Yamaha TRX850 Enthusiasts Forum

had a look on the factory pro site yesterday and they have gained a few bhp on using different bellmouths !! now i have two sets which i got from trixynut , the std blue ones and some short ones made of aluminium , so which one is best or is there no difference ?? the pod filters will go on both

The general thinking is that long bellmouths give better low-mid and shorter ones give better mid-top. MV Agusta had variable inlet tract lengths on its F4-1000 to broaden the spread of power.

Kayla's spot on (never met such an enlightened girl before ).

BUt it's more to it than one thinks. It's unfortunately not so simple to say that x mm longer gives y more BHPs. What I've understood, it has to do with standing waves and stuff (think flutes, or speakers/horns), and will differ quite a lot from engine to engine, depending on revs, airflow. This is something I'll try out next time on the dyno, make up a pair of adjustable bellmouths and test which ones gives the results I'm after (but I bet QT will come in here and explain things in detail any minute now )

HansJ wrote:Kayla's spot on (never met such an enlightened girl before ).

BUt it's more to it than one thinks. It's unfortunately not so simple to say that x mm longer gives y more BHPs. What I've understood, it has to do with standing waves and stuff (think flutes, or speakers/horns), and will differ quite a lot from engine to engine, depending on revs, airflow. This is something I'll try out next time on the dyno, make up a pair of adjustable bellmouths and test which ones gives the results I'm after (but I bet QT will come in here and explain things in detail any minute now )

Haha, thanks Hans I knew that A-level in physics would come in handy some day!

HansJ wrote: (but I bet QT will come in here and explain things in detail any minute now )

You called ?
heh..
Actually, cobby (cobbadiggabuddyblooo) said he was a sound engineer, so he probably has a better understanding about this than i do.

Resonance waves. Get a trombone. The slidey brassmusical instrument. Longer = lower note, shorter = higher note. Its got something to do with the way the air "pulses" into the inlet and the rate at which the piston and valves are moving.
Kayla is 100% correct heh. MV uses infinatly variable inlet tracts (trumpets) to smooth out their power. ALso having orbital valves helps a bit.
Yamaha has a variable inlet tract system. But it only has 2 positions.

Analogy time: (anyone notice i love analogies ?)
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Corridor full of people, closed door at one end. Door opens, people start to push through the door. Door closes. People pile up and push back a touch. There is a "pressure wave" that moves away from the door. It will return slightly, almost like a spring. If the door opens at the exact moment the pressure comes back, you can fit more people through the door. If it opens at any other time, it becomes non efficient.
Now imagine the trumpet and valve in your engine. The longer the trumpet, the longer the compression wave takes to hit the valve again. Good for lower RPM. Shorter trumpets are better for higher RPM.
Its a resonance frequency thing.
Hope i helped (or confused you)

Great analogy Quan. And yes different speaker cabinets are tuned to different speakers(like motors). Take the cover off a look at your speakers on your stereo. You will see a port hole, this is tuned to the resonate frequency of the cabinet(via size and length) and like your elevator analogy as the speaker returns it pushes this out through the port.
Just like the bellmouth what size and length may work for one motor may not be the optimum for another. You may tune the cabinet to give you a little more mid/high responce but ultimately because of the resonate frequency of the cabinet you may gain alot more power down in the mid/low range.
Just like exhaust pulse and size of muffler to dictate the ultimate power gains in which ever range. After fitting FCR'S last week and now looking at a suitable exhaust to complement this myself.. But I havent changed the air intake yet... I now put the question out there to all fellow trx-ians do different filters complement the choice of mufflers..Or am I nitt picking. But I'm sure all these may add up to a gain in h/power or torque

ive just had a look at the bellmouths in the air box and they are a good 3" long !! so it seems like a good idea to run the blue ones ,, how do you fit the pod filters to these ???

steve speed wrote:ive just had a look at the bellmouths in the air box and they are a good 3" long !! so it seems like a good idea to run the blue ones ,, how do you fit the pod filters to these ???

Well, me and others use ramair sockfilters. Problem with that choice is that the don´t provide filtering on the mainair etc.
Would be an idea buying aftermarket one´s that cover mainair jet from allens performance....20£ each.

Yeah, I used to notice this too on the FCR 41s I had on the Guzzi. The 3" long alloy trumpets I bought from Sudco to fit Unifilters or K&Ns on to would give very good low to mid response, and similarly the red shorter open bellmouths would give great mid to top end power.

Bob