Mazda 13B/20B: Our Analysis
A Brief History of the Rotary Engine
The Wankel engine (after its designer Felix Wankel), or rotary engine as it is more commonly known, is an internal combustion engine that instead of reciprocating pistons, employs a system of rotating rotors that are patterned on the Reuleaux triangle, albeit with somewhat flattened sides.
The rotors are housed in an epitrochoid-shaped “engine block”, in which the compression, power, exhaust, and intake strokes occur as the rotor rotates in the housing. Compression and power strokes are made possible by the continually changing shape of the housing in relation to the rotor as it rotates, which has the added advantage that the reciprocating motion of pistons are eliminated since all moving parts rotate in the same direction. This further results in an uncomplicated, yet compact modular design, almost complete lack of vibration, and a very high power-to weight ratio, which in turn is made possible by the rotary engine’s ability to attain very high RPM’s in relation to piston engines.
These advantages make rotary engines viable and successful power plants for cars, motorcycles, racecars, airplanes, go-karts, jet-skis, snowmobiles, chain saws, and auxiliary power units such as power generators, trailer-mounted water pumps, and extraction fans in the mining industry.
Motor Sport Applications.
In the sport of motor racing, Mazda has achieved notable successes with cars equipped with two-rotor, three-rotor, and four-rotor engines. For instance, the Mazda 12A rotary engine equipped Sigma MC74 was the first engine/team combination to complete the 24 Hour Le Mans Endurance Race that did not originate in the USA or Western Europe, and that was in 1974.
Mazda is also the only team not from the USA or Western Europe to win the Le Mans race with a non-reciprocating piston engine with their 2 622 cc or 160 cu/ inch displacement four-rotor 787B rotary engine. However, even though the 787B engine had an actual displacement of only 160 cu/inches, the formula used by the FIA rated the engine at 287 cu/inches, or 4 708 cc.
Moreover, rotary engines in all possible states of development and modification are hugely popular among fans of drifting, spinning, and street racing because of the engines’ low weight and favourable power to weight ratio, despite the fact that rotary engines are often up to three or four times as expensive to buy, maintain, run and repair as comparable piston engines.
13B Rotary Engine Overview.
Contrary to popular belief, Mazda did not pioneer the rotary engine design: the initial design belongs to Felix Wankel, but the first working examples were constructed in 1959 by NSU Motorenwerke AG, a German manufacturer of cars, motorcycles, and bicycles, which was established in 1873. NSU, for short, was acquired by Volkswagen in 1969, and later merged with AutoUnion, which eventually transmogrified into Audi.
Mazda only became involved with the new engine design in 1961 as the result of a co-operation agreement between itself and NSU who needed assistance with resolving major and seemingly intractable engineering and reliability issues. However, Mazda began development of its own version (some say without the knowledge of NSU), but ran into the same rotor apex sealing issues NSU had been struggling with, which made commercial production of the new design impossible.
Nail Marks of the Devil.
The engineering issue that bedevilled the design and manufacturing process was dubbed “Nail marks of the Devil”, because of the scratches, or chatter marks on both the apex of the rotors and the rotor housing surface where contact between rotor and housing was the closest. This caused a serious loss of compression, vastly increased oil consumption, and general misgiving about the viability of the rotary design. Nevertheless, in 1963, the newly formed Rotary Engine Research Department under Mr. Kenichi Yamamoto (who had 47 engineers and metallurgists under his command), tackled the issue head on, and found that the “Marks of the Devil’s Nails”, were caused by the natural vibration of the rotors and apex seals. Resolving the issue was less easy, but it was partly solved by the development of a new type of “cross-hollow” apex seal, but this proved too expensive for commercial production, even though a prototype engine successfully completed more than 300 hours of continuous operation with only minimal scarring of the sealing surfaces.
A more permanent solution was needed, and one was found with the assistance of Nippon Piston Ring Co and the Nippon Oil Seal Co, who in conjunction with Mazda developed lubrication oil that provided increased lubrication, and thus improved sealing of the apex seal surface.
13B Rotary Engine History.
