A BHAF Muffler - Uhh??
I like the Cummins engine, but I try to do anything I can to make the engine quieter. The BHAF air filter flows lots of air, but it also allows a lot of engine air intake noise to be heard in the cab – more noise than the factory air box and the factory air filter. This inspired the idea to experiment with a BHAF muffler on a big honkin’ air filter – in this case, a NAPA 2790.
A BHAF muffler is not for everybody. If you like air intake sounds, turbo sounds, and amplified exhaust brake sounds inside your cab, then this project may not be for you.
For my first muffler I used a length of ribbed plastic drain pipe inserted into the filter. I decided to measure the pressure drop through the pipe on a road test. I hooked up an analog water manometer gauge (mounted inside the cab) at connection port “B” for the test.
All of the following tests were done driving the truck in fifth gear at wide open throttle approaching 2500 rpm. The pressure drop measurements are recorded below:
Port “B” connection
4” H2O @ 27 psi boost @ 2500 rpm WOT – BHAF with NO muffler
20” H2O @ 27 psi boost @ 2500 rpm WOT – BHAF with plastic ribbed muffler
9” H2O @ 27 psi boost @ 2500 rpm WOT – BHAF with steel muffler
Port “A” connection
1” H2O @ 27 psi boost @ 2500 rpm WOT – BHAF with or without mufflers
The plastic pipe muffler failed in terms of pressure drop (20” H2O) probably due to the restrictive 3” inside diameter and ribbed internal surface. The steel pipe muffler is the winner. It is still effective as a muffler, but at a pressure drop of only 9” H2O under the same test conditions. The improved pressure drop is likely due to the increased inside diameter (from 3” to over 3 5/8”) and from a smooth inside wall compared to the ribbed plastic pipe.
So, I scrapped the ribbed plastic muffler (although I ran it for 40,000 miles) and made a steel one. This muffler consists of one part – an 8” length of thin-walled steel exhaust pipe with an outside diameter of 3 13/16” and an inside diameter of 3 11/16” (wall thickness approximately 1/16”) These diameters could vary slightly – in my case the NAPA 2790 BHAF connection has been deformed to a slightly smaller diameter over time because when the filter was new there was nothing in place to support the tension of the intake hose clamp. For more BHAF information, IBMobile wrote an article regarding dimensions and specifications of the most popular BHAF’s – thank you IBMobile!
Check your local muffler/exhaust shops in the area for a piece of exhaust pipe to meet these dimensions. If you can’t find an exact match, select a pipe that is slightly larger in diameter and then modify the pipe as described below.
I made the thin-walled steel muffler piece from a slightly larger diameter exhaust pipe with a wall thickness of approximately 1/16”. After cutting it to the 8” length, I cut out and discarded a piece of the wall and reshaped the remaining pipe to the proper outer diameter with the aid of a vise. I stress relieved the pipe so the cut edges touched each other and then I soldered them together. I made sure that the seam joint was flush and smooth for an airtight seal. I inserted the muffler into the air filter leaving a small edge visible. This will allow for at least 3 inches of air space between the inside back wall of the air filter and the end of the pipe.
I hope that you will enjoy a quieter ride in the cab without the sacrifice of performance.