What is the compression ratio of a Subaru STI?
The most commonly cited compression ratio for the Subaru STI’s characteristic turbo engine is 8.2:1, coming from the widely used 2.5-liter EJ257 engine in many generations. However, STI variants across generations and markets have used different engines with their own compression ratios, so the exact figure can vary by model year and region.
Context: why compression ratio matters in a turbocharged STI
Compression ratio (CR) is a fundamental engine specification that affects performance, efficiency, and the engine’s tolerance to boost. In turbocharged Subarus, a lower CR helps reduce the risk of detonation when boost is applied, enabling safe, reliable power delivery across varying fuel qualities and climates. The STI lineage has prioritized low CR in its turbo engines to support higher boost pressures and aggressive tuning.
Engine configurations commonly associated with the STI
Here is a concise reference for the engines most often linked to the STI name across different eras. Values below reflect widely published specs, but exact numbers can vary by revision and market.
- EJ25-based 2.5L turbo engine (the workhorse of many US-market WRX STI models): compression ratio commonly cited at 8.2:1
- Other STI configurations in various markets and years use different engines (engine-code specific), each with its own compression ratio
In short, 8.2:1 is the touchstone figure for the popular EJ257-powered STI variants; exact values depend on the specific engine code and model year.
How to verify your car’s exact compression ratio
To confirm the compression ratio for a specific Subaru STI, follow these steps:
- Identify the engine code stamped on the engine block or consult the data plate in the engine bay (e.g., EJ257).
- Refer to the model-year specific Subaru workshop manual or official data sheets for that engine code.
- Cross-check credible automotive databases or service documentation that publish OEM specifications by engine code and market.
With the engine code and year, you can locate the precise compression ratio as specified by Subaru for that variant.
Notes on tuning and reliability
Compression ratio interacts with boost, fuel octane, and timing. If you modify boost or fuel, a proper retune is essential to avoid detonation. For any modification plans, consult a qualified tuner and verify all engine specifications before proceeding.
Summary: The STI’s compression ratio is typically low due to turbocharging, with 8.2:1 being a widely cited figure for the common EJ257-powered variants. However, exact values vary by engine code and model year, so verifying your specific engine is essential.
