The reason you need high octane in the 1.8T is due to the high compression level within the engine. Without the higher octane, you would get "engine knocking" and/or "run on" when the engine is turned off. This is caused by the residual gasoline in the cylinders igniting due to the pressure & temp. after the engine is shut down. They say that any improvement in performance from higher (above factory spec) octane gas is largely imagined by the driver. Short of putting in some specialised fuel additive or even "NOS", I wouldn't spend the extra money on the higher octane fuel.
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