We can't really put our finger on just what it is that makes the 409 such an iconic engine with such a huge following. There's more to it than just the song. (And frankly, if we never hear the song again, we'll be fine with that.) There's also more than just being the first big-block Chevy, or those distinctive scalloped valve covers. Unlike the Ford FE and Max Wedge Mopars, the 348-409 W-engine had a relatively short life span. Mopars used the same basic design well into the '70s on the 440, and the FE also enjoyed a long run.
While the 409's star didn't burn too long, it certainly did burn brightly. The 409 was the first high-performance big-block out of the gate, and the others played catch-up. That short couple of years built and cemented the 409's legend. Okay, the stupid song probably helped too.
But there was more to it than that. The 348 with 3x2s came out in 1958 and held its own thru 1960 while all the big cars were being fitted with bigger and more powerful engines; Chryslers had the 392 Hemi but then dropped in power with the 383 and 413, Buicks were never really in the game with their 364-inch and later 401 Nailhead, but Ford busted out the 352 FE and the Mercury/Edsel/Lincoln 430, while Olds had the 371 and 394 Rockets. The Chrysler 354 and 392 Hemi was dominating on drag strips and NASCAR ovals, but that fell off considerably when Chevy turned up the wick with the release of the 409 at the tail end of 1961. The engine was so new, only 143 cars came with them, and there was some question in the minds of NHRA whether or not the 2x4 induction was factory-installed or a dealer option, as they initially classified them as A/FX.
Forty-odd years later, the 409 is enjoying a renaissance. We now have a few sources for aluminum heads, stroker rotating assemblies have pushed the displacement to 480 inches and beyond, new aluminum water pumps are available, and MSD makes a new distributor for them. The only thing lacking for a complete aftermarket 409 engine is a block-and it's severely lacking. Factory 409 blocks come in two flavors: passenger car and a lower-compression truck block. Both are expensive, but fewer passenger car blocks rolled out of the factory than truck 409s, so the car blocks command a premium. And unlike most other overhead valve engines, the drop in compression with the truck engines is caused with the block itself, rather than the head-a large relief was cut into the edge of the bore.
Because the truck blocks are more common and cheaper, builders have come to decide that the truck blocks are more desirable for performance builds because that notch unshrouds the exhaust valve, improving flow. We've asked 409 expert Lamar Walden about this in the past, and his answer is simple: unless it's a supercharged application, compression beats an unshrouded exhaust valve.
If you've never seen a 409 block or head up close, it's obvious why the design was left in the past. It was basically Chevrolet's answer to the flat head, writ large. The underside of the head is flat, with no combustion chamber-just valves on a smooth plane. Rather than the engine block's deck being 90 degrees to the top of the pistons, the deck is angled at 74 degrees to the stroke. The inboard side of the pistons' dome is machined at a matching 74-degree angle to kiss the bottom of the head, while the outboard side of the piston dome remains at 90 degrees. This creates a triangular wedge between about 3/8 of the piston's dome and the bottom of the head. This area is the engine's combustion chamber.
Piston design, ring placement and valve relief will all affect compression ratio, as will head gasket thickness. These are all variable, but that truck block's big notch in the edge of the bore adds space to the combustion chamber that can't be offset, and you lose at least a full point of compression over a passenger car block.