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Old 11-15-2010, 10:21 AM   #9
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Joined: Apr 2009
Location: STL, MO, USA
Oddometer: 1,392
Originally Posted by jscottyk
Does anyone have a diagram of the complete F800 secondary air system? Or a specific explanation of how it work on the F800 motor? I understand the principal of adding oxygen to the exhaust gases to make the for a more complete "burn off" of emissions, but I do not understand where on the F8000 motor this extra air is being introduced in the exhaust system.

There is no diagram of the system I'm aware of on the publically available service DVD, or the dealer only RSD.

There is however a great video complete with animations at BMW university, a web site that's only available to BMW employees. I have no idea why BMW keeps stuff like this secret, but they do.

In any case, YES, at the least the USA variant of the F800GS does have what is commonly known as a "second air" system. The F650GS does NOT.

The system intake is through the right front corner of the air box, so it is filtered air that is injected when the system is operating.

Next up is an on off solenoid valve. This valve is mounted to the right front of the air box. It mounts from the bottom and is clearly visible from the top of the air box. Secondary air would confuse the oxygen sensor were it flowing all the time, so this solenoid valve is switched on by the engine management computer (BMW speak=BMSK) when desirable and off when accurate O2 sensor readings are desired. Some have complained about the click this valve makes, some are louder then others, I've never seen this valve fail, the BMSK has trouble codes to communicate if this valve is stuck open, stuck closed, or unhooked.

Next down the line is a reed valve. This valve is bolted to the valve cover. In essence it is a light weight high speed one way valve. This valve blocks the channel where second air is injected into the exhaust. One side of this reed valve has fresh filtered air at atmospheric pressure any time the second air solenoid is open. The other side has exhaust pressure. Exhaust pressure is not constant. Right as the cylinder exhaust valve opens, pressure goes up and exhaust accelerates through the exhaust system. Exhaust has mass, as the last of the exhaust leaves the cylinder, exhaust with mass is still flowing at high speed through the exhaust pipes. Since the cylinder is empty and exhaust valve closing, there is no longer any inlet to support continued flow and the exhaust begins slowing. The mass of the exhaust that was flowing forms a partial vacuum as it decelerates. This vacuum opens the reed valve and draws in secondary air. This action along with tuned air box, tuned header and exhaust, and secondary air inlet not only reduces emissions by adding more oxygen to promote afterburning, it improves volumetric efficiency and therefore horse power. This is why you will NEVER improve HP and runability through airbox mods, filter mods, header mods, or fancy slip ons. You may increase noise, or peak HP, but always at the cost of mid range torque. No aftermarket manufacture has the wads of cash or engineering brain power to design componants that work better then BMW original equipment.

NOTES: The factory optional Acropovic pipe was engineered by BMW and will improve HP extremely slightly while reducing weight and creating a pleasing exhaust note that drooled out most of the clicks and rattles from the noisy creaky 603 engine.

You CAN increase power some by screwing with the fuel mixture but emissions will go to hell and deposits will build up in the combustion chamber as well as in the exhaust and of course, the O2 sensor will plug.


From the reed valve, the path of the second air circuit is through a channel in the valve cover into a channel in the head. For this purpose, there is a stand alone gasket between the valve cover and head. This gasket is entirely internal. If this gasket leakes, 2 things can happen. 1: exhaust pressure can enter the internal engine and raise crankcase pressure, likely leading to oil leaks. 2: oil can enter the second air circuit creating exhaust smoke and deposits in your exhaust system. For these reasons, this internal gasket should be replaced every time the valve cover is removed! Want to know if your dealer is adhering to this BMW reccomendation? Ask them if they stock this gasket the next time your in for a valve adjustment check. If they don't, I would politely decline to have your service completed untill they do stock this gasket. (note to self, check that my dealer does stock the second air gasket. Just kidding, we do :)
I believe not replacing the second air gasket is one of several reasons F800's develop more oil leaks then the F650's do, and the valve cover gasket is the weak point where the pressure creates oil leakage. Just as common the valve cover is warped, but in either case, using a whole tube of rtv to glue down the valve cover is not the answer ( insert video of bones saying "my god man! drilling holes in his head is not the answer, the artery must be patched" (startrek 4))

Last in the system are chanels in the head they exit right above one or both cylinder exhaust valves. I say "one or both" because I simply can't remember if there's just one or two channels. Sorry, perhaps I have a picture that shows the exits from the one and only rotax twin I've torn down. Will check and post the answer and link to picture if I do.

There you go, likely far more then anyone wanted to know about this system. Sorry for the super long post, I can't help myself. I LOVE this engine and all the super high tech details. Putting iPhone away now and going for a ride
Owned to date. Honda Aero 50, Honda Elite 80, Honda Elite 250x2, Suzuki Katana, Suzuki RF600, Yamaha YZF1000R, Kymco Xciting 500, Suzuki GS500, Suzuki Burgman 650, BMW F800GSx2, BMW S1000RR, Aprilia Scarabeo 200, Aprilia Caponord, Aprilia Sportcity 250
I love and miss you Jeneca and I'm sorry.
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