The Ford GT and Shelby GT500 5.4 motors easily make the most HP from the factory. Both long blocks have sustained 1000 WHP to the ground with only induction mods. So hands down winner but,.. arguably so.

In the 60's Shelby sold 427 Cobra's equipped with Paxton superchargers making 600 - 700hp. Billy Cosby was an original owner of one of those Cobra's. He made a skit about t and how the car scared the crap out of him. He also bought one of only 2 known factory sponsored 68 Shelby GT500 428 cars that had dual Paxton superchargers and the car was supposed to be making 700hp. My memory is foggy about the other car. It was either one of Carrol Shelby's personal cars or it went to a factory rep and was never registered.

And its hard to pass up on some of the classic top list mentions.

The Boss429.
375hp GROSS in factory trim. Thats about 300 NET hp in todays measurements. But Ford deliberately castrated the engine for the street (These engines were built for sustained high RPM on Nascar tracks to compete with the 426 Hemi)(The engines only made it into the Mustangs because they needed 500 cars sold to meet Nascar rules). What Ford did to the street cars was sacralidge. They put peanut sized exhaust manifolds on them, a restrictive low rise dual plane intake with a very small carb on it. The Boss 302 had a larger carb! And the cam in it would not make any power beyond 4800 rpm,.. at 5000 the engines fell on their face. They made little low end torque. A stock 428 car would jump them at the line and then freight train them on the big end. But,.. These engines had forged cranks, rods and pistons, with the right solid lifter cam, headers and the right intake and carb/s. The heads in these engines came to life after 5500 RPM and made gobs of HP at 7000 RPM. No other big block was built to sustain 7000 - 8000 RPM like these motors were built to handle. These engines could make 600 - 650hp NA. No other engine,.. not even the mighty 426 Hemi or solid lifter 454 LS6 from GM could match that. And truth be known,.. the 1970 Buick Stage 3 455 made more hp than the LS6.

I hold a high regard for the 428 and 427. I never had a 427 car but I have owned a 69 Shelby GT500 and a 69 Mach1 SCJ R car. The Mach1 was much faster,.. it had a dual carb 428, 4 spd and 4.30 rear gear. I never ran it down the 1/4 mile,.. but I did make some 7.8/7.9 @93/94 1/8 mi passes. That was bone stock, open headers. Easily an 11 second car in the 1/4.

Another honorable mention should be the early 460 engines. In stock form even with DOVE heads they are pretty weak,.. just torque monsters. But with SCJ heads these engines can make 600 hp NA and have held 700 - 800 hp on stock crank and rods.

Hurst