Forum has been fairly quiet to date. Most of my posts to date have been more on the technical and theoretical side. This is because my '85 1200 FI model is at the "clean up" stage, reducing the amount of extra wires, locating components in more optimum spots and such.
This new thread about fuel and ignition timing is quite interesting, to me anyway. I can safely mention that most of us know about engine ignition timing, advancing-retarding as required, and have dabbled with different octane fuels whether the bike is N/A or has boost.
My research indicates that it is not that simple. I submit that for the most part it is that simple as most of us will not feel a noticeable difference in the engine performance or fuel economy, much like changing out the exhaust system for HD mufflers and such.
I have mentioned in various other threads that it is not the change that is an issue, although it can be, it is understanding what the change is doing. If you understand the technical aspects and still do a change, you are doing the change from an informed perspective.
I will include the usual caveat in that this thread is an opinion piece, and my opinion and understanding from my research. I did not create the information, but think it's nice to know. This type of information is helping with my ECU Replacement/upgrade project as well.
I have a document that is about this topic and will use it to explain my position on this issue.
Ignition timing is used to start the burning of the air-fuel mixture at the appropriate time to ensure the air-fuel mixture is fully combusted at or near approximately 15° after top dead centre (ATDC). The process is:
If the air-fuel mixture is ignited too late BTDC – closer to TDC, ignition timing is said to be retarded, the air-fuel mixture will not be completely burnt before the optimum ignition timing ATDC, but will continue to burn, or not, after the optimum ignition timing degree ATDC, reducing engine power output and fuel economy will suffer.
Ignition Timing Trends
There are engine indications that the engine timing may be off. The most published engine indicators are:
Engine knocking
decreased fuel economy
Reduced engine power
Engine overheating
What differences are there between advancing iengine ignition timing and retarding engine ignition timing?
Advancing engine ignition timing is to compensate for fuel ignition delay. Fuels require a minimum amount of time to start the air-fuel mixture burning until it is in a full burn mode. Some fuels are more volatile and need less time, others more. A high-octane fuel will require additional time to come to a full burn than a fuel with less octane. The following diagram illustrates this:
The air-fuel mixture does not ignite immediately when an ignition event is triggered by the ECU. Advancing ignition timing may be needed to successfully start the air-fuel mixture burning, getting past the air-fuel mixture ignition delay, so that the air-fuel mixture is completely burnt and the pressure pulse from the burnt fuel is at maximum at the appropriate engine timing ATDC. The fuel of choice will also affect the engine ignition timing. Advancing the engine ignition timing should raise high-end power, but at the cost of reducing low-end power.
Retarding engine ignition timing can be used to reduce engine detonation, commonly called engine knock. Using a fuel with a higher-octane rating can assist in reducing engine detonation, pinging.
There are many engine tuning strategies employed by engine tuners, and as such, there is no right/wrong approach to engine tuning. It is recognized that the VE (fuel) table is the first ECU table to be calibrated. The second ECU table to be calibrated is the Spark (ignition timing) table, and third is the AFR table.
The VE (fuel) table is generally calibrated first, need to have the appropriate amount of fuel injected into the engine to match the inflow of air to achieve an AFR of approximately14.7:1. Once the VE (fuel) table is calibrated, follow up with a Spark (ignition timing) table calibration.
Ignition Timing Control – Open Loop
For open loop systems commonly used in a carburetor or mechanical fuel injection:
Initial advance – typically 10 to 15 degrees before top dead centre (BTDC).
Electronic ignition timing is also added for smog control requirements. More recent electronic ignitions modulate spark advance for different driving conditions. This is typical in earlier mechanical fuel injection and lean burn carbureted engines since the late ’60s.
Ignition Timing Control – Closed loop
Timing in more recent ignition systems is computer controlled according to a closed loop ignition timing function. It may be varied for different engine temperatures, throttle positions, and engine loads. A knock sensor can be used to reduce timing when engine knock occurs.
Flame speeds are greater in alcohol fuels than for gasoline fuels in lean, highway fuel mixtures. In one combustion engineering test, methanol flame speed was compared to gasoline flame speed at lean mixtures for each respective fuel. Methanol combustion flame speed was 42% faster than the combustion flame speed in gasoline. Less ignition timing was needed for methanol.
