Hey folks, I just rebuilt the head on my 2019 ATC with a 12.5:1 compression ratio and I'm stuck on the octane question. The factory specs call for 87 octane, but I've heard higher compression benefits from 91. I want the best power without risking knock, especially on steep climbs. Anyone with a similar setup can weigh in on whether sticking to regular is fine or if premium really makes a noticeable difference? Also, any tips on tuning the timing after the rebuild would be appreciated.
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what octane for 12.5 to 1 compression
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Dan WalkerJun 15, 2026, 5:32 PM
I ran a 12.5:1 on my 2020 trail quad and stayed on 87 octane. With the stock cam timing it never knocked, even when I pushed the throttle hard uphill. The key is keeping the ignition timing a bit retarded (around 10-12° BTDC). If you go premium you might get a few extra horsepower, but the gain is marginal and costs more. I'd stick with regular unless you notice pinging.
Sarah CollinsJun 15, 2026, 5:32 PM
I upgraded to 91 octane after a similar compression bump and did notice a smoother response, especially at high RPMs. The higher octane lets you advance the timing a couple of degrees, which gave me a noticeable bump in climb power. Just be sure your fuel pump can handle the thicker fuel and watch for any lean conditions. If you’re okay with the extra cost, it’s a nice performance edge.
Tom HarrisJun 15, 2026, 5:32 PM
From my experience, the biggest factor isn’t octane but proper cooling. I had a 12.5:1 engine that started pinging on hot days with 87, so I switched to 91 and also added a bigger radiator. The higher octane helped, but the cooling upgrade was the real fix. If you’re riding in hot climates, consider both fuel and temperature management to avoid knock.