Sprint Filter P08 Air Filter for Road & Track – Royal Enfield Bullet 350 (2023–2026) CM231S

Two numbers describe an air filter, and they do not lead the same life. Filtration is the first: 80 µm for this medium, a property of the cloth itself, unchanged by anything that happens afterwards. Flow is the second: 5,050 l/m²/sec, measured on a clean element, and it begins to move the first time the engine runs. Nothing an owner does alters the first figure, and everything an owner does shows up in the second. That is the case for the Sprint Filter CM231S on a Bullet 350: a dry polyester element for the standard airbox whose flow figure is restored with an air line instead of a purchase.

Key Features

• A single woven layer of dry polyester: thread from ø 22 microns, aperture set at 80 µm
• Rated at 5,050 l/m²/sec, the medium Sprint Filter designates for road and track use
• Dry from the first fitting onwards — no oil to measure out, none to travel downstream
• About twice the filtering surface of other performance elements, inside the standard housing
• Sits where the paper panel sat: no cutting, no adaptor, no remap
• Cleaned and refitted for the life of the motorcycle, with two years of warranty from purchase

The figure that only ever climbs

A paper panel is at its most open on the day it goes in and never sees that state again. Every hour of running adds a little resistance, the rise is far too gradual to feel from the saddle, and the curve ends only when somebody buys the next one. The later part of each interval is therefore spent on the wrong side of a number printed on the box as a single value. Woven polyester differs in one respect, and it is the respect that matters here: the rise is undone rather than thrown away.

What the second figure does not buy

Not power. A drop-in element does not make a standard 350 single noticeably stronger, and a description promising otherwise has stopped describing a filter. This engine is fed by an airbox, throttle body and fuelling map signed off as a set, and the panel in the box was never the tightest point in that chain. What the CM231S offers is narrower and easier to verify: an intake resistance that returns to its starting value whenever the owner decides it should, and one line of the service list that stops repeating.

Care and cleaning

Compressed air on the clean face, worked outwards, is the whole routine most of the time. When the element is greasy rather than dusty, lukewarm water and mild detergent shift what air will not; rinse until it runs clear. Drying happens in open air and takes as long as it takes; heat guns, radiators and flame all damage polyester. It goes back in dry, with nothing sprayed on it. The maintenance sheet for the CM231S is in the Downloads tab.

Fitment & Compatibility

Cut for the airbox of the Royal Enfield Bullet 350 built from 2023 to 2026. Sprint Filter lists five machines against this element: alongside the Bullet, the Classic 350 of 2022 to 2026, the Goan Classic 350 of 2025 to 2026, the Hunter 350 of 2022 to 2026 and the Meteor 350 from 2021. All five appear in the Fitment tab; check your model year before ordering.