Example: For blown gasoline engines at around 2 atmospheres, 28 degrees of ignition timing is common for best power. For the same blown engine on alcohol at a richer mixture, 32 degrees of timing is common.
Ignition Timing for Fixed Advance (Locked Distributor or Magneto)
Optimum timing from a fixed (locked) ignition advance occurs at only one engine speed. Ignition timing is too advanced at engine speeds below that and not advanced enough at engine speeds above that. Changing the timing value up or down changes the engine speed up or down for the optimum ignition timing. The engine speed operating range affects where the timing is the best. Increasing the timing advance raises the high-end power, reducing the low-end. Decreasing the timing advance raises low-end power, reducing high-end power.
Example: Magneto timing was reduced 6 degrees in our blown alcohol drag racer, and our low-end 60-foot times were quicker by 0.05 seconds from more low-end power. However, the quarter mile ET slowed down by 0.1 seconds, from less high-end power.
Centrifugal Timing Advance
Typically, a spark advance of 1 to 1.5 degrees increase per 1,000 RPM is characteristic of an engine demand. Bill Jenkins and Larry Schreib also reported that range of values in their popular Pro Stock drag racing engine-building book The Chevrolet Racing Engine.
The information presented here is from research and as such, I am not the author but the presenter. There is more to be learned regarding this topic, especially if you are using or contemplating an FI system.
Cheers
This new thread about fuel and ignition timing is quite interesting, to me anyway. I can safely mention that most of us know about engine ignition timing, advancing-retarding as required, and have dabbled with different octane fuels whether the bike is N/A or has boost.
My research indicates that it is not that simple. I submit that for the most part it is that simple as most of us will not feel a noticeable difference in the engine performance or fuel economy, much like changing out the exhaust system for HD mufflers and such.
I have mentioned in various other threads that it is not the change that is an issue, although it can be, it is understanding what the change is doing. If you understand the technical aspects and still do a change, you are doing the change from an informed perspective.
I will include the usual caveat in that this thread is an opinion piece, and my opinion and understanding from my research. I did not create the information, but think it's nice to know. This type of information is helping with my ECU Replacement/upgrade project as well.
I have a document that is about this topic and will use it to explain my position on this issue.
Ignition timing is used to start the burning of the air-fuel mixture at the appropriate time to ensure the air-fuel mixture is fully combusted at or near approximately 15° after top dead centre (ATDC). The process is:
- The spark plug fires during the compression stroke at a set engine ignition timing value before top dead centre (BTDC).
- The air-fuel mixture in the combustion chamber is ignited.
- Pressure from the burning air-fuel mixture increases in the cylinder as the burning gases expand.
- Pressure from the burning air-fuel mixture should be maximized at approximately 10 degrees ATDC.
- Pressure from the burnt air-fuel mixture pushes down on the piston.
- The energy from the burnt air-fuel mixture is transferred to the cylinder piston increasing the downward motion of the cylinder piston improving engine power and fuel economy.
If the air-fuel mixture is ignited too late BTDC – closer to TDC, ignition timing is said to be retarded, the air-fuel mixture will not be completely burnt before the optimum ignition timing ATDC, but will continue to burn, or not, after the optimum ignition timing degree ATDC, reducing engine power output and fuel economy will suffer.
Ignition Timing Trends
- Advancing engine ignition timing results in a more fuel lean condition;
- Retarding engine ignition timing results in a more fuel rich condition;
- As engine RPM increases, engine ignition timing should be increased;
- As engine load increases, reduced engine MAP, engine ignition timing should be retarded;
- Riding profile will dictate a specific engine ignition timing in that the amount of engine timing will be less at cruise speeds compared to the engine timing needs during acceleration;
- For higher octane fuels, more timing is needed due to slower flame speed;
- For large combustion chambers, more ignition timing is needed;
- For forced induction, less timing is needed because of faster flame speed;
- For emission controls, less timing is used to reduce smog compounds;
- For richer fuel mixtures, more timing is needed due to slower flame speed;
- For alcohol fuels that are run richer than gasoline fuels, more timing is typical; and
- For nitro fuels that are richer than methanol fuels, even more timing is typical.
