OS .91 FX - RCCanada - Canada Radio Controlled Hobby Forum
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Old 01-30-2003, 03:51 PM   #1
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I am: Albert P.
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OS .91 FX

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I bought an OS .91 FX to use in my Goldberg Extra 300 that I am building. It comes with a very large muffler. It seems a regular muffler with a 4 cm (aprox. 1 3/4") extension in the middle. The extension can be removed but of course then the bolt running through the middle of the muffler to hold the 3 pieces together is too long.
So, I e-mailed to O.S. support in the States as follows:

"I just bought a .91 FX and received it today. I was planning to put it into a Goldberg Extra 300S. With that massive muffler the engine is supplied with there is no way I can fit it inside the cowl.
1) Is it possible to remove the middle 4 cm. aluminum extension without interfering with the engine performance?
2) If so, what is the part number for the shorter screw required?"

After many automatic e-mail replies saying to look into the FAQ's at their site, etc, I received the following two answers:

Answer 1:
Thank you for your recent e-mail.

Unfortunately, due to liability and warranty considerations, we cannot support or advise on any conversions or modifications for any products we distribute. The halves may line up but not provide enough back pressure for proper operation. Obviously, without input from our design and engineering staff it is difficult to assess how your proposed modification will affect the product's ability to be used as intended. You might try placing a message on one of the Internet lists such as rec.models.rc., www.RCONLINE.com and some of the many other r/c related sites to attempt to locate a fellow modeler who has completed similar modifications.

Answer 2:

Changing the size of the muffler will change the engine performance. You could try a pitts or sports scale muffler.

Item # Description Retail Price

BISG4091 04091 OS .91 FX PITTS MUFFLER GMA 45.00

Any prices proApril are all Manufacturers Suggested Retail Pricing only. As a distributor, we support our dealers by selling directly to them, and we do not sell parts or complete items to the consumer. Please recognize that as a service center we simply are not set up to do so.

Typically your favorite Great Planes authorized hobby shop

(find one now at: http://www.gpmd.com/cgi-bin/wc569010ap.pgm?V=GPM)

Your international distributor if you are outside the US

(find one now: http://www.greatplanes.com/gpdist.html)

or a mail order firm like

Tower Hobbies ( http://www.towerhobbies.com/ )

is able to provide you better pricing and excellent customer service.

I hope you find this information helpful. Should you have any further questions or concerns

So, I decided to call by phone. The person who answered was helpful and thought that the engine may loose some efficiency or the needle settings would be different, but maybe would not be a big problem. Seeing that, I asked about the shorter bolt part number and there we hit a problem. He couldn't give me a part number.

So, now, I am hitting all the Forums I know and I am expanding my original two questions by two more to any user that has had experience with the OS .91 FX.

Question 3: The Bisson Pitts muffler for the OS .91 FX has a much smaller expansion chamber than the OS muffler. How can this be expected to still work OK (and my LHO says it will) if OS in the USA says it will not (first answer). I would calculate the volume of the Bisson muffler to be 1/3 or 1/4 of the original.

Question 4: The OS manual for the engine specifies to use 15x8, 16x6~8, 17x6, 13x11~13, 14x10-11 propellers. The FAQ site for USA OS has an specific entry about overpropping that engine and recommends using a 13x8 or 13x9 specifically. A disclaimer is there also referring to altitude, fuels used in North America, etc.

I think that the e-mail response regarding "don't touch it, warranty!" is a cop-out. They have not answered my question.

I am not happy with the USA site for OS, they took a long time to include the .50 SX Ring engine info even if it was already on sale everywhere.

What say you, fellow fliers?

Thank you

Once my hands and knees stop shaking I take the planes out of the car.
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Old 01-30-2003, 04:31 PM   #2
I am: Boolean21
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Different mufflers can and most certainly do affect engine performance, I doubt that it would affect it enough to truly bother you.

As for the response about not touching it due to warranty, that is a common response by a company as big as Hobbico, who is the parent company for GP, Tower, blah blah blah. They get lots of claims and just try to cover their butts financially. I doubt you would have a ever have a real warranty issue with a muffler.

I would not think twice about changing mufflers or removing extensions that interfere with my plane, I cant see how you would do any damage to the engine.
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Old 01-30-2003, 05:29 PM   #3
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I'll second what can773 said. If you want to remove the centre piece, do so. As far as part number for shorter screws, just buy two shorter length screws from Canadaian tire, or worst scenario, just cut the original ones short.
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Old 01-30-2003, 06:18 PM   #4
Terry Gauvin
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I have an OS .91 FX on a Goldberg Ultimate and use a Bisson Pitts style muffler. Sounds good, works great.
DX18 Stealth, H9 Inversa 33, H9 33% Edge, H9 33% Decathlon.
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