Detailed Data on the Effect of Different Intake Runner Lengths
 

 
The following graphs show the effect on HP and TQ of different length tubes mounted on the intake of the XR1200. 
 
The first comparison is simply a repeat, to show that YES, using the airbox does provide an advantage over simply running the bike without the airbox and air filter.
 
The next sets of data highlight the fact that there is a benefit to properly tuning the intake to enhance HP or TQ.
 

DSCN2931.JPG
Intake Rubber boot

 
Airbox or No Airbox
 

 
 
 
 
 
 
 
 
 
First, just to revisit whether the airbox itself is effective, here is a look at the power output of the XR1200 with the airbox installed, and then without the airbox using just the flared rubber boot that connects to the box.
 
 
 
 
 
 
 
 
 

AirboxVsNoairbox.jpg
Airbox vs No Airbox

 
In the above graph, the run with the airbox (BLACK) shows a clear power output advantage to using the box, over simply removing it (RED).
 
 

 
Effect of the Different Tube Lengths
 

 
Running through from shortest to longest, here is a look at the effects on peak Horsepower and Torque, of extending the nominally 6" long, 50 mm XR1200 throttle body with a tube. 
 
NOTE:  I did not experiment with different diameter tubes.  I kept the tube inner diameter at a constant 2" or nominally 50mm, to match the throttle body.
 
 

 
Right Bend Only
 

1Bend.JPG
Intake Bend

 
 
 
 
 
This right bend provides nominally 4" of extension (along its center line).  It is 2" in diameter, up until its last  1" where it opens up to 2.5" (for mating with extension tubes).
 
Total extension of the throttle body is approximately 4" or 10.2 cm.
 
The total throttle intake runner length (throttle body + bend) is approximately 10" (25.4 cm).
 
 
 
 
 

 
The effect of this extension on HP and TQ can be seen in the following dyno graph.  The curves for the intake with the extension are in GREEN, compared to the power produced with the airbox in BLACK.
 
 

0-ElbowvsAirbox.jpg
Elbow vs Airbox

 
The above graph for the intake tube is in free air - not in an airbox - and does not include an air filter.  The airbox used in the airbox run is modified for a larger K&N air filter, and has had internal obstructions removed or smoothed.
 
 

 
Extension Tube 1
 

1Tube1.JPG
Tube 1

 
 
 
 
 
When this 2' long tube is inserted in the bend, the total intake tube extension length is approximately 5" (12.7 cm).
 
The total intake runner length (throttle body + bend + tube) is approximately 11" (27.9 cm)
 
This tube will fit inside of a modified OEM airbox.
 
 
 
 
 

 
The effect of this extension on HP and TQ can be seen in the following dyno graph.  The curves for the intake with the extension are in GREEN, compared to the power produced with the airbox in BLACK.
 

1-Tube1vsAirbox.jpg
Tube 1 vs Airbox

 
The above graph for the intake tube is in free air - not in an airbox - and does not include an air filter.  The airbox used in the airbox run is modified for a larger K&N air filter, and has had internal obstructions removed or smoothed.
 
 

 
Extension Tube 2
 

2Tube2.JPG
Tube 2

 
 
 
 
 
When this 4" long tube is inserted in the bend, the total intake tube extension length is approximately 7" (17.8 cm).
 
The total intake runner length (throttle body + bend + tube) is approximately 13" (33 cm)
 
This is the longest tube will eaxily fit inside of a modified OEM airbox..
 
 
 
 
 

 
The effect of this extension on HP and TQ can be seen in the following dyno graph.  The curves for the intake with the extension are in GREEN, compared to the power produced with the airbox in BLACK.
 

2-Tube2vsAirbox.jpg
Tube 2 vs Airbox

 
The above graph for the intake tube is in free air - not in an airbox - and does not include an air filter.  The airbox used in the airbox run is modified for a larger K&N air filter, and has had internal obstructions removed or smoothed.
 
 

 
Extension Tube 3
 

3Tube3.JPG
Tube 3 in Airbox

 
 
 
 
 
When this 5.5" long tube is inserted in the bend, the total intake tube extension length is approximately 8.5" (21.6 cm).
 
The total intake runner length (throttle body + bend + tube) is approximately 14.5" (36.8 cm)
 
This  tube could fit inside of a modified OEM airbox, if the air filter were removed or relocated.
 
 
 
 
 

 
The effect of this extension on HP and TQ can be seen in the following dyno graph.  The curves for the intake with the extension are in GREEN, compared to the power produced with the airbox in BLACK.
 

3-Tube3vsAirbox.jpg
Tube 3 vs Airbox

 
The above graph for the intake tube is in free air - not in an airbox - and does not include an air filter.  The airbox used in the airbox run is modified for a larger K&N air filter, and has had internal obstructions removed or smoothed.
 
