RPM Graph Analysis

RPM Graph Analysis Guide

This guide explains how to analyse RPM data from AiM MyChron 5 and MyChron 6 using RaceStudio 3. Understanding RPM behaviour is essential for gearing, tuning, and improving driving performance for kart racers in New Zealand.

Why RPM analysis matters

RPM data helps you investigate engine and drivetrain behaviour. It can help you:

  • Assess the gear ratio against the engine's operating range
  • Investigate over-revving or bogging
  • Compare RPM with braking and corner-entry data
  • Investigate acceleration and corner-exit performance
  • Compare performance between sessions or drivers

1. Understanding the RPM trace

The RPM trace is a line graph showing engine speed over time. Key features include:

  • Peaks — maximum recorded RPM on straights
  • Drops — reductions in engine speed; compare their position with speed, braking and video
  • Rises — increasing engine speed; confirm whether vehicle speed also increases
  • Flat sections — investigate the engine's operating range, limiter behaviour and signal quality rather than assuming one cause

Check that the RPM pickup and RPM factor are correct before making setup decisions. AiM specifies RPM acquisition at 20 Hz or 50 Hz for MyChron 5/5S and MyChron 6. These are RPM-channel settings, not a statement that every channel records at the same rate.

2. Identifying correct gearing

Assess gearing against the engine manufacturer's or engine builder's operating range, the track layout, tyre circumference and applicable kart class rules. Check straight-line RPM and corner-exit performance together; a single trace does not establish that the gearing is correct or incorrect.

Rear-sprocket direction

For a conventional chain-drive kart, with the front sprocket and tyre circumference unchanged:

  • Larger rear sprocket: shorter gearing, greater torque multiplication and potentially stronger acceleration, but lower theoretical speed at a given engine RPM.
  • Smaller rear sprocket: taller gearing, less torque multiplication and potentially weaker acceleration, but higher theoretical speed at a given engine RPM.

Actual top speed and acceleration also depend on available engine power, traction and conditions.

Signs to investigate before changing gearing

  • Reaching the limiter too early: confirm the engine limit, RPM signal and speed trace. If gearing is too short, a smaller rear sprocket makes it taller; check the effect on corner-exit performance.
  • RPM below the useful operating range on exit: check throttle application, engine condition, clutch behaviour where applicable and corner speed. If gearing is too tall, a larger rear sprocket makes it shorter; check straight-line RPM before accepting the change.

3. Analysing corner entry

Compare the RPM trace with speed, track position and any available brake data or video:

  • Sharp RPM drop: investigate braking, lifting, drivetrain behaviour and signal quality.
  • Smooth drop: compare deceleration and driver inputs before judging braking technique.
  • Sudden dip: investigate possible wheel locking, a drivetrain event or a signal dropout; RPM alone cannot distinguish them reliably.

4. Analysing corner exit

Compare the rate of RPM rise with vehicle speed and the engine's useful operating range:

  • Fast rise: confirm that speed increases too; an RPM rise alone does not prove strong acceleration.
  • Slow rise: investigate corner speed, line, throttle application, gearing and engine response.
  • Flat spot: check driver inputs, engine response, drivetrain behaviour and data quality before diagnosing a cause.

5. Comparing two laps or drivers

Overlay RPM with speed and other available channels, using comparable track conditions, tyres and vehicle setup. This can help investigate:

  • Differences around braking points
  • Minimum corner speed
  • Acceleration after the corner
  • Whether a gearing change improved the complete lap rather than one section

6. Common RPM patterns and checks

RPM Pattern Possible causes to investigate First check
Hitting limiter early Gearing may be too short, or the RPM reading/limit may need checking Confirm engine limits and RPM accuracy. If appropriate, a smaller rear sprocket makes the gearing taller; recheck corner exits.
Slow RPM rise Corner speed, driver input, gearing or engine response Compare speed, throttle where available and video before choosing a change.
Sudden RPM dip Possible wheel locking, drivetrain event or signal dropout Check the RPM pickup and compare other channels; do not diagnose braking from RPM alone.
Low peak RPM Gearing may be too tall; engine performance, driver input and track layout also matter Check the engine's useful range and supporting data. If appropriate, a larger rear sprocket makes gearing shorter; recheck straight-line RPM.

7. What RPM should your engine reach?

Use the operating range and maximum RPM specified for your exact engine model, configuration and kart class by the engine manufacturer or engine builder. A broad brand name such as Rotax, Vortex, IAME or Briggs is not enough to establish a safe RPM limit. The MyChron display scale, shift-light setting and recorded RPM are not themselves engine safety limits.

Change one setup item at a time, remain within the applicable class rules and compare the complete lap. If RPM is erratic or the engine behaves abnormally, stop testing and have the signal or engine checked before pursuing a gearing change.

Related guides

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