Impala Tech - View Single Post - 427 Engine rebuild
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post #7 of (permalink) Old 12-01-2019, 12:24 PM
Big Dave
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Join Date: Jan 2006
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If it is the original engine it will all fit, without looking for aftermarket parts. The engine you have now requires premium leaded gas and makes it's power by reving. Think of it as a 350 horse 327 engine only super sized. At 6,800 RPM it will make 480 horse on 100 octane fuel (it was derated not by changing any parts for the insurance company, but by testing the engine power at a much lower RPM).

So your 427 engine is just the way the engineers designed it back in 1963 for NASCAR racing, including it's domed high compression pistons. It will also have the closed bath tub shaped small combustion chamber heads that shroud the valves. In 1967 that head had the walls of the combustion chamber laid back to open the chamber up for free breathing. This dropped not only the static compression, but also the power it makes if you do not use a hemi-spherical dome cut in half piston to fill the void (why the Rat was called a Semi Hemi).

With flat tops it will make more than a 396 with open chamber heads, but not as much as it would with 11.0:1 pistons and 100 octane fuel. The big block was designed to be a high compression deep breathing high reving race car engine. That design and tooling was paid for by also using it as a truck engine, but that truck engine had different heads, pistons, block, and cam to make it a slow reving torque beast that is an additional inch wider and two inches taller than a passenger BBC. (the only common parts between the truck and passenger engine was the water pump and valve covers.

The open chamber 454 replaced the 427 in the production line up in 1971 (introduced in 1970 exclusive to the Corvette and the Chevelle SS). In 1972 the compression dropped in an effort to save the engine for use in cars, but the power drop off was so great that people opted for the 350 small block instead of the smog motor version of the big block. It was last installed in a car in 1973 (carried over to the Corvette as an option through 1974 when it was dropped there also). But like I said the small block had more power and better mileage which was important in 1973 because of the OPEC crude oil embargo. This made gas hard to find in 1973, and by 1976 that gas was unleaded with 87 being the highest octane you could buy.

By 1974 you could buy a 454 SS Chevelle or Corvette for twenty to thirty percent of the resale value of a 427 or 454 version of the same car (Hemi 'cudas and 440 GTX-Road runners were being crushed because no one wanted them at any price). That is why the seventies trough the late nieties were called smog era cars and if you wanted a powerful fast ride you bought a BMW or a Porsche, or a Jap hot rod like the Datsun Z.

It wasn't until GM introduced the redesigned second generation small block that power returned. GM did this redesign of the small block not because people were crying for more powerful engines; but to reduce emissions as mandated by ever tightening standards. Making more horsepower was a side benefit that people quickly noticed. Especially when older muscle cars still on the road had their doors blown off by a 4600 pound four door Impala SS.

The LS-x engines know as generation three small blocks replaced the gen two SBC two years after they were introduced; so you may never have even heard of them. From 1974 and up the only vehicle with a BBC installed under the hood was a light truck, because they were exempt from the emission laws of the era (it had to be a three quarter ton or a one ton truck to be exempt).

Big Dave

Last edited by Big Dave; 12-01-2019 at 12:39 PM. Reason: spleling
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