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Mazda 20B (3 rotor Engine)

Introduction / 20B-Production Engine / 20B-Racing Engine / Performance / 20B Aero Engines / Aero Engine Suppliers / More Information

Introduction to Mazda 3 rotor engines

Be sure to check out the JC Cosmo, 13G, 20B RX7 and 4 rotor engine pages at this site. See More Information below for links.

Mazda have made several prototype and experimental engines over the years ranging from the 6A (a single rotor version of the 12A, intended for use in Japanese "Kei" cars), the 2002 "4 rotor 10A" to the massive R-II 21A with 2x1046cc rotors (You can see the 2002 and 21A elsewhere at this site).

Several 3 rotor prototype and racing engines have been produced over the years. Generally speaking these engines were constructed using the modular nature of the rotary engine (that is to say, the parts can be "stacked" together).
This allowed comparatively cheap production items (rotors, rotor housings, side housings, seals etc) to be used in conjunction with some specially made (expensive) parts (eccentric shaft and centre housing with rotor phasing gear/bearing)
While NSU also had a 3 rotor engine it was not widely available, thus the only mass produced passenger car 3 rotor engine - the 20B, which was a twin turbo fuel injected engine.

Generally speaking, the following 3 rotor engines were "available" from the early 1980s:
13G Racing engine (1984-1989), 13G "Production Prototype" (1984/1985), 20B Production engine (1990-1996), 20B Race engine (1990-now)

20B - Production engine

Cosmo type JCESE (640x480,1024,768)
Cosmo type JCES (640x480)(1024x768)
Bare 20B engine at mazdatrix  (640x480)
Bare 20B engine at Mazdatrix (640x480)
early cosmo20 engine bay  (640x480)
"Early" Cosmo 20B engine
bay (640x480)
20B parts (640x480,1024x768)
Dismantled 20B (640x480)(1024x768)
Dismantled 20B (640x480)
Dismantled 20B (640x480)
cutaway view of 20B (640x480)
Cutaway view of 20B (640x480)
cutaway view of 20B (640x480)
Cutaway view of 20B (640x480)

The 20B engine was only sold in the Japanese market. It was also only available with twin turbochargers.
It was fitted to the "Eunos Cosmo" (Model type JCES) sold from March 1990 until January 1996.
(There was a minor model change, with mainly cosmetic changes about 1993. The first of the cosmos have an engine with plain alloy intake manifold etc. The "updated" model has these parts anodised dark grey. All other mechanical specifications remained the same)
The Cosmo was a high specification, high performance luxury coupe nearly comparable to the Lexus. All Cosmos were 4 speed automatics.
(See my "EUNOS COSMO" Page for more detail about the car).

It seems that the 20B engine is based on the 13B-REW engine of the "3rd generation" (FD3S) RX7.
(Despite the FD3S being released in August 1991, well over 18 months after the Cosmo with it's 20B in March 1990 - However the Cosmo ALSO was available with a 13B turbo, with very similar specifications to the RX7's 13B-REW, except power was down a little (about 20-25 hp less).
Perhaps the Cosmo's engine was used to test how reliable the new design was.. Especially with the upcoming volume selling RX7.)

This engine appears to use many standard 13B components, such as rotors /rotor housings, seals etc. As with the "prototype production 13G" the only obvious custom components are:
*Special second centre housing with gear and bearing
*Eccentric shaft and counterweight assembly
*Intake and exhaust manifolds/turbos
*Ignition system (Distributorless, but uses FC3S/FD3S RX7 type pickup unit)

NOTE: 20B-REW is the actual correct designation for this engine.
This means "20B-Rotary Engine DOUBLE TURBO" according to what I have read (it is easier for a Japanese speaker to say "W" than "DOUBLE")

20B-REW "Production" Engine specifications
Origin Closely based on 3rd generation (FD3S) RX7's 13B-REW
Special 3 rotor parts based on 13G design
Capacity 654cc x 3 rotors = 1962cc
Compression ratio 9.0:1
Turbo Boost 0.7 Bar (=10.29 psi)
Induction Turbocharged (twin sequential)/intercooled (intercooler mounted
near car's radiator), electronic fuel injection (2 injectors/rotor)
Exhaust Peripheral Exhaust Port
Ignition Distributorless electronic ignition (2 plugs/rotor)
Power/RPM 280ps@6500rpm
Torque/RPM 41kgm@3000rpm
Max RPM Cosmo Tacho redline at 7000rpm (scale ends at 8,000)
Dimensions Length 672mm Width 549mm Height 520mm (with accessories)
Weight 350kg With all accessories & manifolds
Special Features Ignition knock control uses one sensor per rotor (on rotor
housing above trailing spark plug)

20B - Racing engine

Mazdaspeed 20B (640x480)
Mazdaspeed 20B

(Picture from 1996/1997 Mazdaspeed catalog Page 104)

