This all seems to be confusing me. The w72 blocks used in 78-79 were a reintroduced 988 casting but many say not to build later blocks due to their weakness. Can anyone clear me up on this? Did these blocks avoid the whole weight savings program?
I don't know who gave you the information, but either they don't know enough or they didn't tell you enough. Like most enduring myths, there is a grain of truth to what you've been told. But the picture is totally different once you hear "the rest of the story".
The original casting number for the early 400 CID Pontiac blocks was the 481988 number John mentioned above. It is a very strong casting and it has been successfully used for many high-power and stroker builds without any block problems at all.
In 1973 the United States had a little problem with foreign relations, and in response the Organization of Petroleum Exporting Countries (OPEC) got all pissed off, so they got together and decided to cut back on how much oil they would ship. This action precipitated the 73-74 "Oil Crises". You're probably too young to remember that, but basically people panicked and suddenly gas stations started running out of gasoline. There were long lines at ALL gas stations, not to mention which caused gas prices to shoot through the roof. People got a little pissed off by that, and in response the government imposed the Corporate Average Fuel Economy (CAFE) regulations and they empowered the EPA to rate ALL new cars for the MPG rankings.
Suddenly FUEL ECONOMY became the #1 issue in new car sales. Performance got dumped like a load of 3 day old fish. Everybody knows that weight is a prime penalizer for automotive fuel economy, so Detroit sent all the designers notices to reduce weight wherever possible.
Pontiac was no exception. The engineers in the drive train division looked at their engines, and decided that they could shave a few pounds out of the 400 block by redesigning it with thinner internal webs. The result of this was the 500557 block. It IS weaker than the 481988 block, but it's plenty strong for most street applications. In particular, it was WAY stronger than the factory needed for the 185 HP engine they were putting out in 75-76.
So... during the 1975 model year, Pontiac switched to the 500557 casting. They stuck with it through the end of the 76 model year. But as car production moved into the 77 model year, the painful memories of the gas lines of early 1974 faded. Pontiac had a few production problems with assembly of the 500557 block engines, and they wanted to emphasize performance a bit more with introduction of the higher-HP option package. Rather than sink the cost of engineering yet another revision to the 500557 block, they decided to simply revert to the 481988 block.
The use of the more robust block continued through the end of the Pontiac 400 CID engine production, during the 1978 model year. A set number of the 400 engines were stockpiled for the 79 model year, and then all the tooling was ditched because that was the end of the Pontiac 400, per corporate edict. The Olds 403 was use for the "big engine" production once the 400 engines were gone (all 400s were 4 speed, all 403s were autos), and for 1980 there were no more "big engines".
A 500557 block is fine for almost any streetable 400 engine you'd want to build, right up to 450-500 HP, and with RPMs under 5,000. It's not the BEST candidate for a stroker engine or a drag engine that's going to see 6,000+ RPM -- the 481988 block is much better for that. But given a choice, most people opt for the 481988 block if they can find one for the same or approximately the same money.
That's the entire ugly story... warts and all.
