Mazda Cosmo

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Mazda Cosmo

1966 - 1972
Twin Rotary Wankel
2000 cc
110-128 bhp
4/5 spd. man
Top Speed:
186-201 km/h
Number Built:
5 star
Mazda Cosmo
Reviewed by Unique Cars and Parts
Our Rating: 5


The Cosmo was Mazda's rather futuristic car and was designed from the ground up. It was perceived as Mazda's first foray into the world of high-performing excitement machines. In most aspects the 110 S was a conventional and stylish sportscar designed to compete alongside the Porsche 911and the Jaguar E-Type. Its capacity equalled about two litres with an output of 110 bhp at 7,000 rpm.

Top speed was 186 km/h and it could reach 0 - 96 in around 10 seconds. It handled terrifically with its DeDion axle at the rear and wishbones at the front. Its braking power relied on discs at the front with drums at the rear. In its five year production span the Cosmo basically stayed the same, other than a closer-ratio five-speed gearbox, longer wheel base and slightly increased power.

Development Of The Wankel Rotary

Using the then latest aids to engineering research and development, Toyo Kogyo Co. Ltd. developed their Wankel rotary combustion engine, in parallel with other licensees, to the production stage in barely six years.

Indeed, the twin rotor Wankel RC engine as fitted to the Mazda Cosmo was the first twin-rotor RC engine to go into production, though NSU, Curtiss-Wright and others did have twin and multi-rotor RC engines in advanced stages of development.

Toyo Kogyo research was focused on improving engine durability and reliability; they made exhaustive studies and comparative evaluations of peripheral versus side inlet ports for single, twin, three and even four-rotor engines, besides investigating the effects of single and dual ignition systems on combustion phenomena.

A formidable 1.87 million road test miles were covered during the development of the Mazda Cosmo and its engine to production standards, and during 1966 numerous cars were in the hands of Mazda dealers and other experts throughout Japan for final evaluuation. Their cogent observations were examined, and wherever possible any ideas which emerged were incorporated in the final version.

A Conventional Sports Car Formula Mated To Rotary Genius

The silhouette of the Mazda Cosmo was rather conventional, although very good looking. The frort mounted engine sat behind the front axle, which in turn drove the rear wheels through a four-speed and reverse all-synchromesh type gearbox, an open propeller shaft and a hypoid final drive. This hypoid final drive made up part of a de Dion rear axle arrangement, which also incorporated' half-elliptic springs. Independent front suspension was by unequal-length wishbones and coil springs. Double acting teleescopic hydraulic dampers were used all round, and there were Dunlop front disc brakes and leading/trailing shoe rear drums.

On the Mazda Cosmo's release in May 1966 all interest was naturally focused on detail engine features and performance characteristics, all of which quickly proved that there was (and remains) a keen appreciation of RC engine peculiarities at Toyo Kogyo. The displacement per shaft revolution was 2 X 491 cc., which made the engine equvalent to a 1,964 cc. four-stroke reciproocating piston engine. The compresssion ratio was 9.4 to 1 and maxiimum power output was 108.5 bhp at 7,000 r.p.m., with maxiimum torque of 96.2 lb. ft. @ 3,500 r.p.m.

Two-Stage Carburettor Ensures Near Perfect Low Speed Top Gear Flexibility

A four-barrel two-stage Stromberg carburettor was matched to the engine, the main feature of which were the double inlet ports into each chamber. The primary inlet ports, each 1.108 sq. in. effective cross-sectional area, were provided to operate at the lower load and speed range, whilst the primary and secondary inlet ports - 2.217 sq. in. - become operative as load and speed rose.

Mazda Cosmo
Tsuneji Matsuda returns from his triumphant trip to the Tokyo Motor Show with two prototype Cosmo's. The welcoming group included Moriyuki Watanabe and Kenichi Yamamoto...
This port arrangement was the main reason the Wankel Rotary afforded excellent flexibility over a wide performance range, and in turn making the Wankel a particularly easy engine to live with day-to-day, particularly when you consider that top gear performance could be as low as 15 mph. on standard grade petrol.

Mazda claimed the Cosmo was capable of covering a standing quarter-mile in 16.3 sec., with 0 to 60 mph. in under 8.7 sec; top speed is 115 m.p.h. The ignition was by conventional coil system, but there were two plugs per rotor, which were spaced 1.18 in. before t.d.c. and 0.708 in after t.d.c. respectively. They fired a short interval apart, about 5 deg. at an idling speed of 700 r.p.m.

High Torque - Low Speed

The brilliant low speed high torque characteristics of the Rotary are easily seen when comparing the Cosmo with 2 European contemporaries of the time, the Mercedes-Benz 250 SE and the BMW 2000 TI. The more interesting figures are the shaft speeds at which maximum torque was developed as a percentage of the speed at which maximum power was developed and the percentage torque developed at 1,000 r.p.m. shaft speed.

The BMW 2000 TI (1,990 c.c.) developed its maxiimum torque at 62.1 per cent of the speed at which maximum power was developed. The Merrcedes 250 SE (2,496 c.c.) ddid the same at 72.8 per cent and developed 74.4 per cent maximum torque at 1,000 r.p.m. whilst the Mazda Cosmo (equivalent to 1,964 c.c.) developed maximum torque at 50 per cent of the speed of maximum power and developed 73.3 per cent of maximum torque at 1,000 r.p.m.

