Mazda 20B-REW: The Three-Rotor Wankel That Powered the Cosmo Unicorn

The 20B-REW is one of the rarest production engines ever built. It's Mazda's three-rotor Wankel engine — essentially a 13B rotary with one extra rotor bolted on — and it appeared in exactly one production car: the 1990-1995 Eunos Cosmo, a luxury coupe sold only in Japan.

About 8,900 total Eunos Cosmos were built across 5 years of production. Roughly half of those were 20B-REW equipped (the rest had the smaller 13B-REW from the RX-7 FD). That makes the 20B-REW one of the scarcest production engines of the 1990s — a unicorn that most JDM enthusiasts have never seen in person.

Factory Specifications

Note: displacement naming is controversial. The "20B" name suggests 2.0 liters, but rotary displacement is calculated differently from piston engines. Some equivalent-effective-displacement calculations rate the 20B-REW as closer to a 4.0L piston engine due to the three-power-strokes-per-rotation math. Mazda officially used "1,962 cc" for emissions testing.

Three Rotors = Unique Smoothness

Adding a third rotor to the 13B's two-rotor configuration produces a fundamentally different engine character. With three rotors, combustion events are evenly spaced every 240° of eccentric shaft rotation — creating an exceptionally smooth power delivery with zero vibration at any RPM.

Two-rotor Wankels (like the 13B-REW in RX-7 FD) have perceptible combustion pulses at idle. Three-rotor engines are silky smooth — they feel more like a turbine than a piston or two-rotor rotary. This smoothness was a key selling point of the Eunos Cosmo, which was marketed as a luxury GT rather than a sports car.

Sequential Twin-Turbo Setup

The 20B-REW uses a sequential twin-turbo system similar to the 13B-REW in the RX-7 FD, but with three rotors feeding two turbochargers instead of two rotors. The primary turbo (smaller) spools fast at 1,500 rpm for instant low-end response. The secondary turbo engages above ~4,500 rpm for top-end power.

The transition between primary-only and twin operation can be abrupt — especially on aging engines with degraded vacuum lines. Most restored 20B-REWs have had the sequential plumbing either rebuilt or converted to a single-turbo setup.

Known Weaknesses

1. Apex Seal Wear (Three Sets Instead of Two)

The three-rotor engine has three sets of apex seals — 9 total instead of 6 on a 13B. This means more wear surfaces, more potential failure points, and more expensive rebuilds. Apex seal wear is the #1 reason 20B-REW engines need overhaul.

2. Parts Scarcity

Because Mazda built so few 20B-REWs, replacement parts are extremely expensive. Rotor housings, side plates, seals — everything costs 2-3x what an equivalent 13B part costs. A complete rebuild on a 20B-REW typically runs $8,000-$15,000 at a rotary specialist, compared to $4,000-$6,000 for a 13B-REW.

3. Oil Metering Pump (Same as 13B)

Inherits the oil metering pump issues common to all Mazda rotaries. Many owners disable the factory OMP and pre-mix 2-stroke oil into the fuel tank.

4. Heat Management

Three rotors produce more heat than two. The Eunos Cosmo has a larger cooling system than the RX-7, but the 20B-REW still runs hot, especially in traffic. Upgraded radiators and oil coolers are common modifications.

Real Tuning Limits

The 20B-REW's bigger limitation is parts and engine builder availability. Few shops worldwide can work on 20Bs. RE Amemiya in Japan is the most famous 20B specialist.

The Eunos Cosmo — The Only Car Ever

Mazda built exactly one production car with the 20B-REW: the Eunos Cosmo. Sold from 1990-1995 in Japan only, the Eunos Cosmo was a 2+2 luxury GT coupe based on the Cosmo 929 platform. It was a soft, luxurious car targeted at wealthy buyers, not enthusiasts — which is why it's such an odd vehicle to feature the most exotic Mazda rotary ever built.

Approximately 8,900 total units built. Roughly half with the 20B-REW, half with the 13B-REW. All JDM only. Clean 20B-REW Eunos Cosmos now sell for $60,000-$150,000 when they appear on the market.

Famous Modifications

The 20B-REW has been swapped into various other chassis by ambitious builders:

• RX-7 FD3S — the "20B swap" is the ultimate rotary build, creating a 1.5x larger engine than factory
• RX-8 — less common but done
• Various drag cars — the 20B-REW's smoothness and power potential make it a drag racing curiosity
• Custom motorcycle — one builder put a 20B-REW in a custom chopper frame (!)

Factory Service Data

• Oil Change: 5,000 km (3,100 mi) — critical for rotary
• Spark Plugs: NGK BUR9EQ (trailing) + BUR7EQP (leading) × 3 rotors = 6 plugs total
• Coolant: Mazda FL-22
• Rebuild Interval: 80,000-120,000 km (50-75k miles)

Conclusion

The 20B-REW is the most exotic Japanese engine of the 1990s. It's a three-rotor Wankel that appeared in exactly one car in exactly one market, and it established a reputation for silky smoothness that no piston engine can match. It's also the most expensive production Mazda rotary to maintain, the hardest to find parts for, and the rarest engine in the rotary family. For collectors, the 20B-REW is a unicorn that will only become more valuable as remaining Eunos Cosmos disappear from the Japanese second-hand market. It deserves a place in the legend book for existing at all.