Posted for "Jonathan Fuller" <jfuller@cox.net>:

 Robert,
    I don't know what a TSIO-550-N is for sure but if memory
 serves, Columbia flat rated their engine down to 310hp to get the longer
 TBO.
 ...so that may relate to their choice of compression ratios, if it is not a
 misprint.
 
 At any rate, there are more than a few TSIO-550's flying now with the higher
 compression ratio.   The advantages of that are: 1) lower CHT's due to the
 fact that the greater expansion factor takes more heat away and 2) 5% or
 better improvement in fuel economy.
 
 Personal belief is that you get to run at higher power settings at high alt.
 cruise because you aren't temp limited.
 
 If I had to do it over again I would certainly use a 8.5:1 setup.
 I think the 7.x:1 idea was that if you mismanage your mixture you have
 a little more margin at 7.x:1 than at a higher compression ratio.
 But the impact of mismanagement on internal cylinder pressure is so
 much higher than the impact of a slightly higher compression ratio, I
 don't see the sense in keeping the lower compression ratio.   If you would
 blow up your engine at 8.5:1 you would probably blow it up at 7.x:1 also.
 
 
 Colyn,
 
 The higher compression ratio engine would actually be expected to have
 higher CHTs, but lower EGTs and TITs.  The reason for the difference is just
 as you say however.  The greater expansion factor takes waste heat out of
 the exhaust and converts it to work, so CHT is higher, EGT lower and brake
 specific fuel consumption is lower.  The way to mitigate the higher CHTs is
 to run the engine LOP, and LOP operation will also yield the lowest bsfc.
 
 I believe the -N model number refers to the Continental engines with tuned
 induction.
 
 Jonathan