LS1 - the first of the Almighty LS family engines. Came in the 98-02 V8 powered Camaros and Trans Ams, the Firebird Firehawk, and the Corvette from 98- unsure but I think 02 or o4. Engine is 5.7L/350 cu in.
LS2 - A revised LS1. Displacement was 6.0L/364 cu in. Made it's way into the GTO and some Corvettes. They are not as popular as the LS1 and LS3 because many say that they tend to run a little hot and they vary greatly in power output from one to the other.
LS3 - 6.2L/376 cu in. Found in the 5th Gen Camaro, Corvettes from 05-13, And the G* GXP. Shares similarities with the L99 engine. Some sources list the LS3 as the High Output and the L99 the standard. L99 has DoD aka AFM. I think the cam profile is also slightly different.
LS4 - 5.3 found in FWD cars...Grand Prix GXP, and Impala SS. has some differences but not sure what besides displacement.
LS6 - Same displacement as the LS1. However the LS6 is a stronger block. I think certain parts on the LS6 (heads, cam) are not interchangeable on the other LS engines. This was found in the Z06 Vettes in like 2002 (not sure on the years).
LS7 - Largest displacement of the LS family engines. 7.0/427 cu in. Also, probably the most powerful NA engine GM has ever made (not 100% sure). Found in the newer Z06 Vettes...up to 2013. The block has some light weight super strong components and the heads, cam, and intake manifold are specific to the LS7...cannot put LS7 heads on a LS3. Other differences in the block bore. Also it is a dry sump oiling system. Meaning oil is not kept in the oil pan.
LS9 - Most powerful production engine in GM history. It is a 6.2L/376 cu in supercharged dry sump engine and is found in the ZR-1 up to 2013. It has forged pistons and a 2.3 L Roots style positive displacement blower. Not sure on other differences between this and the LS3/L99. I also think that each engine is hand built...not 100% sure.
LSA - Found in the Cadillac CTS-V. A lower output LS9. It has hypereutectic pistons, a 1.9 L Roots style positive displacement blower and some other differences.
The Trailblazer SS has a 400 hp engine. I think it is the L99 or a detuned LS3.
The ZL-1 has a LSA engine. But I think there are some small difference between this LSA and the CTS-V LSA. Not enough to make them different engines tho.
Among the LS engines, there is a lot of compatibility with the heads, intake manifolds, etc. They are all sort of based along the same build. But they aren't all basically revised LS1 engines even tho the LS1 engine came out first. What I'm saying is that LS1 doesn't mean that the rest are refreshed or newer LS1 engines. But still, the LS2 was def a revised LS1.
The LT1 engine from the 90s was an aluminum head engine displacing 5.7L/350 cu in. The new LT1 engine is a different engine altogether. I haven't had a chance to read up on all the differences between the new LT1, the LS family, and the old LT1. But I know the differences are substantial. Hope this helps.
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