I saw this topic and started to reply when I reliezed that we exhausted it a while back. I didn't realize how long ago though. Anyways, it was way back in March of 2000, but the information is still solid.
A couple of additions to this as well as the archived list:
The L89 thing is correctly listed sometimes and incorrectly others. It is in fact an aluminum head option and was availble on 396/375's in Camaros, Novas and Chevelles, and 427/435's in Corvettes. Some engines such as the L88's in three years of Corvette only and the ZL1 available in Corvette and Camaro,and the 71 only Corvette only LS6 were only available with aluminum heads, so the L89 designation wasn't used. Some parts books list an engine code for a 70 Chevelle with an LS6 with aluminum heads, but I've never heard of a documented example and don't think the L89 designation would've been used.
There are two designs of L88's correctly referred to as Heavy Duty. Many publications and numbers books now incorrectly list the L88's factory horsepower rating as 435 also and I've even seen it done in recent GM literature. At the time these engines were installed in cars, they were factory rated at 430, intentionally 5 less than the Spec. High Perf. 427's which carried the 435 rating. Even though the L88 was an RPO, GM did everything possible to discourage the general public from ordering it, including the lower HP rating. The first design L88's installed in 67,68 and early 69 Corvettes were closed chamber and had a slightly milder camshaft profile than the second design open chamber engine which came along after the first of the year in '69. The second design engine with it's more radical camshaft and open chamber heads was the exact same as the ZL1 with the exception of the aluminum block. The LS7 engine is nothing more than a second design L88 with the stroke of the 454 substituted for the 427 and it's generally thought that the full crate motors were delivered with open chamber iron heads instead of the L88's aluminum ones. The LS7 was factory underrated at 460 HP but is night and day stronger than the LS6.
Far as the LS5's go, in addition to the 360 horse Chevelle engine in '70, they were placed in Corvettes and rated at 390HP. The two were identical except for the exhaust manifolds. The LS5 also continued into the 71 Corvette and I believe the horsepower rating was lowered to 365. Although I had an orginal engined 71 LS5 Chevelle about 10 years ago, I can't remember the HP rating. (335?????????)
The LS6 was rated at 450 HP in the Chevelle in 1970. The lower intake was in anticipation of it being placed in the 70 Corvette which it wasn't. When the LS6 finally did make the Corvette in 71, it was outfitted with alum. open chamber heads and the rating was dropped to 425. It's widely thought that there was never an LS6 factory installed in a production 71 Chevelle.
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