 380mm discs
 
#10 ·
THETYRANT said:
To the OP what is it your not happy with on the stock brakes out of interest ? maybe they just need a good service, fluid bleed and new pads as in good order they should be plenty for road use, quite often customers come to me complaining brakes are not good enough wanting stronger pads but they have never been serviced in years which is often all they need, brakes despite being the most safety critical aspect of a car are often never looked after correctly :(

If they are in good health and you just want more bite/stopping power then upgrading the pads/fluid can make a huge difference, just depends how far you want to go as if you get some proper high friction pads to give really strong stopping power then downside is you get more dust and possible squeal at low speeds so always a bit of a compromise, lot cheaper and easier than replacing all calipers and discs etc though.
My car is a 68 plate (or it would be in UK ;) ) and has done only 34k kms/21k miles so the brakes condition is as good as new. From the beginning it wasn't as good as I hoped as braking from 160km/h to 90 isn't as effortless as I would like it to be (I believe it's mostly due to overheating as consecutive uses of brakes are drastically less efficient) and believe me people in Poland drive pretty terribly on the highway suddenly changing lanes in front of you which requires a lot of work from those brakes. To be honest I feel like my VW T5's brakes are more adequate but it's driving almost empty most of the time. Also as I mentioned BMW did put slightly bigger brakes in the 125i/125d which aren't that much faster or heavier, but probably were expected to be used in a more dynamic fashion (not that the 120i is slow...).
 
#11 ·
21k miles and near 2years old the brakes wont be "as good as new" but they should be functioning ok, it used to be every 12 months brakes would be part of standard service schedule (strip, cleaned,greased, reassembled) but this doesnt happen these days and due this brakes gradually decline until worn out or seized up. However in your case lets assume the brakes are good and working at least 80% or above of brand new performance so function should be plenty.

Sounds like a pad upgrade and fresh fluid (should be done every 2 years max) would make a significant difference and worth exploring, question is how far to go as if you went for a high performance pad like Carbotech XP or Pagid RS etc which would make huge performance upgrade then there is some downsides like more dust/noise etc, but thats always a trade off to the sort of performance they offer, some people have found EBC yellow give a upgrade over stock so maybe they are worth a try, its not one i use myself as working for Carbotech i have no need to use other brands :D

HTH
 
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