Engine RPM: Tachometer & Idle

Engine RPM is a tachometer that works out engine speed from the exhaust sound, through your phone's microphone. Choose 2- or 4-stroke and the number of cylinders, hold the phone 30–50 cm from the exhaust and read the RPM on a big dial, with a ± value. When the sound is not clear enough, it shows no number instead of a guess.

▶ TARGET RPM: IN, LOW OR HIGH
• Enter the idle and maximum speeds from the engine's manual, for example idle 2,800 ±100.
• The dial marks the target range and a colour band says IN, LOW or HIGH while you turn the idle screw or set the governor.
• Big readout: the number fills the screen in the target colour, readable from where you work.

▶ IDLE STEADINESS TEST
• Let the engine idle for 10, 20 or 30 seconds.
• The result card shows the average, ± spread (standard deviation), variation %, the middle 90 % of readings, drift in RPM per minute and how much of the run gave a steady reading.
• It compares the new test with your previous idle test of the same engine and with your idle target.
• Less than 70 % steady readings? The test says so instead of showing statistics.

▶ ×½ OR ×2? SHOWN, NOT HIDDEN
• Engine sound is full of harmonics, so half or double the true speed can look just as plausible. The app lists the candidates and how well each fits; tap the right one and it is kept for that engine.
• Candidates that would fall outside the engine's range are ruled out.
• Match a known RPM: enter a reading from a dashboard or clip-on tachometer and the app picks the interpretation that fits, or suggests pulses per turn for a fan or propeller.
• LOCKED, WEAK or no reading, with the reason in words: too quiet, no clear engine pattern, out of range.
• If the reading jumps the wrong way while you rev, the lock is dropped instead of showing a wrong number.

▶ BELT SQUEAL AND WHINE
• The Tone tab lists sustained tones from 400 Hz up, with how far each stands out and how much of the time it is there.
• Follow test: idle, then about 1.4 × idle speed. It shows whether a tone tracks engine speed (with its order) or keeps its pitch. These are possibilities to discuss with a mechanic.

▶ ENGINES AND READINGS
• Car, motorcycle, mower, generator, chainsaw / trimmer, outboard, RC nitro or other, each with its own search range.
• 2- or 4-stroke with 1, 2, 3, 4, 5, 6, 8, 10 or 12 cylinders, plus a pulses-per-turn mode for fans, propellers and brushless motors.
• RPM, rotation Hz or firing Hz. Min, average and max of the locked readings, a 30-second chart and Hold.
• Up to 30 engines and 300 saved readings. Pick two readings for a Before / After comparison.
• Share a result card as an image or export a CSV file.

▶ GOOD TO KNOW
• Readings are estimates worked out from sound. How close they come on your engine and phone has not been verified; check critical settings with the engine maker's tools.
• A quiet car muffler at idle may not lock, and measuring from inside a car does not work.
• An idle that swings a lot may not give enough steady readings for the idle test.
• Sound levels in dB are relative to the phone microphone, not absolute.
• The microphone is used while a measurement runs. Sound is analysed on the phone and never recorded or uploaded. No account, no sign-in.
• Never use it while driving or riding. Keep hands and the phone away from belts, fans and hot exhaust.
• Ads appear at app start, on the Garage tab and the guide, and sometimes after you close a saved result. Not while a measurement runs.