This engine is currently (1997) sold by both Mazdaspeed in Japan and Mazda Motorsports in the USA, both of which are officially part of Mazda. At first I presumed this was a racing engine based on the production 20B. However on comparing the photos of the race 20B and the race 13G, plus the Mazda catalogs list this engine as using rotor housings with grooves for the water seals, I think it is still a 13G they are selling.
Also the power output and torque figures are identical (same RPM), and the weight/dimensions are almost the same.
(The grooves for the water seals were moved from the rotor housings to the side housings when the "2nd generation" RX7 (FC3S) was released in 1986)

20B Racing Engine specifications
Origin Possibly same as 13G race engine, but not certain.
Capacity 654cc x 3 rotors = 1962cc
Compression ratio Unknown (But 13G is 9.4:1)
Induction Naturally aspirated peripheral port, electronic fuel injection (2 injectors per rotor)
Exhaust Peripheral Exhaust Port
Ignition Distributorless CDI ignition (2 plugs/rotor)
Power/RPM 450ps@8500rpm
Torque/RPM 40kgm@8000rpm
Max RPM Unknown (But 13G is 9500 RPM)
Dimensions Length 675mm Width 549mm Height 520mm
Weight 143kg
Special Features Dry sump

The Complete Mazdaspeed 20B parts list

These pictures have been scanned from the 1996/1997 Mazdaspeed catalogue, Pages 105 to 121 inclusive.
I could see no copyright notices in the catalogue, so these are here as "free advertising" for Mazdaspeed.

Page 106 shows the rotor housings, and you can see that the groove for the water O-ring is in the
rotor housings (as were all pre-1986 engines), HENCE I think this is ACTUALLY still a 13G engine.
Also, check out the trick throttle assembly in page 116!

By the way, the prices are in Yen, and 1 US Dollar = 123.43 Japanese Yen (March 1997)

[PAGE 105] Rotor diagram and parts list
[PAGE 106] Rotor housing diagram
[PAGE 107] Rotor housing parts list
[PAGE 108] Side housing diagram
[PAGE 109] Side housing parts list
[PAGE 110] Eccentric shaft diagram
[PAGE 111] Eccentric shaft parts list
[PAGE 112] Manifold diagram and parts list
[PAGE 113] EFI computer diagram and parts list
[PAGE 114] Fuel system diagram and parts list
[PAGE 115] Water Pump diagram and parts list
[PAGE 116] Throttle plate diagram
[PAGE 117] Throttle plate parts list
[PAGE 118] Oil system diagram
[PAGE 119] Oil system parts list
[PAGE 120] Ignition/Alt/Starter diagram
[PAGE 121] Ignition/Alt/Starter parts list

Note: All pictures are 768x1024x4 GIF files. This was found to be the best compromise between file size and picture quality, which is about the same as a fax (to get better quality meant much bigger files).

Performance applications of the 20B engine

A second hand production 20B engine is the only multi rotor engine worth considering. The 13G engines are extremely rare (and expensive). The new price of a "20B race engine" as listed above is $US 20,000+ for the core engine alone. Many more 'production' 20B engines were made as they were fitted to a production car rather than being a special racing item.  However 20Bs are far rarer than their 13B cousins; so they are difficult to find and will be expensive. (I would estimate that less than 100 of these have been imported to Australia).

In Australia, the price for these units started at $Aus 12000 in 1991, dropping to a low of about $Aus 4000 in 1997. In March 2001 I was told by someone who had just bought an engine that the going rate is $Aus 4000 to 9000:
*Adelaide Jap Dismantlers $Aus 6550 for front cut (Whole front half of car)
*Ichiban for $Aus 2975 (some damage to engine), $Aus 3995 or $Aus 4995
Refer to my "parts" page for contact information. If you have any updates of prices or supply sources please let me know).

So, assuming a 20B can use standard RX7 13B components (rotor housings, seals etc) a rebuild on a 20B could probably be done for under $Aus 3000.

20B engines used in Australia have been fitted to many cars in the Mazda range - RX2, RX3, RX4, RX5, RX7s of all vintages, 929 sedans and Coupes, I have even seen a mid 80's BMW 5 series with a 20B installed. (See elsewhere on this site for both a 2nd Generation RX7 with a 20B conversion and the Rod Millen 20B rally car).

Because the 20B probably has one of the most complex engine management requirements in the world and the factory computers have complex wiring, these engines generally have the twin turbos replaced by a single large turbo (eg TO4) and are run on a aftermarket fuel computer.
Expensive engine management systems from Motec, Haltech and Autronic and others may be capable of distributorless ignition and/or twin turbo operation however in the past the ignition was typically done via an early RX7 distributor, with a Racing Beat conversion kit comprising of a big clear perspex distributor cap and a new ignition triggering vane.

