Our old iron headed 1971-1975 4.1 log and 2v and 4.1 cross flows from 1976 to 1980 with just 9.15:1 compression would run very badly on 91 octane, great on 96 to 97. Same with any Cleveland 351. Downgrade the octane, and it would detonate. These were 16 thou deck engines, where the piston stopped short 16 thou, not 103 thou like the 250, or 25 thou like the old log 200.
The Classic Inlines is a modern redesign of the 2V 250, and its alloy construction and high swirl mixture motion gives you detonation resistance with your 89 aki pump grade gas. To be safe, use 93 octane, but the cylinder head isn't canted valve, and is made of alloy, and it should handel a lot of compression with even a short duration cam.
The three US pump grade gasolines has more variation than NZ fuel, as you have oxygenated blends now, but all NZ fuel is aromatically inferior, and has less controlls in place for the levels of benzine and aromatics. In Kiwiland, three grades of It can go from the bowesers 91, 95 or 98 call rating to just 83, 87 or 93 in just a six weeks. Our rating is MON+RON/2. Your rating is either AKI or RON. Our fuels are 3 to 5 % aromatic benzenes, a lot more than US blends. The so called octane difference is explained fully in this article
http://www.scribd.com/doc/26804743/How- ... vid-Vizard
*Source:How to Modify Ford SOHC Engines (Pinto/Cortina/Capri/Sierra 1.3- 2.0 litre 1970 to 1982) by David Vizard, published 1984 by Fountain Press.
ISBN 0 86343 9856, Dewy Decimal No 629.2504 Viz
See Chapter Nine on Page 119-124
This is a very old article but it covers most of it. You can blend
1. alcohol race fuel to get a target r+m/2 octane ratio,
2. or use a water injection system with pure water or as Anti Detonation Injection in a 50/50 water/alcohol ratio
3. or use a water injection system as Anti Detonation Injection (ADI) in with a 50/47/3 blend of water, alcohol and acetone with a small amount of fuel conditioner to create a very high compression, long duration cam engine that runs on standard grade gasoline. You can run them like Mustang Geezer used to. I use ADI to operate 12.7:1 on my 3.3 engine, while still using 91 r=m/2octane, our lowest grade.
He noted
It was also pointed out in the cylinder head chapter (CHAPTER 2) that high compression ratios are needed if long- duration cams are used because the valve overlap causes much of the incoming charge to be pushed back into the intake duct at low rpm. This leads to a "soggy" feel of the engine. By utilizing a high numerical compression ratio the cam can be made to work much more effectively
He noted in Chapter 2
In some areas of the world, such asthe U.S.A., the octane rating of available fuels is little better than that of peanut butter. Once compression ratios start exceeding about 9.7-9.9:1 the engine is likely to run into detonation problems, even on premium-grade fuel. Where 100 octane fuel is available, compression ratios up to about 11.5 or 12.1 can be used.
The X-flow and log heads were a lot like the early Pinto 2000 engines, they didn't cary a lot of compression before detonation set in, but the Classic Inlines head has a lot more mixture motion and always is mated with a wilder than the short period 240, 252 or 256 degree cam....everyone seams to go for medium duration cams like the 260 and 270 Schnider, the 260 Comp Cam, or 264 and 274 Clay Smith cams, and they bleed off effective compression. Down here, we go a lot more radical flat tappet and roller cam durations than the US does, and use much higher lift with 300 thou longer 5.16" Holden 308 valves and common deep dish pistons. That's how come you see 250 sixes pushing the 350 to 410 hp barrier often in oval track and drag race engines. The detonation issue drops off with long duration, high lift cams and deep dish pistons. Experts say that the shallowest piston with relief for flame front travel is the best to kerb detonation, but that is for NAscar engines. For the street, whatever gets the compression ratio into the sweet spot for a medium duration cam also yields detonation resistance. The decimlized 87/89/93--->8.7/8.9/9.3 is a good rule for iron heads, not alloy ones. You've got quite a lot more head room to play with when the head is aluminum.
I used for 8 years 100 MON+RON/2 Liquid propane Gas, and it allowed 9.65:1 compression with just a 252 degree Heatseeker cam and 500 cfm Holley 2-bbl and 348 cfm mixer on my 1984 Falcon 4.1 X-flow. 34 degrees total, 9 degrees static, and it was hammered and never detonated due to the compression ratio. It did get some rtv stuck in the carb, and melted a combustion chamber and blew a gasket, but it was easy to reweld the camber, and the pistons and bearings were fine. High compression alloy Canted valve heads and propane are the worst combination for detonation...I never got any. The Classic Inlines head is more like the FE and Windsor, HSC/HSO Tempo/Topaz/ inline four and Big Six head, and they are much more detonation resistant than my X-flow heads were.
I think 9.8:1 with the Classic Inlines head, a 264 Clay Smith cam and a good carburation system with the well tubes that aren't damaged, and power valves working, and the right Power Valve channel restrictions and a good igntion system without more than 34 degrees total and a quick advance with a lot of static timng should like 93 AKI, and the odd fill of 89 AKI.
I've driven in Houston and 9 times out of 10, I'd rather be there than in Laffy! That being said... I probably am safer off sticking with a lower CR. 
