Sprint Filter P08 Air Filter for Road & Track – Honda CB350 (2021–2026) SM289S

A 350 cc single takes one breath every two turns of the crankshaft. In between, the column of air in the intake tract stops moving altogether and has to be set going again from rest. That is a different job from the one a four-cylinder hands its filter, where overlapping intake events keep the air moving almost without pause. It also decides where a restriction is noticed: not at the top of the rev range, which this engine barely visits, but in the first fraction of throttle where the bike spends nearly its whole life. Nothing sold in a box makes a standard CB350 meaningfully quicker, and the Sprint Filter P08 SM289S does not pretend to. What it offers is an intake restriction that starts low and stays low between services, in an element cleaned rather than replaced.

Key Features

• Slots in where the paper element sat — nothing cut, nothing remapped, homologation untouched
• Woven from polyester threads of ø 22 microns and upwards to a calibrated aperture
• Dry from the day it is made; with no oil on it, nothing can creep downstream onto a sensor
• About twice the filtering area of a typical performance element
• A compressor and a few minutes; water only when a season has earned it
• Dimensioned to stay in the bike for as long as the bike lasts

Why one big cylinder is harder on a filter than four small ones

Air flow through a filter is usually quoted as an average, and averages flatter a single. Because the intake happens in separate gulps rather than in a continuous stream, the speed of the air crossing the medium during each gulp is far higher than the average suggests, and a restriction acts on that peak. It is why a partly loaded element can still look respectable on paper while the engine picks up more slowly off a closed throttle. The P08 answers with area rather than a coarser weave: roughly double the filtering surface of other performance filters, so the same volume crosses it more slowly, at 5,050 l/m²/sec through an 80 micron medium.

What a stable baseline is worth here

A paper element is at its best on the day it is fitted and slightly worse every day after. The slope is gentle, which is why nobody notices until the element is changed and the bike feels different for a fortnight. An engine with modest output has no surplus to hide that drift in. Servicing a woven polyester element returns it to its own baseline instead of letting it wander, and the dry medium leaves no oil film to reach the injection sensors.

Care and cleaning

Routine service is compressed air: from the clean side outwards, so the dirt leaves the way it arrived. When more is warranted, lukewarm water and a little mild detergent — rinse until the water runs clear, then leave the SM289S in open air until it is dry right through. No heat, no flame, ever. It goes back in dry and unoiled. Sprint Filter’s maintenance sheet is in the Downloads tab.

Fitment & Compatibility

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