Compression Ratio to PSI & Back
The polytropic formula is the bridge between geometry (CR) and physics (cranking pressure). More importantly, when something is wrong with your engine, the PSI readings are how you diagnose what exactly is wrong — rings, valves, or head gasket.
Compression Test Decision Tree
The Polytropic Formula — Why n = 1.3, Not 1.4
Every textbook gives you the ideal gas law compression formula with n = 1.4. But when you actually crank an engine, the reading is always lower than the ideal predicts. The difference is real-world physics: heat loss through the cylinder walls and tiny amounts of air leaking past the piston rings. The polytropic index n = 1.28–1.35 accounts for both effects and gives you the number you'll actually read on your gauge.
Isentropic (Ideal Gas) — n = 1.4
Zero heat loss, zero leakage. Theoretical upper bound. Predicts 15–20% higher PSI than reality. Useful for heat-exchanger engineering, not engine diagnostics.
Polytropic (Real Engine) — n = 1.28–1.35
Accounts for heat loss + ring leakage. Industry standard n = 1.3 for most engines. Matches actual cranking pressure within ±5%. Race engines with tight rings → n = 1.32–1.35. High-mileage engines → n = 1.25–1.28.
PSI by Dynamic CR — Reference Table (sea level)
| Dynamic CR | Isentropic | Polytropic (n=1.3) | Polytropic (n=1.28) | Realistic Range |
|---|---|---|---|---|
| 7:1 | 224 | 184 | 177 | 200–220 PSI |
| 8:1 | 270 | 219 | 211 | 250–280 PSI |
| 9:1 | 319 | 256 | 245 | 300–330 PSI |
| 10:1 | 369 | 293 | 280 | 340–370 PSI |
| 11:1 | 422 | 332 | 316 | 390–420 PSI |
| 12:1 | 477 | 372 | 354 | 430–470 PSI |
Altitude — The Forgotten Variable
Atmospheric pressure decreases with altitude, and the polytropic formula is directly proportional to Patm. Every 1,000 feet of elevation reduces your expected cranking PSI by approximately 1.5–2%. At 10,000 ft, Patm drops from 14.7 to 10.1 PSI — a 31% reduction that looks like worn rings if you don't correct for it.
A 9.5:1 DCR engine reads 345 PSI at sea level but only 287 PSI in Denver — same engine, no problems, just thinner air. Always correct before diagnosing.
Compression Test — The Correct Procedure
Skip any step and your readings are garbage. Here's the procedure professional technicians follow — every detail matters because compression tests diagnose mechanical integrity.
Warm Engine to Operating Temperature
Drive or idle until coolant is 190–210°F. Cold engine has thicker oil and tighter clearances that give falsely high PSI. Most importantly, valve clearances are designed for operating temperature.
Remove ALL Spark Plugs
Every single one, not just the cylinder you're testing. Removing only one cylinder creates back-pressure in the others and skews the results. Label them if needed for reinstallation.
Disable Fuel Injection
Pull the fuel pump fuse or disconnect the injectors. You don't want raw fuel being sprayed into cylinders during the test — it washes cylinder wall lubrication and can hydrolock an intake valve.
Hold Throttle WIDE OPEN
WOT means no intake restriction — the engine ingests maximum air and compression is representative of actual operation. Partial throttle restricts intake and gives 10–20% lower PSI.
Crank 5–7 Revolutions, Record PSI
Use a fully charged battery — cranking speed matters (200+ RPM minimum). Crank each cylinder for the SAME number of revolutions (5–7 complete rotations of the crankshaft). Record the maximum PSI reached.
The Oil-Squirt Test — Rings vs Valves vs Gasket
When one cylinder reads low, this 2-minute test tells you exactly what is leaking. It's the cheapest, fastest diagnostic in engine building and 90% of mechanics skip it.
Compression Test Pitfalls
A 350 PSI reading on one cylinder vs 345 on another (1.4% differential) is perfect — even if both are 10% below what the formula predicts. A 350 vs 280 (20% differential) is a problem even if 280 is "within range." Differential is everything.
New head gaskets take 500–1,000 miles to fully seat and compress under heat cycling. A compression test immediately after installation will read 10–15% low because the gasket hasn't fully sealed yet. Wait until after the break-in period.
Compression test is dynamic (cranking, measuring PSI). Leak-down test is static (injecting compressed air, measuring leakage rate). They're complementary — compression test says "something's wrong," leak-down test says "where." If a cylinder reads low, do a leak-down test next before tearing anything apart.
