The facts.
DescriptionThe whole story▼
Few motorcycles attract as much modification as this one. The H’ness opened Honda’s classic 350 family and is rarely left alone for long: bars, seats, exhausts, and sooner or later somebody suggests taking the airbox off and hanging an open cone filter on the throttle body. That change is not free. Intake noise is a homologated figure, the airbox is part of how the engine was calibrated, and an open filter usually brings a fuelling problem with it. The Sprint Filter P08 SM289S is the opposite kind of upgrade. It goes inside the standard airbox, in the standard position, and changes nothing anybody can inspect — no remap, no paperwork, nothing to explain at the next roadworthiness test. It is also honest about what it is: an element, not a power part.
Key Features
- Sits inside the standard airbox in the standard place, so induction noise and fuelling stay as they were
- Nothing has to be modified and nothing has to be remapped; homologation is unaffected
- Dry polyester with no oil anywhere on it — nothing to migrate onto the sensors downstream
- A calibrated weave, its threads from ø 22 microns upwards
- Filtering area roughly twice that of other performance elements
- Made to serve the motorcycle for its whole life, cleaned and refitted rather than replaced
The upgrade that leaves the calibration alone
An airbox is not simply a dust cover. Its volume, its inlet and the length of the tract behind it are part of how the engine was made to run, and the injection mapping was written around them. Take that away and the map no longer describes reality: the mixture goes lean where the airbox used to help, and what is gained in induction noise is usually paid for in throttle response. Replacing only the element sidesteps the argument. The P08 sits where the paper element sat, presents roughly double the filtering surface of other performance filters at 80 microns and 5,050 l/m²/sec, and asks nothing of the ECU.
Dry medium, because everything downstream is a sensor
The other tempting shortcut is oiled gauze, and it is the wrong one on a fuel-injected engine. Oil migrates — slowly, invisibly, always in the direction of travel — and downstream lies the metering hardware the injection depends on. The P08 has no oil to give up. Its filtration comes from polyester threads from ø 22 microns woven to a calibrated aperture; the 22 microns is the thickness of the thread itself, not the size of the openings.
Care and cleaning
Service it with compressed air, held to the clean side and blown outwards. If it wants more than that, lukewarm water and a mild detergent, rinsed through until the water runs clear, and then patience: open air, no heat, no flame. The SM289S goes back into the airbox dry, without oil of any kind — that step does not exist for this medium and never did. The maintenance sheet is in the Downloads tab.
Fitment & Compatibility
Made for the Honda H’ness CB350 from 2020 to 2026. The same SM289S element covers five machines built around the same 350 cc single; the complete list is in the Fitment tab.
DownloadsOwner's manual · brochure▼
FitmentWhich bikes this fits▼
- Honda CB350 (2021–2026)
- Honda CB350RS (2020–2026)
- Honda GB350C (2024–2026)
- Honda GB350 / GB350S (2021–2026)
- Honda H’ness CB350 (2020–2026)
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