How to Read a Compression Test Report Before Buying a JDM Engine
Why Compression Testing Beats Mileage as a Health Check
Mileage tells you how far an engine has traveled. It doesn't tell you how it was driven, how it was maintained, or whether it was sitting in a flooded storage yard for two years before import. A compression test tells you something much more direct: whether each cylinder is actually sealing and building pressure the way it should.
What the Test Actually Measures
A compression tester is threaded into each spark plug or injector hole in turn, and the engine is cranked to measure peak cylinder pressure. Healthy cylinders on the same engine should read close to each other — typically within about 10% of one another. A cylinder reading meaningfully lower than its neighbors is the classic sign of worn rings, a burnt valve, or a head gasket issue.
What to Look For in a Report
- Consistency across cylinders — this matters more than the absolute number, since different engines have different healthy baselines.
- No cylinder reading dramatically lower than the rest — a single weak cylinder is the most common red flag.
- Whether it was tested wet or dry — a wet test (with a small amount of oil added) that improves the reading points toward ring wear specifically.
Why We Test Before Listing, Not After You Ask
A lot of sellers will tell you an engine "ran good when it came out" — that's a description, not a measurement. We compression-test units before they're listed so the number you see is the number we're standing behind, not a claim we're making after the fact. Combined with real-unit photography and VIN-based fitment checks, it's how we back the "no guesswork" part of what we do.
Have questions about a specific listing's numbers? Call us at 253-536-6855 or browse compression-tested engines currently in stock.
