The octane rating used to determine the difference between regular, mid-grade, and premium does only one thing... it determines the gasoline's resistance to self-detonation under compression. 93 octane gas is more resistant to detonation than 89 octane which in turn is more resistant to self detonation than 87 octane gas. The only reason to run a particular octane rating is to avoid pre-detonation in your engine... low compression engines do not require a high octane rating to avoid this problem..... however, race engines will require a high octane rating to avoid pre-detonation (when the piston compresses the fuel/air mixture so much that it detonates spontaneously, rather than waiting for the spark from the spark plug). The problem with pre-detonation is that the gas mixture explodes BEFORE the piston completes it's travel to the top of it's stroke, causing the piston to suffer tremendous pressures that it's really not designed to take... you'll know if you start having this problem because your engine will make a "pinging" noise... especially under hard acceleration, or possibly want to "diesel" when you turn it off (the engine will try to continue running).... although the "dieseling" symptom is frequently a sign of other woes.
Your engine is rated to run 87 octane because it is a relatively low-compression engine, and will benefit in no way from a higher octane rating, so there is no point in buying premium gas for your KJ unless you just want to spend more money at the pump.
Luis
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