BMW R850 Maintenance Manual page 88

If you plot these torque and horsepower curves you will notice a much smoother
power delivery across the RPM range and an average power increase of 2.28
horsepower from 3000 to 6000 RPM – right in the meat of where most of us ride.
But would the dyno results pan out on the road? I purchased and installed a set
of these manifolds to find out.
Part numbers and cost for the GS tubes (manifolds) are:
Left: 13 71 1 341 405
Right: 13 71 1 341 406
A reasonable sum, I'd say!
The GS manifolds are 247 mm long and have a 47 mm inside diameter. They
are MUCH longer than the RS variety which measure 127 mm and have a 51
mm inside diameter. You can't confuse them!
To install the new tubes you will find it easiest to remove the throttle bodies from
the rubber cylinder head intake spigots. Loosen the two clamps that secure each
end of the RS intake tubes and then loosen the spigot clamp. Push the
tube/throttle body assembly slightly into the air box to achieve separation of the
throttle body from the spigot. It's now easy to separate the throttle bodies from
the intake tubes and replace the tubes with the GS types. Be careful to align the
non-symmetrical GS tubes with their casting indices aligned up and down with
the throttle body casting marks. The GS tubes are also (as the part numbers
indicate) different for left and right as opposed to the identical RS tubes, which
may be interchanged. Reinstall the throttle bodies into the spigots, observing the
index "tang" alignment. Double check the tightness of all six clamps you
loosened to do the installation. Following installation, perform normal throttle
body synchronization though the adjustment may hardly change. By the way,
this modification will not affect "Zero=Zero" adjustment of the Throttle Position
Sensor if previously performed (re: "Bench Wrenching", BMW Owners News,
May 1997).
So how does this $35 "hop-up" perform? Here are the plusses and minuses:
First, Jon Diaz (who performed the mod to back up my data)...
"Rode to work today. Bike seems livelier from idle to 5K......really seems to jump
up off the bottom. One big thing I noticed was the increase in intake noise....even
with earplugs in, its a lot louder."
$13.99
$19.40
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