Evolution of airbox covers, mods and bellmouths - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 140 (permalink) Old 09-18-2012, 03:19 PM Thread Starter
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Evolution of airbox covers, mods and bellmouths

Apologies, once again, for this over-long post(s). It's also humour-free I'm afraid...I just thought that I might as well do a complete review for future reference by new members.

I have resisted the temptation of carrying out airbox removal and substitution by pod filters on my EFI bike for over 3 years now.

I know there are huge performance gains to be had by doing so, but I always felt that a few simple changes to the existing airbox would net nearly as much power without, to my mind, afecting the integrity of the original layout. I had no proof of this, just the feeling gained by a few mods:

-Snorkel removal
-Some form of bellmouth or modded airbox cover (now TTP's "BREATHE")
-Restrictor plate removal
-A free-flowing DNA drop-in air filter element
-Relocation of the IAT (Intake temperature sensor)
-Relocation of the crankcase breather discharge hose (Details: http://www.triumphrat.net/twins-tech...ree-power.html)
-PJ Glassfibre's vented side covers

Together with:

-An Arrow 2-into-2 free-flowing exhaust which, incidentally, saves over 22 lbs from overall weight(!)
-Complete O2 sensor elimination
-NGK Iridium plugs (it's not much, but every little helps)
-Top quality fully-synthetic 10W-40 oil
-A 17 tooth gearbox sprocket to sharpen acceleration, hill-climbing ability and top gear performance.
-And last, but definitely not least, the essential custom ECU Tune map from TTP (Triumph Twin Power). I have the number 3 option. This takes care of the new injection and ignition parameters required for the after-market changes and O2 elimination (most important), but also incorporates stage 1 ignition advance, the difference that makes has to be experienced to be believed. A description on this post:


The way it now flies through the gears and revs freely tells me the existing, modified airbox is breathing well enough. There has been no loss of driveability, tractability or refinement, quite the contrary, the bike pulls from as low as 1800 rpm with commendable smoothness and vigour, together with close to standard levels of fuel consumption. Not bad considering that power output at the crank must now be nearing 80 bhp from the standard 67, without touching the engine internals. No dyno shops around here so I can't prove this, of course, although only last weekend I invited a local Bonnie T100 EFI owner to take mine for a quick ride and he didn't get back for over an hour with an empty gas tank and a wide grin on his face. He was astonished at the difference compared to his.

The fact that, for some reason, the EFI Throttle bodies and OEM airbox flows a bit better than the carb/airbox combination on older bikes has been confirmed for some time now by such experienced tuners as Carlos at Triumph Performance USA and Mike (aka Pieman) at TTP Triumph Twin Performance in the UK. They both agree that with basic airbox mods it comes to within 1-2 hp of complete airbox removal.

I realise that the frame/engine layout of our neo-classic bikes precludes the fitting of a decent-flowing airbox due to lack of space, unlike modern-framed bikes with their lateral twin-spar frames that enable huge, well tuned airboxes with acoustically tuned resonant chambers and cold-air or ram-air intake arrangements to be accomodated on top of the engines. Unlike our bikes there are not too many improvements that can be made to those. The engineers can design those airboxes for maximum performance together with more than adequate intake roar silencing features to comply with ever more stringent noise regulations.

Our airboxes are little more than a container that holds an air filter element snug and dry, silences the intake noise and still manages to accomodate a tiny and, frankly, inadequate battery and other electrical and hydraulic items nailed all around it. In fact the silencing characteristics are the most important to the Factory, see the rubber split-duct snorkel, multiple internal silencing ribs, and the infamous resonator plate, also known around here as the "restrictor". That's not its purpose, but that's what it does, it's a silencing device that robs you of around 3hp from 4.5k rpm upwards...

This is what it looks like:

Instructions for removal are all over this forum by doing a search, but Pieman has these on his site here:

Legend JJ likes this.

Last edited by Forchetto; 09-18-2012 at 03:50 PM.
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post #2 of 140 (permalink) Old 09-18-2012, 03:23 PM Thread Starter
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Until the arrival of TTP's "BREATHE", a properly researched and proven airbox cover to maximise air flow into the intake, there were all sort of tricks and devices to do this, none seemed to offer any conclusive proof of their effectiveness.

