Understanding Oil Pressure and Relief Valves

Maintaining optimal oil pressure is crucial for the longevity and performance of any internal combustion engine. Oil serves multiple vital functions: it lubricates moving parts, reduces friction and wear, cools critical components, and helps to clean the engine by flushing away debris. When oil pressure deviates from its intended range, either too high or too low, it can lead to significant engine damage. This article delves into the mechanics of oil pressure, focusing on the role of the pressure relief valve and how modifications, such as shimming, can affect its operation. We will also touch upon bearing clearances and their correlation with oil pressure, as well as compare two popular Melling oil pump models.

The Function of the Oil Pressure Relief Valve

The oil pump in your engine is designed to supply a consistent flow of oil under pressure. However, the actual pressure generated can fluctuate based on engine speed, oil viscosity, and the internal condition of the engine. To prevent excessively high oil pressure, which could damage seals, filters, or even the oil pump itself, most engines are equipped with an oil pressure relief valve. This valve acts as a safety mechanism. When the oil pressure exceeds a predetermined threshold, the relief valve opens, allowing some of the oil to bypass the main lubrication galleries and return to the oil pan. This action effectively limits the maximum oil pressure within the system.

Shimming the Relief Valve: The Impact on Pressure

The question of whether shimming the relief valve increases pressure is a common one among mechanics and enthusiasts. The answer is generally yes, shimming a pressure relief valve will increase the system's oil pressure. The relief valve typically consists of a spring-loaded plunger. The spring exerts a force that keeps the valve closed until the oil pressure overcomes this force. Shimming involves adding small washers or shims to the spring. This effectively preloads the spring, meaning it requires a higher oil pressure to compress it and open the valve. Consequently, the system will operate at a higher pressure before the relief valve intervenes.

However, increasing oil pressure by shimming is not without its risks. As highlighted in the provided information, a combination of tighter bearing clearances and increased oil pressure places a greater demand on the oil pump. The pump must work harder to force oil through these tighter clearances at the higher pressure. This increased workload can put additional stress on the distributor gear, especially if it's not designed for such conditions. Overloading the distributor gear can lead to premature wear or failure, a known issue with certain engine configurations, such as the 2.3 Ford turbo.

Bearing Clearances and Oil Pressure

The clearance between engine bearings (like main bearings and connecting rod bearings) and the crankshaft journals is a critical factor in oil pressure. These clearances are precisely engineered to allow a thin film of oil to flow between the rotating surfaces, providing lubrication and preventing metal-to-metal contact. The size of this clearance directly influences how much oil escapes the bearing at a given pressure.

Tighter bearing clearances mean less oil can flow through the bearing. This results in higher oil pressure being maintained within the system, as less oil is bypassing the intended lubrication points. Conversely, looser bearing clearances allow more oil to escape, leading to lower oil pressure, especially at lower engine speeds.

The provided text mentions that ".002 on the mains is slightly tight, but it will be fine." This indicates a bearing clearance of two thousandths of an inch. While this might be on the tighter side of the recommended specification for some engines, it's generally considered acceptable and will likely result in slightly higher oil pressure. The primary downside of overly tight clearances, as noted, is the increased risk of bearing damage if the engine experiences oil starvation or severe oil aeration. In such scenarios, the reduced oil flow can quickly lead to overheating and failure of the bearing.

Potential Downsides of Increased Oil Pressure

While higher oil pressure might seem beneficial, it can have several negative consequences:

• Increased Load on the Oil Pump: A higher relief pressure setting forces the oil pump to work harder, potentially reducing its lifespan.
• Stress on Distributor Gear: As mentioned, this is a significant concern. The increased resistance can lead to premature wear or failure of the distributor gear, especially in older or less robust designs.
• Potential for Seal Leakage: Excessively high oil pressure can find weak points in seals and gaskets, leading to leaks.
• Reduced Fuel Efficiency: The increased parasitic drag from the pump working harder can marginally impact fuel economy.
• Increased Oil Aeration: In some cases, high pressure can contribute to oil aeration, particularly if there are issues with the oil pickup or crankcase ventilation.

