Through his continued research of the W72 Performance Package, hobbyist John Witzke determined that '77 T/A 6.6 engines received common No. 500557 400ci-block castings, while '78-'79 T/A 6.6 engines received casting No. 481988. The latter also had a large "XX" cast into it in several locations, including near the casting number on the rear of the block. Because of this, it is often referred to today as "XX 481988" or "481988 XX."
Not knowing a reason for the casting change or that any internal differences existed, we guessed that the No. XX 481988 was another "thinner" casting of the same era. What we didn't know at that time-and later figured out with key Pontiac engineering documents-was that some W72 engines were experiencing crankshaft failures, and while it may not have been directly related to the block casting itself, Pontiac revived the "better" No. 481988 casting last used in 1974 to eliminate the block as a variable.
By this time, Jim's book had already been released and was selling well. It wasn't long thereafter that the publisher informed him the initial run had sold out, and a second printing was scheduled. They gave him the opportunity to make minor changes, and he contacted his contributors, asking for any updates. I provided Jim with the "latest" known information we had on the No. XX 481988 block, and page 22 of the subsequent printing reflects this.
Paul Spotts later compared an XX 481988 casting to an earlier, vintage No. 481988 block and found that the two are virtually identical. All this information reveals that in 1978, Pontiac engineers felt safer using the "better" block for the T/A 6.6 engine and the No. 500557 in all non-T/A 6.6 400ci applications. Because the T/A 6.6 engines used in 1979 were machined and assembled during the '78 model year, they utilized the No. XX 481988 casting as well.
Having sorted through all of this, I can accurately state that the No. XX 481988 block is indeed a viable candidate for a performance build. While this statement might make the information in Jim's book appear incorrect, his book contains the most accurate information that was available at the time. I suspect that as research on all facets of our hobby continues and we uncover more information, we'll continue to see similar "mistakes" in the various books and articles that hobbyists currently use as reference material today.
Regarding the clarification of the L78 and W72 scenario, "L78" was the UPC that Pontiac used for its 400ci four-barrel engine for several years, and the 180hp version of this was the Trans Am's standard engine in the '77 and '78. The T/A 6.6 was an engine option (generating 200 hp in '77 and 220 in '78-'79) that was included with the W72 Performance Package available on L78-powered Formulas and Trans Ams. The '79 model year was slightly different-the L80 403 Olds was the Trans Am's standard engine and the base L78 400 four-barrel was dropped entirely. Pontiac then considered the T/A 6.6 engine its L78/W72 since a "base" L78 didn't exist. L78 and W72 still, however, appeared on the buildsheet.