Quick question

It could be in the owner's manual.

Unless you have 10:1 or greater compression, you don't need high octane gas.

Bought the XJ very used (330,000+ miles) ... no manual.

And I've been a mechanic, heavy equipment and small engine, so I do understand that I don't need higher octane than recommended ...

and that's why I asked.

I just thought someone here might know but I'll just search for a manual on the 'net.

Budd

The manual (and compression both new and as read) for my 88 MJ indicate that

87 is acceptable. In the 100k miles I've put on it, it has never seen anything higher.

From my 1997 Jeep book, looking at Cherokee 4.0:

HP net brake @ rpm 190 @ 4600 Tq net lb-ft @ rpm 225 @ 3000 Displacement cc(cu) 3956(242) Bore & stroke mm 98.4 x 86.7 Bore $ stroke in 3.88 x 3.41 Compression ratio 8.8:1

Depends on altitude. Use the lowest one that doesn't ping and adjust up if it does. The biggest issue you might have is with 10% corn-juice blends. My 82 C-10 chevy handled 10% fairly well, but some makers are more suceptable than others.

Thanks, Will.

I use 87 octane and my altitude (5400 feet) actually allows an octane or two drop so I'm good.

Budd

And it if does ping, take your foot out of it, and add a gallon or so of the next higher grade. Mine used to ping very slightly on regular at sea level, but at current altitude of 5-10000 feet it could probably run peanut butter.

Sounds right. I was over Cottonwood Pass (12,000+) a couple of weeks back and noticed no issues - or at least nothing not expected up there. Lots of mud and snow for July made the beast struggle a bit.