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The difference between the various types of lube oil filters available today can be confusing. There are conflicting views in the industry as to which is the best type to use. The three most common lube filter designs are full-flow, by-pass and combination.
Full-flow filters are the most common type, receiving and cleaning all the oil before it’s pumped through the engine. Providing essential engine protection, full-flow filters must capture and retain damaging contaminants and have adequate dirt holding capacity to achieve the required service intervals. Synthetic media such as Donaldson Synteq™, provides finer filtration, low initial pressure loss, and increased dirt holding capacity over traditional cellulose filter media. Full-flow filters may also incorporate a by-pass valve.
By-pass filters do not supply the engine with clean oil directly but sit off to the side as a separate circuit or as a kidney loop unit generally accepting only a very small portion of the total oil flow. A by-pass filter has the added advantage of increasing the oil capacity of the engine and incorporates highly efficient media to polish the oil before it returns directly to the sump. This process improves oil cleanliness without compromising the oil flow through the engine.
Some manufacturers have introduced two-stage (or combo) filter designs in attempts to combine the features of both a full-flow and by-pass filter into one unit. The two-in-one design significantly increases restriction, causing shorter filter life and decreased cold flow performance. Poor cold flow performance may result in by-pass valves remaining open for longer, depriving the engine of clean oil during start-up and leaving the engine temporarily unprotected. This may lead to increased engine wear that could result in premature repairs or even engine replacement. Another downside of two-stage filters is that the more efficient by-pass portion of the filter inherently has higher restriction, initially driving most of the flow through the lower-efficiency full-flow media. This translates to longer periods of time delivering dirtier oil to the engine. It is only after sufficient loading with contaminant that the flow through the higher-efficiency by-pass media will increase.
Two-stage filters are often incorrectly referred to as by-pass filters and should not be confused with lube filters that have an integral by-pass valve or that are kidney loop style by-pass filters.
Engine lube systems require by-pass valves to ensure the flow of oil from the sump to the engine is not interrupted. These valves may be built into the filter head or form part of the filter itself. Regardless of its location, a by-pass valve should remain closed for the greatest amount of time ensuring that the full flow of oil is cleaned by the filter. By-pass valves are intended to open on cold startup when the oil is thick and heavy or when the media has become plugged with contaminants that it struggles to accept the full-flow of oil required by the engine.
Every oil filter needs to effectively balance three characteristics: efficiency (contaminant removal), capacity (contamination holding ability), and restriction (resistance to oil flow).
Donaldson full-flow filters incorporating Synteq media technology are extremely efficient high capacity elements that minimize cold start pressure loss. These filters provide the cleaning power of high-efficiency by-pass filters without sacrificing filter life or increasing restriction.
Two-stage or combo filter designs split the oil flow between two media packs and suffer greater cold start pressure loss.
No matter how you stack it – Donaldson full-flow filters provide better engine protection and greater filtration efficiency than two-stage filter designs.
Understanding the difference between common lube filter designs and how they work can help you select high-quality lube filter elements providing the optimal balance of efficiency, capacity, and restriction.