Understanding Rotary Displacement

A single rotor Wankel RC engine completed two thermo-dynamic cycles during every two shaft revolutions, so that a twin rotor Wankel RC engine was the equivalent of a four cylinder reciprocating piston engine with a swept volume per cylinder the same as the displacement volume of one chamber. Geometric considerations meant that the eccentric shaft, which was the equivalent of a crankshaft, turned at three times rotor speed, so that every thermodynamic phase extended over 270 deg. of shaft rotation.

It is a convenient measure of comparison to quote b.h.p. per litre figures for different engines, although the volumetric dissplacement of any positive dissplacement engine, reciprocating or rotary, is not a factor in power measurement. Even the FIA, the international organizaation controlling motor sport, agreed back in 1966 that it was logical to classify Wankel RC engines as twice the displacement volume of a single chamber multiplied by the number of rotors.

Mazda Cosmo
Beautiful and "almost" conventional ...

Design Details

The Toyo Kogyo twin rotor RC engine was manufactured by sandwich construction, and in some respects could be considered as two single rotor designs in tandem using a common eccentric shaft. The eccentric portions were staggered by 180 deg. for even firing intervals, and to keep the engine weight down. There was a staft used to carry two thin shell bearings at either end; these bearings were accommmodated in the stationary phasing gear for each rotor - a meshing ring gear being attached to each rotor.

The rotors also ran on plain bearings which were supplied with a copious amount of oil via drillings made in the shaft and housings. This generous oil flow was not a lubrication requirement, but served the purpose of cooling the rotors. There were radial drilllings from the distribution groove to the cavities within the rotor.

The earliest NSU Wankel RC engines incorporated special extractors for the cooling oil, but it was found that due to the planetary rotation of the rotor, that is, rotation about its own centre of gravity which coincides with the centre of the eccentric, and its simultaneous rotation about the output shaft centre, the oil was circulated and ejected without any external aid. Passing coolant oil through the rotors necessitated two sealing systems, one to prevent excessive loss of oil, and the other to separate the gases in the three chambers round each rotor. The gas sealing grid was, in fact, very similar to that developed by NSU.

The Gas Sealing System

It was essential that the whole grid of the Wankel Rotary engine not only made, but maintained completely uninterrupted contact over its primary and secondary sealing areas. The primary sealing areas were those between the sealing elements and the stationary housing, whilst the secondary sealing areas were those between the sealing elements and the slots, grooves or counterbores containing them.

Only when all these reequirements were met was there considered to be a "complete" sealing grid. Light springs behind every element ensured adequate contact for starting conditions; under all other conditions the requisite contact was assured by gas presssure. Lubricating oil fulfilled the most vital sealing of the grid in the Wankel RC engine, since neither the requisite primary nor secondary sealing areas could have a satisfactory metal/metal surface contact to seal without the presence of oil.

Another indispensible requirement of the Rotary design was that there had to be a pressure difference between the two sides of the grid, and the sealing grid needed to adjust itself automatically relative to wear. The sealing grid as developed for Wankel RC engines was a "complete" sealing grid rather than a "perfect" one, because the latter suggests absolute gas and/or liquid-tight sealing, which was not required, and more importantly would have been extremely expensive to manufacture. In common with the NSU-Wankel RC engine, Toyo Kogyo developed a "nearly complete" sealing grid in support of interchangeability. Three piece apex seals in carbon material were used in order to compensate for a certain amount of tip wear.

The Mazda Cosmo L10A Wankel Rotary Engine
The Mazda Cosmo L10A Wankel Rotary Engine. Without the gearbox the
engine weighed only 225 lb ...

Cooling The Rotary Engine

The cooling fluid circulated in an axial direction, turning in the end covers and making a short detour into the intermediate casing which sepparated the two centre housings. (In the NSU Spider engine, coolant flowed in a peripheral direction round the centre housing). The radiator also contained the oil cooling elements.

Thermal considerations dictated the size of the fan, water pump and, to some extent, of the oil pump. The type 0813 Wankel RC engine as fitted to the Mazda Cosmo followed closely reciproocating piston engine practice insofar as a spur gear drive for the oil pump-similar to the Hoobourn Eaton-and helical gear drive for distributors and fuel pump, are provided. The space between the distributors was taken up by an idler gear, and the customary vacuum advance and retarding device was incorporated. A triangulated V -belt drive was provided for the fan/water pump unit and the adjustable generator.

Conventional oil seals were incorporated at either end of the shaft, and, in addition, an oil flinger was inncorporated at the front end to make sure that no oil escaped at that end. Adequate provision for draining the oil from the seal back to the sump seems was made at the rear. Apart from the carburettor which was hidden underneath the pancake oil bath type air cleaner; the absence of any valves, accessibility was excellent. The shrouded fan and the simple ducting of the air from the radiator clearly demonstrated that the engineers from Hiroshima had an appreciation of worthwhile detail refinements.

The Mazda Cosmo Transmission

As previously mentioned, the Cosmo's transmission was conventional in every respect. All the gears on the third motion shaft ran on thin plain bearing sleeves; these bearings were, howwever, pressure lubricated by a small pump driven off the front end of the layshaft. A long third motion shaft was used, and reverse gear was overhung within the gearbox extension. The hypoid final drive reduction was 4·11 . to 1.

The rear" suspension was unusual in having half elliptic leaf springs clamped to a de Dion tube. Double articulated drive shafts had ball-type splines to permit telescoping with suspennsion movements without jammming under full torque transsmission. At the time the Cosmo was regarded as Mazda's high-tech luxury flagship. Today Mazda remain the only company still currently using rotary engines in its production vehicles, with the most famous being the RX7.

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Mazda Cosmo Specifications
Mazda Commercials
Mazda Production 1960 - 1979
Mazda History
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