A boxer engine is a horizontally opposed engine: its cylinders lie flat in two banks on either side of the crankshaft, and opposing pistons move in and out at the same time, like boxers trading punches. Subaru uses boxer four-cylinder engines in almost all of its cars, and Porsche uses boxer engines in the 911 and 718. The layout gives a low center of gravity and smooth running, at the cost of a wider engine with more parts.
How a boxer engine works
Each pair of opposing pistons moves toward and away from the crankshaft together, so their forces largely cancel. A flat-four has two cylinder heads, one on each side, and the engine sits low in the car because it is short and wide rather than tall.
Boxer engine vs inline engine
| Factor | Boxer (flat) | Inline |
|---|---|---|
| Shape | Short and wide | Tall and narrow |
| Center of gravity | Low | Higher |
| Smoothness | Naturally smooth, especially a flat-six | Inline-six very smooth; inline-four less so |
| Parts | Two cylinder heads and more valvetrain parts | One cylinder head |
| Service access | Spark plugs and some parts are tighter to reach | Usually easier |
Advantages
- Low center of gravity for better handling
- Smooth running from opposed pistons
- Pairs naturally with Subaru’s symmetrical all-wheel drive layout
Disadvantages
- A wider engine bay and more complex packaging
- Two heads mean two sets of head gaskets and more valvetrain parts
- Some routine jobs, such as spark plugs, take longer
Subaru boxer engine problems and reliability
Subaru boxers are generally durable with good maintenance, but some families have well-known issues: head gasket leaks on older naturally aspirated EJ25 engines, ringland and rod bearing failures on hard-driven turbocharged EJ257s, and oil consumption complaints on some later engines. See Subaru head gasket problems, EJ257 problems and oil in coolant.
Subaru engine families
- EJ: the long-running family with belt-driven camshafts, including the EJ20, EJ25, EJ255 and EJ257; see EJ25 vs EJ20 and EJ255 vs EJ257.
- FA and FB: newer families with timing chains, including the FA20 and FA24 in the WRX, BRZ and GR86; see FA20 vs FA24.
Building a stronger boxer
For more power, builders use closed-deck blocks and forged internals to strengthen the cylinder walls and pistons; see closed deck vs open deck. Our IAG Performance and Turn In Concepts EJ25 and FA20/FA24 short and long blocks are on our Subaru EJ25 and FA20 and FA24 pages; see also timing belt for EJ maintenance.
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