What is the difference between the LQ4 and LQ9?

Both are Gen III iron-block 6.0 truck engines; the LQ9 is the higher-output version.

Engine	Size	Years	Power	Torque	Compression	Block	AFM
LQ4	6.0 L	1999–2008	300–335 hp	355–375 lb-ft	9.40:1	iron	No
LQ9	6.0 L	2002–2007	345 hp	380 lb-ft	10.00:1	iron	No

The LQ9's higher compression is the main difference in the spec sheets we have: 10.00:1 vs 9.40:1, for about 45 hp more than the lowest-rated LQ4s. Both use the 24x crank reluctor (Gen III), so they run on the same generation of computer. Note that 1999–2000 LQ4s have iron heads, and some 1999–2000 6.0s have the long crank flange — see why early LQ4s need a different flexplate.

Compare every engine in the donor finder.