Do Velocity Tubes Make a Difference?

To explore the effect of the airbox, I took the bike to the dyno and compared baseline dyno runs with the airbox, to those with the airbox removed, and then replacing the airbox with velocity stacks of different lengths. 
First, I took a look at removing the airbox entirely and letting the throttle body draw air without having to pass through the air filter.
Next, I took a look at the intake manifold length, and based on calculations from a velocity stack length calculator, tried adding a couple of  short extension tubes to see their effect.
And finally, I added a long tube like is often used when changing to a large cone filter or used in a bigger airbox.

The Tubes

I fabricated the test pieces from plastic tubing, cut and shaped to fit to the intake/throttle body.  (The rubber bell for smoothing airflow at the end of the stack is the OEM boot from the stock airbox.)  Here is a look at the pieces...

Various tubes

I ran through all of these options in one afternoon on the dyno at American Cycle Performance in Bowie, MD.  The various options were tuned using a combination of the Thundermax, and the AFR sensor on the Dynojet dyno. 
Here is a look at the effect of these changes...

Removing the Airbox

Some people do not believe that the XR1200's airbox is well designed.  I had already modified the airbox to clean out the internal mounting box for the air flapper, smooth out some of the internal edges, and install a larger air filter - so instead of trying to further modify the box, I looked at the effect of running without it and replacing it with velocity tubes. 
For this test I removed the airbox and installed the OEM rubber flare  that smooths the air flow, directly on the throttle body.  This change  simply removes all obstruction that might be caused by the box and air filter, and lets the the engine breathe through a simple velocity stack.
Here is a look at two dyno runs done on the same day - one with the airbox and filter(BLUE), the other with the airbox removed (RED).

With Airbox vs Without Airbox

The airbox does appear to improve performance.  Running without it causes a about a 1 hp loss of power and a very broad 2 to 4 ft-lbs loss of TQ.  So, simply dumping the airbox does not appear to be a great idea.

Short Velocity Tubes

While the airbox itself appears to contribute to the engine's performance, the nominally 7" (17 3/4 cm) long intake and throttle body (with the OEM rubber bell installed) appears too be to short to optimize the XR1200's engine performance.  There is a whole science behind optimizing the length of the intake runners to match induction wavelengths that I won't even try to explain.  But, the intake length can be set to match the waves at different RPM points (frequency), and in different numbers of  cycles of the waves.  If the length is set right, it will reinforce air flow through the intake.  Set it wrong, and the intake will not flow efficiently and can hurt performance.
I tried a couple of experimental tube lengths to add length to the intake and see the effect. 

I cut a 1" (2.54 cm) and a 1.5" (3.81 cm) extension tube to add length to the intake.  (Had there been enough  room under the gas tank, I would have added these tubes together to try about a 2.5' extension but space did not allow.)  The exact lengths of the tubes when installed varied slightly from their initial length, because of the shoulders that I needed to cut to have them mate with the ends of the throttle body and OEM rubber bell.

Extension Tubes

Here is a look at the intake with an extension tube and OEM rubber bell in place on the throttle body.

Extension tube installed

With the extension tubes trimmed and shaped to mate with the intake and rubber bell, their length was not exactly 1" or 1.5", but the effect of increasing the intake length to about 20.25 cm or 21.5 cm did not improve performance.  The next graph shows the dyno runs with no airbox with just the rubber bell in place (BLUE), with the nominally 1" extension in place (RED), and witth the nominally 1.5" extension in place (GREEN). 
It can be seen that changing the intake length does produce changes in the efficiency of the intake, and that they may not be good changes...

Effect of extension tubes on Power

Adding these short lengths to the intake started to seriously hurt power output at higher RPM.  It was interesting to see that they also started to increase TQ in a narrow band around 3,500 RPM.  This effect was simply due to the induction wave frequencies (RPM) that the intake runner length had started to optimize, and the other waves that it started to interfere with.
This is something to think about, if one is considering changing from the airbox to a tube type intake with a cone filter.  It is not that the tube will not work, but without some dyno time and experimentation to determine the optimum length, the tube length itself could actually hurt power output, even though a larger cone filter might be capable of flowing more air than the panel filter in the airbox.

Long Intake Tube

After looking at the impact of the short extension tubes, I did not know what to expect from the longer tube (nominally 14.5" = 36.83 cm) that I had cut.  Added to the nominally 6" long intake and throttle body, this tube should give an intake runner length of about 52 cm.  According to the velocity stack length calculator, this intake runner length should have provided good induction wave matching at about 5,000 rpm and have been reasonably well matched down to about 3,000 rpm.  I was skeptical though since the short tubes had hurt performance and I thought the longer tube might also restrict air flow.
Here is a look at the long tube in place.  While this long radius bend is not practical for use on the street, I was trying to minimize the constricting effect of a 90 degree bend, while being able to move the gas tank only minimally to be able to run the bike.

Long tube intake mod

I was thoroughly amazed by the effect of this tube.  Here is a look at a comparison of the engine with the airbox (BLUE),  to a run with the long tube in place (RED).  Look at the impact on TQ from about 2,000 to 5,000 rpm...

Effect of Long Tube

So, while this  particular tube length killed power above 5,500 RPM, look at the way it improved air flow into the engine below 5,000 RPM!  From about 2,500 to about 5,000 RPM this tube length boosted TQ from about 6 to 8 ft-lbs!!
There could actually be a touch more TQ available with this tube since I did not tune the AFR perfectly with this mod.  The air flow was so much better, that I had to make three major adjustments to the baseline mapping of the Thundermax, to allow the base map to add enough fuel.  This tube caused so much air to flow in the TQ band, that the engine went radically lean and was pinging badly until I got the engine retuned.  It was also pretty amazing how this tube length affected air flow so negatively over 5,500 rpm.
Now, while this improvement is on my built engine and dependent on a number of things, including the cam duration, it does show that a significant TQ improvement can be had by optimizing the intake.  Alternately, one can kill performance by an incorrect choice of length.


From my quick look at the XR1200's airbox and intake, it appears that:
  • the airbox does improve performance over operation using a filter without an airbox.
  • a properly designed air tube, or "velocity stack" can further improve performance.

It is also evident that simply extending the intake with a tube and cone type filter could actually hurt performance if the tube length is not set correctly, even though a large cone filter can flow more air than the panel filter in the airbox.


Optimally, it would seem that an airbox incorporating a properly tuned velocity stack should improve the XR1200's performance over the stock airbox.  There would be tradeoffs in putting the tube in an  airbox on the XR1200 becuase of the the tighter radius tubing bend that would be needed and the constraint on air tube length, but the use of the airbox could produce a broader RPM benefit .


After working with the velocity stack length calculator a bit more, the calculations indicate that an intake tube that was either 5 to 15 cm longer, or 10 to 15 cm shorter, would support airflow up to 7,000 RPM while staying reasonably well matched down to about 3,000 RPM.  The longer tube could be more ideal for performance because it appears to better match multiples of induction waves at various RPMs, but the shorter tube looks like it could give good results and be possible to incorporate in a modified stock airbox.  I'll be experimenting with the longer and shorter intake tubes matched for higher RPM airflow in the future, and will supplement this information when I have more data on what can be done to improve the stock airbox.


Alternately, one might want to follow the work of Dris at Twin Motorcycles of the Netherlands, to see the work that he is doing in trying to design an improved airbox for the XR1200 from the ground up...



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