Alrighty... i have a question for some of the more well versed people here.

I want to learn how to tune a car. I'm talking specifically about tuning the fuel delivery and ignition timing. Wanting to know when i should advance/retard the timing, when i should add/lessen the amount of fuel, how to find the ideal AFR.

The physical mechanics on how to work an EMS system, I think i can pick up from reading a manual.. but right now im more intersetd in the theories behind tuning.

I'd like to start from Square 1, and learn how to do this. Not just for tuning my 8, but for future uses as well with whatever car I have. So, does anyone have some helpful information availalbe. Like websites that discuss this or any other sort of documentation.

Thanks in advance for the help.

-Harry

 

http://www.microtechefi.com/forum/ <-- Information Overload

I guess I know what I'm doing in my spare time now

 

I'll contribute in a little while.

 

An Engine Management System (EMS) is defined as a device or module that, through the use of a graphical or mechanical interface, controls the combustion process of an engine, through manipulation of the basic fuel delivery and ignition timing. For our use here, the EMS is understood do be a seperate entity than the Powertrain Control Module (PCM) or Engine Control Unit (ECU), which is the system that is employed by the OEM to do the same, though the roles of each are often blurred or combined.

The interface can be as simple as a set of potentiometers or as complex as fully graphical, 3D plotting system with complex mathematical models for all engine parameters and systems. The most popular systems today utilize the Microsoft Windows environment to render the various fuel and ignition calculations as 2D tables and, often, 3D graphs that allow the tuner to plot the fuel delivery and ignition timing as a tangible "spreadsheet" of values. This interface also manages the "back end", controlling the interpretation of the tuner's inputs into usable instructions that can be executed by the actual EMS and handling the management of the resultant data stream to and from the EMS module.

Despite the differences in the user interfaces and the degrees of resolution of "granularity" of the modeling, all EMS devices operate on the same basic principles. The EMS takes a "snapshot" of the engine's condition at every phase of combustion by looking at all the available sensor outputs and assesses the fuel and spark requirements based on the tables as entered by the tuner (often in real time) and activates the fuel injectors and ignition devices at the appropriate time depending on the position of the crankshaft (eccentric shaft), the measured mass of air, the operating temperatures and the engine's RPM. If the engine's actual operating condition at that exact moment in time fall somewhere between the values in the available tables, it will interpolate the requirements based on a set algorithm to provide and intermediate value.

Though this process can seem hopelessly intricate and time consuming, it is often expedited by a variety of tools available, such as closed-loop monitoring of the air-fuel ratios (AFRs) via and external wide band oxygen sensor, pre-configured calibrations available for similar or identical engine setups, interpolation schemes that "guess" the best values for the tables by comparing a range of load points and even complex modeling software that can take the basic parameters of an engine (it displacement, RPM range, predicted volumetric efficiency and fuel delivery capability) and generate as set of start-up values that may be very close to the actual operating requirements of the engine.
The process is aided further by the fact that optimum combustion actually occurs over a range of fuel and spark values, rather than a specific quantity and time. The air-fuel mixture can be ignited over a fairly wide range and the optimum values will occur between the lean best torque (LBT) and rich best torque (RBT) ratios, that fall anywhere from 11:1 to 13.5:1, depending on the engine's volumetric efficiency and resistance to detonation and pre-ignition.

 

Combustion is the rapid release of energy from a fuel - in this case gasoline.
A finite quantity of gasoline contains an equally finite amount of energy, which can be released by combining it with a specific quantity of oxygen at an absolute ratio.
1 kilogram of gasoline contains 43 megajoules of energy that can be released via oxygen at a ratio of 1:15.179 (a ratio referred to as "stoichiometric"). That is about 41,700 BTUs. Plenty.
The trick with an internal combustion engine (ICE), whether it be rotary or other wise, is to control that combustion in space and time.
We do this by causing the combustion process to occur in the combustion chamber at a precise time and OVER a precise quantity of time to convert that heat into torque.
To effect this level of control, we must take a fixed quantity of space (about 650 ml in the case of the 13b-MSP), fill it with a quantity of gas and air as proscribed by the above ratio, compress it to a precise degree and ignite it at precisely the right time as to cause the maximum pressure increase resultant from that combustion to occur at the time of maximum delta for the combustion chamber's swept cyclic volume (that is to say at the precise moment that the combustion chamber is starting to get bigger again after it just got done getting smaller to compress the charge).
The beauty of this process is that it can occur completely independent of any change in factors in the outside world - temperature, pressure, altitude, pollution, humidity, whatever - as long as we can be assured that these conditions inside the combustion chamber are constant.

