Just been reading through the Porsche Mechanical Fuel Injection Check Measure Adjust booklet.

Noticed a note regarding YM72 onwards 2.4 motors & the use of high octane fuels :

'From Model '72 on, all engines use 91 octane fuel.

If these engines are run on higher-octane petrol, the fuel will not be properly utilized.

Reason : the higher the anti-knock characteristic of a fuel the less easy it is to ignite.

Thus if a 2.4 liter engine is operated on 98 octane fuel, the CO contant in the exhaust gas will be high and very difficult to adjust properly.

When difficulties arise adjusting the CO content, ask the customer what kind of gasoline he is using.'

Just thought it was interesting