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Acro II engine choice

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Well-Known Member
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Jun 12, 2007
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Seattle, WA
So, this is an interesting question. I'd like to build an Acro Sport II, and two engines seem to immediately suggest themselves:

Lycoming O-320 at 150 or 160 HP

Lycoming O-360 at 180 HP

Fortunately (unfortunately?) there's an army of RV builders out there debating between these two engines, which has been most informative to read. I wouldn't say it's clarified anything, but it's been very informative.

My thoughts are now revolving more around biplane-specific questions than engine-specific questions.

* How does a biplane benefit from more HP, once you're within a reasonable range? Is it in increased climb? Increased top speed? Increased gross weight? My guess is that climb goes up noticeably, gross weight goes up a little bit (assuming the gross has been reduced for lower HP, and that one doesn't exceed the structural gross weight of the design), and top speed might go up, but not very much (and not enough to be a good reason in and of itself). Does that sound right?

* How does a biplane benefit from less HP? This is basically the converse question, but what would the plusses be of going with the 160 HP motor? The O-320 and O-360 seem to be within a dozen or two pounds of each other, making the weight differential minimal, although enough to be noticeable. Does the lighter motor entail lighter accessories or firewall forward components? Ie, does total weight change for the whole engine package, or is it really just that 15 lb difference?

Contrary to what I would have expected, most people seem to claim that fuel burn at equivalent speeds and airframes favors the larger engine. The reasoning seems to be that the smaller engine is working harder to make the same speed under the same conditions, and fuel burn goes up quickly with % power (probably not exponentially, but more than linear). That argues that a larger engine makes more sense from a fuel burn standpoint, if I have the self-discipline to run at the lower speed. If I'm just going to run the throttle all the way forward, a smaller engine will be cheaper to run. The bigger engine leaves me with a greater margin if I need to get out of a short/high/hot airport.

The price of the two engines, particularly with 2000+ hours on them, seems to be about equivalent. I'm certainly capable of rebuilding a motor, so I'm likely to go that way to save my pennies. Apparently an O-320 used to cost more than an O-360, which may explain in part why the O-360 is such a popular motor. Even now, I guess they're nearly the same price for a new or rebuilt motor.

I'm all ears. What do I have wrong? What do you think?


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