I am currently breaking in two MVVS 26IFS gassers that I bought from Pe'. In another post on the Gas Engine Forum Pe' said about the MVVS 45 and an after market pitts style silencer:

"That is the muffler Just engines sells for in cowl applications.
These in cowl muffler all cause some substantial power loss, unless there is at least 15 times the engine displacement volume available. In addition, the restricted breating increases cylinder temperature in two strokes, which reduces engine life expectancy. If the cooling of your engine is not adequate, it may even mean the difference between engine survival and disaster. "

My question is about the 15 times rule for my MVVS 26IFS. The stock silencer #3270 is 16.779 cu inches which is about 10 times the displaced volume of the engine. A Brisson Pitts is about 18.28 which is slightly better that the stock silencer but performs worst than the stock silencer. I have designed and fabricated an incowl silencer for my OV10 that is 14.13 cu inches. My first tests with the custom can indicates that I am getting about equal performance to the stock MVVS silencer. The engine cylinder head will be outside the cowl and should get good cooling.

What are your thoughts on my concerns?

Gaines,
please place a picture of your setup air intakes and air exits. I have seen too many remarks about "good cooling" that when scrutinized prooved very lacking.
The 1.60 is a powerhouse, and as such must get rid of a lot of waste heat.

Pe,

Attached are some photos of the installation on the right nacelle. The silencer can is 2" diameter by 4.5" long .030 wall aircraft grade chome moly steel tubing. The flange attachment to the engine is the 22 mm K&S fittings you got for me. There is a perforated baffle brazed inside the silencer that has 3/16 diameter holes equal to 1 and 1/2 times the area of the exhaust outlet on the engine. The exhaust outlet is 3/4" chome moly stub out with 2024 T3 .035 wall aluminum tubing. The 3/4" exhaust tubing exits the nacelle through a 1 1/8" dummy exhaust tube for cooling and scale affect. The silencer area is cooled by air coming in from the upper front of the cowl and exiting through a scale baffle on the top rear of the cowl. Initial test runs indicate that I am getting about the same rpm as the stock MVVS silencer with a bit more noise. Of course the real test is how the engine performs on the Bronco. It is not to late to make changes so tell me what you think.

Gaines

Looking good. The carb and cylinders get lots of fresh air. Air may get stagnant behind the cylinders though.
Try to get the air outlet at least twice the air inlet, maybe by using the nacelle end width as total flap area.
Pay attention to the pulse line connection. The engine will get hot, and the line softens and may leak. That will cause the carb to malfunction. An extra silicone tubing ring around the tygon line will keep the tygon in place on the nipple even when it gets warm. For this pulse line, I like neoprene tubing better, though it softens with heat as well.
I also would suggest large steel washers under the backmount stand-offs to spread the load, and prevent wood crushing of the firewall.

Pe',

Thanks for your input. I will try to open up the outlet air for better cooling. I nomally use the second tubing ring to secure fuel tubing. I will do the same with the impulse line. I have not had any problems with the factory tubing to date. What is that tubing made of?

I will add fender washers under the standoffs when I do the final install. I just bolted the engine on the nacelle to show you what I had.

Am I OK with the exhaust system?

Gaines

Your muffler design looks ok to me. Like all in cowl mufflers, they take away a bit from the power, but in a twin plane the double prop wash is incredibly effective, so you have power reserve. Just take good care with cooling of the engines, and you will be fine.