OK what am I missing here? Why do some high performance brakes use up to 6 pistons and some stick with one?

 

More pistons provide more consistent pressure across the pad surface.

 

That's what I thought - so why are all the MINI kits single piston? I don't see the big advantage to a single piston 'upgrade' beyond a small amount of surface area addition. Seems like just going to a multiple piston caliper would produce better results with a stock size rotor wouldn't it? Why aren't there 4 or 6 pistons kits out with these larger rotors?

 

Ok, this is my type of q.

Calipers come with from one to as many as ten pistons that I've seen.

In the case of calipers, more does not mean better. At least not in the most common way of thinking. Meaning that a six pot is not really any more or less brake than a four or a single. More pistons do not make for shorter stopping distances.

The issue comes down to piston area and how the pressure is distributed over the pad surface. Think of your single piston. When pressure is applied the pad turns to a banana of sorts as the pressure is all in the center.

Now go to the twin pot production caliper. Think PBR. Equal pressure onto the pad top and bottom. No distortion.

Next consider pad shape; a square pad for the single is far less effective than the long slendor one with multi pistons at a greater ER. What's ER? Effective Radius. A square pad puts the 'average' load at a shorter distance from centerline. The tall skinny pad moves this out from center and appies the same total pressure- assuming equal piston area. Far more effective.

Let's talk fixed vs floating. Floating is stock. Cheap to produce, very wheel friendly. Requires one piston. PBR's two. Moves on guide pins or such. Applies pressure to the inside of the rotor first then pulls the outside into contact. Time consuming. Fixed calipers apply equal pressure to BOTH sides of the rotor at the SAME time. Quicker action and release of brake pressure.

Now move onto piston area. A single, twin, four, six or more CAN all have the same total clamping ability. However a lot of builders don't both with this. They take what is on the shelf and make it fit. Here's where things can make or brake, er break you.

Piston area is that of one piston on the stock car. Simple enough. The area of a four pot however is calculated as half of the totoal or only the two INSIDE pistons for example. Huh? Yup. Remember the foater pulls the outside in as well. So they perform the same work. Or you could double the numbers on the single and get the same results for the same reasons. Don't rack your brain over this, just hang with me here.

Now take your stock caliper; single piston, foater and maybe 3.5 sq" of piston. (don't have the notes here at home sorry) and suppose you put on a multi piston caliper. That zoomy gigantic brand X might look cool. Ok, it's a four piston off a Pcar maybe. Total piston area (one side only remember) is 4.2sq". Yea baby, that's gonna stop like no tomorrow! Not exactly. Let's assume you put it onto a 13" rotor. We've increased the rotor diameter AND the piston area. All good right? Nope. You have now moved all the brake work to the front of the car. Rear is along for the ride. You have so much front brake the rears never see any pressure nor do any work.

Recent conversation on a Marauder; "man, this X kit is great! I hit the brakes and this tank nose dives 'til it damn near stands on its front bumper!" Too bad he doesn't understand that this just means the fronts are now doing all the work. On the other hand- Magnum owner; "You know, I like both the pedal feel being firmer and the look, but what I really like is the whole car 'hunkers down' under braking now when I stop". !!! He get's it! Braking with four wheels will always beat two.

So....what does all that have to do with the q of pistons? Lots. Generally speaking a well designed kit will reduce total piston area when you increase rotor diameter. "More OD, less area, tune with pad" you heard it here. Granted the choice of what and how on the rear of the car does come into play as well. And tire choice. And suspension set ups etc. etc.

Why do follks put X calipers on to a car without thinking about these things? Because people will buy them. And in America- more is better.

*Yes, I know there are folks here who have other kits and are very happy with them. That's fine, if you like what you have now that's all that matters. The math, data, and dynamics are what they are however.

When considering multi piston calipers you need to review the body size for wheel clearance, piston size for bias, pad size for life and heat, how it's balanced to the rotor it is fit too. Other than that it's a piece of cake!