Hey folks, I’m dialing my 4-stroke ATV’s compression down from the stock 12:1 to about 1:1 for a custom cam swap and I’m not sure what octane rating I should be running now. The manual says 12:1 needs 93 octane, but with the lower compression I’ve heard you can drop to regular pump gas. Has anyone made a similar change? Did you notice any loss in power or heat, and what fuel do you fill up with now?
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Mike HendersonJun 15, 2026, 5:32 PM
I ran a similar 12:1 to 10:1 conversion on my Yamaha Grizzly a couple of years back. I stuck with 91 octane because the lower compression still needed a bit of knock resistance, especially on hot days. I didn’t notice any real power dip, but the engine ran a tad cooler. If you go down to 1:1, regular 87 should be fine, just keep an eye on spark plug color and listen for any pinging.
Tom HarrisJun 15, 2026, 5:32 PM
I’d be cautious about dropping octane too much. My brother lowered his compression on a Can-Am and switched to 87, but after a few weeks he started getting pre‑ignition on steep climbs. He went back to 89 and the issue vanished. It might be a trade‑off between fuel cost and engine safety, especially if you push the throttle hard. Maybe try a mid‑grade fuel first and see how it behaves.
Ryan EdwardsJun 15, 2026, 5:32 PM
From a safety standpoint, don’t forget the fuel system’s tolerances. I once used 95 on a low‑compression build and got excellent response, but the carburetor needed re‑jetting for the richer mix. If you stay with 87, you’ll likely need a leaner jetting to avoid fouling plugs. Also, double‑check your compression after the swap; sometimes the numbers don’t drop as much as you think, which could still demand higher octane.