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The following explains my experiences of fitting Porsche 911 (type 996) brakes to my S2. The brakes on the front of the Porsche 996 are 318mm diameter x 28mm thick. These discs are clamped by a pair of 4 pot (piston) monobloc calipers manufactured by Brembo - the worlds leading brake manufacturer (debateable perhaps, though Brembo do own AP!).

CALIPERS

Porsche 996 C4 monobloc
Seat split caliper with integral crossover

Monobloc brake calipers just mean that the caliper is made from one piece. Traditionally, multi-piston calipers are made in two halves which are bolted together along the centre seam. This is done to make it easier to machine out the fluid channels inside the solid caliper half. With new technology, it has been possible to do all the machining from the 'inside out', thus meaning that there is no need to have a mechanically jointed seam. This, we are told, reduces the weight and increases the calipers resistance to flex under load. All calipers with pistons on both sides of the disc need to have a transport mechanism to allow the fluid to act on all pistons equally. Most calipers have what are known as cross-over tubes which provide this function - though it is possible with two part calipers to have internal crossover channels. This is commonly seen on the Brembo 'Gran Turismo' range and is also frequently seen as standard fitment on various Seat 'R' vehicles.

DISCS

The monobloc caliper I used was, as previously mentioned, designed for use with the corresponding porsche 318 x 28mm disk. Needless to say, the Porsche disc is unsuitable for this application for a mumber of reasons - dimensional incompatibilities to put it simply!

As a result, other people have investigated and discovered that Audi A8 (European) discs are an ideal solution. These discs are 314mm in diameter and 30mm thick. They are also considered ideal becaust they are cheap, easily sourced and dimensionally perfect for the conversion we are performing. Dimensions we need to consider are: number of bolt holes and 'pcd', centre bore size, hat height, disc thickness and diameter.

With these discs sourced, there are two further items which are needed for the conversion:
1) a mounting solution to hold the calipers in place.
2) a brake line to connect the new caliper to the existing brake lines.

BRACKETS

When Porsche designed and built their limited edition RS2 vehicle, they made a number of modifications to the vehicle's specification. One of the things they changed was the brakeing equipment. They too added Porsche brakes and came across similar mounting and connecting issues.

Their solution was to have made a cast mounting bracket to convert the Audi struts to enable them to mount 'radial' type fitting brake calipers. It just so happens, that these RS2 calipers can also be used to mount 996 type calipers over the audi A8 disks.

With these brackets bolted up and the discs held in place, it was possible to do a visual check to ensure everything lines up. And it did. At most there was about 1-1.5mm of overhang where the pads overlapped the edge of the disc. When I eventually got round to fixing the brakes in place, I did notice that whilst the whole system worked well together, the calipers were very close to the inside edge of the standard 16" wheel.

PIN MODIFICATIONS

It was noticed at this point that there are pins which the pads sit on to keep them aligned. These pins are approx 31mm apart and on a system which uses 28mm thick disks this is fine. But using the 30mm Audi discs, there is a need for more clearance. This was achieved by using a grinder to shorten the pins by a millimetre or so. I have also recently discovered that it may be possible to simply press the pins back through the caliper housing, though this is something I've not done myself.

CLEARANCE ISSUES

I tested the setup for a few days to monitor the situation using my standard 16" Avus alloy wheels. When I checked the system, I was somewhat unsurprised to see some score lines, both on the inside of the wheel and also on the outside edge of the caliper. At this point I decided to make some adjustments to fit the car better.

To bring the caliper radially inwards away from the wheel, I had to do two things. Firstly I had to reduce the overall diameter of the disc, and secondly machine the bracket/caliper interface surface to physically move the caliper in. Not knowing much about brakes and safety margins, I e-mailed the big brake manufacturers to see what sort of clearance I should be allowing. AP specify a minimum of 3mm. Wilwood recommend a minimum of 0.100". Bremsport recommend a minimum of 0.125".

