There's a guy in our club who's been independently experimenting with his 4 strokes to find the optimum compression for them to run on zero nitro and he's also found that 13.5:1 is ideal. We were both surprised when we discovered we'd both been playing around with compressions for the same purpose.

Early Saitos originally had very high compressions so they could be run with zero nitro but Americans started complaining that they didn't run very well or overheated etc because they automatically used fuels with high nitro. Saito then had to reduce the compressions to suit the Americans because it was the biggest market.

I suspect that the 4 stroke with the lowest compression is most likely the YS because apparently it needs about 30% to run properly even with their supercharging system. This is a kind of win/lose situation because while high nitro definitely adds power a low compression makes an engine less efficient even when using a high powered fuel. The edge goes to nitro but at the expense of very high fuel consumption.

I guess a rule of thumb could be made by saying that if 13.5 is ideal for zero nitro and 7.5 is ideal for 30% then a compression half way between these two at 10.5:1 should be ideal for 15% nitro. In fact this is probably quite close.