Air Box

[ty_steve]

Perhaps someone could explain the science behind an airbox, and why a simple air filter isn't adequate enough. I have a 76 TY175 and have read on the forums here that the stock airbox does not provide adequate flow and should be "opened up". After taking a close look at my airbox I agree it could benefit from some extra venting but am considering removing it all together in exchange with a filter - no box at all. As this seems all to simple I pause to wonder why it isn't more common place? Is there some science involved with the airbox or what??

Thanks from a newbie!

[shercoman2k8]

Perhaps someone could explain the science behind an airbox, and why a simple air filter isn't adequate enough. I have a 76 TY175 and have read on the forums here that the stock airbox does not provide adequate flow and should be "opened up". After taking a close look at my airbox I agree it could benefit from some extra venting but am considering removing it all together in exchange with a filter - no box at all. As this seems all to simple I pause to wonder why it isn't more common place? Is there some science involved with the airbox or what??

Thanks from a newbie!

The airbox protects the air filter, and air intake from direct mud and water contact, quite common on trials bikes... Acts as a barrier to keep crap away from the actual filter. If they're still putting airboxes on bikes today, they obviously have a use... therefore best keeping your airbox on!

[tony27]

There is a science to airbox design to a point, they can't restrict airflow too much & generally act to give a nice large area of still air for the motor to draw from.

I have seen photos of TYs running a pod filter but they have always been in dry countries, the owners put a plate in front of the filter & basically end up boxing it in to keep some of the crap off the filter.

The 250c I'll be riding next weekend had no airbox so we fabricated one that fills up all the area that the original airbox & oil tank took up, the biggest problem that came from doing this is that with the whole top of the airbox being open with the filter across it was that the bike was seriously lean on the main jet due to there being almost no restriction to airflow, disconnect your airbox from the carb & you'll see what I mean. Have a look back through previous postings & you should find the photos. The topic to look up is titled Airvalve on carb

[ty_steve]

Thanks Tony,

I realize it sounded like a daft question, but being new to motorcycles I'm trying to figure out a few things, namely the engine and carburetor. I have seen the pod filter set up and thought it was a very neat, tidy and simple solution. I like simple. As different people will take differing amounts pride in their machines I can see why an air box in a production bike is a must. If a manufacturer does not include the air box it's only a matter of time before one runs into problems. So, in theory if kept clean wouldn't a pod filter be a better solution than an air box?

[feetupfun]

Issue 1

The airbox prevents water/mud impinging directly on the filter media. If liquid water touches foam filter media, it will wet the foam, increasing the pressure drop across the filter. When the airflow rate through the filter media gets high enough, the liquid water will be drawn through. It may travel harmlessly through the motor, or it may cause problems by bridging the sparkplug gap, getting into the float bowl, or if enough gets into the motor in a short time, it may cause great mechanical damage to the motor by preventing the piston from going past top dead centre.

Issue 2

A pod fitted directly to the carby inlet (ie close to where the air goes into the carby) on a two stroke with piston skirt controlled inlet timing will become soaked with petrol, causing problems. This issue is not relevant to TYs as they have reed induction, greatly reducing the reverse pulse through the carby.

Issue 3

Some bikes like the TYs have the carby supported by the connectors/hoses. If you don't have the airbox hose holding the carby up, and make the carby even heavier by fitting a pod, the connector between the carby and the cylinder will be overloaded on landings.

[jse]

Thanks Tony,

I realize it sounded like a daft question, but being new to motorcycles I'm trying to figure out a few things, namely the engine and carburetor. I have seen the pod filter set up and thought it was a very neat, tidy and simple solution. I like simple. As different people will take differing amounts pride in their machines I can see why an air box in a production bike is a must. If a manufacturer does not include the air box it's only a matter of time before one runs into problems. So, in theory if kept clean wouldn't a pod filter be a better solution than an air box?

If you are looking for simplicity a pod will work. If you are looking for long-range performance a well designed airbox is probably better. Besides insuring better quality air, the airbox/boot system helps shape the incoming air mass in a way that allows the carb to make better use of it.

Some time ago I ran a series of dyno tests on several 600 SuperSport roadrace bikes and used several types of individual aircleaners (foam and cotton gauze)/tuned flared tubes (four carbs) and the stock airbox. Although the tuned intake tubes gave a slightly better top end HP reading, the airbox consistantly gave better low-end, out of the slow turn pull type of torque that wins races.

Jon

[stork955]

Gday, further to what the other guys are posting, airboxes only need the same size inlet as the carb and manifold in terms of area. So many people open these holes up for "extra breathing" that only upsets carburettion and allows dirt and water more entry places. If you think about it, the carb and inlet manifold are the restriction, so if the airbox can keep up with this it is sized correctly. Manufacturers dont get this wrong! Another thing is that all carbs exhibit a standing wave - if you watch an open one running there will be a mist of air/fuel just out of the inlet side - this is what feetup was referring to. It seems that the carb is working "in reverse". The inlet tract and airbox help control this. On an off-road bike there are no good reasons not to use one.

Cheers,

Stork