I recently received an inverted pitts Bisson muffler for my 3W-35. Muffler looks great and fits perfectly. Only problem is the engine runs 500 RPM less with the Bisson when compared to a normal downdraft muffler. Both mufflers have (2) 5/8" OD exhaust tubes but the Bisson muffler has the ends of the tubes rolled inward. Does this restrict the engine's performance in any way? I'm thinking of cutting the rolled portion of the tubes off of the Bisson muffler in hopes of gaining the 500 RPM back. Any of you out there have had this problem or can think of any reason not to cut the rolled ends off??

Ryan

is this on a broken in motor? i changed out mine in the break in stages to the peacekeeper because i hated the straight pipe sound as compared to the peacekeeper.

Sound makes no difference where I'm at. We have no restrictions. I'm most concerned about power.

Ryan

It's been my personal experiance that Pitts mufflers do not flow as well as the side type, and the side type dont flow as much as just a header, and the header doesn't make the power of a tuned pipe. Cutting the ends off won't help {I tried it}. Be sure you readjust the carb after the change, or it will be on the rich side.

The rolled in ends do make a difference. It may make the engine quieter, but the larger the opening the better. Try running the engine with and without the muffler. You will see a big difference in RPM (and sound!).

After careful consideration, and speaking with Jim Bisson, I decided to cut the rolled ends off. In fact, I completed the task last night.

I was told that the exhaust end open area should be around 60% of the exhaust port open area (which is .6 sq in). With the rolled ends I was at 42% (.25 sq in) running 6600 RPM. The down draft muffler was at 72% (.43 sq in) running 7000 RPM. Now that the rolled ends have been cut off of the Bisson, the open area at the exhaust end is 80% (.48 sq in).

I will run the engine this weekend to see what gains I will make in RPM. I will be sure to post it for all of you. This may be a valuable experiment for you who have rolled ends on your mufflers.

Ryan

Let us know as well what the relative db difference is - I'd be interested in trying this mod myself...

Thx

David

Ryan,
What did Bisson have to say was his reason to roll the ends of the muffler tips?

Jim said it was for sound suppression. He also mentioned that it helps with throttle transition. When I spoke with him of my concerns, without hesitating he said to "cut the ends off."

From my experiences so far, the rolled ends are much quieter than the non rolled ends.

Ryan

The results are in: I have gained 500 RPM by cutting the rolled ends off of the Bisson muffler. Did not change anything between test. Same fuel, same prop, same needle settings, etc.

Stats below using a Zinger 18 x 10 on a 3W-35:

Bisson muffler w/ rolled tubes: 6500-6600 RPM
Bisson muffler w/ rolled tubes cut off: 7000-7100 RPM
Regular B&B style downdraft muffler: 6900-7000 RPM.

Ryan

I must be doing something wrong, I only picked up about 100 RPM on my Brison 2.4 when I cut the pipes on the Bisson. How are you guys getting 500 or what am I doing wrong?

IN your opinion, how much louder was the engine when you removed the rolled ends?

Outcast,

You may have alreadt had the 60% or better area before cutting the ends. AllI did was cut the ends off. Nothn else.


Geistware,

The engine definately was louder but not as loud as the downdraft muffler. It had a smooth sound rather than a cracking sound if you know what I mean. By the way, we have no sound restrictions where I fly.

Ryan