Initial Sound Testing

The team conducted sound testing and found that two Hushpowers in series was clostest to the dampening of the stock muffler.

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Prototype Design

The basic idea is that the flow enters the muffler at the inlet. Then it flows through the two Hushpowers in series which are connected with welded chambers. Lastly the flow passes through a shell which houses the CAT and exits the muffler.

The CAT is inserted by sliding it into the shell where it hits a stop ring. Then a removable end cap is bolted to the shell which holds the CAT in place. To change the CAT you need to take the muffler out of the sled and remove four bolts.

Prototype Construction

The muffler was constructed using sheet metal, two fully assembled Hushpowers, and a cylindrical shell provided by Hushpower. Three parts had to be machined on the mill and the assembly was welded together. The full drawing package can be found on the documents page.

Prototype Sound Testing

The sound data came back very positive with the Stealth CAT muffler being 2.5 dBA quieter than the stock muffler without panels. The picture shows the prototype mounted to the sled during a testing pass.

For a true evaluation of performance more than just the sound pressure level must be examined. The reason the new muffler was quieter than stock was because it limited the performance of the engine. The engine could only rev to 6000 revolutions per minute (RPM) instead of the normal 8000 RPM so much less noise was produced resulting in a quieter run.

Flow Testing Results

To determine why the engine performance was limited the team turned to flow testing. The team used both the newly constructed flow bench as well as Solidworks flow simulation. Both mufflers were flow tested on the flow bench and the results compared. The picture shows their pressure drops over a range of flow rates.

From this test the team learned that the new muffler had much higher back pressure than the stock muffler. This was limiting the performance of the engine.

Next the team looked for ways to increase the muffler’s flow rate. The current Hushpower used in the muffler had an inner diameter of 1.5 inches in diameter. Hushpower also makes a 2.5 inch muffler that could be used to increase the flow rate through team’s muffler.

The team looked to Solidworks flow simulation so see if this idea could produce around stock backpressure. Both Hushpower mufflers where modeled in Solidworks as well as both muffler configurations. The picture shows an example of the Solidworks flow simulation.  

The graph below shows the results from the flow simulation. A muffler made with 2.5 inch Hushpowers will have similar back pressure to the stock muffler.

The graph below shows the results from the flow simulation. A muffler made with 2.5 inch Hushpowers will have similar back pressure to the stock muffler.

Prototype Conclusions

The Stealth CAT prototype muffler had too much back pressure for the sled to run correctly. Building the same muffler with larger inner cone diameter Hushpowers will increase the backpressure and allow more flow.