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I believe they were rated at 175 'net' Horsepower.
 

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I believe they were rated at 175 'net' Horsepower.
Agreed, that was the factory net rating with single exhaust (all that was available from the factory in 71/72 with the Chevelle L48). The L48 was rated at 200 net when installed in the Nova SS & Camaro SS because they used dual exhaust systems.
 

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SAE gross horsepower
Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower (bhp), frequently referred to as SAE gross horsepower, because it was measured in accord with the protocols defined in SAE standards J245 and J1995. As with other brake horsepower test protocols, SAE gross hp was measured using a stock test engine, generally running with few belt-driven accessories and sometimes fitted with long tube (test headers) in lieu of the OEM exhaust manifolds. The atmospheric correction standards for barometric pressure, humidity and temperature for testing were relatively idealistic.

[edit] SAE net horsepower
In the United States, the term bhp fell into disuse in 1971-72, as automakers began to quote power in terms of SAE net horsepower in accord with SAE standard J1349. Like SAE gross and other brake horsepower protocols, SAE Net hp is measured at the engine's crankshaft, and so does not account for transmission losses. However, the SAE net hp testing protocol calls for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold.

[edit] SAE certified horsepower
In 2005, the SAE introduced a new test protocol for engine horsepower and torque.[12] The new protocol eliminates some of the flexibility in power measurement, and requires an independent observer present when engines are measured. The test is voluntary, but engines completing it can be advertised as SAE-certified.

A few manufacturers such as Honda and Toyota switched to the new ratings immediately, with multi-directional results; the rated output of Cadillac's supercharged Northstar V8 jumped from 440 hp (330 kW) to 469 hp (350 kW) under the new tests, while the rating for Toyota's Camry 3.0 L 1MZ-FE V6 fell from 210 hp (160 kW) to 190 horsepower (140 kW). Much of the drop can be attributed to Toyota now rating engines on 87 octane, compared to Lexus which uses 91 octane. This is why the same 3.3 L 3MZ-FE V6 engine in the Lexus ES330 and Camry SE V6 did not show equal declines. The ES330 and Camry SE V6 were previously rated at 225 hp but the ES330 dropped to 218 hp while the Camry declined to 210 hp. The first engine certified under the new program was the 7.0 L LS7 used in the 2006 Chevrolet Corvette Z06. Certified power rose slightly from 500 hp (370 kW) to 505 hp (377 kW).

Effective (true, wheel) hp
Effective horsepower (EHP), true horsepower (thp) or wheel horsepower (whp) is the power converted to useful work. In the case of a road vehicle this is the power actually turned into forward motion as measured on a chassis dynamometer. Power available at the road is generally 10% to 20% less than the engine's bhp rating due to coastdown losses, most of which are due to the vehicle's rubber tires rather than true transmission losses. Front-wheel drive cars (provided a transverse engine layout is used) suffer slightly lower coastdown losses due to the absence of the bevelled crown and pinion gears used to change the drive direction in the back axle of a RWD car.[14]

For railway locomotives the terms drawbar horsepower or equivalent drawbar horsepower (EDHP) refer to the power available to haul a train. This is synonymous with the Effective horsepower.[15] This figure takes into account the horsepower needed to move the locomotive, which is not available for hauling the train. The rail horsepower is the power at the wheels of a locomotive, directly comparable to the wheel horsepower of a road vehicle.



From Wikipedia. As you can see it's not a simple answer.
 

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Here's a simple answer. In 1971 the L48 was rated at 270 'gross' horsepower and I'm pretty sure the engines were identical 1971/1972.
 

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Agreed, that was the factory net rating with single exhaust (all that was available from the factory in 71/72 with the Chevelle L48). The L48 was rated at 200 net when installed in the Nova SS & Camaro SS because they used dual exhaust systems.
My 72 Heavy Chevy came with an L-48 and dual exhaust.
the build sheet is on my website
 
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