Here they are:

Some of us tried to modify the snorkel itself. The extra noise created probably fooled us into believing this was an effective measure:

This shows the "trumpet" portion of the snorkel cut off

This is with the inner part that fits inside the air filter element removed:

Most people ran with the snorkel removed altogether. There's some dyno evidence that this worked OK but an improvement could be made by opening the oval hole to the maximum permitted by the standard filter element. Here's such a mod:

It was noted that some sort of bellmouth could help in obtaining a smooth airflow into the airbox and that a rubber bellmouth from a Polaris snowmobile showed promise. I've had one for over two years and it seemed to work well. A few after-market sources exists for these and they can also be obtained directly from Polaris dealers. Sellers make great claims for these but offer no real proof. They cost around £24 from the likes of Jenk's Bolts.

Here's one fitted to the airbox cover, note that due to the thickness of the rubber the overall size of the opening remains the same as with the OEM snorkel:

You can see it here: http://jenksbolts.motoevolution.co.u...sCsid=flwfnmro

A member cleverly adapted a Rageon loudspeaker air-port. This needed the airbox cover enlarging and it's reasonable to assume it flows better than the Polaris, having a larger entry area.

Jenk's Bolts also offers this alloy, riveted-on bellmouth. I have not yet heard of anyone fitting this, at least I don't remember reading anything about it. Once again unsubstantiated claims are made but, looking at it, it seems unlikely seeing as it restricts the opening appreciably. It's also very expensive at £62+tax:

You can see it here: http://jenksbolts.motoevolution.co.u...sCsid=flwfnmro

Last edited by Forchetto; 09-18-2012 at 04:08 PM.
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post #3 of 140 (permalink) Old 09-18-2012, 03:28 PM Thread Starter
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I created and experimented with a unique combined bellmouth and cold air intake which gave me very good results, albeit only measured with the arse dyno...I thought it looked good as well. I was surprised when I looked at Triumph-online.co.uk site the other day to see that they're marketing such a mod and, on following the link to the instructions, it turns out that it is to my post on this forum describing the mod...nobody asked me!. This is a link to where it's being marketed:


See my write-up here: http://www.triumphrat.net/twins-tech...ir-intake.html

That, rather indiscrete mod, was rendered obsolete by the fitting of PJ Glassfibre's subtle and elegant vented side panels. Lovely, well-made bit of kit, it funnels lots of cold air into the intake:

More about it here: http://www.triumphrat.net/twins-tech...ide-panel.html

Incidentally, Peter has a new, updated website that's a bit easier to use than the old one, although it contains lots of apostrophe abuse...:


Up until now the most elaborate and expensive one was offered by Norman Hyde. This takes the form of a heavy and IMHO over-engineered alloy casting that substitutes the airbox cover completely and incorporates a bellmouth profile. It seems reasonable to assume that due to the work and investment in tooling needed to create this, proper airflow studies were made and indeed it flows better than most of the others according to Pieman's own research. Unfortunately on Normans site it just says that it "improves performance". No proof or details are given. Strangely, for EFI aplications it says that no re-mapping of the ECU is needed. This is somewhat suspect as any appreciable increase in airflow has to be entered in the tune map tables to gain anything. The cost is rather high at £76+tax.

See it here: https://normanhyde.co.uk/hinckley-tr...bellmouth.html

Mike at TTP (Triumph Twin Power) has studied most of these examples and as a result of long, painstaking research and considerable investment in time and cash, has come up with what I think is the definitive performance airbox cover. Not only does it flow better than all the others, and he has the figures and dyno charts to prove it, but it's much lighter and cheaper than the nearest rival at £59+tax, as well as flowing nearly 30% more air.

See it here:


As with the OEM cover, it's made of plastic, but of much better quality, finish and appearance. It maximises the airbox opening consistent with proper sealing around the filter element. Together with a DNA filter and restrictor removal he can show important power gains, specially when combined with the usual exhaust mods and a re-map of the ECU.

I can't say how this works for carbed models though, some re-jetting will be neccessary but, being an EFI man, I can't say.

As an added refinement that none of the others contemplate, he supplies a rain guard to be added to the side cover to stop any water getting into the airfilter. I did not fit mine for two reasons: 1.- I don't ride in the rain if I can help it and 2.- The PJ Glassfibre vented side cover wraps around the frame and airbox opening a lot closer than the OEM cover and stops any water getting in.

