Summary of the Effect of Intake Tubes on the XR1200's Power and Torque

After my initial dyno testing of velocity tubes on the XR1200, I took a more in depth look at the effects of different length velocity tubes on the power ourput of the XR.  While tube lengths calculated from a velocity stack calculator are most accurate for intakes using individual intake runners and multiple throttle bodies, I did use the velocity stack calculator to cut a number of tubes that were tuned in the 2,000 to 7,000 rpm band of the XR1200.  I picked lengths that matched several different multiples of induction wave cycles from 2 to 7, and either emphasized the 2,000 to 5,000 range or the 3,000 to 7,000 rpm range.  Considering the single EFI  throttle body on the XR1200's intake, the tube lengths that I cut were really just starting points to see what the practical effect on air flow into the engine would be.  For these tests I used 2" PVC pipe.  I custom fit a tight radius bend to be able to keep the pipe noiminally in the space of the XR's airbox, and then trimmed various extension tubes and a bell-type flare to vary the overall tube length.  The throttle body extension varied from about 4" to 17.5". 
Here is a look at the effect of the tubes on the XR1200's performance.

First a Baseline Comparison for a Tight Bend Tube

Since I was uncertain of whether a tight radius bend in the intake tube (needed to position a velocity tube under the gas tank) would itself restrict air flow, the first velocity tube length that I cut matched the overall tube length of the gradual bend tube that I used in my initial test. 
With the tube and extension in place, I pulled a few dyno runs and did some tuning with Smartlink on the Thundermax to get the fuel curve dialed back in.  With everything retuned and set, here is a comparison of performance of a tightly bent 2" ID air tube, with the gradually bent 2" air tube I ran a week before.

Gradual Bend vs Tight Bend on 14

In the above graph the RED and BLUE lines show the effects of a nominally 14" long, 2" diameter tube on the power output of the XR1200. 
The minor differences in the two curves could be due to a number of factors - but there is not a major difference in the effect of the intake tube when bent in a gradual radius, or in a clean tight bend. 
For comparison, the GREEN line shows the XR1200 power output with the airbox installed.

Flared Bell

A flared bell on the end of a velocity stack is intended to smooth the airflow into the end of the stack and help improve performance.  It is tough to get any appreciable flair on a 2" tube for use inside the XR's airbox, so I wondered what the effect of running without the bell, and just some minor tapering at the end of the velocity tubes.  To test this I tried dyno runs with the flare, and without it, as shown below...

Tube With Flared End

Tube without Flare

In back-to-back dyno tests with and without the flare on the end of a tube, there was no noticeable difference in power output or torque produced.  The flared end did not appear to make any difference in the dyno runs.  So, all of the following test runs were conducted without the flare.

Summary of the Effect of the Different Length Velocity Tubes

The below dyno chart shows the effect of all of the tested velocity tubes, ranging from just the 90 degree elbow shown above (about 4" of extension), through the elbow with a long extension tube (total extension approx 17"), on the HP and TQ produced by the XR1200.  
The XR1200 was re-tuned on they dyno with the TMax and Smartlink for each tube length.  The runs were run back-to-back on the same day, separated only by the length of time it took to re-tune the engine.  The same Vance and Hines Widow exhaust was used for all runs.

Comparison of All Intake Tubes

The above graph shows the effects which can be achieved in terms of boosting TQ and/or HP with different length tubes. 
One can see that the effect of the boost on either TQ or HP is related to the length of the stack installed on the throttle body.


While my dyno testing clearly showed the impact on power and torque of using velocity tubes and how proper selection of tube length can affect different RPM bands, there is much more that can be looked at in terms of the best tube length to be used in an airbox on the XR1200.
  • It appears that engine performance with the OEM airbox is better than performance when simply removing the airbox and letting the throttle body suck unrestricted air.
  • It is evident that power can be increased on an XR using a velocity tube in place of the airbox.
  • Initial data also points to the possiblity tht the OEM airbox and/or throttle body is too small to support peak power much over 100 HP.

While all of my data is collected off of a built XR1200 motor, the build is not extreme.  While tube lengths might vary for a stock XR1200, the benefits of a velocity tube on a stock XR motor should also be significant. 

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