Project Specifications

The two main goals of this project were to make the snowmobile cleaner and quieter than it currently was. The team succeeded on both of these fronts. Adding a CAT reduced sled emissions and the Stealth CAT muffler reduced the noise of the sled by 2.27 dBA. Even with this success the team did not meet the project specifications for sound and emissions.


Sound Testing Results

The final muffler was mounted in the sled and ran though the J-192 sound testing procedure. The Stealth CAT muffler was tested with and without a CAT and compared to the stock muffler.

Changing out the mufflers and running six sound passes takes time and conditions often change. This team compensated for this by running a control sled, a sled that not is not changed during testing so it produces the same dBA every time it is ran, in between a few of the tests. On the testing day the control sound increased by 2.5 dBA. The scores on the graph were normalized with respect to the control sled in order to account for the changing conditions.

Emission Testing Results

The Stealth CAT muffler may not have made it to the 2011 competition, but the CAT the team received did. Below is a comparision of the results.


(all units in g/kW-hr)
As you can see the U of I CSC teamed passed all of the competition requirements but they still have a lot of work to do to meet National Park Standards. This year the team was lower in every category but CO which also hurt the Escore.

The team did well compared to the other teams at competition. The University of Idaho sled had the second lowest in-service emissions and the fourth lowest laboratory emissions.

Flow Testing Results

As you can see the Stealth CAT muffler has about 140% of the backpressure of the stock muffler. This did not seem to hinder the sled performance but more testing will be done before the 2012 competition. The stock muffler’s backpressure fits somewhere within a range of pressures that the sled will operate within and the final muffler is close enough to fall in the upper pressure range.

The final muffler had more backpressure than predicted because of two reasons. First the team knew from the 1.5 inch simulation that Solidworks under predicted backpressure in full muffler runs due to getting the exact muffler geometry correct. Second the team added half of a Hushpower cone to the space above the CAT. This increased backpressure but decreased sound. The team evaluated the engine performance with and without the extra cone and decided that the extra backpressure was worth the increased sound reduction.

Design Failure Mode and Effect Analysis (DFMEA)

The purpose for creating a design failure mode effects analysis (DFMEA) spread sheet is to reduce the risk of failure for a proposed design project. With a well-developed spread sheet, a design team can easily identify and quantify possible areas of risk and determine possible design controls in order to effectively reduce and/or eliminate potential design flaws. The outcome of this type of analysis should result with an improved product with a reduction in functioning risk and increased usability. The teams DFMEA table is included in our final report.

Our DFMEA analysis covers two main categories of possible failures: the muffler components and the catalytic converter. The results of the spread sheet analysis deemed that excess heat production from the catalytic converter as our highest risk for component failure with three categories receiving a 20 point score for the risk assessment number (RPN). RPN is the product of the severity, occurrence and detection numbers. Items with high RPN numbers should be taken as the most critical for analyzing failure but the severity, occurrence and detection numbers should also be taken into account individually when determining solutions for possible component failures. Possible solutions for our team are: designing an open or closed loop cooling system for the catalyst chamber and using welds for heat shrinking the muffler end caps in order to stiffen the chamber walls.

Estimated Costs

The estimated cost of producing the final muffler as design and constructed by the Stealth CAT team is detailed below.

The estimated MSRP (manufactured suggested retail price) is detailed below:

The team cost for making the muffler and the final MSRP are below the project specifications of $300 and $700. Our muffler would be cheaper than buying a stock muffler which costs $547 and with a CAT $707.