24/02/2005
Understanding Oil Consumption in the Mazda RX-7
The question of how much oil a Mazda RX-7 uses is one that often sparks lively debate amongst rotary engine enthusiasts. At Rotary Performance, we believe in providing clear, definitive answers. The truth is, depending on the specific circumstances and how the car is used, both those who believe premixing is essential and those who rely solely on the factory oil injection system can be considered 'right'. This article aims to demystify the topic, explaining the 'why', 'who', 'how much', and 'what type' of oil when it comes to keeping your rotary engine lubricated and healthy.
Why Do Rotary Engines Need Oil Injection (or Premixing)?
The core of understanding oil consumption in a rotary engine lies in its unique design and operating principles. Unlike conventional piston engines, rotary engines operate on a cycle akin to a two-stroke engine. Every time a rotor face approaches the spark plug, combustion occurs. This is in stark contrast to a four-stroke engine, where the exhaust and intake strokes provide cooler periods, reducing the thermal load on engine components. In a rotary, heat builds up and is retained more readily within the engine's parts.
Adding to this thermal challenge is the apex seal. These seals are critical for maintaining compression within the rotor housing. They travel the longest distance of any internal engine component, performing a constant 'whipping' and 'wiping' action against the housing. This action, combined with the inherent heat, leads to significant wear over time. While advanced metallurgy and clever design have been employed to mitigate this, a thin boundary layer of oil is ultimately required to ensure the longevity of the apex seals.
From its inception, Mazda has utilised an oil metering system to automate the delivery of this crucial lubrication. A small quantity of oil is drawn from a dedicated reservoir (or the engine sump in some cases) and injected into the fuel and air mixture. This provides the necessary 'sacrificial' lubrication for the apex seals and other critical components.
However, meeting increasingly stringent emissions regulations has presented a continuous challenge for Mazda. To comply with these standards, the amount of oil injected has, at times, been reduced. This 'stingy' injection profile, especially when combined with driving styles that place higher demands on the engine, can lead to accelerated wear and premature engine failure. This is where the concept of premixing comes into play.
What is Premixing?
Premixing, in essence, is the act of adding a specific two-stroke or rotary-specific lubricant directly to your fuel tank. This supplements the factory oil injection system, ensuring that an adequate amount of lubrication reaches the apex seals and rotor housings, particularly under demanding conditions. For the layman, it's simply adding oil to your petrol to reduce wear.
Who Should Consider Premixing?
The decision to premix falls into three main categories:
1. Those Who Have Deleted the Oil Metering System
This is the most obvious group. Some owners choose to remove the factory oil metering system for various reasons: to simplify the engine bay, for dedicated motorsport use, or because the existing system has failed and deletion was the chosen solution. For these individuals, premixing is not just recommended; it is absolutely vital. Failure to premix with every tank of fuel will lead to permanent and irreparable engine damage. Given the critical nature of this system, we strongly advise against its removal, as it provides a valuable fallback for less demanding, everyday driving.
2. Owners Using Their Cars for Occasional Sporting Events
Many RX-7 and RX-8 owners use their cars for weekend track days, autocross events, or other performance driving activities. These events often involve sustained periods of high engine load and heat, which significantly increase the rate of wear. In these scenarios, adding a small amount of premix to the fuel is an excellent way to supplement the factory lubrication and provide an extra layer of protection. This includes participants in driver training days, club events, and even occasional drag racing or drifting.
3. Owners of High-Risk Models
Certain RX-7 and RX-8 models were known to have particularly conservative oil injection rates from the factory, making them more susceptible to premature wear. These include:
- 1989-91 RX-7s: These were the first RX-7s with electronically metered oil injection, and the oil flow was significantly reduced compared to earlier models. Automatic transmission variants were particularly prone to accelerated wear.
- 2004-11 RX-8s: Mazda strived to balance performance with emissions for the RX-8. This led to the adoption of a thinner oil, smaller apex seals, and a very low oil injection rate. The automatic transmission models from 2004-06 were at the highest risk. Premixing, along with a few other minor modifications, can effectively double the lifespan of an RX-8 engine.
How Much Oil Should You Premix?
The correct ratio of premix to fuel depends on several factors, including whether your car is turbocharged, the intensity of your driving, and the type of fuel you're using. However, we can establish some general guidelines based on typical oil consumption.
Under ideal street driving conditions, a healthy RX-7 or RX-8 will consume approximately one quart of oil every 1,500 to 2,000 miles. To determine a recommended premix ratio, we can use this as a benchmark. Let's assume an average fuel economy of 21 miles per gallon (MPG) and split the oil consumption difference at 1,750 miles.
Calculation:
1750 miles / 21 MPG = 83.3 gallons of fuel consumed per quart of oil.
