Pinging on 91 fixed by E10 94
 

I recently noticed my XJ was pinging (pre-detonation) quite badly

Spark plugs are recommended Champion 12RYC, about 4yrs old but only about 40k, they are whitish and the ground electrode is eroded out to about .060" from stock .035"

I change them and pinging is reduced, but still present

Italian tuneup further reduces pinging, but it is not altogether gone,
most folk would not notice, but I know burned piston is a thing on XJs

I have always used 91 which is stated ok in Handbook, as is E10

I decide to fill with E10 (94), as it is cheap on that day

Pinging is gone

Continue to use E10, no more pinging

(was previously present under high throttle & load situations only)

I have not tried it on 95, but probably will, just to see

With E10 @ $1.75, and 95 @ $2.15, I will be just using E10 in future

(in theory, if an engine is deemed suitable for 91 Octane, increasing octane content should give no better power or economy)

Pre-detonation is unequivocally bad for an engine

Has anyone else experienced pinging on '91 ?

Ethanol has a much higher octane rating than regular gas, most of us older guys that have classic car toys often use E10 in the warmer months to eliminate detonation which is directly related to intake charge temperature.

I too found this with our '95 when it was stock but as we wanted to do the cape and other regional travels another solution was required.
What worked for ours was a simply TB spacer, now I know the interweb experts will all say they make no difference but it did in ours, an even bigger improvement was the addition of the '99 intake with the TB spacer, no more pinging even on hot days travelling slow offroad towing the camper trailer on standard '91!!!