The problem is, we can't.

Because the process of getting a fixed volume of O2 into the combustion chamber at a proscribed density (meaning temperature via Avagadro) is complicated by the fact that this air is supplied by the available atmosphere, we are straddled with the effects of varying density on the combustion charge.
What that means is we must compensate for the volatility of gasoline as it responds to the varying charge densities. At differing charge densities, the amount of energy necessary to start the combustion process and the time it takes to complete the combustion process changes in a not so linear fashion.
So, what we do is vary the amount of fuel we add and ignite the process on a adjustable schedule based on what information we can obtain about the conditions of the air going into our ICE.
What we measure in the case of the RX-8 to know these conditions are these:

Air Flow
Intake Air Temperature
Barometric Pressure
Coolant Temperature
Throttle Position
Eccentric Shaft Position

By computing all of these measurements together, the engine control unit (ECU - sometimes called PCM for powertrain control module) can determine APPROXIMATELY the density of the air charge in the combustion chamber at any given time. It is a shame, really, that there is no way to measure the density directly or we could forgo all of this.
Two factors that can't be measured by the above methods are important to the whole equation as well.
First is volumetric efficiency (VE) or the amount of air, as a percentage of maximum, that the engine actually ingests as a result of the physics of mass and inertia. This number is fixed to some degree and changes at different RPM.
The other is the latent temperature of the actual combustion chamber as a result of the combustion cycles that proceed the cycle under scrutiny at that moment. This changes as a result of RPM as well, but it is also tied to 'load' or the increase of RPM over time as a proportion to charge density.
What that leaves us with is a very crude measurement of the total charge density.
How do we compensate for that?
By conservative 'hedging' on the bet that is ignition timing through advancement and retardation of the onset of the spark and by introducing elements into the gasoline that seek to stabilize its volatility. That is what octane is for and how much it affects the combustion process is measured by various methods including, but not limited to, Research Octane (RON), Motor Octane (MON) and the Anti Knock Index (AKI - and average of the RON and MON numbers). Unfortunately for the average motorist, many other ingredients are added to the fuel we use to affect its environmental impact that are not directly computed into the AKI. Ingredients are added to lower the boiling point and vapor point, reduce the hydrophilic nature of gas and reduce the amount of oxides of nitrogen after the combustion process. Many, if not most, of these ingredients change the combustion process in ways that may not be consistent from sample to sample. They also alter the total energy content of the fuel itself.

Having the ignition process starting at the wrong time (especially too soon) is a bad thing and can (especially in the case of the rotary ICE) quickly destroy a motor. So what is done, more often than not, is to err on the side of safety and bracket the combustion process with extra fuel and start the ignition with a slightly delayed spark. What this does is lower the temperature of the intake charge and insure that the combustion process is slower and later than optimal and never faster or sooner. Raising the AKI of the fuel used will accomplish the same thing but since the manufacturer of the vehicle can't insure that the fuel used will always have the proper AKI to achieve this or won't contain additives that adversly affect the ignition onset, they don't depend on their octane recommendation alone.
What is done by "tuners" then, is to take into account this margin of error and dial some of it out for more power which can be achieved by charge composition that is closer to optimal. To achieve this, they depend on the operator to use fuel that takes up the slack in AKI and remove some of this extra fuel and spark retardation.
Really, that is all there is to it. How good a tuner can be is dependant on his or her ability and knowledge with respect to the events within the combustion chamber in question.

 