Note that the link given above also contains instructions on fitting and the history of its development, worth reading. There are also comparison charts showing its effectiveness and relevant dyno charts. More dyno charts are shown on a recent thread discussing the BREATHE airbox cover, here, see posts 20 and 30:


I've made my own comparison chart to show the percentage airflow gains:

Airflow figures in CFM (Cubic feet per minute)

Type of air intake---------------------CFM---------- % Improvement over OEM

OEM cover with snorkel....................215
OEM cover without snorkel...............422..................+96% over OEM
OEM cover with Polaris Bellmouth.....448..................+108% overOEM
Aluminium Bellmouth.....................623................. .+189% over OEM
TTP Breathe..................................793...... .............+268% over OEM

This is what it looks like fitted. Photo courtesy of the first member that has fitted one, Ric from Bristol (aka m2MQP):

Don't forget to transfer the sealing ring from the OEM cover to ensure no unfiltered air can get past the new cover:

I know that Pieman won't be too pleased with my carving up his cover to incorporate one of my favourite mods, but it was very easy given that the material is very easy to cut and the curved section is hollow, the cover being moulded in two pieces bonded together.

It involves the relocation of the IAT sensor to the entrance of the airbox rather than being buried in the depths of the airbox, behind the blind end of the air filter element, where it's over-heated and transmits erroneous temperature info to the ECU. I don't understand why the Factory has done this, I imagine they just placed the sensor in the same place as the carb heater thermostat was fitted on carbed models. Very poor this. I only have to look at where Mazda fitted their sensor to my MX-5 (Miata) to see that I'm on the right track, it's right on the end of the cold air intake ducting, right on the entrance to the airbox, where it should be, to provide accurate data.

The way ours is fitted works OK for the first 10 miles or so, beyond that the sensor attains a much higher temperature than the real air coming into the engine and not only weakens the mixture but also retards the timing to avoid detonation due to what it thinks is a high ambient air temperature. You can see this in real time using TuneECU or a hand-held code reader/scanner. This mod retains the same level of performance and driveability that's as good at the end of a journey as at the start.

If you fancy reading more about this mod there are two links to look at:



Last edited by Forchetto; 09-19-2012 at 02:21 PM.
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post #4 of 140 (permalink) Old 09-18-2012, 03:49 PM
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Hmmm, now that's probably your best yet!!! (And one of your most interesting, hehehe) writeups!

Thanks Forchetto - I was looking at TTP's "Breathe" the other day and was honestly quite awed by Piemans creation.

BTW if anyone is looking to pick up a Polaris Bellmouth please drop me a Private Message as I have three or four lurking around in my garage (leease: Trastero)


07 Thruxton: TEC 2 into 1, K&N, TTP Breathe Bellmouth, 135 mains, 40 pilots, std. needles, 1 shim (0.65mm). All baffles removed. Bitubo Fork Cartridges. Bitubo Rear Shocks. TEC Steering Damper. 3 Pot Nissin (Honda) brake caliper.
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post #5 of 140 (permalink) Old 09-18-2012, 05:01 PM
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Another incredible post! Thanks for memorializing this for the community!

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post #6 of 140 (permalink) Old 09-18-2012, 05:30 PM
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Freakin mastermind.
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post #7 of 140 (permalink) Old 09-18-2012, 05:37 PM
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Thank you Forchetto, it's like you can read my mind and answer my questions before I even know to ask them!
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post #8 of 140 (permalink) Old 09-18-2012, 08:52 PM
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I fell asleep but okay now. Nice job. I put the expensive
aluminum bell mouth on. It didn't want to run right with
just that. Ran rough. After some jetting and screw turning it
cleared up. Not very scientific but it had changed something.
Better? Worse? I don't know.

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post #9 of 140 (permalink) Old 09-19-2012, 12:03 AM
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Very well written post, enlightening even for us carb guys. I just ordered a Bella Corsa DIY airbox modification kit. I'll start hacking away at mine soon.
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post #10 of 140 (permalink) Old 09-19-2012, 05:21 AM
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This thread by Ventura has tons and tons of great information about successive airbox-mods, with hard data


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