Now that fuel costs in this country are starting to become like
that in most of the world- generally (inconsistencies betwixt the
manufactures inclusive), what is the safe minimum octane I can run
my virtually all stock (save for the Unilite distributor) 8:1 CR
4.2 engine?
AND what is a good timing setting considering the burnability of
the recent fuels? 5 BTDC is my stock recommendation. I am willing
to put up with a bit of poor cold running, if need be.
Thanx, Huff–
69 2+2 color- London Fog
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In reply to a message from Huff sent Sat 2 Apr 2005:
Huff,
Correct timing is 10BTDC. An unmolested (never rebuilt, head
milled, etc.) 4.2 may run on 89 octane, but it’s unlikely it’ll
go below that. In most case, 91 octane is needed. If the head’s
been milled, or the block decked, odds are good you’ll need 91
octane or better. If it runs without pinging under load (foot to
the floor at freeway speed on a hot day), your octane is OK.
Mild pinging can be cured by backing off timing, but going more
than a few degrees (below 5BTDC) will significantly impact
driveability. If you can’t eliminate pinging without backing off
the timing too far, you have limited options. One is find better
fuel. Think AvGas, 100+ octane at $4-5/gallon. Not legal but sure
works! You can mix it with pump gas to increase octane. The best
permanent fix is to reduce the compression ratio. This can be done
by putting in a thicker head gasket, which is not terribly
expensive or difficult. I had to do this to mine last year, and it
was more than worth the effort and expense. Don’t waste your time
or money with those crappy over-the-counter ‘‘octane boosters’’.
They are expensive, and virtually useless. When they claim
to ‘‘increase octane by 3 points’’, they don’t mean from 91 to 94,
but rather from 91 to 91.3, an insignificant difference. IMHO, any
problem that can be fixed with that stuff can be fixed for much
less money by simply backing off the timing a degree or two.–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Huff sent Sat 2 Apr 2005:
Hi Huff
I’ve been running an occasional 89 w/no pinging, but most of the
time 91 octane. My timing was at the stock 5 degrees but the car
ran better at 10 BTDC. Haven’t had a pinging issue.
Have you taken a compression test on your engine?
Dave–
David Ahlers
Skaneateles, NY, United States
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