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.
Questions:
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



Albert

Most mfg.s have to be very careful in the advice they give out. There are a lot of people out there that will screw up an engine quicker than you can say OS. I've been in R/C for 20 yrs. and believe me that's true. They have to protect themselves from all the village idiots in the world. And that's a whole bunch!!

I have the Goldberg Extra, and the stock muffler, no matter if you shorten it or not, will stick out of the cowl too much. You are better off getting a BCM Pitts muffler for the 91 FX. It should have more power than the stock muffler.

Is this the mid-wing model? I don't think it's labeled a 300S, which is a low wing. I thought it's just called the 300.

On a different topic. I measured the width of the cowl. With luck, you may be able to fit the engine head completely in the cowl of you offset the engine w/ mount to the left of the firewall. You may need to do this anyway in order to get some right thrust AND to have the spinner come out at the centerline. Of course, this is assuming you side-mount the engine.

Good luck!

The .91fx and the .61fx are the same muffler, but the .91 has the steel insert in the middle for more volume. It won't run as good if you take it out. The screw for the .61 will fit after taking the center section out though.
Suptter

An alternative for the 91FX is the Jett Turbo in cowl muffler, which should not only fit but increase power.

http://www.jettengineering.com

We have a number of the 91FX's and have found that they leak at the center ring which we fixed with high temp silicone. We've also replaced broken muffler bolts with threaded rod using double nuts on each end.

Good luck!

91fx is a fine engine ,yes it.s sensitive to settings before properly run in, we run ours with a variety of mufflers mostly the pitts/ slimline types and they work well also produce more rpm, we have also increased the tank to needle fuel tubing size and drilled the clunk. we never use the stock mufler as for reasons above. the 91 flies great in our yellow extra300 and seagull edge 540 with lumps of power and reliability.
ps. we only use os8 plugs and morgan omega 5% .