Eventually, through steady advances in design, lubrication, and the development of new materials, Mazda produced the 13B engine after a succession of less successful designs. With this design, most, if not all of the engineering issues that plagued previous designs were eliminated, or at least reduced to manageable proportions, and the first units went on sale in December of 1973 in the RX-4, or the “Luce”, as this model was known in Japan and elsewhere. This variant of the two-rotor 13B displaced 1308cc, which was accomplished by increasing the width of the rotors by 10mm over that of its immediate predecessor, the 12A engine.
Subsequent, naturally aspirated variants of the 13B was fitted to next generation RX-4 and RX-5 models, 2nd and 3rd generation RX-7’s, as well as the JC Cosmo and somewhat re-worked Luce models that were sold only in the Japanese market. Within a few years however, the 13B engine was fitted with fuel injection and a single turbo, then twin turbos (13B-REW variant) in a sequential arrangement, and finally with a six-port induction system that is very similar to that used by Honda in their VTEC system. The 13B engine remains the most sought-after and easiest rotary engine to modify, and hundreds, if not thousands of fuel injected and turbo’d 13B engines have found new applications in almost anything from older RX models to Ford Escorts and Capri’s.
The “REW” in the 13B-WER designation refers to, Rotary Engine Double Turbochargers. The designation, “RED”, for Rotary Engine Double [Turbochargers], did not appeal to the Mazda marketing people, so they chose to use the letter “W” (double-U), as a compromise to denote the two turbos, hence the designation, “REW”.
13B Engine Specs. (Third Gen Specs: 93-95)
Max horsepower: 255 Hp (187.7 KW) @ 6500 RPM
Max torque: 217.00 Ft-Lbs (294.3 NM) @ 5000 RPM
Max torque 13B-REW variant released in 2002: 280 Hp
20B Rotary Engine History.
The 20B is a direct descendant of the 13B, but with one rotor added. Due to the extra rotor, which is the same width a the rotors in the 13B, the 20B engine is more responsive, more powerful, and delivers its power smoothly at lower RPM’s, which makes it markedly more fuel efficient than its predecessor, the 13B. However, the 20B engine is available only in the JC Cosmo, which makes it rare and very expensive to buy when a unit can be located. Prices for used 20B engines vary, but on average they command around $10 000.
20B “Production” Engine specs.
Origin: Closely related to the third generation RX-7 13B-REW engine, but employing special three-rotor parts derived from the 13G variant.
Max torque: 297 lb/ft (402 Nm) @3000 rpm
Max boost: 500 mm hg
Max RPM: 7000 RPM
13B vs. 20B Rotary Engines?
Comparing a 13B engine to a 20B is essentially the same as comparing an inline six-cylinder engine to a V8- it cannot be done in any way that compares apples with apples. Both engines deliver their power in a relatively smooth power band, but that is where the similarities end. Modifications to turbos and induction systems on both engines can result in either severe turbo lag, or sudden, and/or large power surges, which largely makes this kind of modification a haphazard affair if the by now established body of knowledge on what works and what does not, is not fully investigated and made use of.
However, rotary engines are available in a great many variants, and particularly in the case of the 20B-REW, that has arguably the most complex engine management system of any car in production today. This makes modification not only difficult, but also very expensive, which can place building a 1200+ HP 20B engine out of the reach of most rotary enthusiasts.
Nonetheless, in standard, or near standard form, both engines have known reliability issues with rotor apex seals, apex seal springs, corrosion in water jackets, and excessive oil consumption in older engines. Of course, the level of modification to boost, ignition, and fuel systems have a direct bearing on how soon apex seals and other components will fail (or not). Nevertheless, this is also related to the purpose of the modification, and it is therefore not possible to give even approximations in terms of mileage for every possible level of modification, or possible application.
As a rule of thumb, however, a drastic modification such as supercharging an engine that is not “turbo adapted” will not see many hundreds of miles before failing catastrophically, because the standard fluid cooling systems cannot cope with the elevated temperatures and internal pressures caused by a supercharger.
The only direct comparison that is possible between a 13B and a 20B relates to the cost of modification. However, since the 20B is already the more powerful engine, it takes less time and money to build a super powerful engine from a 20B base, than is possible from a 13B platform, but in both cases, the modifications are tricky, expensive, and NOT recommended as a DIY project.
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