AC Delco RAPIDFIRE TM Spark Plugs
For Corvette Engines
 The AC Rapidfire is a premium, high-performance spark plug intended
for street high-performance and "mild" racing applications. It combines the
high-performance of a racing spark plug, the RFI suppression of a resistor spark plug and
some of the reliability/durability features of a platinum-tipped plug. Since the Rapidfire
does not use platinum tips, they cost much less.
AC Rapidfire spark plugs use a nickel/silver
composite side electrode. AC claims this side electrode cannot be bent to regap, however,
practice has proven that untrue. For many applications, they are gapped properly at the
factory, but they can be regapped if necessary.
The following chart is Rapidfire data for Corvettes and is
based on data from the 1999 AC Spark Plug catalog, however, in some cases the data has
been revised according to the experience of the Vette Magazine
Technical Staff. For example, during the 1970s AC recommended either the RF#2 or the
RF#10. One is an extended tip plug the other is a standard tip plug. After 1971, all
engines were low compression and can use the extended tip plug which improves idle
stability.
| Model Year |
Engine Type |
Rapidfire Plug Number |
| 1955-1968 |
Small-block Base and/or
Low-performance Engines |
RF#7 |
| 1957-1968 |
Small-block High-performance |
Not Available ! |
| 1965-1969 |
All iron-head, Big-block |
RF#4 |
| 1967-1969 |
All aluminum head, Big-block |
Not Available ! |
| 1969-1970 |
All small-block |
RF#7 |
| 1970 |
All big-block |
RF#10 |
| 1971-1985 |
All |
RF#2 |
| 1986 |
All Iron Head |
RF#2 |
| 1986-1991 |
Aluminum Head (VIN 8) |
RF#5 |
| 1990-1995 |
LT5 (VINJ) |
RF#92 |
| 1992-1996 |
Gen II small-block LT1/LT4
(VIN P) |
Not Available ! |
| 1997-1999 |
Gen III LS1 |
Not Available ! |
General notes:
Whenever the RF#2 is specified, if a slightly colder plug is desired for racing or very
aggressive street driving, the RF#1 can be substituted. Whenever the RF#5 is specified, if
a slightly colder plug is desired for racing or very aggressive street driving, the RF#9
can be substituted.
Specific notes:
1) Some small-block, high-performance engines may be able to use the RF#4 depending on
clearance between the piston dome and the spark plug when the piston is at
top-dead-center.
2) The original LT5 spark plug was the FR1LS, a 14mm,
5/8-in. hex, 3/4-in. reach,
flat-seat plug with a .035 gap. The FR1LS was an AC one-heat-range
plug deemed appropriate by Lotus and GM engineers for the LT5's
very high output. Right at the start of production in 1990, as a
result of some concerns about plug fouling
in pre-delivery situations at dealers,
GM superseded that with the FR2LS which was one heat range hotter.
In 1993, GM converted the LT5 to the double-platinum tip, 41-907,
a 14mm, 5/8-in. reach, taper seat plug with a .050 gap. In 1995,
the LT5 was upgraded again to the 41-913
with a .050 gap. In 1996, all GM
spark plug data for 90-95 LT5s was superseded to the 41-913. The RF#9
is equivalent in heat range to the FR1LS and
can be used in the LT5, however,
you should open it up to .050 gap. In some cases there maybe a very
small increase in idle stability in doing that. The RF#9 is a 14mm,
5/8-in hex, flat-seat plug and will fit all
LT5 cylinder heads by virtue of
their universal spark plug seats that accommodate both flat seat and
conical seat spark plugs.
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