After reading Rob Lentini's email about his experiments swapping out R1100RS intake tubes with GS tubes, I decided it was worth a try. I don't have a dynamometer to check the results, but after a recent 6400 mile trip, I would agree the GS tubes give the bike more grunt in the mid-range. However, there is definitely more intake noise--nothing unpleasant, just noisier. I didn't swap out RS the cat-code for a GS as Rob has also suggested. Thought I'd try one thing at a time.
Rob is a current Director (1998) in the BMW Owners of America (BMWOA) . Here is an excerpt from his message on the Internet BMW Riders' (IBMWR) group list:
"When I learned about a simple way of getting almost 5 more peak mid-range horsepower from my '94 R1100RS while increasing and smoothing power delivery from 3000 to 6500 RPM for less than $35, I was all ears! Here are the facts:"
"Anthonie Mans posted an interesting test on the Internet BMW Riders/Oilheads lists performed by "Das Motorrad", a Euro motorcycle magazine. All that was done to a stock R1100RS (same engine as in the RT) was to give it a dynamometer baseline run and then replace the intake manifolds (black tubes connecting the airbox to the throttle body intakes) with those from a GS. Then they dyno'd the RS again. Here were their reported results in Newton Meters of torque and Metric Horsepower:"
"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."
Rob also has a technical article about the GS tubes on the Internet BMW Riders' website at: http://www.ibmwr.org/r-tech/oilheads/R11pwmod.shtml
Here is the dynamometer chart from Das Motorrad:
| RPM | Torque (GS tubes) | Torque (RS tubes) | HP (GS tubes) | HP (RS tubes) |
|---|---|---|---|---|
| 2000 | 86 | 87 | 24 | 25 |
| 2500 | 85 | 86 | 29 | 30 |
| 3000 | 92 | 89 | 41 | 39 |
| 3500 | 90 | 89 | 44 | 43 |
| 4000 | 92 | 83 | 52 | 47 |
| 4500 | 96 | 96 | 62 | 62 |
| 5000 | 102 | 99 | 73 | 72 |
| 5500 | 106 | 102 | 82 | 80 |
| 6000 | 101 | 96 | 86 | 81 |
| 6500 | 92 | 92 | 87 | 87 |
| 7000 | 86 | 91 | 86 | 90 |
| 7500 | 79 | 85 | 84 | 90 |
Not only is the modification inexpensive, it is also a very simple process. I thought I'd show some pictures to make it interesting.
Note: Pictures were taken with a Apple QuickTake 200 .
Click on image to view full-sized. It will open in a new browser window.
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PARTS LIST
TOOLS AND SUPPLIES NEEDED
Notice the GS tubes are not only smaller in diameter, but also much longer than the RS tubes. |
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Loosen the three clamps on the throttle body and intake tube. |
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Disconnect the front of the intake tube from the throttle body and slide the tube back into the air box. |
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Disconnect the throttle body by pulling it back and out of the heavy rubber (throttle body-to-head) gasket. There is no need to remove any of the connections (throttle cables, TB position sensor wire, or fuel line) from the throttle body...you just need to be able to move the unit out enough to make room for the tubes to slide past. |
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Remove the RS tube and insert the new GS tube. The old clamp will work on the new tube. |
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Attach the throttle body to the cylinder being careful to line up the tab on the throttle body with the notch in the rubber sleeve. |
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Attach the intake tube to the throttle. Insure the throttle body's rubber o-ring is not twisted and the alignment marks match on the tube and throttle body. Tighten the clamp on the air box at the rear of the intake tube. |
Repeat with the other side.
Check the throttle cables to insure they have not slipped out of their pullies. Check the throttle body synchronization (take a look at my page on the TwinMax).
File Creation Date: August 18, 1998
File Last Modified: November 13, 2002
