Hi Kirby,
I understand why you would believe that higher octane is required from
your personal experience with your 83 model, but I can assure you that
the “Drivers Handbook” for my 89 model (Lucas CEI – change to Marelli
was in mid-89 model year run), and my “XJ-S H.E. Handbook” BOTH specify
87 AKI regular unleaded. The 85 Handbook weasels it a bit and states:
“You should use unleaded gasoline with an octane rating of at least 87.”
As previously stated, 87 AKI regular is the ONLY fuel I have ever used
in either of mine, and I have never heard either one knock or ping
under any conditions, EXCEPT on the one occasion, about 15 years ago,
when my 85 blew a coolant hose in city traffic on a hot summer day, and
it took me about 30 seconds to get to a spot out of traffic. The temp
gauge went about 1/3 of the way from the N to the H, and that caused
some slight knock for several seconds before shutdown.
No engine damage occurred, and 15 years later that engine has never
even had the cam covers removed. I must add that I have tested and/or
rebuilt my distributor vacuum and centrifugal advance mechanisms,
regularly check timing at 3k RPM and idle, with and without the vacuum
advance connected (idle speed), to make sure they continue to work
properly, use injector/intake cleaner every 3 to 6 months, regularly
accelerate to 6k RPM in 1st gear to blow out carbon, and otherwise
drive mine fairly hard to prevent carbon buildup, even though my normal
commute is ~ 15 minutes each way which is a common cause of carbon
buildup (short trips at low RPM). At least 5 minutes of that 15 minute
commute I am going ~ 60 MPH, so it isn’t “lugging” too badly all the
time.
The reason I posted back with all the qualifications and explanations,
is that anyone else’s mileage may vary from my own, and as Roger
Bywater states in his discussion, perhaps the single biggest variable
is how much carbon buildup has occurred in a particular engine, which
can’t be assessed by any reliable means I know of short of removing a
head (unless a scope exam might produce some accurate findings)?
For those interested in a discussion of the variables related to knock,
ping or detonation, I would suggest reading the following post by Roger
Bywater in the archives, or the entire thread, which I believes also
contains a post from Kirby 
George Balthrop, Clifton, VA USA
85 & 89 XJ-S Coupes; 89 XJ40 VDP-----Original Message-----
From: Kirbert palmk@nettally.com
‘‘That said, your 1988 V-12 should run just fine on 87 AKI/91 RON
unleaded regular…’’
I’d disagree with that. My '83 w/ 11.5 compression and Lucas CEI
ignition would knock on regular. It would sometimes run OK on mid-
grade, sometimes not, so I just ran it on premium all the time.
Now, if the '88 has Marelli ignition, perhaps the timing maps are a
bit different. But the Marelli also has a jumper for using lousy
gas. Several owners will confirm that, given the choice between
running with the jumper in place on premium or with the jumper
removed on regular, you should always choose to run on premium. The
performance and fuel economy lost with the jumper pulled means it is
only to be used when you accidentally get a tank of bad gas.
– Kirbert
// please trim quoted text to context only
