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why dont you use 305 x 28mm 5x98 disc from new gtv. Then you wouldnt need to grind the caliper. I think that 3mm diametre difference wouldnt decrease braking performance too much

After a lot of measuring and mock-up work, I concluded that 15" Compomotives will accept a max of 304mm diameter disc when the big series Brembo calipers are used, provided they too are reduced by 2.5mm at the bridge area (down to 21.5mm from 24mm). The small series Brembo calipers can be used as they are with 310mm discs. However, the braking difference between the two is phenomenal for the same car -both using DS2500 pads and new parts. I just finished my second big-caliper set which is going into the 24v car (currently with the smaller 4-pot reds. Pad area difference is >50%.... The gtv 305mm disc can be used now IF the swept area width can accommodate the the 55mm wide big brembo pad, instead of the smaller 50mm 4-pot. I'll have to find a disc and measure it. In other words, the inner edge of the pad must be ON the swept area also and not hanging past it. Now something strange: 8x15MO wheels have LESS room inside than 7x15MO wheels! Oh well, I found out as I tried both for the new brake set! That's how the 304mm came about.
All parts were bought new. Calipers/pads (Italian ebay) 350€, discs 100€, fabricated adaptors 120€, steel rings holding studs 50€ for a total of 620€ + my labor (lots of it). I think the price is great by any standard. Some pics below.
Jim K.

What makes the braking so much better with the brembo? Bigger piston size? If so then you'd need a bigger MC, no?

No, the MC remained the same. Note that we are comparing two 4-pots, small Brembo to large Brembo. I don't know piston sizes but I believe its the 50% bigger pad area. I can't quantify pedal pressure difference but the improvement is eye-opening, no 'ifs and maybes'. We have a 3-day track weekend on March 13/14/15 and I want to see how it behaves there.
Jim K.

JK,

Can you take some vids please. I wanna see how effective the brakes are, like if you stomp on it, will your denture hit the windscreen? Oh wait, you don't have dentures, I must have been thinking about MD

Dentures and a glass eye -or two glass eyes, judging by how you drive! Floor's all yours MD... start mopping, heheheh!
Jim K.

To choose a disc, beside the most obvious dimensions (diameter x width) you must consider the total 'hat' height, sometimes called offset and its relation/suitability to your wheel design/offset. In my case, the caliper sides are just a few mm from the wheel spokes. It looks like the gtv discs will not make it in this respect as the hat is shallower than the 166 discs, pushing the caliper against the spokes.
Jim K.

My disc "hats" are custom aluminum and bolted onto coleman discs. Larry Jr made them for me. I think the disc size is only 300mm with 944 Turbo rear (small) calipers. I want to see if I can fit bigger calipers from an EVO or even the later Boxsters.

If the disc thickness is not adequate for the caliper, would using shims be ok?

Evo and scooby use the same caliper/pad but it mounts with lugs rather than radially.

if you mean spacing the caliper wider for thicker discs yes but there is a limit without it requiring longer bolts

Got a hot tip for ya for Saturday night JK...

You would think so but it has been tried exactly as you see it in 4 cars and the pedal is rock hard. When you pressure bleed the brakes there is no stationary air in small bore tubes as it gets carried around with the fluid. You can actually SEE it happen by substituting a thin transparent pipe bent to the same shape for curiosity's sake. The air just goes around and finally out the bleed valves. I must admit, I thought it might be a problem when I first tried it but it worked out well. I believe the reason is the small brake line bore. I would opt for much shorter lines without loops but this is the std Alfa pipe reshaped - I don't have a pipe flaring tool to do a perfect job by cutting to proper length and I dislike banjo fittings for some reason.
Jim K.

MAxiboy,

No mate, I meant if the disc was 24mm thick but the caliper was meant for 28mm discs. So I was wondering if I could shim it with 2mm stainless shims on each side of the pad backing.

you could just add 2mm to the minimum pad thickness and change them that much earlier

Are these Brera/159 calipers?

So Jim do you run the standard 19mm master cyl with the 4 pot calipers?