What/how does compression ratio affect an engine?

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Hi,

So I was fantasizing the other day (as I do quite a bit) about my
latest dream-plane-to-build: an RV-9A. (Note that this is fantasy in
the extreme as I'm not a pilot yet AND I have no money!)

I was thinking of how to power this plane. A Jet-A burning diesel
would be great but that's another story. I spotted an ad for
Superior's XP-series engines in Kitplanes. The website has a great
"build your own engine" feature where you get to change all the bits
and customize the engine.

One of the things you have to choose is compression ratio: 7:1
(150hp), 8.5:1 (160hp), or 9:1 (165hp).
Going for the 7:1 option (from the default 8.5:1) adds $100 to the
price! I'm assuming this is a supply/demand issue.

So my question (finally) is: what is the effect of a higher or lower
compression ratio? I believe TBO for all three engines is still 2000
hours.

- Is there a difference in wear?
- Would maximum power be produced at the same RPM for all three
engines? In other words is there a relationship like (power) =
(compression ratio) x (RPM) such that these engines all operate at the
same RPM? In which case wear would be the same...(?)

For the RV-9A 150hp would be fine. I guess I'm trying to understand
what benefit is to be had by spending the extra $100 to go for the
lower compression pistons. All in my fantasy

Thanks,

Michael

The extra $100 is probably because the 7:1 pistons are a special order item,
as opposed to the 8.5:1 pistons which are the standard.

Max RPM is the same for all 3 engines, and the only wear difference should
be on the connecting rods. Apparently the difference isn't enough to change
the TBO, which is a theoretical figure anyway.

As to autogas vs 100LL, plenty of people running 8.5:1 compression engines
are running autofuel. A bigger issue with autofuel (at least in engines
with a compression rapto of 8.5:1 or less) is vapor lock, rather than
detonation.

Kyle Boatright
160hp (8.5:1 Lycoming) RV-6