I have been wondering what is the exact purpose of the airbox snorkel? I know it has an effect on reducing induction noise. I have been doing some testing with a fuel booster while fitting new pipes and a de-cat (detailed here - Verdict On Fuel Booster) and have come to the conclusion that it also has an effect on low to midrange power. This seems to correlate with info I have found that suggests a longer intake tube helps boost torque at the low to mid rev range. Looking at the shape of the snorkel on the Speed Twin it seems to restrict the flow of air into the airbox and extends well into the airbox beyond the edge of the filter itself. Also, the air is pulled from right behind the throttle bodies and cylinder head. I thought this might have two influences, the flow is reduced constraining top-end power and the air is warmed from passing by the engine leading to a slightly higher temperature being read at the temperature probe in the top of the airbox under the back of the tank and it's not fully utilizing all the available flow of the air filter. I have done short test runs with the snorkel in place and with it removed since the de-cat install and have noticed the following results. With the snorkel in place the low to mid range is robust but the top end feels a little restrained compared to runs with the snorkel removed however the mid range doesn't seem as robust but the engine definitely feels like it's flowing more freely and revs faster from 4k up. I've decided to re-install the snorkel and have drilled two 1 inch holes in the part inside the airbox and a 1 inch hole on the side of the snorkel to allow a faster route to the airbox. Has anyone else done any work on their Speed Twin they can share what they've done and the results they feel they have achieved? I'm guessing the next obvious step is a remap, and I'd rather not do that until I have no other choice with the warranty as the constraining factor.
You are in danger of asking the impossible because everything works in concert to a specific purpose. The snorkel will variously suppress induction noise, keep the paper filter dry etc., but it probably also plays a role in velocity as much as outright volume aligned to throttle body sizing right through to exhaust gas pulses. You can experiment but you might find a degree in thermodynamics and mechanical engineering is a good place to start!
Snorkels/induction box volume are a dark art/science, and are tuned to a particular motor and desired ridability. Better add fluid mechanics knowledge into the mix, along with the realization that the really high velocity air column entering the head also bounces backwards through the throttle body when the intake valve slams shut, producing what is known as "fuel stand off" which can be visible droplets of fuel hovering sometimes over a downflow carb. Induction system. Cold air is always good. Butt dyno is not very reliable, much less repeatable.
Considering all the variables isn't as easy as it used to be in the days of carburetors or even early DFI to tune the engine to get better power delivery because you had more room to play. Bikes were not engineered with fuel efficiency and emissions as a primary focus, so it was much easier, but now because of the need to comply with these newer standards the actual effect you can make is reduced, and the changes have to be more specific to have the desired result. In the end it probably comes down to the feeling that you've made an improvement and made the bike sound better, or easier to ride with regard to snatchy throttles and unpredictable throttle response rather than outright power gains. If you feel good about the changes, then it's probably worth the effort.
Blue trace is stock, red trace is without the snorkel. Power loss below 3300 rpm, increases for the next 3,000 rpm and then no gains (or losses) from 6250 upwards. Guess it depends where you feel it would suit you best. There's certainly more to gain than lose by running without it.
Wow, those charts tell a story alright. I may have to adjust my butt dyno and include some timed runs. Thanks for sharing the info. Bristle eh? I used to live off Wells Rd near St Nicks.
I’m with T100rider, what are the other lines??? Super intrigued cause I’ve been having debates with myself for the last year+ on what to do about the snorkel.
The upper green line is the increase after a Dyno remap, the other two are for an Xpipe and free flow silencers although I can't recall which is which. When I saved the graph a few years ago I didn't save the accompanying text and have not been able to find it since. However this graph is for a 1200HP engine and the differences with snorkel removal and where and how they fall, as has been found with the Xpipe, are likely to vary between capacity and model. So for some engines like the T120, dependent on riding style it may actually be more beneficial to retain the snorkel than remove it..For the Speed Twin (and Thruxton) as a standalone modification for more power its almost a total win win.
