Innovate Motorsports ST-12 - Manual (Page 20)

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the next time. In vacuum/boost measurement mode the ST-12 will then subtract the measured value from
the MAP value internally.
Function 3 of input 4 of the ST-12 measures vacuum with a range of 0..30 inHg (gauge) This is the range
intended for normally aspirated engines where the maximum intake pressure can be atmospheric
pressure.
Function 4 of input 4 of the ST-12 measures vacuum and boost pressure with a range -14.7 to 29.4 PSIg
This is the range intended for boosted engines where the maximum intake pressure can be up to 29.4
PSI above atmospheric pressure. Vacuum is typically measured here as negative PSI. But you can set
up any metric you want with LogWorks 2.
5.4 Measuring Ignition Advance
Warning:
Measuring ignition advance is NOT a simple plug-and-play process. The variables are many and
you need to know what you are doing to do it right. Read this chapter multiple times before
attempting this and try to understand what you are measuring.
The ST-12 expects the spark reference pulse on input 1 and the crank reference pulse on input 3.
Input 1 is simultaneously still used to measure RPM, but MUST be set to measure RPM.
The ST-12 can measure ignition advance between 10 degrees ATDC to 50 degrees BTDC. ADTC
numbers will be negative, BTDC numbers will be positive. The LogWorks equivalents are 0V = -10
degrees, 5V = 50 degrees.
Ignition advance is typically measured in degrees. This is the number of degrees before Top-dead-center
of a piston where the spark fires.
When the spark in an engine fires the mixture in the combustion chamber starts the burn process.
Because it takes time for the fire to consume the mixture, it has to be lighted before the piston hits top
dead center. During that burning process the pressure and temperature rises. The pressure and
temperature rise results not only from the energy released by the burning mixture, but also the piston is
still moving up, compressing the burning gas. At some point in this process the pressure in the cylinder
peaks and then falls off. The position of this pressure peak (in crank angles) depends on the engine
geometry (bore-stroke ratio, stroke-rod length ratio and so on), but NOT on engine load or RPM. For
many engines the ideal peak pressure position to extract the maximum energy is between 14 and 20
degrees ATDC.
The time the mixture takes to burn is dependent on many variables. AFR, mixture density, temperature
and so on are some of the variables. The point of ignition advance is to time the spark such, that the
peak pressure point is reached at the ideal position. Earlier or later looses power.
An engine typically does not have a “crank degree” sensor output of sufficient resolution. Therefore
ignition advance must be measured as a time measurement. An engine crankshaft rotates at 360
degrees per revolution. So, by measuring the time between the spark pulse and a reference pulse, the
ignition advance time can be calculated.
For example if the reference pulse is at a 90 degree crank angle and the spark happens at 20 degrees
BTDC at 6000 RPM, the engine rotates at 36000 degrees per second. So, the time difference between
spark pulse and reference pulse is 0.003055 seconds.