How do oil filters work?


Early internal combustion engines did not use oil filters and, coupled with the poor quality of oil available at the time, vehicles required frequent oil changes. Eventually, the first full-flow oil filtration system was developed. Basically, this arrangement allowed for the oil to flow through the filter before it reached the critical working components inside the engine.

So far so good, but there was (and still is) a big caveat: The vast majority of pressurized lubrication systems found in internal combustion engines incorporate some form of filter by-pass to protect the engine from starvation under certain circumstances. A good example is very cold weather. In this situation, if the oil is too thick, it is allowed to bypass the filter. Oil can also bypass the filter when the filter is plugged. Because of these events, oil is sometimes not filtered, even when the engine is fitted with a full-flow oil filter.

In operation, oil enters the oil filter through a series of small holes on the outer edge of the base flange. The oil is then directed through the filter, eventually making an exit into the engine through the large center hole. Most modern oil filters are equipped with an anti-drainback valve. This is often some form of rubber membrane that covers the perimeter holes in the base flange. The membrane is forced aside as oil enters the filter case. When the engine is not running, the rubber membrane covers the holes. Obviously, the anti-drainback valves maintain oil within the filter. In turn, they prevent engine dry starts (when the engine is started with no oil).

Early oil filter designs

Early oil filter designs were based on a replaceable element fitted inside a metal housing. When changing the filter, one removed the housing, discarded the element, cleaned the housing, added a new filter and re-installed the assembly to the engine. By the mid-20th century, spin-on filters gained popularity. Here, the filter element and the cartridge are self-contained. You simply remove the works, discard it, and screw on a new filter during the oil-change process. Today, there’s been a move back to the earlier oil filter design. In this system, oil is filtered through an element contained within a separate housing, because the replaceable filter element may be more environmentally conscious than a spin-on filter. Keep in mind that today’s motor vehicles require far fewer oil changes than those of yesteryear.

Today’s oil filter types

There are many types of oil filters available today, and there are likely an equally large number of tests in which various filters are cut apart and diagnosed. The truth is, all oil filters are not created equal. The bottom line: You usually get what you pay for.

But are there any real differences between standard filters, high-performance filters, race filters and synthetic filters? Absolutely.

You have to first consider the mission of the motor vehicle. Case-in-point is a racecar. Here is something that will seldom, if ever, experience cold starts (in many cases, the oil is warmed prior to starting). Oil is changed frequently, simply because the engines are inspected and regularly disassembled. Oil in racecar engines was once far thicker than that found in passenger cars, but today it’s just the opposite. Racers have discovered the benefits of light oil.

Without going into detail, it’s not uncommon to find race engines filled with oil as light as zero grade. Racing filters are engineered to work with those oils. Some race filters are not fitted with drainback valves.

On the other hand, many racing oil filters are engineered with an internal media that is resistant to high temperatures and water levels in the oil that can plug standard oil filter media types. Many racing oil filters are engineered to provide high levels of oil flow with low restriction. Certain racing oil filters engineered for use in endurance applications (for example, 12- or 24-hour races) contain a different media that is designed to trap smaller contaminants.

Some race or high-performance filters are built with more robust cases to protect against damage from track debris. Heavier baseplates are also incorporated in some of these filters. This ensures that the filter body does not flex under high-pressure conditions. Some are constructed so they can be safety-wired to prevent accidental loosening. A few of the high-performance filters also incorporate rolled threads instead of cut threads to ensure the filter doesn’t strip during installation.

Paper or plastic?

Filter media is another difference. Some filters are engineered with synthetic filter media (rather than pleated paper-based media). The synthetic media is said to be capable of trapping small contaminants over a longer period of time (higher miles). Additionally, some synthetic filters include special blends of rubber for gaskets and drainback valves. The purpose? Like the filter media, they’re designed to last longer. Finally, some synthetic filters actually have larger (typically longer) bodies than conventional filters, which means they have more capacity. Because of these factors, some synthetic oil filters have service lives of anywhere from 7,000 to 25,000 miles.

As you can see, there are plenty of variables found inside oil filters. So what’s right for your car, truck, motorcycle or other vehicle fitted with an internal combustion engine? The answer is, it depends. You really need to carefully study the specifications of each filter to determine the suitability for your particular application. If your vehicle is newer, you should also give some serious consideration to the warranty. Some filters may not be deemed compatible by the vehicle manufacturer, and that’s important when it comes to a warranty claim.

The reality is, you might want to think twice before using high-grade synthetic oil and a top-of-the heap high-performance oil filter in a thrashed beater. And, similarly, it doesn’t make much sense to use the cheapest oil and filter possible in a collectable Ferrari. In the end, selecting a filter is much like selecting oil. Pick the one that best fits the application and your budget.

The filtered facts

  • The car oil filter required for the vintage six-cylinder Buick is far different from the one found on the 7,000 (or more) horsepower Top Fuel dragster. In both cases, the mission is the same: Keep the oil clean.
  • Filters are not created equal. The intended application for a given oil filter has a big impact on the design and engineering of the component. Internally, a racecar’s oil filter is far different from a passenger car’s filter.
  • In a typical passenger car engine, oil enters the filter at the filter pad, most often part of the engine block. From here, it circulates through the series of smaller holes in the outer perimeter of the filter. At this point, oil is forced through the filter element (from the outside, in) and eventually is forced toward the center of the filter (going back into the engine through the large threaded hole below the pointer).
  • When a filter does its job, internal engine components – such as the connecting rods, camshaft and valvetrain – are protected from contaminants that can cause major damage. These components are expensive. Picking the right oil along with the right filter can definitely promote long engine life.

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