The internal combustion engine is just an air pump. It moves air from one place to another - from the intake manifold to the exhaust manifold. It is driven by heat - heat that is generated by bonding oxygen to parts of hydrocarbon molecules. Just like any pump, it has a volume it can move in a given time and that volume is affected by a range of different variables like the time per stroke, the restriction of the inlet and outlet and the very inertial properties of the air it is moving. We call this "volumetric efficiency" (Ve) and it is a percentile rating of how much air the engine can actually move compared to its maximum, theoretical volume.
Internal combustion engines are rated by displacement, measured in cubic inches or liters. If an engine has a displacement of 1.3 liters or 80 cubic inches (like the Renesis), its Ve would be 100% whenever the motor is able to take in that full amount of air on an intake stroke. The reality is that a typical street engine will only have a very limited range in which it can induct air at 100% efficiency without the help of something beyond the pressure of the atmosphere that surrounds us. That range almost invariably lies at the rated torque peak of the engine. At all RPM points above and below the torque peak, the Ve will drop below 100%, in some places as low as 60% (like at idle).
As noted, many factors will affect the Ve of an engine - the shape and length of the intake runners and their resonant properties, restrictions in the exhaust system, the size and shape of the intake and exhaust ports and the degree to which the throttle is opened, just to name a few.
In most cases, torque and Ve change in a manner consistent with changes in RPM. Normally aspirated (NA) engines, which have been optimized to run at high RPM with resonant tuning of the intake and exhaust at the expense of lesser low-RPM output are not uncommon today as displacement is being replaced with RPM capacity. These motors tend to have a near-linear increase in Ve as the RPMs rise to the torque peak and may actually exceed 100% at the torque peak because of these mechanical tuning techniques.
Since the Ve of an engine is not constant, the amount of air in the combustion chamber at any given load is not often completely predictable. At the torque peak, the injection pulse width will be at maximum because air flow is at maximum, but above and below this point optimum injection pulse width will be less. Maximum horsepower will be measured at a higher rpm than the RPM of the torque peak because the engine is producing more actual power strokes per unit of time, although the combustion cycles above the torque peak are less efficient and will require less fuel to be injected for each because of this declining Ve.
However, even though the torque curve is changing over the full range of operational RPM, given the relatively wide range of A/Fs at which an engine will produce near-peak torque, an injection pulse curve that approximates a straight line may still produce relatively good performance even with no corrections for the changes in Ve. A properly tuned engine will always use the most fuel per time at peak power.

 

Jeffey, You do tuning for locals?

 

Depends on what you have going on. PM or e-mail, please (to keep this thread on topic).

 

Traditionally, the greatest tendency to knock was near 13.5:1 air-fuel ratio, but was very engine specific. Modern engines, with engine management systems, now have their maximum octane requirement near to 14.5:1. For a given engine using gasoline, the relationship between thermal efficiency, air-fuel ratio, and power is complex. Stoichiometric combustion ( air-fuel ratio = 14.7:1 for a typical non-oxygenated gasoline ) is neither maximum power - which occurs around air-fuel 12-13:1 (Rich), nor maximum thermal efficiency - which occurs around air-fuel 16-18:1 (Lean). The air-fuel ratio is controlled at part throttle by a closed loop system using the oxygen sensor in the exhaust. Conventionally, enrichment for maximum power air-fuel ratio is used during full throttle operation to reduce knocking while providing better driveability. An average increase of 2 (R+M)/2 ON is required for each 1.0 increase (leaning) of the air-fuel ratio. If the mixture is weakened, the flame speed is reduced, consequently less heat is converted to mechanical energy, leaving heat in the cylinder walls and head, potentially inducing knock. It is possible to weaken the mixture sufficiently that the flame is still present when the inlet valve opens again, resulting in backfiring.

 

SWEETNESS!!! I'm gonna get absolutely no work done today at the office. Thanks for the help MM. I'll be replying with many questions after i read this.

-hS

 

A fuel's octane rating represents its ability to resist detonation and pre-ignition. Factors influencing an engine's octane requirements are listed below:

• Effective compression ratio
• Atmospheric pressure
• Absolute humidity
• Air temperature
• Fuel characteristics
• Air/fuel ratio
• Variations in mixture distribution among an engine's combustion chambers
• Oil characteristics
• Spark timing
• Spark timing advance curve
• Variations in optimal timing between individual combustion chambers
• Intake manifold temperature
• Combustion chamber coolant temperature
• Condition of coolant and additives
• Type of transmission
• Combustion chamber hot spots.

When an engine knocks or detonates, combustion begins normally with the flame front burning smoothly through the air/fuel mixture. But under some circumstances, as pressure and temperatures rise as combustion proceeds, at a certain point, remaining end gases explode violently all at once rather than burning evenly. This is detonation, also referred to by mechanics as knock or spark knock. Detonation produces high-pressure shock waves in the combustion chamber that can accelerate wear of an engine or actually cause catastrophic failure. Pre-ignition is another form of abnormal combustion in which the air/fuel mixture is ignited by something other than the spark plug, including glowing combustion chamber deposits, sharp edges or burrs on the surfaces of the combustion chamber, or an overheated spark-plug electrode. Heavy, prolonged knock can generate hot spots that cause surface ignition, which is the most damaging side-effect of knock. Surface ignition that occurs prior to the plug firing is called pre-ignition, and surface ignition occurring after the plug fires is called post ignition. Pre-ignition causes ignition timing to be lost, and the upward movement of the piston or forward movement of the rotor on compression stroke is opposed by the too-early high combustion pressures, resulting in power loss, engine roughness, and severe heating of the piston crown or rotor face. It can lead to knock or vice versa.

