Clearances And Oil Weight
Different types of engines—street, drag race, circle track, naturally aspirated, and forced induction—are set up with different types of bearing clearances and ring gaps. They also operate at different temperatures, and all these factors affect which grade of oil is ideal for each type of engine. Since all oils get thinner as they get hotter, the key to selecting the right viscosity is oil temperature. "The oil temperature in an NHRA Pro Stock engine only runs about 100 degrees Fahrenheit, so these engines use a 0W-5 grade motor oil. The operating viscosity of a 0W-5 grade motor oil at 100 degrees is the same as a 20W-50 at 240 degrees, so oil temperature is a very important factor," Lake explains. "Another major factor is bearing oil clearance. The larger the clearance, the thicker the viscosity that's required. For example, a 500hp, 350ci engine that runs 220-degree oil temps can use a 10W-30 with 0.0025-inch bearing clearances, but that same engine needs a 15W-50 if the bearing clearances are opened up to 0.0030. The difference between a multi-grade conventional oil and a straight-grade conventional oil is the use of viscosity modifier additives. Straight-grade conventional oils do not use viscosity modifier additives, and as a result, they have a lower viscosity index. Synthetic oils break these rules. Synthetics tend to have an inherently high viscosity index, so synthetic oils can be multi-grade without using viscosity modifiers. For high-horsepower applications, it is important to stay away from oils that use a lot of viscosity modifiers since they tend to break down."
The growing popularity of oil coolers in OE engines reinforces the fact that oil temperature and oil life are closely related. All oils gets thinner as they get hotter, but oil can protect an engine beyond 300 degrees as long as the viscosity does not drop too low from the heat. The real question is: For how long can oil protect an engine at elevated temperatures? "For roughly every 20-degree increase in oil temperature, oil's useful life is cut in half, so for the sake of oil longevity, you want to keep oil temperature under control. Oil temperatures below 200-degrees won't burn off much fuel and moisture through evaporation, so low oil temps are also detrimental to oil life," says Lake. "Keeping your oil temp around 220 degrees is pretty much ideal because it's not too hot or too cold. It is important to realize that above 320 degrees your engine bearings will not be happy, as the bearing alloys do not like temps that high. Keeping your oil temp in the 220-degree range helps extend the life of your engine parts as well as the oil. Never forget that oil does about 40 percent of the cooling in your engine."
Synthetic vs. Conventional
Engine builders discovered many years ago that synthetic oils offer superior protection and lubrication than their conventional counterparts, but what is it that makes them so much better? "The simplest way to describe the difference is that a conventional oil is refined down to a group of molecules whereas a synthetic oil is built up to a very specific molecule structure. The process of building up the molecular structure allows a chemist to tailor the performance properties of the oil," Lake explains. "As a result, synthetic base oils feature improved thermal and flow properties that make them a better choice in extreme conditions. However, conventional oils still have a place in performance applications. Engines built with rope-style seals should run conventional oils to reduce leakage. Likewise, engines that see high levels of fuel dilution should also use conventional oils. That's because the best thing for that type of engine is frequent oil changes, and the lower price of conventional oil makes it more affordable to change the oil more frequently."
As a testament to the advanced science involved in formulating race oils in modern times, engine shops rely on different types of oils for different types of races on the calendar. In sanctioning bodies that allow running separate qualifying and race engines, qualifying oils are used to increase power output during brief qualifying sessions without harming the engine. Since these qualifying sessions are short in duration, the oil can be optimized for the sprint conditions of qualifying compared to the marathon conditions of a race. "Qualifying oils use special components that work best in these sprint conditions, whereas restrictor plate race oils are also unique in that they're used in engines that need to produce maximum horsepower yet also survive 500 miles at wide-open throttle. Fortunately, restrictor plate engines are restricted airflow engines, and that reduces cylinder pressure." Lake explains. "The constant high-rpm and lower cylinder pressure allows for the safe use of lower viscosity oils, which free up horsepower. Consequently, the bearing clearances and oil cooling system in these engines are designed to run 0W-20 down to 0W-10 grade oils. One of the issues with engines that run at wide-open throttle continuously is heat, and these low viscosity oils actually help to reduce heat generation. High viscosity oils generate more heat as they are pumped through an engine, so using a lighter oil helps to prevent excessive temperatures. Again, the chemistry employed in a restrictor plate engine oil is optimized for the unique needs of that type of engine."