This translates to 32 ounces of oil per 83.3 gallons, or approximately 0.38 ounces of oil per gallon. This equates to a ratio of roughly 330:1 (fuel to oil). Therefore, an ideal premix ratio for a street-driven RX-7 or RX-8 is approximately 4 ounces of oil per 10 gallons of fuel.
This 4oz per 10 gallons ratio is also recommended for the 'high-risk' models mentioned earlier (1989-91 RX-7s and 2004-11 RX-8s) due to their factory-limited oil injection rates.
Weekend Warrior / Occasional Track Use:
If you plan to participate in events that place higher demands on the engine, such as road course driving or autocross, increasing the premix ratio slightly is advisable. For these situations, a ratio of 2 ounces of oil per 10 gallons of fuel is a good supplement to a properly functioning factory oil metering system. This provides cheap insurance against increased wear during spirited driving. If you're already premixing at 4oz per 10 gallons for an RX-8 or 89-91 RX-7, you can stick with that.
Dedicated Race Cars:
Purely race-prepped cars often have the oil metering system deleted entirely. Furthermore, they may run alternative fuels like ethanol or methanol, which have different lubrication requirements. Premixing for these applications can be more of an experimental science. As a minimum, expect to use 6 ounces of oil per 10 gallons of fuel. For alcohol-based fuels, consider an additional 2 ounces per 10 gallons to counteract the drying effect of the fuel. Some racers opt for even richer mixtures, such as 1 ounce of oil per gallon of fuel. However, be cautious, as excessive oil can foul spark plugs. It's advisable to start with a richer mix and gradually reduce it while monitoring spark plug deposits. Excessive oily deposits indicate the mixture is too rich, while a lack of lubrication is far more detrimental than a slightly too-rich mixture.
What Type of Oil Should You Use?
For Gasoline Engines:
When choosing a premix oil for gasoline, keep these three key points in mind:
- Use a Dedicated Premix Oil: Avoid using standard 30-weight motor oil. While some older enthusiasts may recall using it, regular engine oil does not mix well with gasoline and can stratify in the fuel tank, leading to uneven lubrication.
- Synthetic Premix is Preferred: Synthetic premix oils offer consistent quality and performance. Products like Idemitsu's synthetic premix are highly regarded for their clarity, consistency, and low smoke output. The clarity is also beneficial as it avoids the dyes found in some oils.
- Avoid Lawn Mower Oil: Oils designed for line trimmers and leaf blowers, often purchased from hardware stores, work adequately but are optimized for air-cooled engines. They typically contain dyes that can discolor fuel system components and fuel jugs over time.
For Alcohol-Based Fuels (Ethanol, Methanol):
When your fuel mixture is over 50% alcohol, different lubrication principles apply. Alcohol has significantly lower lubricity than gasoline, requiring a specific type of lubricant. The standard for alcohol fuels is castor oil.
Castor Oil: This is a vegetable-based oil known for its excellent lubricating properties, especially under high heat and stress. A popular choice is Klotz BeNol. It's readily available, though it might be harder to find in physical stores. Be aware that castor oil has a distinct, strong smell that some associate with RC racing fuels. Due to the increased lubrication needs of alcohol fuels, it's wise to keep a good stock of castor oil if you regularly run these fuels.
Conclusion
Understanding your RX-7's oil consumption and the role of premixing is crucial for maintaining the health and longevity of your rotary engine. While not every driver needs to premix, for many, it's an essential practice to prevent premature wear, especially in specific models or when engaging in spirited driving. By following these guidelines on who should premix, how much to use, and what type of oil is best, you can ensure your rotary engine continues to perform at its peak for years to come.
Frequently Asked Questions:
- Q: Does my RX-7 automatically inject oil?
A: Yes, all factory RX-7s are equipped with an oil metering system that injects oil into the fuel and air mixture. - Q: When should I start premixing?
A: You should consider premixing if you have deleted the oil metering system, if you participate in track days or spirited driving, or if you own a model known for low factory oil injection (e.g., 1989-91 RX-7s, 2004-11 RX-8s). - Q: What is the ideal premix ratio for street driving?
A: For most street-driven RX-7s and RX-8s, a ratio of 4 ounces of oil per 10 gallons of fuel is recommended. - Q: Can I use regular 10W-30 oil for premixing?
A: No, it is strongly advised against using standard engine oil for premixing as it does not mix well with gasoline and can lead to uneven lubrication. Use a dedicated rotary or two-stroke premix oil. - Q: How can I tell if I'm using too much premix?
A: Excessive oily deposits on spark plugs are a primary indicator of too much premix.
Important Note: Always consult your vehicle's owner's manual and consider the specific recommendations from reputable rotary specialists when making decisions about your engine's lubrication.
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