 

 
Extension Tube 4
 

3Tube4.JPG
Tube 5

 
 
 
 
 
When this 6.25" long tube is inserted in the bend, the total intake tube extension length is approximately 9.25" (23.5 cm).
 
The total intake runner length (throttle body + bend + tube) is approximately 15.25" (38.7 cm)
 
This  tube could fit inside of a modified OEM airbox with its hood scoop, if the air filter were removed.  The tube could stay inside the screen of the air scoop keeping the stock cosmetic appearance.
 
 
 
 
 

 
The effect of this extension on HP and TQ can be seen in the following dyno graph.  The curves for the intake with the extension are in GREEN, compared to the power produced with the airbox in BLACK.
 

4-Tube4vsAirbox.jpg
Tube 4 vs Airbox

 
The above graph for the intake tube is in free air - not in an airbox - and does not include an air filter.  The airbox used in the airbox run is modified for a larger K&N air filter, and has had internal obstructions removed or smoothed.
 
 

 
Extension Tube 5
 

6Tube5.JPG
Tube 5 inside of Scoop

 
 
 
 
 
When this 6.875" long tube is inserted in the bend, the total intake tube extension length is approximately 9.875" (25.1 cm).
 
The total intake runner length (throttle body + bend + tube) is approximately 15.875" (40.3 cm)
 
This  tube could fit inside of a modified OEM airbox with its hood scoop.  The tube would stay inside the screen of the air scoop keeping the same cosmetic appearance as the stock airbox, but would effectively eliminate the airbox since the opening would be near the screen of the scoop.
 
 
 
 
 

 
The effect of this extension on HP and TQ can be seen in the following dyno graph.  The curves for the intake with the extension are in GREEN, compared to the power produced with the airbox in BLACK.
 

5-Tube5vsAirbox.jpg
Tube 5 vs Airbox

 
The above graph for the intake tube is in free air - not in an airbox - and does not include an air filter.  The airbox used in the airbox run is modified for a larger K&N air filter, and has had internal obstructions removed or smoothed.
 
 

 
Extension Tube 6
 

7Tube6.JPG
Tube 6 vs Airbox

 
 
 
 
 
When this 11.25'" long tube is inserted in the bend, the total intake tube extension length is approximately 14.25" (36.2 cm).
 
The total intake runner length (throttle body + bend + tube) is approximately 20.25" (51.4 cm)
 
This  tube is too long to fit inside the space of the stock airbox and scoop.  Additionally, while I did not experiment with larger diameter tubes, by the time one reaches this length the diameter should probably be increased to 2.5" to decrease flow resistance.
 
This was the tube that I used to compare the effects of narrow and wide radius turns on the intake air flow.
 
 
 

 
The effect of this extension on HP and TQ can be seen in the following dyno graph.  The curves for the intake with the extension are in GREEN, compared to the power produced with the airbox in BLACK.
 

6aTube6vsAirbox.jpg

 
The above graph for the intake tube is in free air - not in an airbox - and does not include an air filter.  The airbox used in the airbox run is modified for a larger K&N air filter, and has had internal obstructions removed or smoothed.
 
 

 
Extension Tube 7
 

 
When this 13.5" long tube is inserted in the bend, the total intake tube extension length is approximately 16.5" (41.9 cm).
 
The total intake runner length (throttle body + bend + tube) is approximately 22.5" (57.15 cm)
 
 
This  length was picked simply to show what is possible in terms of tuning for Torque - but for tubes of this length a larger diameter tube would likely reduce air flow resistance. 
 
Also, a tube of this length is not really practical, unless one wants to try mounting an air scoop on the front forks. 
 
Horsepower may suffer with a tube of this length, but there is some killer Torque from 3500 to 4500 RPM.
 

 
The effect of this extension on HP and TQ can be seen in the following dyno graph.  The curves for the intake with the extension are in GREEN, compared to the power produced with the airbox in BLACK.
 

7bTube7vsAirbox.jpg
Tube 7 vs Airbox

 
The above graph for the intake tube is in free air - not in an airbox - and does not include an air filter.  The airbox used in the airbox run is modified for a larger K&N air filter, and has had internal obstructions removed or smoothed.
 
 

 
 
Conclusion
 

 
These intake experiments were simply to show what is possible in terms of altering the HP/TQ performance of the XR1200 through the use of a tube on the intake or "velocity stack."   To truly optimize the intake performance there are many more factors that should be tested, including use of a larger airbox, and larger diameter intake tubes, but there are clearly gains which can be obtained with minimal parts cost.
 
Based on an individual's riding style/performance requriements the intake could be tuned to enhance HP or TQ in the proper range.
 
As time allows, I'll fit the shorter length intake tubes inside of the airbox and test the effect of the tube inside of the OEM airbox - but that is for the future...

 
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