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Permalink Reply by Ernest Christley on March 4, 2021 at 9:16am

I flew a 601 with a 2850 for a year or so, and then switched to a 3100.  I wouldn't switch back.  The extra power makes a world of difference, and it is only a few extra pounds on the nose.  Because everyone is using a 5th bearing anyway, I don't think it causes any measurable stress that would impact longevity.  

The case against the 3100 is that it requires a modified case, and there is concern for finding parts (heads specifically) in the case some need to be replaced.  I don't give much weight to this concern, because it isn't like there are that many places you'd be able to get Corvair parts in the first place.  You're NOT going to be able to get a replacement head at a remote FBO.

I've got a 2850 and a 3100 sitting on a shelf if you're in the market.  Neither is in flying condition, as they require accessories (carb, generator, etc.)

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Permalink Reply by John Mazzolini on March 4, 2021 at 1:55pm

Thanks for your input...I'm still building and not yet ready for the motor...but you never know

Please the particulars on what you have on those 2...you can email me at jmichael1904@gmail.com

John

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Permalink Reply by Bob Pustell on April 6, 2021 at 12:11pm

In my opinion it depends on how much you can afford to spend and what type of flying you plan to do. Generally speaking, more power is always better (up to a point). That point is when the extra power starts to increase weight and/or fuel burn dramatically, and/or starts you into exceeding the performance limits of the airframe.

With that said, and based on having flown a Rotax powered and a Lycoming O-235 powered XL, I would go for either the three liter or the 3.3 liter stroker Corvair. There is no significant weight or fuel burn penalites in having the more powerful variants of the Corvair conversion and the improvement in takeoff and climb performance I experienced between the 100 HP Rotax and the 115 HP Lycoming powered XL's was dramatic. Once you have met the minimum horsepower required to get a plane to fly, any surplus power ALL goes to improved takeoff and climb performance so even a modest power increase makes a dramatic improvement in performance where surplus power is needed.

Just to pull fictional numbers for example - if an airframe takes 70 HP to just barely take off and climb, it will do so but will be very modest, even scary, in performance. An 80 HP engine would give it 10 more to climb with, a 100 HP would give it 30 more to climb with, etc. The extra power ALL goes into acceleration and climb improvements. However if you put a 250 HP engine into that little plane it most likely would be uncontrollable at full power and would have a fuel burn that would drain the tanks in half an hour. Everything has its upper and lower limit.

With all that said, my XL will have either a three liter or a stroker Corvair in it when completed. If my finances will permit the stroker that is what I will go with - more power with no incrase in weight or cruise power fuel consumption means I will put the strongest Corvair conversion I can afford up front. If my retirement income will not support a stroker, it will be a three liter as my personal minimum - I want decent climb performance. The 2850 or the 2700 is just not enough power to provide what I want in the plane. In something like a Peitenpol or a Cub Clone the 2700 or 2850 would be plenty of engine, but not in the XL/650 in my opinion.

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Permalink Reply by Ernest Christley on April 6, 2021 at 5:01pm

1hp = 550ft-lb/s

In rough numbers, once the base takeoff power is met, an extra 2hp will lift the typically loaded XL an additional foot each second.  Ignoring the shorter takeoff run length for simplicity, I calculated that on the 1900' grass strip I was flying out of, with a climb-out speed of 60mph, the extra 20hp of a 3100 over a 2700 gave me an additional 50' over the Carolina pine trees at the end.

When you feel like you're barely clearing the trees in the first place, that extra 50' makes them feel like a world away.

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