A standard Mazda transmission will bolt onto the 20B without modification, another popular choice are 5 speed Toyota Supra gearboxes. Some drag cars are using ancient 2 speed Powerglide transmissions (why they would use one of these rather than a modern 4 speed is beyond me...)
The differential used is almost always a Ford 9 inch.

20Bs have been fitted to later model RX7s (FC3S/FD3S) without too much problem. (I suspect the RX7 engine bays MAY have been originally designed to hold a 3 rotor engine, particularly in the FD3S). These conversions seem to retain more of the original hardware.

Potential power outputs:
*The standard power output of a 20B turbo is 280HP.
*With turbos removed (naturally aspirated), these engines produce 250hp stock, 320hp if ported.
*Apparently with the mere addition of a boost controller the power jumps to around 400HP.
*Rod Millen's Pike's Peak race car made about 500HP with a nearly stock engine.
*Japanese modifiers get about 700HP without too much trouble (I suspect this would be at a level where some engine porting and severe turbos would be required).
*Racing beat's Bonneville racer makes 900+ hp with a triple turbo peripheral port 13G
*The above figures were all mid-late 1990s. In 2002 some drag racers are claiming 1200hp.

20B Aeroplane engines

Rotary engines have been used by many people in the past as aeroplane engines.
For reliability, these have generally been stock, non-turbo 12A or 13B engines. However there is an increasing trend to using 20B rotary engines due to their high performance for their size and weight (even in stock form), reliability and relatively low cost.
Some are using them in highly modified form (500 - 700hp. Even up to 900 hp is possible)

20B aeroplane engine (595x398)
20B Aeroplane engine
Supercharged 20B aeroplane engine (650x451) (978x689)
20B Airboat engine
Racing Beat's 3 rotor engine (900 hp) for a twin engine plane
Racing Beat's 900hp 20B
aero engine (459x258)

Note with the engine pictures above, the one on the left is a turbocharged 20B, with a propeller speed reduction unit (PSRU) bolted onto the engine where the stock gearbox would normally go. This gearbox allows the propeller speed to be kept under about 2000 RPM, because over this speed the tips of the propeller go supersonic, which creates un-necessary noise and decreases efficiency. (The plywood is part of a test rig, however in the plane this would be the bulkhead).

The one in the middle is a supercharged 20b with a Holley carburettor and a Racing Beat 3 rotor distributor. It was rebuilt and modified by Dave Atkins in Puyallup Washington, USA, for use in an air boat (using an airplane style propeller). The large black pipe in the rear of the picture is an exhaust that has a water cooling sleeve around it, engine cooling water is sucked up out of the lake.

The engine on the right is a Racing Beat's 900hp supercharged 20B (It uses a turbo style compressor driven through step up gearing, on the left of the picture). This engine was anticipated to be fitted to a twin engine plane but I have no further details. I would assume that the basic engine is built to a similar specification as their Bonneville racer.

Note both of the airplanes engines use forced induction for improved power at altitude (aero engines can be non-forced but are generally limited to around 10,000 feet). Carburettors are good for aero applications as they are reliable compared to Fuel injection. When fuel injection is used it is typically a simple mechanical system or occasionally twin electronic units.

These pictures were from the Aircraft Rotary Engine electronic Newsletter
You can subscribe (free) to the by sending email to:
with only one word. "subscribe"

Aero Engine and Parts suppliers

There is a Company in New Zealand that sells 20B engines for use in aircraft. Here is a copy of his info:

Rotary Engine Power, New Zealand
Contact: Grant S. Nicholls, 12 Aristoy Close, Conifer Grove, Takanini, Auckland, New Zealand
Ph / Fax 64-9-2988848
E-Mail Address or

Thank you for your enquiry about Mazda 20B, 3 rotor, rotary engines. The special features of this engine are its light weight, small size, two spark plugs per rotor, two fuel injectors per rotor, small number of moving parts, runs exceptionally smoothly and is considered to be more reliable than a piston engines Piston engines that are raced are usually reconditioned after every race, rotary engines will last a racing season.

All engines are ex used vehicles with low kilometres. All engines are checked for integrity prior to shipping. Parts availability and supply is not a problem. Most 20B engine components are the same as in twin rotor 13b engines. The basic core of this engine without turbos or fuel injection can produce over 250 hp normally aspirated, with porting 320 hp. As supplied to you, 350 hp with turbos and fuel injectors. This engine if triple turbo charged and ported can produce up to 900 hp but is at present limited by current propeller speed reduction units over 600 hp.

This engine will fit inside most existing fire wall forward dimensions with little modification. This engine installation will normally weigh less than current production lycoming engines of similar horsepower. The total engine installation is around 400 pounds complete. the basic core of the 20B would weigh approx 250 lbs. (There are aluminium housings for this engine that replace the cast iron housings between rotors should weight be a problem, however these are expensive.)

