Is high test really necessary?

Originally Posted by Jordo!

Yes, and no. Higher compression piston heads tend to be more likely to have flame kernels dissipate irregularly, creating hot spots that may lead to knock by way of pre-ignition of the next combustion event.

This is one of the reasons that it is generally easier to tune a boosted car with more boost but lower compression pistons, rather than just assuming more pressure however its achieved is better. You still have to deal with knock and that's generally easier on a low CR high boost motor, at least using port injectors (in theory you can do more with less using direct injection).

In either case, the ignition advance tables were tuned based on the presence of 91 AKI, so to run optimally and safely, especially under load, going below the recommended octane is potentially risky.

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