Baer SS4 Plus vs 6P Brake Systems for Muscle Cars

By Core Brake Parts Tech Team — Circleville, OH warehouse

The short answer: Baer's SS4+ systems pair a four-piston S4 caliper with an 11 inch or 13 inch two-piece rotor and are built to fit behind the 15 to 17 inch wheels most muscle cars actually run. The Pro+ systems use the six-piston 6P caliper on a 13 inch or 14 inch two-piece rotor and need at least a 17 or 18 inch wheel. Pick SS4+ when wheel size or a period look is the limit, and 6P when you have the wheel room and drive the car hard.

Both are complete bolt-on kits, both use two-piece slotted rotors with stainless hardware, and both are machined in Phoenix, Arizona. The real differences are rotor diameter, caliper design, pad shape and what has to fit inside your wheel. This guide walks through each one with numbers taken straight from Baer's own kit pages.

How Baer Names Its Systems

Baer sorts its big brake kits by series. The four-piston families are SS4 (one-piece rotor), SS4 Plus (two-piece rotor) and Track4. The six-piston 6P caliper appears in the Pro and Pro Plus series, plus the 13.5 inch 6T line that Baer aims at trucks and off-road vehicles. Above those sit the Extreme lines with monoblock six-piston calipers.

"Plus" in Baer's naming means the rotor is a two-piece design: an iron friction ring bolted to a separate hat. So when people compare "SS4+ vs 6P," they are really comparing the SS4 Plus series against the Pro Plus series, since both carry the same two-piece rotor style. That keeps the comparison fair: the caliper and rotor size are what change.

• 11 inch SS4 Plus - four-piston calipers, two-piece 11 inch rotors, for 15 inch and larger wheels
• 13 inch SS4 Plus - four-piston calipers, two-piece 13 inch rotors, for 17 inch and larger wheels
• 13 inch Pro Plus - 6P six-piston calipers, two-piece 13 inch rotors, for 17 inch and larger wheels
• 14 inch Pro Plus - 6P six-piston calipers, two-piece 14 inch rotors, for 18 inch and larger wheels

SS4+ vs 6P Side by Side

The table uses four real Baer kits. Rotor thickness changes by application, so treat those figures as examples, not a rule for every kit in the series.

Every kit in the table includes stainless braided hoses, mounting brackets and hardware. The front Mopar kits also come with billet aluminum hubs and bearings. The Fox Mustang rear SS4+ kit adds a backing plate that holds a one-piece parking brake shoe, so the car keeps a working parking brake.

Why Rotor Size and Piston Count Matter

Brake torque is simple physics. Brembo's engineering explainer puts it as the product of three things: the rotor's effective radius, the caliper's clamping force and the pad's friction coefficient. A bigger rotor moves the pad farther from the hub, so the same clamp force produces more torque. More pistons, or bigger pistons, raise the clamping force the caliper can put on the pads.

That is why the jump from an 11 inch SS4+ to a 14 inch Pro+ is a bigger deal than the piston count alone suggests. You are adding leverage, a thicker rotor with more iron to soak up heat, and more piston area all at once. On a car that sees repeated hard stops, like autocross, open track days or long mountain grades, that extra mass and leverage is what keeps the pedal consistent as the rotors heat up.

For a car that mostly cruises and sees an occasional hard stop, the SS4+ is not "the weak one." An 11 inch or 13 inch two-piece rotor with a fixed four-piston caliper is a large step up from the single-piston sliding calipers or drums many classics left the factory with.

Inside the S4 and 6P Calipers

S4 four-piston caliper (SS4+)

Baer lists the S4 as a two-piece caliper CNC machined from US-sourced forged or billet aluminum, with hard anodized aluminum pistons, stainless abutments, noise suppression clips and dual seals (a dust/weather seal plus a pressure seal). Front S4 kits use DR1 or HB100 shape pads, and Baer sells its own DR1-C Street pad as the service part.

6P six-piston caliper (Pro and Pro+)

The 6P is also a two-piece forged aluminum caliper, but it is held together with six cross bolts to keep the body from flexing under load, according to Street Muscle's coverage of the 6P Pro+ release. Baer uses stainless steel pistons in staggered sizes, meaning the pistons across the pad are not all the same diameter. Baer says this minimizes uneven pad wear, the tapered pad a caliper with equal pistons can leave behind. Brembo makes the same point from the engineering side: the number and diameter of the pistons set how much clamping force the caliper can deliver.

The 6P takes the D0731 pad shape. Baer's pad page cross-references that shape to the 1997-2013 base Corvette, so replacement pads in different compounds are easy to find. Baer's own D0731 Street pad comes four to a set and is meant for daily driving, spirited weekends and mild track days, not extreme-duty racing.

Which One Fits Your Muscle Car

Start with the wheel, not the caliper. Baer says plainly that with so many wheel brands and styles, it cannot guarantee a kit fits a specific wheel, and it publishes a printable template for every system so you can check spoke clearance before ordering. Minimum wheel diameter is only the first test; spoke shape and offset decide the rest.

