The original Rectimo 1200 engine (stroke 64mm, bore 77mm, capacity 1192cc) had a compression ratio (CR) of 7:1
So its combustion chamber volume is 42.57cc

Converting that to a Rectimo 1400 (stroke 64mm, bore 83mm, capacity 1385cc) with a combustion chamber still of 42.57cc gives a CR of approx 8.1:1

Rectimo's AR1600 had a CR of 8:1

Limbach's 2000EO & EC have CRs of 8.4:1, but their L2000 EAI, EBI and EOI have CRs of 8.7:1

The Rollason Ardem 4CO2 1500 had a CR of 8.4:1

The early Revmaster 1700, 1834, 2100 all had 9.5:1 CRs (although their current 2300 runs 8.0:1)

Aeropower Engines of Australia used a max CR of 8.9 but specified only using Avgas.

Great Plains direct drive engines all use 8.0:1 CRs, specifying 96 octane fuel.

Lycoming runs CRs as high as 10:1 on some of its helicopter engines (although most are in the 8.5:1 to 8.7:1 range)

Continental runs CRs up to 11.4:1, but again mostly in the region 7.5:1 - 8.6:1

My engines both have CRs of 8.2:1, with no overheating, pre-ignition, detonation, running on or any other problems. I run them on at least a three-to-one mixture of 91ron mogas to Avgas

To increase power at little cost and effort I am considering raising the CRs to 8.5:1

Has anybody else done this? Were there any problems?

Yours, Bob

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My Rectimo is 8-1 and I run Auto 91-92 octane fuel seems to run fine. I am rebuilding Bob Brocks Rectimo and installing 69mm crank with 40hp rods, 83 bore. I am planing on 8.5-1 for his engine.