Myth busting: Do stock air filters really compromise engine performance?

As bolt-ons go, swapping out your stock air cleaner for a tasty performance filter is a pretty easy mod. But is it worth it?

Fortunately, replacing your stock air filter with an ‘upgraded’ performance item needn’t come at significant additional cost. As air filter renewal is a component of almost every bike’s service schedule, why not ditch the OE unit for a performance one? But hold on, surely if these non-genuine parts were any better than stock, the bike manufacturers would just use them in the first place?

We press-ganged the guys at Dynotech Ecosse into lending us their dyno for the day and we set about figuring out how to unclip all the plastic trim and lift the tank off a GSX-S750.  But before we scooped out the contents of the Suzuki’s airbox, we ran her up on the rollers to establish power, torque and air-fuel ratio’s for the stock air filter.

Suzuki may claim 113bhp, but we could only find 104.3 horsies hiding inside this engine; that’s the difference between claimed crank and true rear wheel horse power figures. One thing we did notice was how rich the bike ran at full throttle. Although not ideal for power (or economy) it is common to find overly rich fuelling on bog-stock bikes. Manufacturers do this as a safety precaution – the extra fuel helps to keep pistons and valves that bit cooler when the engine is at full chat. Plus, aftermarket exhausts and filters can tend to lean the fuelling out. Setting the mixture a little rich from the factory can give the engine a margin of safety.

K&N filters are one of the most popular choices for upgrading your filtration over standard. These replacement elements simply locate in place of the factory filter, with no adjustments or modifications necessary. K&N claim superior to stock filtration from their filters, along with improved airflow.  Whilst confirming the former is beyond the scope of this test, we did measure a modest increase in power; an extra 1.2 bhp became visible. The GSX’s fuelling curve also edged a little leaner; these two factors both indicate improved air flow over the factory filter.

Corrupted by the extra power, we got greedy for more. We pulled out the K&N air filter and fitted a rather tasty MWR filter element this time. Rather than provide simple plug-and-play replacements like the K&N, MWR filters are designed with optimum airflow and racing in mind. Their filters are designed to work in conjunction with airbox modifications to alter the way the intake system handles intake airflow. MWR’s instructions for the GSX-S were to junk the middle two long throttle body intake trumpets and fit new shorter trumpets as used by the outer two cylinders. As we couldn’t source these Suzuki parts in time for the test, I decided to cut the centre trumpets down, then ‘bell mouth’ the ends with a little heat and brute force….

The little Suzuki seemed to like the rough stuff, it was singing along nicely. The bike made a little more power again, peaking at a respectable 107.3bhp. An extra 3bhp over stock, a nice gain, making the 30 minutes of fiddling with the Suzuki’s airbox worthwhile I’d say. It’s also very likely that buying a couple of outer trumpets as instructed by MWR would have yielded a slighter better result. The Suzuki’s torque (and therefore power) curve responded as predicted with four (roughly) equally short intake trumpets: there was a very slight loss of torque between 5000 and 8000rpm, but the engine held onto its torque significantly better from 9,500 to the 11,500rpm redline.

The MWR filter along with the required airbox modifications had an additional impact on full throttle fuelling too, leaning out the mixture more than the K&N did. Although not too weak to cause an issue or hurt power on this otherwise standard Suzuki, other mods like an end can or especially a full exhaust system could see the mixture stray to the lean side of ideal. Weak mixtures not only hurt power and rideability, they can lead to engine damage such as burnt valves or melted pistons. Quite simply, if you intend to modify your ride to this extent, a fuelling remap either by re-flashing the ECU or fitting a Power Commander is definitely the order of the day.

On the road, I couldn’t feel a huge difference in power between standard and the K&N, but the Suzuki had an appreciably edgier induction howl with the K&N in place, plus the throttle response felt a bit choppier at low rpm. But the GSX-S has never been great in this regard.  Euro-4 compliancy is probably to blame – the bike runs pretty lean at small throttle openings and the higher flowing K&N was very slightly exacerbating this. However, the bike did feel more responsive to the throttle once over 7,000 rpm.

Riding the Suzuki with the MWR filter and shortened trumpets, was eye opening though. It’s newly acquired slight kick in the torque curve was evident on the road and its induction noise different; quieter overall but raspier near the redline. In fact, intake noise reduction is an important feature of some MWR filters. They claim an intake noise reduction of up to 8db for some of their High Efficiency filters. Very handy when you consider that quite a number of sports bikes exceed drive-by track day noise limits as standard. The Suzuki’s progress may have been a little more hushed, but the way the bike now revved-out was impressive. The Suzuki’s ‘retuned’ K4 GSX-R750 motor had got back some of the top-end rush Suzuki deemed unbecoming of their midrange naked. If nothing else, that counts as a win in my book. So, are aftermarket air filters worth the investment? The answer’s got to be yes, but just don’t expect to see a world of difference.  

Filter types…

Pleated paper:

Standard air filters for road bikes are almost always made from either dry, or oil-impregnated pleated paper. Sometimes they incorporate a flat wire-mesh to add mechanical strength and support. When new, pleated paper filters perform their function perfectly well, but over time dust particles block the element and it becomes an ever increasing restriction to airflow. The service replacement interval is a compromise between cost/convenience and filter performance.

Oiled fabric gauze:

Popularised by K&N, many aftermarket performance air filter elements are made from a washable oiled fabric gauze. Although frequently pricier than the OEM item, cost savings over the long term are possible because the element can be washed out and re-oiled, returning the filter to as good as new. The thin fabric, usually cotton, gauze is held in a pleated design with wire mesh.  As with paper elements, pleating increases surface area, effectively packing a large filter into a smaller space. Makers of this type of filter point out that suspending the trapped dust and dirt particles in the element’s oil, rather than within the material like a paper element, reduces pressure drop in use, allowing for superior filtration and extended service intervals. They still need cleaning though and servicing oiled fabric gauze elements takes a bit of common sense. They should not be washed in a flammable solvent, such as petrol and proper filter oil, in the correct amount must be reapplied once the element has thoroughly dried out from washing. Unless you actually want to set your bike on fire or fill your street with blue smoke.

Foam filters:

Off road machines invariably employ foam filter elements from the factory. Mainly because these bikes are exposed to really dusty operating conditions and foam filters have an excellent particulate retention to airflow profile – meaning they can catch a lot of dust without restricting airflow. Plus, they are robust enough to be washed out and re-used repeatedly. Foam filters are usually oiled, but some are designed to work dry. Oiling improves filtration quality at the expense of service time, not a huge problem with off-road competition machines, but stripping your air filter out on most modern road bikes every 1,000 miles is going to get old pretty quick. Foam filters are also popular with road racing teams, a thin foam element offers adequate filtration with minimal restriction to airflow and the reduced operating time between cleaning isn’t a problem when you’ve got a team of mechanics on staff…