Which AP Racing Pro5000R caliper is best for me?
CP9668 vs CP9660: similarities and a lot of confusion
- Intro
- Common base
- Differences between the 9668 and 9660
- Selection criteria
- Conclusion and best choice
Introduction
What follows covers the factors in choosing between a CP9668 and a CP9660. The CP9665 has deliberately been left out to keep things simple, but for us, apart from its different mounting centres from the CP9668, it's its twin brother. We'll try to keep it simple to make your choice easier in the big-brake jungle.
When you're thinking of switching to a big brake kit, there are plenty of brands, such as Brembo, Alcon, adapted Porsche brakes, PFC and of course AP Racing.
While a Porsche caliper is made by Brembo, it's generally quite different from the Brembo you'll find on a Civic Type-R or a Mégane RS, because in the industry everything is dictated by the specifications, and therefore the price behind them!
Once you've established that, you quickly see that you can't reason in terms of a Brembo vs AP Racing vs Alcon vs PFC comparison. To make an informed choice rather than one dictated by fashion, you'd need to get into the detail of each caliper's advantages, and why it's really a competitive advantage.
We won't go into that level of detail here, because the AP Racing calipers in the Pro5000R range offer the best of their know-how at a price that's still accessible, but we will explain why there are 2 calipers that are the front-runners and often a source of debate among track enthusiasts.
As each caliper offers its own unique advantages, we'll break them down to help you see clearly and make the most relevant choice for YOUR use.
Common base
The CP9668 and CP9660 share a common DNA: design through computer simulation that optimises loads, heat exchange and mass distribution while maintaining strength.
The result is a completely asymmetric caliper, stiffer under dynamic loads, but also better ventilated than conventional calipers. These advances come mainly from AP Racing's involvement in racing.
It took years to bring these cutting-edge technologies from competition to the general public, thanks among other things to the latest forging techniques. These 2 calipers form the backbone of the Radi-Cal range with their 2-piece design.
The resemblance is pretty obvious and the similarities are many: 2 halves bolted together, a quickly removable bridge, stainless steel pistons with a concave base, low-friction seals, a hard-anodised finish, internal channels that eliminate the bridge pipe for the brake fluid. Even the piston sizes are identical, and the pad shapes are the same too.
They also share equivalent disc sizes, but that's where the list of common points ends.
Differences between the 9668 and 9660
Because while they're interchangeable on the same disc, the 9660 is narrower, and the 9668 is more than 25 mm wider in total either side of its axis, which naturally means you'll need to be careful when choosing wheels.
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Pads
This extra caliper width allows for the major difference between our 2 contenders: a 25 mm thick pad for the 9668 and an 18 mm thick pad for the 9660.
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Pad retaining bridge
In terms of visual differences, apart from its size, the 9668 also offers an optional quick-release top bridge in place of the traditional bolted bridge.
CP9660 with traditional bridge:
CP9668 with optional quick-release bridge:
Selection criteria:
Now that the differences have been laid out, even if things are clearer, you don't necessarily have the criteria to make the wisest choice, so here are the deciding criteria we suggest:
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Inner wheel clearance:
Very often, choosing a caliper comes down to matching your choice of brakes with your favourite wheel design; for many track enthusiasts these days, looks come into play.
The 9660 won't take up much more space than a single-piston floating caliper, and can even fit inside the original wheel if the disc size allows; at worst, a spacer of a few millimetres will be the only constraint.
Team Lupos tip: beyond a 12 mm spacer, we recommend switching to wheel studs!
But the 9660 will often fit without a spacer with current wheel designs. On older models with large wheel offsets, there's a good chance it won't fit without spacers.
The 9668, on the other hand, will require you to check clearance inside the wheel carefully using a template (which we can provide), and perhaps even buy a set or two of wheels, studs and spacers. It's a point to consider when buying, because these extra costs can rule this option out fairly quickly.
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Length of sessions or races:
The 9668 was designed for endurance racing; the 25 mm pads and the quick pad-change option are the perfect illustration. Objectively, 25 mm pads will last you longer.
However, if you only drive 15- or 20-minute sessions, qualifying or short races, then the 9660 will be more than enough.
While it's practically impossible to predict the life of your pads given the multitude of factors involved, it's certain that 18 mm pads will last several trackdays, and proportionally even more for the 9668's 25 mm pads.
And given that most OE pads are less than 18 mm thick and have a smaller surface area, there will inevitably be a gain in pad life.
If longevity is your priority and you hate changing pads, then the 9668 is the champion in this category.
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Pad prices
Pad price is another selection criterion. 25 mm pads logically cost more than their 18 mm cousins, as most manufacturers broadly charge by thickness. However, the predominance of 18 mm thicknesses for decades means that the higher volumes allow more aggressive prices.
So per millimetre, the 25 mm pad is very slightly more expensive than the 18 mm.
If you don't do your own maintenance, you'll save on labour for replacement (even if, objectively, changing the pads on these calipers takes barely longer than changing a wheel, they're that well designed!).
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Availability of pad compounds.
As the shape is the same, if you ran out of 25 mm pads you could always throw in a set of 18 mm ones; it would be as if you were fitting a spare set from a previous track day.
From this point of view, the 9668 won't leave you without options and offers more flexibility.
On the other hand, because the 25 mm pad is more endurance-oriented, you'll more easily find compounds dedicated to that discipline, so you might not have the most aggressive pads available in 25 mm, forcing you to fit 18 mm in that case. Whereas 18 mm pads are more sprint-oriented, so the choice available leans more towards soft compounds with a high friction coefficient.
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Unsprung mass
This point can be seen as the corollary of the types of pads available. Compared with an OE single-piston brake system, or even a classic 6-piston one that often weighs almost double on average (3.6 kg to 5.4 kg):
The 9660 weighs approximately 2.8 kg
The 9668 weighs approximately 3.1 kg
The 9660's pads are on average 40% lighter, so the 9660 is the lightweight champion if you're chasing every gram, particularly in unsprung mass.
View of the range, from left to right: CP9668, CP 9660, CP94XX
side view, in reverse order:
Conclusion and best choice:
Simplified decision matrix:
- I really don't want to have to buy a set of wheels, spacers at most = 9660
- I drive 20-min sessions or races, I'm chasing lap times with aggressive pads = 9660
- I want to optimise weight saving, I want the lightest = 9660
- I race endurance, I want maximum durability = 9668
- I hate changing my pads = 9668
- I'd rather have lower pad wear and lose a few tenths with a less aggressive pad = 9668
In the end, whatever choice you make, with Pro5000Rs there's little chance of getting it wrong. As explained, it's racing technology within anyone's reach, and to date even PFC, a cutting-edge brand used in endurance on GT3 Cups, doesn't offer an equivalent.
And on track, these brakes combined with good pads guarantee a crazy amount of fun, and even in case of slightly overflowing "optimism", the braking reserve is absolutely insane. Your first laps with these brakes usually produce slower lap times because they brake so hard, you'll need to readjust your reference points!
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