The single most important internal engine characteristic that requires specific fuel characteristics is compression ratio, which generally increases the ONR +3 to +5 per one ratio increase (in the 8-11: 1 compression ratio range). High compression ratios squash the inlet air/fuel mixture into a more compact, dense mass, resulting in a faster burn rate, more heating, less heat loss into the combustion chamber surfaces, and consequent higher combustion chamber pressure. Turbochargers and superchargers produce effective compression ratios far above the nominal compression ratio by pumping additional mixture into the combustion chamber under pressure. Either way, the result is increased density of air and fuel molecules that burn faster and produce more pressure against the piston or rotor. Another result is an increased tendency for the remaining gases to spontaneously explode or knock as heat and pressure rise.

Keep in mind, high peak combustion chamber pressures and temperatures resulting from high compression can also produce more NOx pollutants. Lower compression ratios raise the fuel requirements at idle because there is more clearance volume in the combustion chamber that dilutes the intake charge. And because fuel is still burning longer as the piston descends or the rotor progresses, lower compression ratios raise the exhaust temperature and increase stress on the cooling system.

Until 1970, high-performance cars often had compression ratios of up to 11 or 12 to one, easily handled with vintage high octane gasoline's readily available in the 98-99 ((R+M)/2) range. By 1972, engines were running compression ratios of 8-8.5:1. In the 1980's and 1990's, compression ratios in computer controlled fuel injected vehicles were again showing up in the 9.0-11:1 area based on fuel injection's ability to support higher compression ratios without detonation, coupled with the precise air/fuel control and catalysts required to keep emissions low. Race car engines typically run even higher compression ratios. In air unlimited engines, maximum compression ratios with gasoline run in the 14-17:1 range.

Ratios above 14:1 demand not only extremely high-octane fuel (which might or might not be gasoline), but experienced racers use tricks like uniform coolant temperature around all combustion chambers, low coolant temperature, and reverse-flow cooling. Extremely high compression ratios also require excellent fuel distribution to all combustion chambers, retarded timing under maximum power, very rich mixtures, and probably individual combustion chamber optimization of spark timing, air/fuel ratio, and volumetric efficiency.

 

The tendency to knock increases as spark advance is increased. For an engine with recommended 6 degrees BTDC (Before Top Dead Center) timing and 93 octane fuel, retarding the spark 4 degrees lowers the octane requirement to 91, whereas advancing it 8 degrees requires 96 octane fuel. It should be noted this requirement depends on engine design. If you advance the spark, the flame front starts earlier, and the end gases start forming earlier in the cycle, providing more time for the autoigniting species to form before the piston reaches the optimum position for power delivery, as determined by the normal flame front propagation. It becomes a race between the flame front and decomposition of the increasingly-squashed end gases. High octane fuels produce end gases that take longer to autoignite, so the good flame front reaches and consumes them properly.
The ignition advance map is partly determined by the fuel the engine is intended to use. The timing of the spark is advanced sufficiently to ensure that the fuel-air mixture burns in such a way that maximum pressure of the burning charge is about 15-20 degree after TDC. Knock will occur before this point, usually in the late compression - early power stroke period.
The engine management system uses ignition timing as one of the major variables that is adjusted if knock is detected. If very low octane fuels are used (several octane numbers below the vehicle's requirement at optimal settings), both performance and fuel economy will decrease.
The actual Octane Number Requirement depends on the engine design, but for some 1978 vehicles using standard fuels, the following (R+M)/2 Octane Requirements were measured. "Standard" is the recommended ignition timing for the engine, probably a few degrees BTDC.
The actual ignition timing to achieve the maximum pressure from normal combustion of gasoline will depend mainly on the speed of the engine and the flame propagation rates in the engine. Knock increases the rate of the pressure rise, thus superimposing additional pressure on the normal combustion pressure rise. The knock actually rapidly resonates around the chamber, creating a series of abnormal sharp spikes on the pressure diagram.
The normal flame speed is fairly consistent for most gasoline HCs, regardless of octane rating, but the flame speed is affected by stoichiometry. A 12:1 CR gasoline engine at 1500 rpm would have a flame speed of about 16.5 m/s, and a similar hydrogen engine yields 48.3 m/s, but such engine flame speeds are also very dependent on stoichiometry.

