Briggs and Stratton Engine Compression Chart(For All Engines)

Understanding compression is essential for troubleshooting and maintaining a Briggs & Stratton small engine. A compression test measures how well the engine’s cylinder holds pressure during the compression stroke, which indicates the health of piston rings, valves, head gasket, and overall engine sealing.

Briggs and Stratton Engine Compression Chart

Engine Category	Model / Horsepower	Expected Compression (PSI)	Notes
Small Horizontal Shaft Single	3–4 HP (push mowers, tillers)	75–100	Older or small displacement engines may read slightly lower.
Small Horizontal Shaft Single	5–6 HP	80–110	Common for residential equipment; decompression mechanisms may lower readings slightly.
Mid-Size Single Cylinder OHV	7–9 HP	90–115	Residential lawn mowers and garden tractors.
Mid-Size Single Cylinder OHV	10–11 HP	95–120	Slightly higher due to larger displacement and compression ratio.
Mid-Size Single Cylinder OHV	12–14 HP	95–125	Common for larger garden tractors, riding mowers, and small generators.
Larger Single Cylinder OHV	15–18 HP	95–130	Typical readings for overhead valve engines used in commercial and heavy-duty equipment.
Twin Cylinder V-Twin	18–20 HP	120–155 per cylinder	Residential and commercial riding mowers; consistency between cylinders is important.
Twin Cylinder V-Twin	21–22 HP	120–160 per cylinder	Larger garden tractors and commercial mowers.
Industrial OHV	22–25 HP	125–160 per cylinder	Heavy-duty engines for commercial applications.
Industrial OHV	26–30 HP	130–160 per cylinder	Larger industrial or utility engines.
Older L-Head / Flathead Single	3–5 HP	70–100	Flathead design and decompression valves reduce compression readings.
Older L-Head / Flathead Single	6–8 HP	75–110	Typical older small engines.
Industrial L-Head / Flathead	10–14 HP	80–120	Some vintage commercial engines still in use.
Vertical Shaft OHV	5–7 HP	85–115	Small engines used in pressure washers, generators, and pumps.
Vertical Shaft OHV	8–11 HP	90–125	More robust vertical shaft engines for larger equipment.
Vertical Shaft OHV	12–16 HP	95–130	Commercial vertical shaft engines for heavy-duty mowers, generators, and utility equipment.
Twin Cylinder Vertical Shaft	18–25 HP	120–160 per cylinder	Industrial or commercial equipment; even readings across cylinders are key.
Industrial Horizontal Shaft OHV	15–30 HP	115–160 per cylinder	Designed for industrial machines, generators, and construction equipment.

Note:

PSI values are approximate, typical readings, not official factory specifications.

Consistency between cylinders is critical; differences over 15–25 psi suggest valve, ring, or gasket issues.

Compression below 80 psi on most small engines typically indicates a mechanical problem.

Flathead or older engines usually show slightly lower readings due to design differences.

Why a Compression Chart Matters

Engines with low compression may:

Fail to start
Run rough
Lose power
Smoke or misfire

A compression chart provides a reference for evaluating engine health and determining whether maintenance or repair is required.

How to Perform a Compression Test

Warm up the engine.
Remove the spark plug.
Attach a compression gauge to the spark plug hole.
Hold the throttle wide open.
Crank the engine several revolutions.
Record the highest steady reading on the gauge.

Compression readings help identify potential problems such as worn piston rings, leaking valves, or a blown head gasket.

Normal Compression Ranges

Compression readings vary based on engine type and size. As a general guide:

Most small engines show readings of 90–120 psi.
Larger engines, such as V-twins, may show 120–160 psi per cylinder.
Older flathead engines often read lower, around 70–110 psi.

Compression below the typical range may indicate an engine problem, while differences between cylinders greater than 15–25 psi can suggest sealing issues.

Interpreting Compression Test Results

Healthy Engine

PSI readings within the expected range
Smooth operation
Even readings across cylinders

Potential Problems

Low compression (<70–80 psi) may indicate worn piston rings, leaking valves, or a blown head gasket.
Large variation between cylinders (>25 psi) may suggest valve or ring issues.

Troubleshooting Tips

Perform a wet compression test by adding a small amount of oil to the cylinder and retesting. An increase in compression indicates worn piston rings.
Inspect valves and seats if compression is low.
Check the head gasket for leaks or damage.

Practical Compression Test Tips

Always hold the throttle fully open during the test.
Ground the ignition system to prevent a spark during testing.
Compare readings to another similar engine if possible.
Engines with compression relief mechanisms may show slightly lower readings, which is normal.

Example Case Studies

Example 1: Briggs 8 HP Single Cylinder

Compression reading: 95–105 psi
Condition: Healthy engine

Example 2: Briggs 21 HP V-Twin

Front cylinder: 135 psi, Rear cylinder: 150 psi
Condition: Within typical range; slight variation is acceptable

Example 3: Older Flathead 5 HP

Compression reading: 60–80 psi
Condition: Lower readings are normal for older flathead engines with decompression mechanisms

FAQs

How much compression should a Briggs and Stratton have?

Most Briggs & Stratton small engines should read between 80 and 160 psi, depending on engine size, type, and design. Single-cylinder engines typically range from 80–130 psi, while twin-cylinder and larger industrial engines often measure 120–160 psi per cylinder. Consistency between cylinders is more important than the absolute number.

What is good compression for a small engine?

A good reading for a small engine is generally 90–120 psi for most residential engines. If readings are within this range and consistent between cylinders, the engine is likely in a healthy condition. Readings significantly lower than this may indicate worn piston rings, leaking valves, or a blown head gasket.

What is normal engine compression psi?

Normal compression varies by engine type:

Small push mower engines: 80–110 psi
Larger OHV engines (12–18 HP): 95–130 psi
Twin-cylinder engines: 120–160 psi per cylinder
Older flathead engines often measure slightly lower, around 70–110 psi.

How to check compression release on Briggs and Stratton?

To check the compression release:

Remove the spark plug.
Attach a compression gauge.
Slowly crank the engine while observing the reading.
If the compression release is working, the gauge will show a slightly lower peak than a standard compression test, as the release allows easier cranking without changing the engine’s overall health.

What are signs of bad compression?

Signs of low or bad compression include:

Engine fails to start or struggles to crank
Rough running or misfiring
Loss of power during operation
Excessive smoke or unusual exhaust
Significant difference in compression between cylinders

Final words

A compression chart is a valuable tool for assessing engine health, but it should not be the only diagnostic method. Always combine compression readings with: