I've always flown my planes with 2 cycle engines but am considering buying a OS FS91 II Surpass. I have an OS 60 Fx and an OS 91FX and I'm not sure if a 4 cycle OS is as easy to tune and as reliable as the 2 cycle ones.

What 4 cycle OS would be comparable to the OS 60FX and to the OS 91FX for 3D flying?

An OS 60 2-stroke will produce 7 - 8 lbs of static thrust. Your 91 2-stroke will produce 10 - 11 lbs. An OS91 surpass will produce about 10 lbs - I have one, that is what I get. I like about 1.5 or more thrust to weght for 3d. If I were in your situation I would get a Saito 91 or maybe a Magnum 4-stroke. My Surpass is a good engine but I like my Saito a little better and it is lighter.
In general 4-stokes run better in the half to 3/4 throttle range, smoother resopnse and quicker spool-up than comparable 2-stroke engines. I also have aYS 63 that is smooth in transition, light and powerful. I fly both 2-stroke and 4-stroke engines and don't fuss much with tuning either.

Don't know about OS, but the equivalent Saitos will be the FA-100 and FA-125 respectively. BTW, shouldn't it be the 61FX?

For what reasons are you looking to go to 4-stroke? If its to fly 3-D better,forget about it,a good running,over-powered 2 stroke will do an excellent job. Ive been there,I switched a few years back when a lot of people thought 4 strokes with larger props was the way to go. I have now gone full circle and back to 2 strokes,now that I know a little more and my skills are better,I'm doing the best 3-d I've ever done, and the sky's the limit,sometimes,the ground is the limit.

I think the new generation 2 strokes (AX series) operate at a lower RPM than the old style 2 strokes. This allows the biger props for larger "wash" which is better for 3D. I know the 120AX is rated at 9000 RPM. That's very similar to 4 strokes. I'm guessing the engines are becoming similar. The 4 strokes are turning more RPM. Maybe we should call all the new generation engines 3 strokes.

Me...I don't think any engine will compete with the YS110 in it's class. At least when it runs. Which is most of the time, unless it doesn't.

Thanks
Barry

Oh I almost forgot...another advantage of the 4 stroke is it will allow you to burn the expensive high nitro fuel.

Thanks
Barry

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Why would anyone act proud that they don't tune their engines? That is like bragging that you are illiterate and proud of it. (shakes head in disbelief)

Whether its 2C or 4C only you can tell. I enjoy both but with a 4C the throttle response is different a little more linear. A 2C engine, generally, gets its tirque at the upper end of the RPMs so you have to push her unless like Barry says, over power it. And yeah YS110 is the best in its class!!!

Artisan - maybe he means he does not have much trouble in getting them tuned?

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Let's hope so. If I had it to do over again, I wouldn't...<G>

Hey Artisan,
The guy was worried he would have to fuss with tweaking his engine all day if he got a 4stroke. I said I don't fuss a lot with mine. I didn't say I have someone else tune them. As a matter of fact no one tunes my engines but me and I am the field 4-stroke 'expert'. My engines don't require a lot of tuning and work fine for me. I shake my head at the guys that think they have to reset all of the needles every time they fire up an engine. I would if I was trying for a speed record or something but for just having nice flying 95% of perfect is plenty. I spent 30 years as an engineering analyst developing advanced propulsion systems and know something about tweaking things that need it.