 

Since gasoline isn't a single chemical compound but a mish-mosh of different complex hydrocarbons, it doesn't always have the same combustion characteristics under all engine operating conditions. If we were to ignore that (which we are, to some extent, forced to do since we can't predict the changing qualities of gasoline), we can make a few basic observations of the chemical properties of gasoline as it relates to proper combustion.
Gasoline requires oxygen to burn. It requires a specific amount of oxygen to burn completely. Since air is only about 12% oxygen and mostly nitrogen, it takes about 14.7 pounds of air to burn one pound of gasoline - a ratio that is referred to as stoichiometric or "stoich" (pronounced "stoke"). If combustion were to proceed perfectly, mixing those amounts of air and gasoline together and igniting them would create only carbon dioxide and water. The nitrogen would pass through unaltered.
Unfortunately, the reality quite a different result. Because the time for mixing of the air and gasoline is very limited and is a bit like trying to fill a drinking glass with a garden hose from fifty feet, not every drop of gas will find air with which to mix and vice versa. Additionally, some of the nitrogen in the mix will get oxidized (which is just pollution) and some of the gasoline will just be broken, but not oxidized (also pollution). Since the mix is not perfect, we are confronted with three different potential strategies, the choice of which depends on our goals.
For maximum power, it is a good idea to have more gasoline available than the stoich ratio to ensure that every bit of oxygen is burned. For maximum economy, it is the other way around with the goal to have all of the gasoline oxidized. For best emissions, however, the chemically correct stoichiometric ratio is best, even though it is a compromise in that leaner mixtures (with more air) produce less broken gasoline molecules and richer mixtures (with more gas) produce less oxidized nitrogen. Staying with the stoich ratio just produces the best average of both. For our purposes here, we will assume that you are looking for the most reliable power, rather than fuel economy or low emissions. I promise I won't tell the EPA. For our examples, we will also assume that the motor is completely warmed up and the appropriate air to fuel ratio is being selected for a stable load point, which is to say the motor is at a set RPM and ingesting a steady amount of air (as opposed to an engine that is rapidly accelerating or decelerating).
The optimal air-fuel ratio for an engine to produce its best torque output is actually a range of ratios from "rich best torque" (RBT) to "lean best torque" (LBT). Right in the middle would be "mean best torque" and this is the ratio you would shoot for on a naturally aspirated, non knock-limited motor. RBT tends to fall at 11.5:1 and LBT at 12:8:1, so MBT would be approximately 12.2:1. This would be the A/F you would shoot for across the entire RPM band at wide open throttle to provide the optimum average torque and power output from the motor.
It is important to note that the requirements for a normally aspirated motor are somewhat different than those of a motor equipped with some form of forced-induction - particularly so of a motor that was not originally so equipped. Because N/A motors tend to have higher compression ratios and are usually thermally optimized to a razors edge as delivered, the added load from aftermarket forced induction must be bracketed with more conservative A/F ratios and ignition timing.
On a motor that is equipped with a turbo or some other form of supercharging, you would never want the A/F ratio to be any higher than MBT and you would probably be best advised to shoot for RBT if only for the margin of safety and the added benefit of fractional charge cooling that the extra fuel would provide. Running a boosted motor this rich is not a drawback at all since the flame front during combustion is actually accelerating as you move to ratios closer to 11:1. Richer than that and the flame front starts to slow down again. A faster flame front is less prone to abnormal combustion, detonation or knock.
At cruise and idle, the stoichiometric ratio is probably best, though it is often possible to run a bit leaner. Mid throttle power should be in the range from stoich down to 13.5:1. On decel, it is a good idea to cut fuel completely, or at least shoot for ratios as close to 20:1 as possible to prevent loading and backfires.

 

Wow - its a book!! great stuff MM

 

Are you just typing this stuff off the top of your head.....?

In any case, great info!

 

Nah, I've been accumulating this stuff over the last few months as I produce the tuning video.

 

I hate you MM !

 

Sorry.

 

I need to get a copy of this video

 

Can you advise on the amount of ignition advance to run for NA rotary? Thanks

 

Go check what Fred said in the ignition discussion thread.

The OEM PCM goes for up to 40° off-load and 23° to 30° on load with the split going from 15° to 8° in the same conditions.
Idle is -3° and 5° L/T.
At peak torque (and the most knock-prone RPM range around 6k), it goes as low as 19° with an 8° split.

Coming this Christmas to a PayPal near you!

 

Yesssss!!!!!!!!!!!!

 

i move to sticky this


now... back to reading

 

One of the best threads I've seen in a long time. What a great write up, thanks.

 

MM,
I finally got a chance to read over the material... AND HOLY SH*T!!! I can't believe how valuable it was. I have a few questions I'll post up later, which you may want to see as questions coming from someone who knew nothing about tuning. Might help with your video.

and yes... let me know when the video is available... i'll be first in line with payment waiting.

-hS