Our Porsche brake caliper mounting kit grew out of an idea to put together a brake upgrade with a rigid 4 piston caliper that would fit inside the 15″ wheels that many use on track with 80s and 90s Hondas. Like many of our products, it started out with the goal to make our own cars faster and more enjoyable on track.

Critically any brake modification had to preserve the OEM front to rear brake bias, incorrect bias is a sure way to ruin stopping distances. Additionally we wanted to keep both the up front and running cost of the kit down, by using commonly available parts allowing consumables like pads and discs can be obtained from many suppliers.

After exploring disc options, we’d identified the Mini Cooper S JCW part as a great candidate for an easy to fit disc, at 294mm diameter (11.6″) compared to the original 262mm (10.3″) disc and 22mm thick compared to OEM 19mm. Much more heat capacity, and a straight bolt on fitment to the Honda centre bore and stud pattern. No 2 piece disc here, just $30 discs you can get from your local parts store which are perfectly suited for the majority of track cars.

Having decided on our disc, our ideal caliper would have a reduced piston area compared to the the OEM caliper, which when matched to a larger disc giving greater leverage to end up with the same overall brake torque. Without doing a full front and rear upgrade this is the best way to ensure good front to rear balance, by effectively maintaining it exactly as Honda intended just with more heat capacity, and a better caliper. The section below gives a quick summary of our process to confirm the Porsche Boxster caliper would work well.

If you’re not interested in the calculation side of things, feel free to skip to the install below.

Design

Considering an individual front brake, the following equation applies:

Individual wheel brake torque = PA * A * µ * R

Where:
P = System pressure
PA = piston area
µ = pad coefficient of friction
R = effective radius

Effective radius is the distance from the centre which the pad clamps, acting as a lever about the axis of the hub. For our purposes we can assume it’s at the radial midpoint of the pad. A bigger radius is a bigger lever for the caliper and pad to act on.

The brake torque is directly proportional to each of those variables. If we want to keep brake torque constant to maintain correct F:R bias when the disc diameter increases, the piston area must decrease. This is a critical aspect that many brake upgrades ignore; more and bigger pistons isn’t always better!

So to determine what’s needed to maintain correct bias, we can ignore pads and system pressure; assume they’re constants. We simply compare piston area * effective radius between each caliper and disc arrangement that we considered. The table below summarises the data comparing the stock caliper to the Porsche 986 Boxster caliper on the 294mm disc, assuming constant pressure and pad friction coefficient:

OEM caliper Porsche caliper
Piston diameter (per side) 57mm single 40 and 36mm
Piston Area (per side) – PA 2552 mm² 2275 mm
²
Piston area: % Change compared to OE -12.3% less
Disc diameter 262mm 294mm
Disc effective radius – R 107mm 121mm
Disc radius: % change compared to OE 13.1 % more
PA * R 273064 275275
PA * R: % change compared to OE 0.81 %

We can see that the Porsche caliper turns out to be pretty much exactly what we need! Sometimes things just work out, and we won’t sweat the 0.8% difference.

The image below graphically illustrates the net effect of changing both piston area and disc diameter, as the lever arm gets longer the piston area and thus clamping force applied by the caliper decreases proportionally; resulting in the same brake torque for a given input pressure.

The scale of piston sizes and discs are exaggerated somewhat for clarity

Install

With the maths out of the way, here’s a run down of the install. The test vehicle lucky enough to receive our first production kit is a well known and thoroughly Honed EK4 Civic VTiR belonging to our mate Rhys.

Up until now the car’s brake system was completely unmodified, except for upgraded pads, and like a lot of OE sliding calipers they’re starting to look a bit tired.

First part of the install is to get the calipers ready, which is the only part any different from just swapping between OE Honda calipers:

The caliper bleed screws and the bridge hoses that pass between each caliper half have to swap position. The calipers are installed on the rear side on the Porsche install compared to the front on our Hondas. To make it possible to bleed them correctly the hoses have to swap positions. The caliper ends up installed in the same corner of the car as on the Boxster, with the larger trailing piston to prevent tapered pad wear.

With the fittings torqued up it’s then time to get to work on the car. We pulled the old caliper off, and hung it out of the way. This allowed us to quickly swap the hydraulic hose over later once we have the caliper mounted to limit mess and fluid going everywhere. Here’s the adapting bracket in place, with supplied flange bolt with all metal lock nut for safety.

Then it’s simply slide the disc on, bolt up the caliper with the provided hardware and slip the pads in, then fit the retaining spring and pin. Connect up the brake hose with the supplied adapter, bleed the brakes and done.

Rhys can now enjoy the rock solid and consistent pedal feel the fixed caliper gives, with much less binding and tapered pad wear compared to a sliding caliper, and the larger disc will give greater consistency in the braking zones with larger mass to absorb heat. Check out the products here: Caliper kits

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