SAE gross horsepower is an outdated method of measuring the maximum power output of a production engine, typically measured at the flywheel or crankshaft using a dynamometer. This standard was developed by the Society of Automotive Engineers (SAE) and was widely used prior to 1973.
Key Characteristics:
- Measured with the engine mounted on a test stand, free of many power-draining accessories.
- Includes only components essential to engine operation, such as:
- Fuel pump
- Oil pump
- Ignition system
- Excludes accessories like:
- Alternator
- Water pump
- Cooling fan
- Power steering pump
- Exhaust system
- Air cleaner and emission control devices
This testing setup creates ideal laboratory conditions, allowing the engine to operate without external parasitic loads, thereby yielding inflated horsepower figures compared to real-world use.
Historical Context:
- Used until 1972, after which it was replaced by the SAE net horsepower standard, which measures power with all standard production accessories installed.
- The change to net ratings caused a notable drop in published horsepower figures, even though the engine’s internal performance had not changed.
- Example:
The Cadillac 500 cu in V8 was rated at 400 hp SAE gross in 1972.
Under SAE net rating in 1973, it dropped to 235 hp, without any major mechanical changes.
- Example:
Comparison:
| Feature | SAE Gross Horsepower | SAE Net Horsepower |
|---|---|---|
| Accessories Attached | Minimal (bare engine) | All production accessories |
| Exhaust System | Excluded | Included |
| Cooling System | Excluded | Included |
| Realism | Overstated power | Realistic vehicle power |
| Era Used | Pre-1973 | 1973 onward |
Related Terms:
- Brake Horsepower (bhp): The measured output at the crankshaft without drivetrain losses.
- Horsepower (hp): A general unit of power; often used interchangeably with bhp in automotive contexts.
Summary:
SAE gross horsepower provides a best-case scenario power rating of an engine, useful for lab comparison but not representative of installed, real-world performance. It has been replaced by SAE net horsepower, which reflects true engine output under normal vehicle operating conditions.
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