As a result, I took the components to a trusted local engineering machine shop and took 4.5mm off the caliper mounting surface. This brought the calipers 4.5mm away from the wheel inner surface. I then had to machine the disc down to match. Since there was already a 1mm pad overlap, I decided to take the disk radius down by only 3.5mm. Doing this meant that the pad edge and the disc edge should align perfectly. Reducing the radius by 3.5mm reduced the overall disc diameter down to 307mm.

With testing, this proved more than adequate and proved no further issue.

BRAKE LINES

The brake hoses were something I tried hard to find a solution to. In all I had three sets of custom goodridge brake lines made up. The third set worked well but was still not perfect and were fairly expensive solution. Since completing this conversion, I have found out that the RS2 hard lines (hard copper brake pipes as opposed to soft flexible hoses) are able to be fitted in place of two of the S2's original hoses. These hard lines are approximately £16 each and are designed for the job. Although the fluid ports are in different positions on the RS2 calipers and the 996 versions, the lines apparently have enough flex in them to address this difference.

DISC OPTIONS

323mm Audi S8 discs.
As we have already said, there is some pad overlap using the above components in standard form. Other owners have completed this same conversion and have reported back that it is quite possible to replace the 314mm disk with a 323mm disc from an S8 vehicle. Doing this does not involve moving the caliper at all. All it means is that the edge of the disc will be much further inside the caliper than before. It has also been reported that this means the disc edge runs very close to the inside edge of the caliper, and with heat generated by braking, the hot & expended disc can foul the caliper itself. Some suggest that this can be avoided by modifying the caliper itself, but this I know nothing about firsthand.

PAD OPTIONS

Pads are available from a number of sources. Clearly they are available from the Porsche dealer network but this comes at a price. The original pads are made by a company called Textar, who also make pads for the aftermarket. A set of Textar pads will cost around £30 from a motor factor such as GSF or Euro Car Parts. Moving up the scale, there are offerings from EBC and Hawk to name but a few, but from personal experience I can thoroughly recommend Ferrodo pads and their DS2500 compound.

CALIPER SOURCES

New calipers rather unsurprisingly come from the dealer network. Used calipers can come from a variety of sources including specialist breakers and e-bay.

CALIPER VARIATIONS

The calipers used here are 996 items from a non-turbo'd car.
They are available on the front of 996 Carrera 2 in BLACK,
on 996 Carrera 4 (four wheel drive Carrera) in SILVER,
and also on the 986 Boxster 's' in RED.

It may also be possible to use the calipers from the rear of a GT3 / turbo 996, though these would certainly need the crossover pipes moving from one end to the other (as the rear calipers on a Porsche are mounted in a leading position).

Rear calipers from any other vehicle are not advised as they are usually designed for much thinner discs. Rear calipers from the Boxster and 996 can usually be identified by four lugs on the outer caliper casting (see photo with lugs circled in red)

Front calipers from th 2.5 or 2.7 Boxsters are also not advised for the same reasons.



audi parts:
qty. part number description rough cost (ukp)
2 441.615.301.aa euro audi a8 disc 314mm £20 ea.
2 8a0.615.125.aa rs2 caliper bracket £75-£150 ea.
4 n.100.880.03 bracket mounting bolts m12x1.5x25 £1.5 ea.
porsche parts:
qty. part number description rough cost (ukp)
1 996.351.425.(11) left caliper (without pad) £275 ea.
1 996.351.426.(11) right caliper (without pad) £275 ea.
4 999.067.041.09 caliper mounting bolts m12x1.5x72 £1.5 ea.
4 996.351.088.01 pad damping plate £5.5 ea.
1 996.351.959.01 pad mounting kit £18 ea.
other parts:
qty. part number description rough cost (ukp)
4 frp 3051 ferodo ds2500 pads £150 set.
4 bespoke - click for specs goodridge hose set £65 set.

The ebay thread I bought my calipers from.
Seller knocked off £30 for the damaged crossover pipes!