• Stock-size 15 inch wheels or Rally-style rims: the 11 inch SS4+ is built for this. Baer describes it as the same design as its larger systems in a smaller package for 15 inch wheels.
• 17 inch wheels, street and weekend use: the 13 inch SS4+ and 13 inch Pro+ both fit most 17 inch wheels. Choose Pro+ if you do track days or autocross; SS4+ if the car is a cruiser or show car.
• 18 inch or larger, pro-touring or track: the 14 inch Pro+ is the one to look at. Baer aims it at performance street cars, autocross and open-track builds.

Also check your spindles. Many Baer front kits are matched to a specific factory disc spindle, like the 1973-80 Mopar big-bearing spindle for the 11 inch SS4+ kit above, or the 1970-72 E-body two-piston spindle for the 13 inch Pro+ kit. Drum-brake cars may need a spindle swap before either kit bolts on.

What to Replace at the Same Time

A big brake kit changes how much fluid the calipers need and how pressure is split front to rear. Plan for these parts on the same job:

• Master cylinder. Baer recommends a master cylinder upgrade with its Fox Mustang rear SS4+ kit and sells a 15/16 inch bore master cylinder package for it. Older cars with a small-bore or single-circuit unit should get a modern dual-circuit replacement from our master cylinder selection.
• Brake booster. A tired vacuum booster makes any caliper feel weak. If the pedal is hard before the upgrade, check the booster and replace it from our power brake boosters if needed.
• Hard lines and hoses. The kits include braided hoses, but 50-year-old hard lines are often corroded. Replace rusty lines with new brake lines while the system is open.
• Fluid. Flush the whole system with the fluid type your master cylinder cap or service manual calls for.

If a full kit is more than your car needs, a fresh set of loaded brake calipers with new brake rotors will restore factory stopping power for far less work.

How Hard Is the Install

For someone with jack stands, a torque wrench and a free weekend, both kits are bolt-on jobs with Baer's instructions. Front kits that include hubs mean you also set wheel bearing preload, which is a careful step, not a hard one. Rear kits on a C-clip axle mean pulling the axle shafts. Torque every caliper bracket bolt and lug to the values in Baer's instructions, bleed the system fully, and bed the pads before you drive hard. Upgrading both axles, or adding a new master cylinder, adds time because you will be bending or fitting lines and bleeding the whole system.

Frequently Asked Questions

Is the Baer 6P better than the SS4+?

It has more capacity, not a better purpose. The 6P brings six staggered stainless pistons and 13 or 14 inch rotors, which suits track days and heavy cars. The SS4+ fits smaller wheels and is plenty for street driving and cruising.

Will Baer SS4+ brakes fit my 15 inch wheels?

The 11 inch SS4+ is designed for 15 inch and larger wheels. The 13 inch SS4+ needs at least a 17 inch wheel. Always check your exact wheel with Baer's printed template, because spoke shape and offset matter.

What pads does the Baer 6P caliper use?

The 6P uses the D0731 pad shape, which Baer cross-references to the 1997-2013 base Corvette. The S4 caliper in SS4+ kits uses DR1-type pads.

Do I need a new master cylinder with a Baer big brake kit?

Often, yes. Baer recommends a master cylinder upgrade with some kits, such as its Fox Mustang rear SS4+ system. Check the notes on the kit you choose and confirm your pedal feel after bleeding.

Can I run SS4+ on the rear and 6P on the front?

Baer sells front and rear kits separately, so mixing is possible, but brake balance changes when you change rotor size or piston area at one end. Follow Baer's recommended front and rear pairing for your car rather than guessing.

What does the "Plus" in SS4+ and Pro+ mean?

It means the kit uses a two-piece rotor, an iron braking ring mounted to a separate hat, instead of a one-piece rotor.

Sources

1. Baer Brakes - Brake Kits by Series - series lineup, piston counts, rotor sizes, minimum wheel sizes
2. Baer Brakes - 13 inch SS4 Plus Disc Brakes - four-piston aluminum calipers, 2-piece rotors, fits most 17 inch wheels
3. Baer Brakes - 14 inch Pro Plus Disc Brakes - 6P calipers, intended for performance street, autocross and open-track builds
4. Baer Brakes - 11 inch SS4+ Front Kit 4141049B - S4 caliper specs, 11.000 x 1.020 in rotor, 15 inch wheel, Mopar fitment
5. Baer Brakes - 13 inch SS4+ Rear Kit 4262271B - 13.000 x 0.810 in rotor, park brake, Fox Mustang fitment, master cylinder recommendation
6. Baer Brakes - 13 inch Pro+ Front Kit 4141039B - 6P staggered stainless pistons, 13.000 x 1.100 in rotor, E-body fitment
7. Baer Brakes - 14 inch Pro+ Front Kit 4261211B - 14.000 x 1.150 in rotor, 18 inch wheel, S197 Mustang fitment, wheel template guidance
8. Baer Brakes - D0731R Replacement Pads - 6P pad shape, Corvette cross-reference, Street pad intended use
9. Street Muscle - Baer 6P Pro+ Six-Piston Forged Caliper Sets - two-piece forged body, six cross bolts, stainless pistons
10. Brembo - 5 Factors of a Braking System - brake torque from effective radius, clamping force and friction; piston number and size