compression ratio + pump gas
So the octane rating system has changed since the "old days"? I had an Olds in high school that had 10.25 CR and ran fine on pump gas even under heavy loads. I guess I gotta try to get it into my head that what "used to be" + what "is" are not the same :(
There's a reason for that. There was a big difference between advertised compression ratios and actual compression ratios. That 10.25 Olds was probably lucky to have one full point lower. I had a friend whose father had a 68 Olds Delta 88 with a 425. It said "Ultra High Compression" on the air cleaner. It was advertised the same as yours was at 10.25. I can remember the same thing. He ran Ammoco 92 in it and it never spark knocked. Just like all of the Chrysler engines. You can disassemble any original Chrysler engine and blueprint the compression ratio and it will fall way short of what it was advertised to be.
Now after all that hot air, I will try to answer the question.....as I am building my slant right now and am in the process of figuring it out myself. The answer is "it depends". If you want to spend the extra money and time, you can certainly build a slant that will run on pump premium with probably a max of 10.5:1. With excellent quench, good polishing of the combustion chambers, profiling the pistons to remove any sharp edges. But all that takes time and a LOT of money to get someone to set it all up. Everybody throws it around that you can run really high compression on pump gas and no one ever details how to do it. Let me explain.
To properly set up a quench engine, The piston must be about .035"-.040" away from the part of the combustion chamber where the valves are NOT. On a slant, you would need a domed piston to acheive quench, or one of those Argentine closed chamber heads. Good luck with either. But lets suppose for a minute that we DID have a closed chamber head to use.
That means the piston needs to be between .035"-.040" away from the flat surface of the head at TDC to acheive proper quench. Ok. No problem, just get the right piston.....like for instance what I am using, the KB239 flat top that can acheive zero deck height. Not so fast. Each combustion chamber is a little different. Each connecting rod may not be exactly the same length. Each crank throw may not be indexed the same. Each piston head may not be the same. When you're measuring in THOUSANDTHS of an inch, there can be a LOT of variation. There's a LOT of EXPENSIVE and tedious machine work involved getting the quench distance the SAME in every cylinder. So, when you see people say "yeah you can run 10.5 on pump gas", they are usually shitblowers, OR they are leaving out a LOT of info.
So, to try and further narrow down your answer. With completely untouched combustion chambers, pistons and valves, the absolute most you can probably get by with on pump premimum is around 9.2:1....and that's going to be with a camshaft with a decent amount of overlap to bleed off some cylinder pressure. Put too mild of a cam in to jack cylinder pressure up and you've got spark knock.
Now, take that same engine and polish up the combustion chambers to get all the sharp edges out that might start detonation, profile the pistons to do the same and you've just assured that you will probably NOT spark knock at that same 9.2. But that doesn't mean to ADD compression since you did all that work because then you're in the same boat. You're just assuring by all the polishing and profiling that you will NOT have spark knock at 9.2.
Can you possibly take the unmodified slant and run over 10.1? Yeah. With a ridiculously huge cam that bleeds off tons of otherwise useable cylinder pressure, sure you can. But why would you want to when you can build a 9.1 engine that will make more power with a smaller cam that's utilizing more of the cylinder pressure?
Let me put it this way. Go look at my ported head thread. Look how smooth those chambers came out. I am going to do my pistons the same way. But, I am actually afraid I am still going to have too much compression. I am sure that I will have to have a custom head gasket made much thicker. Although I have not measured anything yet, I really don't have to, I already suspect it. Even with good polishing and everything else, without quench, which in a slant is impossible without a domed piston or close chambered head, running on anything above 9.2 on pump gas is risky.