Is high test really necessary?

Jordo!

I mentioned it

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.

Sort of. It will aggressively retard ignition timing and may also pump more fuel to control what is determined (correctly or incorrectly) to be knock -- HOWEVER, this state of affairs is far from ideal in terms of performance or fuel economy, and will not necessarily preserve the engine.

It's not really simply a secondary map, where you just make less power, as it almost certainly is for an engine where the manufacturer only "recommends" higher octane fuels, but claims it will neither damage the engine nor void the warranty if you use a lower octane -- if nothing else, assume that they don't want to have to keep replacing motors for customers, so they have some confidence the "low octane" spark maps are perfectly fine for normal driving.


Well, true, provided you are high enough that pressure levels are sufficiently lower in the cylinder -- but without a way to monitor what the motor is doing, it is still probably a bit of a crap shoot...

And... if something does break, evidence of the use of a lower than required octane may void the warranty...

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