FILTERING OUT THE TRUTH
MYTH: DO STOCK AIR FILTERS REALLY COMPROMISE ENGINE PERFORMANCE?
Last month we focused the Bull Busting microscope on additives; this month it’s air quality. Namely that little bit of paper or gauze designed to catch the flies, dust and other crap that would otherwise wreak havoc with your motor.
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?
Get your tank off
Once again 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 our long- term 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.
Extra oomph
K&N filters are easily the most popular choice 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. While confirming the former is beyond the scope of this test, we did measure a modest increase in power; an extra 1.2bhp became visible. The GSX’s fuelling curve also edged a little leaner; these two factors both indicate improved airflow over that provided by 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, resigned to the fact that dyno testing is part of life on the FB long term fleet. 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.
Leaner and meaner
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&Nwas very slightly exacerbating this. However, the bike did feel more responsive to the throttle once over 7,000rpm.
More character
Riding the Suzuki with the MWR filter was eye-opening though. Its newly acquired kick in the torque 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. And for that simple reason I’d argue filters are a worthy investment.