I specifically said same "era"; the M5 engine was available in 1998, not exactly a thousand years from the 1996 of the SVT Cobra.
In 1965 they were using the SAE gross horsepower rating. This was a, excuse the pun, "grossly overrated" measure of the engine's power. In the early 70s they changed to the SAE net horsepower rating. The difference between these two ratings is massive - some engines that achieved 400 HP before the change to net produced 235 HP after the change. The Chevrolet Nova did not have 300 HP by any of today's standards.
Every American car since the 1970s has had to meet emissions requirements and be measured using a power rating that is remotely reasonable i.e. the engine is expected to have the exhaust attached, which it wasn't for SAE gross, and they can't have big "road simulation" compressors forcing air in to the engine. If you look at the power figures for American engines just before the early 70s you'll see that they are mentally overrated.
Because power relative to engine capacity is the whole purpose of this debate?
From the opening post - a question of why American engines don't seem to produce much horsepower for their size.
We are talking about power efficiency in terms of per unit of cylinder capacity. This is flagrantly obvious from reading the thread.
Americans do have premium fuel. You can get premium fuel at the pumps that is as good as ours, short of special fuels such as BP Ultimate 102, which you can of course still get somewhere if you are willing to pay. Lum explains it in more detail.
The reality that American engines generally produce less power per litre than European engines is undeniable.
...and I am by no means the only one. the Americans love that sort of thing, hence ...their engines.