How can I know if Ill be needing to replace the stock muffler on a O.S 75AX to fit it in the cowl of a Great Planes Extra 300 (60)? I could sure use the $40 of the pitts muffler for something else...

I don't know if its true or not...I read a post a few days ago that said the 75AX will not run with a pits muffler.
Now mind you I'm just repeating what I read..I have no personnal experence with this engine...

Could someone confirm this? Anyone know anything about this issue?

The accessory list in the O.S website has got the Slimline Pitts as one of the possible accessories for this engine:

http://www.gpmd.com/cgi-bin/wgpacg100p?&T=4&I=OSMG0575

its possible but that dosent make much sense,ive read that some engines need one pipe plugged on the pitts style mufflers.the st engines i think it was.

The Tower 75 runs great with a pitts...not sure about the OS. The Tower would easily fit in the cowl.

For anyone interested:

Gustavo Machado (This is me)

to [email protected] (It is the one listed on the O.S website)

I have been investigating the features of the 75 AX engine for airplane use that you manufacture. Someone however, suggested that the engine will not work with the Slimline Pitts muffler that some distributors like tower hobbies carry for this particular engine. Specifically the Slimline Pitts Muffler O.S. .61 SF ABC ( http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXFG84 ) is the one I'm interested in coupling with the 75 AX. Is this incompatibility true? I noticed that in the accessory list for the 75 AX on the O.S website you do have this muffler listed but I want to make sure that it will work.

Thanks in advance.

Gustavo Machado (Me, again)

__________________________________________________ ____

Engine Support <[email protected]>

Hello,

Thanks for your email!

The muffler should work. It fits fine and should not cause any running problems.

I hope you find this information helpful. Should you have any further questions or concerns, please do not hesitate to contact us at this email address. (Please be sure to copy all previous emails into any future questions.) You can also reach our product support technical team at 217-398-8970, or via fax at 217-398-7721.

DID YOU KNOW that our web pages now all contain FAQs? (Frequently asked questions) Please drop by and take a look! We hope you'll find the information helpful and valuable to you.

Thanks Again,

Nick A.
Product Support


_________________________


Hope it helps someone else.

First the Pitts style mufflers work very well with the 75 AX. No problems what-so-ever.

As to how you "know"... that's basically up to you.

A Pitts style muffler will give the plane a better "look" and may require far less cowl cutting, which is a good thing, but you could also probably install the stock muffler, at worst using an extension.

Much like with ANY ARF, you have to play with the fit to make it work.

If you've never done this before, you may want to enlist the aid of a more experienced builder to help you out, as cowls are easy to ruin.

I read the manual of the GP extra on their website and it does in fact recommend a pitts muffler for a .61 which is very similar in dimensions to the .75 AX so that's that ill be doing.

75AX Powerbox 6.0oz and the slimline pitts 2.6oz. I have not run the engine yet so I can not comment on performance.

I can attest to the performance with the Power Box muffler installed. It is one great engine. I have it installed on a Goldberg Protege and it has more than enough power for that plane. And as for engine performance, well, I am using a 13.5 x 6 Bolley Prop and this thing has pretty much unlimited vertical. Takes off in about 20 feet on grass with the stock wheels (those puney 2 1/4 inch things) and will fly "all day" at less than half throttle.

I put it on the plane, checked the CG.. right smack on, by the way without any nose weight so the engine was perfect for this plane (won't be the same with others, but based on my experience with some of the Goldberg ARF's, there has been an issue with excessive nose weight to get a correct CG). I then headed out to the field. I fueled it up, primed the engine by going to full throttle, put my finger over the carb and rotated the prop five revolutions. I then attached my glow driver, checked the needle valve and set it to 2.5 turns out from the stop, and hit it with the chicken stick. One flip. ONE FLIP.. and it was running.

I ran it very rich at about 1/4 to 1/2 thorttle for about two to three minutes, then slowly advanced it to full throttle with a very rich mixture and ran it that way for a few minutes.

In other words, I broke the engine in correctly.

After the first tank on the ground, I refueled it, and started it up again. I adjusted the mixture for good full throttle RPM but on the wet side of lean, and checked the transition from idle to full. Perfect. I set the idle, and it was at about 2000 RPM and ran just fine. So, I put it in the air, with only one tank of fuel for break-in. It ran perfectly. Absolutely perfectly. And the power. Wow!!!

A great engine.

I own three OS 1.20 AX engines and all three run exactly like that. Great power, great throttle response, and start very easily.

CGr.

Do you know what RPM's you're getting with the Bolly 13.5x6 CGr?

Looking at the suggested prop sizes in the manual i was going to start with a Bolly 13.5x8 in my Super Skybolt but now you have me wondering.

Hey, beginners,
RCU has an excellent engine forum. We also have an OS subforum that is monitored by at least one factory rep. The guy is excellent.