Rotary Engine FAQs
Yes. A rotary engine is a modified 4-cycle engine that recommends the use of an API-licensed motor oil for street applications.
Unlike in a piston engine, in which intake, compression, combustion and exhaust happen in the same space, a rotary engine relegates each of these four tasks to separate areas of an oval-shaped chamber, separated by the corners of a triangular rotor. The rotor moves around a spirographic path and forms three distinct volumes of gas.
In a rotary engine, the oil lubricates the eccentric shaft bearings, thrust needle bearings and rotor bearings (similar to a crank and rod bearing of a piston engine). It also is injected into the combustion chambers to lubricate the apex seals, corner seals and side seals, all of which helps to create the sealing mechanism (the equivalent job of the piston rings).
Royal Purple provides outstanding protection for the e-shaft, rotor bearings and thrust bearings and is suitable for the oil injection system, as it has proven to run cleaner than other oils and is an excellent choice for rotary apex seals, corner seals and side seals.
Royal Purple has performed seal compatibility testing on the components used in Mazda rotary engines with excellent results — including older rotary engine seals dating back to the Cosmo (mid to late 1960’s). Royal Purple’s former Technical Services Manager had owned and raced rotary engine cars using synthetic motor oils since 1985 with excellent results. Extensive research by the Royal Purple Technical Services department has been unable to determine a credible and valid technical explanation for this recommendation against the use of synthetic engine oils in Mazda rotary engines. Please note that review of vehicle owner’s manuals for the Mazda RX8 shows no warning or note about not using synthetic engine oils. In the early development of synthetic engine oils decades ago, there were purportedly some seal compatibility issues. Today’s synthetic oils do not have the compatibility issues of the old oils. There is no substantiated evidence of seal compatibility issues with Royal Purple.
Here are some facts:
- The Mazda Factory racing departments recommend and use ‘synthetic’ oils including the winning 1991 Leman’s 20-G 4 rotor Mazda 787B.
- The MazdaComp USA printed manual (now Mazdaspeed) recommends the use of synthetic oils for racing conditions.
- Royal Purple Motor Oils have been used in rotary engines (both race and street) for 10+ years with excellent results.
- Royal Purple Motor Oil is compatible with the bearing material, sealing elastomers and combustion seals used in a rotary engine.
If this was a problem with synthetic motor oils in general, then all internal combustion engines using a ‘synthetic’ would experience increased deposits on internal surfaces. The opposite is actually the norm.
Conventional four-cycle motor oils will typically leave deposits of carbon and ash when injected into the rotary apex seal, corner seal and side seal areas. Royal Purple’s motor oil burns cleaner because its synthetic base stock is free of contamination, and many of its additives are “ashless”. This may not be true for all synthetics but Royal Purple has been proven to work extremely well in rotary engines.
Royal Purple’s formulation of synthetic hydrocarbon motor oil burns at the nominal combustion temperatures experienced in both street and racing applications, whether normally aspirated, turbocharged or supercharged. (500 – 1700%°F idle to race rpms typical combustion temps).
No. Royal Purple’s motor oil is fully compatible with the elastomers found in rotary engines as well as more conventional piston engines. The oil seals, housing seals and other elastomers used in rotary engines typically consist of Buna N, Nitriles, Neoprene or Viton materials, which are also commonly found in piston engine cars.
No. If an engine’s sealing surfaces are in good condition, synthetic oil should not cause any leakage. However, if an engine has marginal seals, there is some chance the seals will leak. A synthetic motor oil is going to have similar viscosity to that of a conventional motor oil—except at extreme temperatures. Due to a flatter viscosity curve, at low temperatures it will not thicken as much (easier winter cranking) and will not thin out as quickly at higher operating temperatures (better oil film at higher rpm).
Royal Purple recommends that the maximum oil drain / filter change interval listed in the owner’s manual be followed while under warranty. For all applications that OEM warranty compliance is no longer a concern, the oil change interval can safely be extended 2 to 3 times what is listed in the owner’s manual, up to 12,000 miles/12 months for Royal Purple API-licensed synthetic engine oil or up to 15,000 miles/12 months for Royal Purple HMX, HPS and XPR synthetic engine oils. Since the rotary engine injects oil through the use of a metered oil pump, either adding oil into the carb base plate air / fuel mixture or directly injecting oil into the rotor housing, rotary engines will consume oil of one quart per 1,000 – 3,000 miles. It is important to periodically check and maintain the proper crankcase engine oil level in your rotary engine if you decide to extend oil drain intervals.
In an ideal world, the rotary engine metered oil pump should inject an ashless oil designed to burn in the combustion chamber and use a 4-cycle oil in the crankcase for the eccentric shaft, rotor bearings and thrust bearings. For the street, Mazda simplified the OE system to use just one oil—a typical 4-cycle oil for both the e-shaft as well as the combustion chamber. Royal Purple recommends using our standard HP 2-C if the metered oil pump is still enabled. The 2-cycle oil being added to the fuel tank is in addition to what Mazda designed to inject and acts as a supplement or insurance. Depending on the kind of engine, the level of modifications (street port, Bridgeport, peripheral port, nitrous turbocharged) and application, the typical mix ratio could vary from 200:1 to 800:1.
For a pure racing application in which the metered oil pump has been disabled or removed, again based on the actual engine and modification level, the ratio could vary from 150:1 to 600:1. For this application, we recommend our HP 2-C engine oil.
A stock FD twin turbo 13B with the MOP oil injection system can typically use about one quart per 1,500 miles under hard street driving. If this vehicle is getting 15 mpg, the gasoline to oil ratio is 400:1. If the oil consumption on this vehicle reduces to 1 quart per 2,500 miles and fuel efficiency increases to 20 mpg, the gasoline to oil ratio increases to 600:1. The stock metering oil pump is a great system as it varies with throttle position (load on the engine). Premixing has to be calculated for the “worst case” that will be seen by the engine for that fuel load. Under racing conditions, that’s wide open throttle at racing rpms. This means that at idle, the ratio may be slightly fat (rich).