Compression Test & Polytropic FAQs
Isentropic vs Polytropic — why two formulas?+
How much does altitude affect compression test results?+
Top QWhat's the correct procedure for a compression test?+
Top QWhat is a 'good vs bad' compression test result?+
Can I use compression test results to determine my compression ratio?+
Why does my compression test give weird readings after an oil change?+
Two adjacent cylinders reading low — what does that mean?+
Related Guides & Resources
CR to PSI Calculator
Convert compression ratio to cranking PSI and back. Polytropic vs isentropic, altitude correction, visual CR-to-PSI table 7:1–15:1.
ECR with Boost
SCR or DCR plus boost PSI → effective compression ratio. Turbo vs supercharger ECR difference, target octane → max safe SCR, detonation risk matrix.
Metric CR Calculator
Pure metric-unit compression ratio calculator. Bore/stroke in mm, altitude in meters, results in bar and kPa. European and Japanese engine presets.
Octane by DCR
Enter dynamic CR, altitude, head material, and quench to see if 87, 89, 91, 93, E85, or race fuel is right. Altitude and aluminum-head correction built in.
Piston Comp Height
Bore, stroke, rod length, and dome volume in — required piston compression height and deck clearance out. OEM vs aftermarket reference values for stroker builds.
Clearance Volume
Calculate clearance volume from displacement and compression ratio, or reverse-solve the missing Vc component — chamber, gasket, deck, or piston dome cc.
Gasket vs CR
See how gasket thickness and head milling move compression ratio. Bore + Δthickness → ΔCR live output, gasket material effects, D16 Honda worked example.
2-Stroke CR
Two-stroke compression ratio calculator with corrected CR from port timing. Predator 212 and TZ250 presets, 2-stroke vs 4-stroke CR comparison, small-engine octane.
Racing CR Presets
One-click compression ratio presets for 350 SBC, 383 stroker, 440 Mopar, LS1–LS7, Coyote, Hemi, 351W, and Pontiac 400 — each with DCR and octane recommendation.
What Is Compression Ratio in an Engine? Complete Guide with Formulas & Examples
Complete guide to engine compression ratio — physical meaning, formula, every variable explained, BDC vs TDC, swept vs clearance volume, 8:1 vs 12:1 comparison, thermal efficiency.
Static vs Dynamic Compression Ratio — Why the Difference Is Everything for Fuel Choice
Formula-level deep dive: SCR pure geometry vs DCR real physics, IVC angle, slider-crank derivation, rod ratio effect, octane decision matrix, real LS3 stock vs cammed example.
How to Calculate Compression Ratio — Step-by-Step with Real Engine Example
Complete hands-on guide: tools list, step-by-step measurements (bore mic, stroke, burette chamber cc, piston dome, gasket thickness, deck clearance), full worked Chevy SBC 350 example, error propagation.
Compression Ratio Formula — Every Term Explained with Engineering Context
Engineering-level formula derivation: CR = (Vd + Vc) / Vc from Otto Cycle thermodynamics, Vd = (π/4)bore²stroke geometry, Vc four-component breakdown, Metric vs Imperial conversion, polytropic pressure P ≈ 14.7 × DCR^n.
Effective Compression Ratio with Turbo & Supercharger — What ECR Means for Detonation
Boost physics: why turbo/supercharger create pre-compression, ECR = DCR × (boost/14.7 + 1) derivation, Turbo vs Supercharger intake temp, detonation window visualization, 2JZ-GTE E85 case study.
Head Milling & Decking Block — How Much Does Each Mill Change Compression Ratio?
Machining physics: head milling chamber CC reduction by 0.010" (0.8-1.2 cc varying by hemispherical/wedge/pentroof shape), decking block clearance change, gasket thickness CR tuning efficiency, 350 SBC case study 8.5→10.5 CR.
Cam Timing and Dynamic Compression Ratio — Why a Wild Cam Lowers Your Octane Requirement
IVC angle (Intake Valve Close) DCR dominance, why late IVC = lower DCR, cam duration vs IVC relationship, using wild cam to drop octane 93→87, Harley Twin Cam 1550cc stock vs custom case study, cam card reading guide.
How to Estimate Compression Ratio from a Compression Test Reading
Reverse engineering CR from PSI: DCR = (PSI/14.7)^(1/1.3) polytropic inverse, why only DCR not SCR, reverse SCR from DCR using known cam IVC, PSI reading limitations (temperature, RPM, seal), step-by-step procedure.