Hi Ron,
Sorry but I think you may be confused on the compression ratios. The normally aspirated IO-550 has 8.5:1 pistons. The TSIO-550 (and most other turbocharged Continentals) have 7.5:1 pistons.

From John Deakin/ AvWeb (Pelican's Perch #35) "Those Fire Breathing Turbo's (part 5):

"However, if you really, really must know the HP you're pulling, there's an incredibly simple trick to find out. On the flat, horizontally opposed, air-cooled aircraft engines with 8.5:1 compression ratios (including TCM IO-520/550 and Lycoming IO-540), simply multiply fuel flow in GPH times 14.9. The result is HP. If you insist on percent, divide that by the rated power of the engine. For the same engines with 7.5:1 compression ratio (most factory-installed turbos), the numbers are worse, and the multiplier drops to about 13.7."

Ref: <http://www.avweb.com/news/columns/182106-1.html>

Ron Laughlin
Legacy 156

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