As the topic states, I'm looking to upgrade my brake discs to some larger diameter ones but have zero clue what is the max I can fit inside the rims.
I recently acquired some 15 inch alloy wheels off of a MK1 Focus LX (like pic attached) and I wanna get some slotted Brembo rotors to fit onto my shitbox. Before anyone guts me like a fish asking "What do you need bigger brakes for, you've got a 1.4 liter under the hood" Alas, I've invested in a decat exhaust manifold, full sports exhaust, an actual  cold air intake with a fog light delete to route air into it and soon to go for a ecu reflash to hopeeefullyy push it over the edge of 100hp (praying hard), alongside a set of quality coilovers with eibach springs and some thick tyres.
If anyone has any experience with this type of rims I'd be more than happy to listen to their advice on brake sizes. (and off-topic, if anyone has a guide on what I'd need for a drum to disc conversion for the rear wheels that'd be absolutely amazing)

 

You don't need to upgrade the brakes on a MK6 until you get anywhere near ST150 levels of power.

Just fit drilled grooved if you want better performance

You need these:

That seems like a lot of time and expense for 100bhp.  Why not just buy an ST?  Bigger brakes on this will just make acceleration worse due to the extra weight.

But to answer the question, 300mm discs just fit under some 15's, though you may have to grind a little off the caliper.  I don't know whether they'll fit these particular rims.  278mm discs will fit.

To answer the question "Why not an ST150" I asked myself the same thing and the circumstances answered a firm but clear "No" so I work with what I have so far. Thank you for the genuine answer Tom, I'd say Im just future-proofing the car. I plan on mounting a turbo later on, after upgrading internals and getting a standalone, there'd be no point to put power in and brakes afterwards, considering brakes are the safety factor. You don't see the Mk6 as a project car at all so I picked it as it seemed interesting to me.

39 minutes ago, TomsFocus said:

That seems like a lot of time and expense for 100bhp.  Why not just buy an ST?  Bigger brakes on this will just make acceleration worse due to the extra weight.

But to answer the question, 300mm discs just fit under some 15's, though you may have to grind a little off the caliper.  I don't know whether they'll fit these particular rims.  278mm discs will fit.