The Problem ~ Noise vs. Performance
Modern motorcycle manufacturers have to comply with stringent noise emission regulations. This problem is much easier to overcome with water-cooled engines as they have the water filled jacket wrapped around the cylinders/heads of the engine and this absorbs and baffles a fair amount of engine noise.
The Triumph 790/865 twins are blessed with a beautiful air cooled engine and are inevitably noisier. Triumph have designed the engine with this in mind and added rubber soundproofing in the clutch casing to reduce clutch noise and anti-backlash gears on both camshafts which significantly reduce top-end noise, but further noise reduction is still required to pass the noise regulations.
So, if Triumph can't dampen the engine noise any further, they would need to reduce noise from other areas. There are two obvious areas where noise reduction can be achieved relatively easily if you're not too concerned with performance, the induction and the exhaust systems. Triumph did a typically efficient job on both. The Triumph twin airbox design is an excellent noise baffle device which holds the air filter and delivers air to the engine in the quietest possible way. The internal "baffle" and the external "snorkel" of the air box reduce induction roar to a whimper. The factory silencers are extremely quiet, annoyingly so to many owners and replacement is usually one of the first modifications a new owner will carry out to his bike. But if extreme noise reduction is what's required of Triumph to get the air cooled twins through the rigorous manufacturers noise emission tests, then they did an excellent job and we have our beloved twins.
Our objective was to remove the strangulation the airbox has on the modern Triumph twin and allow the engine to breathe and produce power with as little restriction as possible. To release this power we determined that we needed to:
- replace the airbox cover/snorkel with something far less restricting
- remove the complete internal airbox baffle, again to reduce restriction
- replace the factory paper air filter
We have instructions on how to remove the internal airbox baffle HERE
and our testing on air filters HERE
had determined which filters to use. So, on to number 1, the replacement of the airbox cover/snorkel.
The Triumph Twin Power "Breathe" airbox intake cover was designed and developed to be extremely free flowing and give the least amount of resistance to the air being sucked in to the airbox and through the filter, thereby giving the engine a chance with correct fuelling to produce as much power as possible from the given airbox design.
The initial design criteria were:
- A cover that reduces restriction and increases airflow as much as possible.
- A cover that was durable and was a direct replacement of the OEM unit, so it could be easily installed.
- A cover that was the same weight or lighter than the OEM cover/snorkel.
Many early experimental designs were tried, tested and then thrown away due to very little increase in airbox intake airflow efficiency. The main problem was the distance between the airbox intake cover and the side panel of the bike (Bonneville, Thruxton & Scrambler, less so with the America and Speedmaster). Some good designs with the side panel off the bike, showed little increase in flow when the side panel was in place, as the side panel became the restriction to flow due to clearance. So, number one of the initial design criteria had to be modified to:
- A low profile cover that reduces restriction and increases airflow as much as possible.
and the "Breathe" was concieved.
The base of the unit was the easiest part of the design as it only had to emulate the OEM cover. Three lugs for the bolts to secure it to the airbox and an inner profile that maintained a good seal and held the air filter in place. So the base of the initial prototype was CNC'd in aluminium with a flat top similar to the OEM cover with the snorkel removed, but with a larger central hole so it wouldn't restrict flow. This base could then be built upon with modelling clay to test various designs on a flow bench. The final design was made in to a mould, filled with car body filler and attached to the base to give a hard design that could be used in real world road testing on the bike. The hard design could then be changed if needed, in a subtle way by sanding.
As the design moved on we wanted to see how the finished item would look, feel and perform in plastic. So we used a silicone medium to make a mould and then used a syringe to inject cold liquid plastic in the mould. This DIY solution was fraught with problems. If injected too fast, the liquid plastic sloshes around and creates air bubbles which don't rise to the vent hole quickly enough before the liquid plastic starts to harden. If injected too slowly, the liquid plastic will harden while the mould is only half full and you end up with half a product and a solid syringe. As you can imagine this process was time consuming, extremely messy and costly as the liquid plastic is very expensive and we got through loads of it, but we did succeed. In the end we had one very good example which gave us exactly what we wanted, a realistic prototype for road testing.
In the final design stages we transferred the design to CAD ready for the CNC tooling of the final product. There was another change in shape we wanted to try, to give a tighter top radius to the bell mouth and a tighter opposing radius to the outside ramp. Before finalising the design and as we now had the design on CAD, we could use fast prototyping for a quick result. This involved using a printer that prints a liquid plastic, layer by layer until the design is complete. The finished fast prototype had quite a rough finish where the layers had built up, so it was sanded with wet 'n dry to a fine finish before air flow testing. Unfortunately, this design flowed less air so it was demoted to the failed prototype box.
We carried out air flow testing on a SuperFlow SF-600E flow bench. All the covers were tested, starting with the OEM cover with snorkel through to the final Breathe product, and the figures speak for themselves. The stand alone test jig was made from steel plate with a hole cut into it exactly the same size as the inside of the factory air filter and three threaded holes to secure the covers. The flow rate of the base plate on its own was 518cfm @28" H²O and the final Breathe design improved airflow over and above the stand alone base plate by 53% and 368% over the OEM cover/snorkel.
We know that Triumph Twin Power have developed and produced an airbox intake cover that will be hard to beat with regard to it's final design and air flow ability. It fulfils all four parts of the design criteria completely.
Have a look at the dyno charts on the right, they show various comparison traces. The first dyno is of an EFI 865 Bonnie in factory spec with Staintune reverse cone silencers fitted, this is compared with the same bike, but with the Breathe air intake cover, DNA filter and TTP EFI tune intalled. You can see from the dyno traces, the excellent mid range boost of 14% torque and power this combo gives the bike. The convergence of power and torque at around 6,500rpm is due to the air box baffle still being installed.
The second dyno trace is of an EFI 865 Bonnie running the Breathe intake cover, DNA filter, Mega silencers and TTP EFI tune compared against the same bike with the airbox removed and DNA pod filters fitted. It shows the two configurations are near enough neck and neck for power and torque up to around 6,000rpm and then the traces start to diverge as the pod filters ability to flow more air comes to the fore. It also shows a slight advantage between 3,500 - 4,500rpm of around 1ftlb of torque in favour of the airbox over pod filters.
The final dyno graph shows a comparison of what can be achieved with the with the Breathe cover and DNA filter installed. The traces are from an OEM factory bike with just the AI removed, and then two traces again from the same 790 bike with a TTP Breathe intake cover, high performance air filter, TTP Ignition Advance, Mega reverse cone silencers with the airbox baffle installed and the second, without the airbox baffle installed. As you can see from the dyno chart, the OEM 790 gave 54.43bhp and 40.86ftlb and when it was modified as listed above with the airbox baffle removed it gave 70.91bhp and 53.06ftlb which is an increase in bhp and torque of around 30% each. Even more impressive is the torque increase in the mid range. This is the area of the rev range you use the most in every day riding, for example when overtaking or when accelerating out of a bend. If you look on the dyno graph you can see at 4,500rpm there is a big difference between the OEM bike and the modified bike. The dyno figures show that at 4,500rpm the OEM factory bike is producing 35.8ftlb of torque and the modified bike is producing 53.05ftlb of torque, this is a massive increase of over 48% right where you need it. We think this is an excellent £$£$ per bhp improvement.
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