Ignition Curve Analysis Guide

1.0 Ignition Timing Objectives Summary
1.1 Overview of Mechanical Events
1.2 Timing Curve – Layering into Regions and Points
1.3 Tuning Analysis Categories
1.3.1 Minor Timing Regions
1.3.2 Transitional Timing Region
1.3.3 Crucial Performance Regions
1.4 Analysis Process
1.4.1 Minor Timing Region Analysis
1.4.2 Performance Analysis preparation1.4.3 Performance Analysis
1.4.4 Final Curve Generation



2.0 Combustion and Detonation
2.1 Principals of Combustion
2.2 Principals of Detonation
2.3 Detecting Detonation
2.3.1 Visual detection
2.3.2 Cylinder Head Temperature Measurement
2.3.3 Electronic Detonation Devices




1.0 Ignition Timing Objectives Summary

Optimization of the ignition timing isto result in maximum torque throughout a given RPM range. It’s easy to get close, but to really extract the best results takes an understanding of the various events and side effects.

Part of the RPM range requires enough spark advance to run on the edge of detonation of a high compression motor. Detonation results in damage to the components of the motor from erosion and pitting of thematerials in contact with the combustion media, to bearings and other reciprocating components. Part of the RPM range requires retarding the ignition to alter the temperatures of the exhaust, resulting in adding over-rev. At any point in between, other chaotic events may occur such as a harmonic reed flutter that may invite a retarding or advancing action for very short intervals.

The SPI system hasbeen designed to provide the most accurate control of timing available. Together with the intuitive programming option, you can finely tune additional power out of your motor without risking detonation damage if you have a good understanding of the ignition events to work around and how to detect detonation.

1.1 Overview of Mechanical Events

Port timing and pipes are designed to accommodate ahigh power peak across a narrow range of RPM. There are various known events that will determine the basic layout of the ignition timing profile. There are also various chaotic or unknown events that will need to be factored that this guide will identify. For the purposes of this example, we will look at a motor tuned to operate between 8800 and 12,000 RPM with a flat line ignition, and a torquepeak of 10,500 RPM.

As the cycle time changes with RPM, the events of port timing and pipe design change in sequential order. The pipe design has a huge impact on 2 stroke power, and the ignition timing can be used to compensate for the changes somewhat. By dialing more retard in at higher RPM, more heat is passed to the exhaust, and higher temps simulate a shorter pipe. This works as thespeed of sound (the key factor in the acoustical characteristics of the pipe) increases with higher temperature. Balancing the added heat in the pipe with the loss of efficiency of a later spark can result in extending the usable RPM range – overrev – by over 500 RPM.

There are certain chaotic events that may occur at any given RPM. Typically some reeds have a harmonic bounce that may occur at say10,200 – 10,250 RPM, resulting in a richer mixture. A little steeper advance may offset this problem. A pipe may have a resonant point at 10,850-10,950 RPM that requires a slight retard across this range.

1.2 Timing Curve – Layering into Regions and Points

Viewed graphically a timing curve is a set of points that is broken down into operational layers of evaluation. At the first layer are theRegions that determine the major functions of the motor. Minor Timing Regions are not a significant subject of performance and are more of a convenience for tuning.

As the Regions can be broken down into points of 100 RPM increments, a second layer of analysis is at this finer increment. To manage development of the best curve takes a layered approach to defining regions for the curve.

The...