SAE Horsepower

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SAE Horsepower
(Also called rated horsepower)

SAE horsepower refers to a calculated engine power rating based on a simplified formula, not an actual measurement of power output. This value was traditionally used for taxation and vehicle licensing purposes, rather than performance assessment.

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Formula:

SAE horsepower=(bore diameter)2×number of cylinders2.5\text{SAE horsepower} = \frac{(\text{bore diameter})^2 \times \text{number of cylinders}}{2.5}

• Bore diameter is measured in inches.
• The result provides a nominal power figure that was used to classify engines for legal or administrative reasons, not for technical accuracy.

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Characteristics:

• Does not account for:
  • Stroke length
  • Compression ratio
  • Engine RPM
  • Valve timing
  • Fuel delivery system
• As a result, it does not reflect the engine’s real-world performance or brake horsepower.

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Usage:

• Historically employed in the early 20th century by vehicle registration authorities, particularly in the United States and United Kingdom.
• Mostly obsolete today, replaced by more accurate power rating standards like SAE gross horsepower, SAE net horsepower, and DIN horsepower.

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Comparison:

Type	Method	Accuracy	Primary Use
SAE horsepower	Calculated from bore & cylinder count	Low	Licensing/taxation
SAE gross	Measured at crank, few accessories	Medium (idealized)	Pre-1973 marketing/performance
SAE net	Measured with full accessories	High (realistic)	Standard since 1973

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Related Term:

• Horsepower: General term for power output, often measured as brake horsepower (bhp).

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Summary:

SAE horsepower is a legacy calculation once used for regulatory classification. While historically important, it is not reliable for evaluating actual engine performance. For technical or performance contexts, refer to SAE gross or SAE net horsepower instead.

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