The 20B engine core diameter is 13 inches or 320mm the same diameter as the 13b engine. Length is 26 inches or 666mm. Sump to top of manifolds 28 inches or 700mm. Centre of crank shaft to out side turbo housing 13.5 inches or 340mm. With the turbo chargers the engine would have a width of 320mm, height 700mm and would require little modification. Without the turbo chargers and normally aspirated the engine would have a width of 320mm height 320mm excluding accessories.

This engine can be mounted on its side. 350 hp is approx 6000 rpm, rotor speed 2000 rpm. Piston engine fuel consumption point 4, rotary engines point 5. The rotary weighs less than a piston engine and produces more power, therefore fuel consumption for the same distance is about equal. A tbo of up to 3000 hours could be expected - overhaul is very inexpensive compared to certified engines. We do not supply engine to fire wall mounts.

The p.s.r.u we recommend is a ross aero propeller speed reduction unit. for psrus over 400hp ph fred geschwender 001-402-4383930 usa. We also have a new Barnett 2/1 psru rated at 600hp at around $3000 us.

We can also provide with 20B engines as accessories; propeller speed reduction units, dual ignition distributors, and electronic ignition fuel computers, coils and power packs. The basic engine package does not include ignition, fuel computers, ignition power packs, coils, radiators or heat exchangers. All these components need to be sourced specifically for your fire wall forward requirements.

List of component prices:
*Information pack $US 10
*Mazda 20B engine $US 3950
*Electronic ignition and fuel computer $US 1350
*Coil (3 combined in one unit) $US 160
*Power pack $110 each x 3 required $US 330
*Freight (eg New Zealand to Los Angeles) $US 300 approx, add $100 us to ship mid states.
*psru's (more info - ring Chris at Ross Aero, Tucson Arizona USA) ph 602-7477877

More Information

Racing Beat and Mazdatrix can probably help get you the parts you need, but don't waste their time unless you are serious. The Mazdaspeed/Mazda Motorsports catalogue are the only place I have ever seen the parts actually listed (with the prices).
Apart from those and the occasional article in a rotary magazine, there are a few books that have some pictures in them (specifically I remember seeing a book about racing car engines from around the world that had 2 or 3 pages about the Mazda multi-rotor engines, but I couldn't afford it at the time and can't find it now)

This page has everything I know and have been able to find about the 20B engine, so I probably can't answer any specific questions.

Further reading and acknowledgements:
* Special thanks to Mr Nishizawa in Japan for sending me a magazine about the Cosmo and 20B
* Special thanks to Asmic for sending me the Mazdaspeed catalogue.
* Thanks to Mazdatrix for letting me take a photo of their 20B when I visited.

Other relevant reading at Craig's Rotary Page (Please go via the HOME page):
* Eunos Cosmo page (20B powered car)
* 13G engine page (the 13G is a predecessor to the 20B)
* 4 rotor engines page
* My 1994 USA trip (Pictures of the Rod Millen 20B RX7 and Mazdatrix 20B engine)
* 20B RX7 conversion
* Parts page (Suppliers of 20B engines, in Australia)
* There are several other semi relevant pages like the NSU page, Hurley page, John Deere page. See the HOME page

Other relevant sites on the Internet (Please go via the LINKS page):
* has a lot of pictures and videos of 3 rotor cars (But, little tech info)
* Asmic's page has some pictures of the 13G engine
* The Hitman has a 3 rotor RX7 (Custom made from 13B parts), also expert on Haltech injection.
* Hurley Motor Engineering has made 3 and 4 rotor engines since the mid 1980s

This page last updated 16/7/2002
Update History:
16/7/2002 - Added links in more information section and changed references to index page to be called home page.
16/3/2001 - Converted all text to new standard (Headings as Heading1, Some sub-headings (e.g. tables) as 14 point normal bold italic, Most text as Normal, Internal page links at top not all uppercase)
11/3/2001 - Minor content update Engine prices, update to 20B aero engine section. Changed from Netscape to FrontPage and from EWK3.HTM to PG20.HTM (& all sub files). EWK3.HTM changed to redirection page. Background image changed to PG00_02B.JPG
17/11/1997 - Previous known update (May have been some before this)

Special thanks to Mr Nishizawa from Japan
for sending me some info, and to
for helping translate it for me!!!
And, Thanks to ASMIC for Mazdaspeed Catalogue.

Mazdaspeed drawings Produced by scanning A4 picture at 768x1024, Black&white photo, 123 Brightness 133 Contrast.
Then in Paintshop pro, "Sharpen more", Posterise 2 bits, crop to 768x1024 (if necessary), save as GIF 89A Interlaced.

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