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OS .75 AX vs. OS .61 SF ABC

Old 11-21-2011, 11:24 AM
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KDBrown
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Default OS .75 AX vs. OS .61 SF ABC

Greetings,

I just purchased a brand new (NIB)OS.75 AX for $165.00. Ihave an OS.61 SFABC(Long stroke) that is older, but Icarefully broke it in when it was new, and it only has about 8 flights on it. I bought this new around 1987. Everything looks and feels new on the engine. How do these two engines compare? Was this a good price for the 75 AX? Will it pull better than the .61 SFABC?

Thanks!
Old 11-21-2011, 11:35 AM
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Sport_Pilot
 
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Default RE: OS .75 AX vs. OS .61 SF ABC

I would certainly think it would out pull the SF since it has .14 cubic inch's of displacement over it.
Old 11-21-2011, 02:29 PM
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Strykaas
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Default RE: OS .75 AX vs. OS .61 SF ABC

Try them out with different props, that would be an interesting comparison !
Old 11-21-2011, 03:18 PM
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Rendegade
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Default RE: OS .75 AX vs. OS .61 SF ABC

I recently fitted a 61SF to a nemesis thinking it'd go like stink. It did but wasn't fast enough.

I then grabbed a 91FX, fitted the same carb (7F) and the SF muffler and tried again. It was only marginally faster, but got very unhappy swinging an 11x11, where the 61 seemed OK with it.

All in all I think there was only about 1000rpm different between the 61 and the 91.
Old 11-21-2011, 05:40 PM
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delman
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Default RE: OS .75 AX vs. OS .61 SF ABC

The 61SF muffler is too restrictive for the 91FX to perform at its best. The 91FX muffler has the extension in the middle for the increased exhaust volume.
Old 11-21-2011, 07:10 PM
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Default RE: OS .75 AX vs. OS .61 SF ABC

the 75 is a powerhouse....but the 61sf will outlast it


my friend has a 75ax on a 60 sized trainer with a mas 14x6 and it revvs it like crazy... i cant see any 61 doing that
Old 11-21-2011, 10:24 PM
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KDBrown
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Default RE: OS .75 AX vs. OS .61 SF ABC

Thanks. I typically run my engines on the rich side, so maybe that will help the 75 AX a bit. Idon't lean them out in order to squeeze every rpm I can get. Oh... and thanks for actually responding to the topic...
Old 11-22-2011, 06:49 AM
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MinnFlyer
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Default RE: OS .75 AX vs. OS .61 SF ABC

I've got both of these engines and the 75 blows the 61 out of the water.

I use a Graupner 14x7 prop on my 75AX
Old 11-22-2011, 08:16 AM
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Default RE: OS .75 AX vs. OS .61 SF ABC

Put a 12x7 or 8 on both of them and then trying pulling a nine pound model straight up (Hobbistar 60). I think you will see the roles reversed.


Ed Cregger
Old 11-22-2011, 05:23 PM
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Ernie Misner
 
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Default RE: OS .75 AX vs. OS .61 SF ABC

Do they both weight about the same? If not, that would be a factor, but seems that 61's weigh the same usually as the 75, which weighs the same as a 91..... so maybe trade both of them for a 91! (I think a Saito 125 is slightly lighter than all three) I'm sure you are pleased with the deal you got on the 75.

Let us know....

Ernie
Old 11-22-2011, 07:19 PM
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KDBrown
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Default RE: OS .75 AX vs. OS .61 SF ABC

Yes, I was originally looking at the 91 AX as my first choice, but when I saw the 75 at that price, couldn't pass it up.
Old 11-23-2011, 06:28 AM
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Default RE: OS .75 AX vs. OS .61 SF ABC


ORIGINAL: Ernie Misner

Do they both weight about the same? If not, that would be a factor, but seems that 61's weigh the same usually as the 75, which weighs the same as a 91..... so maybe trade both of them for a 91! (I think a Saito 125 is slightly lighter than all three) I'm sure you are pleased with the deal you got on the 75.

Let us know....

Ernie
Many of the early .61's weigh less than the present ones. However OS had heavy engines (back then)since the mid 70's or so. Webra's, K&B,and others were much lighter back then.
Old 11-23-2011, 01:53 PM
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Default RE: OS .75 AX vs. OS .61 SF ABC

I've had 2 OS .61SFs, 1 ABN & 1 ring, and also a .46SF ABN. Both ABN OSs had the liners peel. I still have the OS .61SF ring and have used it in at least a dozen airplanes. I would recommend a castor blend fuel in the ABN engines. I haven't had any trouble with my OS engines after I started using castor. I also set for a slight smoke trail after takeoff.

The 75AX is a horse. I don't own one, but I've seen them. Don't think you need a small 12" prop or even a 13-6. It will turn a 14" prop just great with tons of thrust.
Old 11-23-2011, 05:40 PM
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Default RE: OS .75 AX vs. OS .61 SF ABC

Thanks. I also use a castor blend. Omega typically on my ABC(ABN) engines. Never a problem with peeling.
Old 11-23-2011, 09:46 PM
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Default RE: OS .75 AX vs. OS .61 SF ABC

A buddy of mine runs his OS .75AX with a 14x8 Master Airscrew and it really hauls the mail. It tosses his 9 pound Extra 300 around like a feather; unlimited vertical if you want it at half throttle. The engine does seem to do better with a bigger prop and load, although if you reduce the prop, it really screams. I'm impressed...
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