Sprint Filter P08 F1-85 Air Filter for Racing – Honda CB350 (2021–2026) SM289S F1-85

The useful question about a racing air filter is not how much air it can pass, but whether the engine was ever asking for that much. A 350 cc single draws one charge every two turns of the crankshaft and reaches its ceiling early. The volume it wants in a second is modest by any standard. The P08 F1-85 passes 15,000 l/m²/sec where the standard P08 passes 5,050 — and it filters at 85 microns where the P08 filters at 80. That second number is a step in the wrong direction, and it is the currency the first one is bought with. On a Honda CB350 the flow is capacity the engine has no use for, and the filtration is something it does use.

Key Features

• 15,000 l/m²/sec from the racing medium, about three times what the standard P08 passes
• An 85 micron aperture — wider than the 80 microns of the road medium, and wider on purpose
• Polyester, dry, unoiled: no film can find its way onto the intake sensors
• Filtering surface runs to roughly double that of other performance elements
• The standard airbox takes it without modification and the ECU without a remap
• Cleaned with compressed air from the clean side and used again

When air flow actually becomes the limiting term

Flow begins to matter when the time available for each intake stroke gets short and everything downstream has already been opened out: high revs, a free exhaust, larger throttle bodies, mapping written to suit. Under those conditions the filter is genuinely the last thing in the way, and a medium that passes three times as much air is doing real work. A long-stroke 350 single sits at the far other end of that scale. The pressure drop across a clean standard element is already a small term in a small sum. Tripling the capacity of something that was never full does not appear at the throttle.

What the coarser weave costs on a road bike

Racing accepts 85 microns because a race engine is opened on a schedule counted in running hours. Whatever a wider aperture lets past is found at the next strip-down, long before it turns into wear anyone pays for. A CB350 is owned for the opposite reason: it is a motorcycle you ride and do not take apart, often for years. Filtration is the one thing an air filter is unambiguously for, and this medium hands some of it back. For a registered road bike the P08 version of this same element is the better purchase; where the air carries dust, the P14 at 14 microns and 1,800 l/m²/sec is better still.

Care and cleaning

Between sessions the SM289S F1-85 wants only compressed air, held to the clean side and blown outwards, then refitting dry. A wash is the deeper option: lukewarm water with a mild detergent, rinsed until the water runs clear, then dried right through in open air — never with heat, never near a flame. Oil is not part of the procedure, before washing or after. Sprint Filter’s maintenance sheet sits in the Downloads tab.

Fitment & Compatibility

Made for the Honda CB350 from 2021 to 2026. The same SM289S F1-85 element covers five machines built around this 350 cc single; the complete list is in the Fitment tab.