Mazda 20B Rotary Engine: Everything You Need To Know

The Mazda Eunos Cosmo was the first (and only) production Mazda to use the highly sought-after triple-rotor 20B-REW engine when it debuted in 1990. 

Mazda’s initial attempt at a rotary engine, the 1960’s L10 Cosmo, was continually improved upon, and the Cosmo went on to incorporate the highly sought-after 2-rotor 13B and 3-rotor 20B engines during its career. 

The 20B was innovative because it was the world’s first production engine to utilize a twin-sequential turbo system. Still, its higher torque output and enlarged displacement frequently make the 20B the most sought-after rotary on the market.

Welcome to the world of Mazda 20B rotary engine. It’s an amazing time where we get ready for all your questions and learn everything there is about these much-loved engines. 

What are Mazda 20B Rotary Engines? 

Mazda’s remarkable 20B engine is the company’s sole 3-rotor production motor. 

Although Mazda’s 13B-REW is frequently the most sought-after rotary for engine swaps, this is due in part to the 20B’s scarcity, with the twin-turbo variation only marketed within the JDM (Japanese Domestic Market) under the hood of one car in their lineup. The 20B racing engine is a sought-after roadster, but it’s not sold under the hood of any production model.

While the iconic 13B is a fantastic engine, the larger displacement and additional rotor with the 20B allow for significantly enhanced torque, which the 13B lacks, and substantially less stress when it comes to higher-output capability due to the additional displacement. 

Further, the rotary 20B was the sole mass-produced 3-rotor engine, but there was an older counterpart – the 13G – to the naturally aspirated 20B race engine that never made it to the public market. However, we will not focus on the 13G but more on the rotary 20B. 

Engine Specifications and Design: 

  • Production Run: 1990 – Present 
  • Configuration: Rotary
  • Displacement: 2.0 L (1962 cc)
  • Compression Ratio: 9.0
  • Weight: 315 lbs.
  • Maximum HP: 440 HP at 8,500 RPM
  • Maximum Torque: 289 lb-ft at 8,000 RPM

Engine Design: 

Many believe Mazda employed the 20B-REW as a test to evaluate how the engine performed in regular use before potentially using the 20B in the significantly more popular RX7. The 20B-REW shares many components with the popular 13B, with the primary modifications being a second intermediate housing and a 3-rotor eccentric shaft. 

Mazda sold the Eunos Cosmo from February 1990 until September 1995. During the five years that Cosmo included the 20B-REW and a 13B variation, 4,000 20B-REW machines were sold.

In addition to that, Mazda opted to put the highly wanted 20B-REW engine in an unsuspecting ‘halo’ vehicle, the JC Eunos Cosmo, since it was a practical alternative for mass manufacturing. They were lowering the production cost and overall efficiency of making them. 

The Cosmo’s 3-rotor 20B-REW engine has a displacement of 1,962 cc (119.7 cu in) with a displacement of 654 cc each rotor and a combined displacement of 1,962 cc, with the “20” in the designation now referring to its 2.0L capacity. 

When the three-rotor 20B-REW engine initially appeared on the market in 1990, it was the first twin-turbo-powered automobile and the first three-rotor engine to enter mass production.

Racing Specs for a Rotary

Mazdaspeed Japan and Mazda Motorsports USA both marketed the 20B racing engine in addition to the 20B-REW. 

Although the racing engine looks to be based on the 20B-REW, it appears to be a re-branded 13G, with nearly similar specs and power outputs and the stock dry-sump. 

Racing 20Bs appear on the market from time to time, so if you’re looking for something even more distinctive than the 20B-REW, it may be an intriguing choice.

1. R20B Renesis

Racing Beat developed a subsequent version of the engine for the Furai concept vehicle, known as the ‘R20B RENESIS,’ which was presented in December 2007. 

Despite great hopes for the R20B, the ethanol-powered 20B caught fire during a Top Gear photoshoot. Because the team was unable to manage the fire, the fascinating project was ultimately extinguished. 

2. 20B Turbo Design

The groundbreaking twin-turbo architecture of the 20B meant that the first, smaller turbo would spool rapidly to eliminate as much turbo lag as possible. 

The second turbo would spool up in the background, waiting to be unleashed from 3,500 rpm and beyond, with the turbos finally reaching ten psi. 