There are engine indications that the engine timing may be off. The most published engine indicators are:
Engine knocking
decreased fuel economy
Reduced engine power
Engine overheating
What differences are there between advancing iengine ignition timing and retarding engine ignition timing?
Advancing engine ignition timing is to compensate for fuel ignition delay. Fuels require a minimum amount of time to start the air-fuel mixture burning until it is in a full burn mode. Some fuels are more volatile and need less time, others more. A high-octane fuel will require additional time to come to a full burn than a fuel with less octane. The following diagram illustrates this:
The air-fuel mixture does not ignite immediately when an ignition event is triggered by the ECU. Advancing ignition timing may be needed to successfully start the air-fuel mixture burning, getting past the air-fuel mixture ignition delay, so that the air-fuel mixture is completely burnt and the pressure pulse from the burnt fuel is at maximum at the appropriate engine timing ATDC. The fuel of choice will also affect the engine ignition timing. Advancing the engine ignition timing should raise high-end power, but at the cost of reducing low-end power.
Retarding engine ignition timing can be used to reduce engine detonation, commonly called engine knock. Using a fuel with a higher-octane rating can assist in reducing engine detonation, pinging.
There are many engine tuning strategies employed by engine tuners, and as such, there is no right/wrong approach to engine tuning. It is recognized that the VE (fuel) table is the first ECU table to be calibrated. The second ECU table to be calibrated is the Spark (ignition timing) table, and third is the AFR table.
The VE (fuel) table is generally calibrated first, need to have the appropriate amount of fuel injected into the engine to match the inflow of air to achieve an AFR of approximately14.7:1. Once the VE (fuel) table is calibrated, follow up with a Spark (ignition timing) table calibration.
Ignition Timing Control – Open Loop
For open loop systems commonly used in a carburetor or mechanical fuel injection:
Initial advance – typically 10 to 15 degrees before top dead centre (BTDC).
Electronic ignition timing is also added for smog control requirements. More recent electronic ignitions modulate spark advance for different driving conditions. This is typical in earlier mechanical fuel injection and lean burn carbureted engines since the late ’60s.
Ignition Timing Control – Closed loop
Timing in more recent ignition systems is computer controlled according to a closed loop ignition timing function. It may be varied for different engine temperatures, throttle positions, and engine loads. A knock sensor can be used to reduce timing when engine knock occurs.
Flame speeds are greater in alcohol fuels than for gasoline fuels in lean, highway fuel mixtures. In one combustion engineering test, methanol flame speed was compared to gasoline flame speed at lean mixtures for each respective fuel. Methanol combustion flame speed was 42% faster than the combustion flame speed in gasoline. Less ignition timing was needed for methanol.
Example: For blown gasoline engines at around 2 atmospheres, 28 degrees of ignition timing is common for best power. For the same blown engine on alcohol at a richer mixture, 32 degrees of timing is common.
Ignition Timing for Fixed Advance (Locked Distributor or Magneto)
Optimum timing from a fixed (locked) ignition advance occurs at only one engine speed. Ignition timing is too advanced at engine speeds below that and not advanced enough at engine speeds above that. Changing the timing value up or down changes the engine speed up or down for the optimum ignition timing. The engine speed operating range affects where the timing is the best. Increasing the timing advance raises the high-end power, reducing the low-end. Decreasing the timing advance raises low-end power, reducing high-end power.
Example: Magneto timing was reduced 6 degrees in our blown alcohol drag racer, and our low-end 60-foot times were quicker by 0.05 seconds from more low-end power. However, the quarter mile ET slowed down by 0.1 seconds, from less high-end power.
Centrifugal Timing Advance
Typically, a spark advance of 1 to 1.5 degrees increase per 1,000 RPM is characteristic of an engine demand. Bill Jenkins and Larry Schreib also reported that range of values in their popular Pro Stock drag racing engine-building book The Chevrolet Racing Engine.
The information presented here is from research and as such, I am not the author but the presenter. There is more to be learned regarding this topic, especially if you are using or contemplating an FI system.
Cheers
