15 to 1 Compression on Alky

No solvent additive is necessary - so please don't mention it anymore. Just add 25% race gas to methanol and nothing more. Problem solved.

How do you accurately measure the 25% at home in your home garage ? With methanol, the less time you have your can open, the better - it takes up moisture. Years ago, many service stations had a manual pump which used to dispense a measured amount of fuel from a glass container. I would not try to mix using measuring cylinders - you would be exposed to all the risks.
Of all people, I am probably better equipped to blend fuels, but I would not do it at home. I also probably have more to gain by using a blend, than most other people. With straight methanol, there are no complications - as long as you drain the carbs after every race meeting.
What really surprised me was that by using straight methanol, my 850 motor is quick enough to win against some pretty formidable opposition, even with low comp. It really has not had much done to it. The cam and ignition timing are advanced and it has a 2 into 1 exhaust. Without methanol, I would probably not be able to do that. The exhaust system or a piston might melt.
The only real problem the bike has got, is that it is too loud.

I suspect that with modern bikes, the valve timings used are limited by the noise levels. If you play with the timings, you can get a lot more power, but the noise levels get too high. Exup was not developed for nothing. They claim it improves torque - but an overall increase in power with the top end stifled also means an increase in mid-range power. When I first started racing my 500cc short stroke Triton, it had separate pipes with megaphones and would rev easily to 10,500. But it was unrideable. I fitted a two into 1 exhaust and that immediately cut 2000 RPM off the top end. I cut the collector back and fitted bigger tail pipes, until I had only lost 1000 RPM off the top. Then I started getting decent lap times, but that bike was also too loud.

At the risk of returning to the original topic, has anyone any figures on the point at which gains from increasing compression lose out to the losses from the less than optimal combustion chamber shape? Assuming methanol and a LSR or drag race motor, big cams etc and no problem with supplying fuel volume. The OP said that the sidecar outfit would drop to 13- odd compression so I'm guessing that improved flow from the new head is more important?

In Australia, the usual comp. ratio at which methanol is used is 12 to 1. Doing that with a big Norton Motor, compared with a Triumph 650 motor, is chalk and cheese. Even the 500cc two-valve Jawa motors have squish bands. However any two valve motor is never as efficient as a four-valve. One of my friends had a 650 Triumph motor fitted with a Rickman 4 valve head - said he could not tell the difference between it and using the two-valve head. But with methanol, most guys run it rich - so they never get the best out of their motors anyway.
My brother has an 880 JAP sidecar outfit on speedway. It is on 17 to 1 comp. - he uses Dell Orto speedway carbs, but he has fitted needles into them. He recently took the bike to a meeting and as he turned onto a straight, gave the motor a handful. The bike stood up and did a quick right-hand turn into the infield. He is accustomed to tuning 3-cylinder 750cc Kawasaki two strokes on methanol, so he knows how to get the bikes going. With methanol, you can race very well if you are jetted rich, but lean makes a big difference. At lower comp. ratios in four-stroke motors, you have more leeway as you lean off the carburetion. At high comp. , it is more critical to get it right.

I met a guy who had an 900cc Triumph triple on 14 to 1 comp. with methanol fuel. Apart from pulling the back out of the motor, his main problem was in getting the ignition system to be reliable.