The dual-exhaust arrangement allowed the gases to travel through the dual, twin-exit exhaust pipes at the back, with two open at all times and the remaining two fully open under high throttle, allowing the 20B to maximize its full potential.

Transmission

The Cosmo’s 4-speed automatic transmission will be one of the first things to go for most potential 20B-REW buyers. 

Although the sophisticated four-speed claimed further world-firsts, with manual selection capabilities inside the first three gears, a ‘PWR’ button activated a sportier mode, allowing for higher-rev gear changes. 

Unfortunately, the transmission limited the engine to 7,000 rpm instead of its proven 9,000 rpm capability. But, let’s face it, you wouldn’t maintain the antiquated auto box if you were going with a 20B-REW in the first place.

Power Production 

The Japanese 20B-REW is a city racer with more power than its factory stats claim, and as such, it’s no wonder that many claimed this to be significantly higher. With an alleged 276 HP stock form for the engine itself, some people got up close estimates of 300hp or even 320 on their dyno sheets. 

Applications of Mazda 20B engine:

  • JC Eunos Cosmo Coupe

Tuning Potential 

Adding a turbo to the rev-happy naturally aspirated 20B race engine may seem like the easy way out. Still, you will find it easier if your ultimate goal involves forced induction. The rarity factor of this variant makes for an especially interesting project with plenty of challenges. 

1. Turbocharging Upgrades

The Mazda 20B race engine thrives at high rpm, but turbochargers power the 20B-REW. 

When it comes to increasing power on the 20B-REW, you’ll quickly meet the limits of the standard turbos at roughly 14 psi of boost at around 8,000 rpm and 300 horsepower with an aftermarket ECU. 

Some have claimed 400 horsepower, but we would be extremely careful about using this method, especially with a rotary engine. Alternatively, you may gain more power with the factory system by removing the sequential turbos, resulting in greater latency and more power.

Also, you can upgrade both turbos or replace them entirely with uprated alternatives. The simplest method for high power setups is likely to opt for the single turbo route. Given that this car has more significant displacement, it’s capable of eliminating lag when compared against smaller rotaries like those found in 20B-REW kits.

If you’re looking for the ultimate horsepower, then a 700HP Japanese tuner is your best bet. However, if these kinds of substantial power numbers are what will ultimately satisfy your thirst and hunger for top-tier performance from an engine rebuild along with further supporting upgrades that can take it even farther than before.

2. Swaps 

The 20B-REW engine is one of the most prevalent 20B engine swaps, and it is used in the Mazda RX7 FD. 

Although we’ve seen many of them, and you may think it’s an easy drop-in conversion, that’s not the case. 

To prevent changing the transmission, you’ll need a particular subframe assembly for the most convenient FD installation. You’ll also avoid needing to trim parts like the frame or firewall. 

Problems Surrounding Mazda 20B Rotary Engine.

It’s no secret that rotary engines need significantly more maintenance and particular attention than standard piston engines. 

As a result, they’re not everyone’s first option, but for those willing to put in the time and work, they’ll be rewarded with one of the most remarkable engines ever constructed.

Once you’ve spent far too long tracking down your 20B, the first thing that should come to mind is what kind of life this engine has had. With its age, you’ll want a thorough inspection for any signs of neglect or abuse so as not to damage such a rare piece of engineering. 

If you’ve found one and assume it’s had a rough existence, but there’s no history to back it up, we’d recommend a rebuild. 

We would anticipate new apex seals and seal springs, side seals, side seal springs, inner and outer o-ring seals, and new gaskets. 

Given the nature of rotary engines, we recommend having a rebuild performed by a respected workshop rather than attempting to undertake the work yourself. It will undoubtedly help, especially when it comes to selling.

Summary

Despite being frequently exorbitantly priced, if you can get your hands on one, you will be rewarded with one of the most magnificent engines the world has ever seen. 

Sure, the 13B-REW is the more frequent option, but let’s be honest: they would have chosen a 20B if they could.

While a 13B isn’t insufficient by any means, the 20B improves on the 13B masterpiece. 

For those who aren’t interested in massive power, this beast’s sheer sound is definitely worth the outrageous price tag, and few engines on the earth sound better on full-song. 

The Mazda 20B Rotary engine is an excellent choice for those looking to stand out and make a statement. This car has plenty of power under the hood and can perform in drag, street, or drift builds. 

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