Back in '93, when I was designing the Lycoming engine installation for my IV-P,  I was asking the same questions about compression ratio. In researching the topic and talking to engineers at Lycoming and Continental I uncovered an interesting fact; the exhaust system is frequently considered part of the airframe and not part of the engine. In those cases, the design of the exhaust system is the responsibility of the aircraft manufacturer, not the engine manufacturer. This caused the engine manufacturers to be very conservative regarding detonation margins and therefore, compression ratios.

My investigation and analysis lead to the conclusion that a higher compression ratio (8.5 :1) and lower max manifold pressure (35 inches) combined with intercoolers would result in an engine with superior thermodynamic efficiency and lower exhaust temperatures. I hired a retired Garret engineer (designer of the Mirage turbo installation) to assist in selecting specific components (turbos, waste gate, controller and relief valve). I  designed an  Oil Sump with integral engine mounts and Oil Suction Strainer, a Oil Baffle Plate, Tuned Induction Plenum and Intake Runners, Intercoolers, Intercooler Flow Combiner, Turbocharger Mounting Brackets,  Exhaust System, Baffling and ducting.

I integrated the components on a used IO540 K series engine core and sent the assembly to Barrett Performance Aircraft  for overhaul and dyno testing. The BPA SAE calibrated dyno test data shows the engine producing 360 HP at 2700 RPM and 35 inches MP. Power tests were run 150 degrees rich and TIT never exceeded 1500 degrees.  Monty Barrett was impressed by the engine and still features a picture of it in his print ads in TAP. After nearly a dozen years and over 900 flight hours (and several races) I can report that  the engine is still going strong with all of its original parts. Compression is still in the mid 70s. I  cruise at 75% power, 2,500 RPM, 28 IMP and 21 GPH.

A couple of years ago I shared the details of this installation and flight experience with the engineers that  went on to produce an STCed high compression and sealed plenum modification for the Mirage.

My experience with the higher compression ratio and lower maximum manifold pressure has been positive however I must caution that the ultimate success of an installation is determined by the details of the design, not the "big" parameters like compression ratio.

Regards
Brent Regan