Fuel Recommendations
Octane Ratings
By Rich Acre (Takeittothelimit)
I have noted a need for some information on John Deere Snowmobile
gasoline recommendations with an emphasis on octane ratings. I will
approach it somewhat from a historical frame work.
In the fall
of 1971, when the 400 & 500 were first introduced, Regular fuel was
recommended. Key is that Regular was 90 Octane or higher. In Deere's
SM-2100 General Tune-Up and Adjustment (10-20-1) it is stated
"IMPORTANT: Gasoline must be of regular or premium grade with an octane
rating of 90 or higher. For continued high speed operation, Premium
fuel is recommended."
When the 1974 295/S was introduced,
Premium fuel was recommended. This also held true with the introduction
of the 1975 340/S and 1975 Model 800 & JDX8. The JDX Operators
Manual states "Important: PREMIUM gasoline ONLY should be mixed with
the oil for use in the JDX8 Snowmobile. If premium is not available and
regular grade gasoline must be used, DO NOT operate the snowmobile at
peak performance because engine damage will result. Gasoline for the
JDX4 and JDX6 Snowmobiles must be regular or premium grade with an
octane rating of 90 or higher. Premium grade is recommended for
continued high-speed operation."
A Consumer Products Service
Bulletin (CP75-1-2) explained that "Premium Grade Gasoline (of at least
95 octane) is required for the JD295/S, JD340/S, 800 and JDX8 (30,001-
), to prevent detonation damage". This was again explained in a Service
Information Bulletin 21 November 1974 (CP75-C) and again in a Service
Information Bulletin 19 December 1974 (CP75-G).
The
Modification Program No. 200 (CDI installation) for the 1975 Model 800
& JDX8 was intended to reduce engine failures by keeping the engine
timing more consistant and reducing the spark plug preignition. "These
varriables combined with high performance engines which operate at
extreme specific outputs and high temperature can cause engine failures
through preignition and detonation." This Modification, however, did
not reduce octane requirements.
Introduction of the 1976
Cyclones and Liquifires continued to recommend Premium fuel. In a
Service Information Bulletin 18 December 1975 (CP76-C) which reviewed
causes for "Scoring of Liquifires"...item #7 was "Using fuel other than
Premium Leaded". Please note that Liquifire carb jetting changes were
also made.
The 1978 Cyclones and Liquifires were built with
lower compression and Regular gasoline was finally recommended for
these models. Questions about earlier models came up and a "White
Letter" from Deere explained that "certain models described below can
be converted for operation from Premium grade to Regular grade fuels.
Models not discussed cannot be converted and must continue with Premium
fuel. Those that are converted should not be raced or subjected to
sustained high speed running...Examine plug color regularly for the tan
shade and ride within reasonable limits." In short, 1976 Cyclone 340
& 440 required precise setting of the timing, QN-1 plugs and NEW
Heads; 1976 Liquifire 340s required the precise setting of the timing
and QN-1 Plugs; 1976 Liquifire 440s required installation of 1978
cylinders, heads and exhaust, as well as the precise setting of timing
and QN-1 Plugs!!!
When the "New Breed of Deere" (known as the
Fires Series to most of you), recommendations for fuel became: "Use
regular leaded or unleaded gasoline with anti-knock index of 88 or
higher. Never use gasoline that has been stored for a long time.".
Deere understood what was happening in the fuel industry and detuned
the Fires by lowering the compression ratios. This, in fact, is one of
the main reason a Liquifire has different ignition timing and heads
than the Kawasaki's. To help you understand that it was compression,
not HP, that required the change in octane recomendations I will give a
couple of examples here: 1972 Model 400 compression ratio 8.2:1...1980
Trailfire 440 compression ratio 6.5:1.
A quick guide is that
engines with a 7:1 compression ratio require a minimum of 89 octane;
those with 8:1 a minimum of 91 octane; those with 9:1 a minimum of 94
octane. Ignition timing is another factor but this is a quideline. From
this, we would conclude most of the early Deere's need between 90 and
91 octane, just as was recommended.
The bottom line is that you
need enough octane (resistance to burn) to prevent detonation and
preignition. BOTH of these are abnormal combustion and lead to engine
failures! It is also important to note that current fuel is refined to
ONLY hold the octane rating for 6 weeks! This makes fresh fuel even
more critical.