On Tue, 11 Dec 2007 01:23:01 GMT, Matt Whiting
wrote:
wrote:
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 
I'm not sure, but I don't think the difference in compression ratios
will have a significant affect on wear or where the power peaks. The
main difference is that lower compression engines are more detonation
resistant and thus you have more margin to run low octane auto gas.
Matt
And that is only true to a point.
A case in point.
The Corvair engine with the low compression ratio "smog" or "open"
heads is significantly MORE detonation prone than the non smog or
"closed chamber" high compression heads. Also, lowering the
compression ratio of the high compression engine by using thicker head
sealing rings (gaskets) actually CAUSES detonation, rather than
reducing it. The difference in quench and squish is AT LEAST as
significant as compression ratio as far as detonation is concerned.
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