20B 3 Rotor Engine

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) There has really only been one mass produced passenger car 3 rotor engine – the 20B, which was a twin turbo fuel injected engine. Note, NSU also had a 3 rotor engine though I’m not sure if this was mass produced.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

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 anodized 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

(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 (e.g. 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.