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Why does low-speed preignition occur at low rpms and high load a.k.a engine lugging?

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At low RPM with an open throttle ( lugging) , there is nearly atmospheric pressure in the manifold . Then you get maximum compression in the cylinders so if the fuel octane is not high enough for that compression , you get preignition. Of course if the timing is too advanced you can also get preignition even with good octane.

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LSPI is occurring on many late model direct injection w/turbo cars. It's pretty serious and the car companies and oil companies are working on a solution for the problem. The current theory is that the high pressure squeezes oil droplets out from the ring lands and causes them to ignite and cause detonation. See this post on LSPI

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Sounds like a timing fault. Suspect the auto advance and retard is faulty. The distributor is worth a look at because its easily inspected. Or if you have a timing light check it that way as well.

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  • Welcome to Motor Vehicle Maintenance & Repair! How does this answer the question? The OP is looking for an answer on a theoretical question. He's wondering how it occurs, not that there is any fault in the first place. Aug 29 '19 at 21:37

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