Honed’s Porsche brake caliper mounting kit came from a desire to develop a brake upgrade with a rigid four-piston caliper that would fit inside the 15-inch wheels that many ’80s and ’90s Hondas use on track. Like many of our products, it began with the goal of making our own cars faster and more enjoyable.
Crucially, any brake modification had to preserve the OEM front-to-rear brake bias, as incorrect bias is a sure way to ruin stopping distances. Additionally, we wanted to keep the cost of the kit down by using commonly available parts, allowing consumables like pads and discs can be obtained from many suppliers.
We identified the Mini Cooper S JCW front brake discs as a great replacement condidate. At 294 in diameter and 22mm thick, it has much more heat capacity than a standard 262 x 19mm discs and is a bolt-on fitment to the Honda’s centre bore and stud pattern. No two-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, so that when it was matched to a larger disc giving greater leverage we would end up with the same overall brake torque. The result of our investigations was the four-piston caliper from the Porsche 986 Boxster.
Without doing a complete brake upgrade this is the best way to effectively maintain the standard Honda balance but with more heat capacity and a better caliper. The section below gives a quick summary of our process to confirm the Boxster caliper would work well, but if you’re not interested in the calculations, feel free to skip to the install below.
DESIGN
To calculate the individual wheel brake torque (IWBT) on an individual front brake, the following equation applies:
IWBT = P * PA * 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. To maintain the correct front-to-rear bias we need to keep the brake torque constant, therefore if 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!
To determine what’s needed to maintain the correct bias, we can ignore the pad coefficient of friction and system pressure by assuming they’re constants. This leaves us with piston area * effective radius (PA*R) to be considered. The table below summarises the calculations comparing the stock disc and caliper to the larger Mini JCW disc and four-piston Porsche 986 Boxster caliper.
| – | OEM | Honed kit |
| Piston diameter (per side) |
57mm single | 40 and 36mm |
| Piston Area (per side) – PA |
2552 mm² | 2275 mm² |
| 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 % |
As you can see, the combination of the Mini disc and Porsche caliper is exactly what we need – sometimes things just work out and we won’t sweat the 0.8% difference.
The image below 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 have been exaggerated somewhat for clarity.
Installation Guide
With the maths out of the way, here’s a rundown of the install. The lucky test vehicle lucky receiving our first production kit is a well known and thoroughly Honed EK4 Civic VTiR belonging to our mate Rhys.
Until now the car’s brake system was completely unmodified, except for upgraded pads, and like a lot of OE sliding calipers they were starting to look a bit tired.
The first part of the install is to get the calipers ready, which is the only part of the process that differs from swapping between OE Honda calipers.
The caliper bleed screws and the bridge hoses that pass between each half of the caliper have to swap position. Because the calipers are installed on the rear side of the discs on the Porsche, but transfer to the front of the disc on Hondas, this would result in them being upside down.
To make it possible to bleed them correctly, the bleed screw has to be relocated to the top of the caliper, with the bridge hose then relocated to the bottom. The caliper ends up installed on 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 fluid going everywhere. Here’s the adapting bracket in place, with the supplied flange bolt and all metal lock nut for safety.
Then it’s simply slide the disc on, bolt up the caliper with the provided hardware, slip the pads in and 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. Honed makes Porsche Brake Caliper Mounting Kits for 262mm disc/10.3-inch knuckles, 242mm discs/9.5-inch knuckles and later DC5 Integra/EP3/NA2 Civic.