Protecting the Distributor Gear

The concern about the distributor gear is valid. The suggestion to drill a small hole in the plug behind the distributor gear is a common modification aimed at improving lubrication to the gear itself. This provides a direct and consistent oil feed to the gear teeth, reducing friction and wear. Running a bronze distributor gear and a solid roller camshaft, as the author does, are also excellent strategies for improving durability and reducing the risk of gear failure. Bronze gears are inherently more wear-resistant and can better handle the increased loads associated with higher oil pressures and tighter clearances.

Melling Oil Pump Models: Melling 10832 vs. Melling M83

The question about the difference between the Melling 10832 and Melling M83 oil pumps is a practical one for those looking to upgrade or replace their oil pump. Both are popular choices, but they cater to slightly different applications and performance characteristics:

The Melling 10832 is often referred to as a "high-volume" or "high-pressure" oil pump. These pumps are designed to deliver more oil flow at a given RPM compared to a standard OEM pump. They are commonly used in performance applications, racing engines, or engines with significant bearing modifications (like looser clearances) that require a greater volume of oil to maintain pressure.

The Melling M83, on the other hand, is typically considered a standard replacement or a "performance upgrade" pump that offers improved flow and pressure characteristics over a stock pump but may not be as extreme as a dedicated high-volume unit. It's a good choice for many street performance applications where increased reliability and consistent oil pressure are desired without the potentially detrimental effects of excessive volume on other components.

The key differences often lie in:

When choosing between them, consider your engine's specific needs. If you have significantly modified bearing clearances or are building a high-RPM, high-stress engine, a high-volume pump like the 10832 might be necessary. For a street-driven performance vehicle with stock or mildly modified clearances, the M83 often provides an excellent balance of improved performance and reliability.

Frequently Asked Questions

Q1: If my oil pressure is low, should I shim the relief valve?

A1: While shimming can increase oil pressure, it's often not the best first step. Low oil pressure can be caused by worn bearings, a worn oil pump, a clogged oil pickup screen, or the wrong oil viscosity. It's crucial to diagnose the root cause before resorting to shimming, which can mask underlying problems and potentially cause new ones.

Q2: How much pressure should my engine have?

A2: Recommended oil pressure varies significantly by engine manufacturer and model. Generally, a good target is around 10 PSI per 1,000 RPM, but always consult your vehicle's service manual for specific specifications. For example, many engines aim for 30-60 PSI at operating temperature.

Q3: Can a tight oil filter cause low oil pressure?

A3: A clogged or restricted oil filter will impede oil flow, leading to lower oil pressure. However, most modern oil filters are designed with a bypass valve that opens if the filter becomes too clogged, preventing complete oil starvation. Still, a dirty filter will reduce overall efficiency and pressure.

Q4: What is considered "aerated oil"?

A4: Aerated oil is oil that has become mixed with air. This can happen if the oil pickup is too close to the oil surface, if there are leaks in the suction side of the oil pump system, or due to excessive splashing and churning within the crankcase. Aerated oil has significantly reduced lubricating properties and can lead to increased wear.

Q5: Is it always bad to have slightly tight bearing clearances?

A5: Not necessarily. As discussed, slightly tighter clearances can lead to higher oil pressure, which can be beneficial in some performance applications. However, it does increase the risk of bearing damage during periods of oil starvation or extreme operating conditions. It's a trade-off that needs careful consideration based on the engine's intended use.

Conclusion

Understanding the interplay between oil pressure, the relief valve, and bearing clearances is fundamental to engine health. Shimming the relief valve is a direct method to increase oil pressure, but it must be approached with caution, considering the potential stress it places on other engine components, particularly the distributor gear. Careful diagnosis of any oil pressure issues is paramount, and modifications should be made with a thorough understanding of their consequences. Choosing the right oil pump, like the Melling M83 or 10832, also plays a significant role in ensuring your engine receives the optimal amount of lubrication for performance and longevity.

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