My 502 Has a 9.8 compression ratio with aluminum heads. I will be running with FW cooling and a 160 stat. I'm not sure of the octane requirement needed to prevent piston knock. I don't have a knock senser type ignition system, and my engine is carburated. I doubt knock can be heard with the noise from open exhaust. I'm sure 93 octane would be sufficient, but I would rather not pay for something I don't need. The idea that higher octane will produce more power is a misconception! Anyone have any thoughts on this?

93 is a waist of your money, my whipple engines dont require 93.........you're good at 87-89

I agree with f2speedy. Aluminum heads you can get away with 1/2 to 1 point of compression more than that of cast iron heads. I would have no issues running close to 10.5:1 max with aluminum heads NA build. I use to run 9.5:1 on NA 89 octane for miles on end with cast heads and never ever a problem. Now a days with inconsistent fuels I'd do a perodic cylinder wall and piston tops with a bore scope just to be certain. My plugs always looked great and no signs of detonation. The aluminum heads transfer the heat better and a little cooler.

Might be an old wise tale however when I was young I rember some saying if you mix premium with 87 half and had good results. Have no clue if true or not. Regardlesss you'll be fine but just do a periodic check. If you don't have a bore scope get one. Cheap now a days and a good multi purpose tool to have.

depending on your area - premium may be the only fuel w/o ethanol

So true, a very good point

A lot of this is in the tuning. Many "rules of thumb" stem from years ago, where carburetors, locked timing, and reading plugs was the norm when tuning an engine .

Today, you'll see cars coming from factory with high compression ratios. My Challenger scat pack was 10.9:1 static. Ran on 91, or even 87. The key here, is engine management systems. If I ran 87, you would see significant spark retard by the ecm, with feedback from the knock sensors. 93 Octane, minimal or no knock retard from ecm.

Keep in mind, these modern engines, use sophisticated EFI fuel systems. They aren't like an old bbc with a dual plane intake carb setup, that likely has 1 cylinder running at 14:1 afr, and another at 12:1 afr. The intake ports are symmetrical and balanced, unlike old bbc design.

I don't get too wrapped up in rule of thumb numbers, because every build is different. A poor tune can kill a 9:1 engine running 93 octane, and a good tune can keep a 10:1 engine running well on 89 octane. My only advice I can give here, is to make sure your fuel tune is good before getting crazy with your timing curve. Get the air fuel ratio in line, and then start playing with your timing curve. One thing I learned data logging modern OEM stuff, is the fuel curve, for the most part, doesn't change a whole lot. The timing curve however, is constantly adjusting for conditions.

My wife drove a '98 Z 28 V8 SB. The only fuel that made it happy was Sunoco 94! On 93, it would rattle when she punched it. My engine is old school, no ECM. I don't intend to try and push the timing to it's limits. Basic factory specs are okay with me. 32 - 34 at top end. I wish I could get fuel without ethenol, I hate that crap! Farmers can still get leaded fuel, but does it require a special permit? I hauled petroleum for a living. I wonder if most folks know that mid grade is only 1/3 premium? I will just need to keep an eye on it after a few hard runs with regular. Maybe at worst I made need a 50/50 mix.

Fuel is dirt cheap right now, and will be for the summer, they are running out of places to store it! Are you really going to risk harming your engine over .20-.30 cents a gallon? On my last boat I had 525 EFI engines, these were 8.75:1 with aluminum heads. With a stock tune, you could run 87 octane. Mine had the Whipple stage one tune, which changes the fuel curve and timing curve. I was told 89 will work, but 91 was strongly recommended.

In your case you are 9.8:1, old school parts, and it sounds like you don't know what your AFRs are. There is no way I would be running 87 to save a few bucks. 89 is probably safe, but I would be running 91+, and not be worrying about my octane rating while out on the lake. I went too far once with my timing on a 10.25:1 engine. I heard spark knocking while getting on plane, and that was all it needed to take out a piston.

Ive made better power with some engines on my dyno with 89 and even 87 octane BUT they had fairly low compression. I WOULDNT personally run a 9.8-1 motor without a knock sensor on 87 octane if thats your other choice vs premium. Very easy to get inaudible detonation if your marginal, fwiw, Smitty

There's always been a big debate on octane going back to the high school days when you in reality didn't know as much as you thought until you did your own trial and error when you thought your cool muscle car would run totally rad on 110 octane in reality it made absolutely no difference at the strip. Boats are a whole different animal while being under a constant load unless at idle. To this day I fill my 421 ci z28 up at our local airport. WHY? cause I love the smell of burned high octane. Sh.t I'd bottle that smell up and sell it as cologne to those who'd appreciate it. Lol... What always amazes me is that static compression is always the rule of thumb for most when dynamic compression is often over looked especially with old schools HP builds. My rule of thumb was typically be on the fine line based on static compression with always filling at the pump with 91-93 and run without worries and when limited to lower octane at the marina while on the go just run sensibly while fluctuating rpm's to help cool cylinders. Regards to OP what cam you running? I totally agree with the price of fuel and even when it goes up. Gotta consider how much more per season you'd actually spend. Probably not as much as one would think unless your running an old trip SC boat 300-500 miles a weekend. Even then how many weekends even able to get out with our crap weather.