I was referring to "as factory built" meaning as delivered from GM. One of the guys on this site had an L72 on an engine dyno and it made something like 429 hp. It had a 2" carb spacer which won't fit under a Chevelle hood. It had dyno headers which don't come on the factory cars. It was running racing gas which isn't practical for the street if you run 3K+ miles a year like I do. I would assume that the carb and timing were also optimized.
Our L78 Chevelle is factory stock except for tuning and headers. It is capable of 13.3s. It has no options to add any substantial weight. That might equate to 320 hp at the rear wheels. There are formulas on the internet where you can roughly figure hp and et. With flowed heads, good streetable cam, headers, and lots of little tricks, a friend's '70 L78 Chevelle runs 11.70s. There is a big difference between "as delivered" and tuned. Not that I have a lot of faith in the old magazine road tests, but the L78 cars were usually mid 14s in the quarter. I road in a few of these back in the day. Our COPO Chevelle came with#68 primary jets and lean surge at highway speed. The advance curve was also for emissions. On top of that 5 cylinders had rings that didn't seat. It was a dog. A re-ring, good valve job, headers and some tuning and it was night and day difference.
I believe that GM tested their engines on an engine dyno without accessories. These numbers will not be indicative of what the engine will produce in a car through the drivetrain. Plenty of losses there.
In the end, the quarter mile is the real test.
