Most muscle car enthusiasts know that the horsepower ratings of the engines were deliberately set lower than what they actually made. Usually for insurance reasons and so the cars could run in a more favorable class in drag races.

Noted auto journalist Roger Huntington wrote an article about what these engines actually put out; here is the list: (All are gross hp & torque figures.)

Engine------------------Advertised----Rated----------True
------------------------HP @ RPM---- Torque@ RPM-- HP @ RPM

Buick 455 Stage 1-------360@5000----510@2800------420@5400
Camaro Z/28 302--------290@5800----290@4200------310@6200
Chevelle 396 L-78-------375@5600----415@3600------400@5600
Corvette 427 L-88-------430@5200----450@4400------480@6400
Mopar 340-4 bbl---------275@5000----340@3200------320@5600
Mopar 440-Magnum------375@4600----480@3200------410@5400
Mopar 440 Six-Pack------390@4700----490@3200------430@5600
Mopar 426 Street Hemi---425@5000----490@4000------470@6000
Mustang Boss 302--------290@5800----290@4300------310@6200
Ford 351-4 bbl Cleveland--300@5400----380@3400------340@5600
Mustang Boss 351--------330@5400----370@4000------360@6000
Mustang 428 Cobra-Jet---335@5200----440@3400------410@5600
Mustang Boss 429--------375@5200----450@3400------420@5600
Oldsmobile 455 W-30-----370@5300----500@3600------440@5600
Oldsmobile 350 W-31-----325@5400----360@3600------350@5800
Pontiac Ram Air 400------366@5100----445@3600------410@5600

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Notes:
-The most underrated was probably the 428 Cobra Jet engine... its official rating of 335 hp was a joke, especially considering that the 390-4 bbl engine was also rated at 335 hp.

-The second-most underrated was probaby either the Mopar 426-Hemi or the 340-4 bbl. The 340-Six Pack was not listed, but with an advertised hp of 290@5000 rpm and torque of 340@3200 rpm, I would guess about 335 hp @ 6000 rpm.

-The actual power output of that 455 Olds W-30 is very impressive... 440 hp!