I recently aquired this research paper and have obtained permission from the owner to post it here on our forums. I urge everyone to read this. It is very informational to say the least. I was unable to post the tables because of the format that the original paper was written in. If you want a copy of this, email me. waachback@hotmail.com

 

Introduction:
The purpose of these papers are to discuss in more detail taming and increasing the power of the turbo-charged engine by addressing some of the issues they raise when creating that power.
These papers assume you have a working knowledge of the concepts behind today's performance cars overall and more specifically gasoline powered internal combustion engines and turbo-chargers. There are many very well written articles on the basics available across the internet and I would recommend those sites for that level of information. Additionally, I am not an engineer and may not explain or word everything perfectly accurate in a technical sense, however I believe it to be generally accurate for the basis of arriving at conclusions based not only on my layman understanding but also based on empirical observation as well. I also rely heavily on those who have much more technical training in this area and will delve to credit them where appropriate and welcome additional constructive input from those who take the time to go through these papers.

Why Turbo-Charging:
The primary attraction of modern turbo-charged engines to automotive performance enthusiasts are their high power ratios. They have extremely high power to engine weight ratios, power to engine displacement ratios and power to cost ratios. The high power to engine weight ratio allows vehicles to be lighter, increasing both the turning and braking performance of the vehicle while still providing high levels of power output. The high power to engine displacement ratio allows these performance cars to achieve similar power outputs to their large displacement cousins while maintaining relatively good levels of fuel economy during low levels of acceleration and cruising speeds. The high power to cost ratio is easily appreciated and simplest to explain ... more bang for the buck. The nature of these relative differences in power to weight and power to displacement can frequently be measured in factors.
With good engine management and tuning today's turbo-charged engines while not completely eliminating the old saying that "there is no substitute for cubic inches" does put a significant dent in it. The only reason it is not fully eliminated is because even a turbo-charged engine would gain performance-wise from additional displacement but at the expense of additional weight, lower economy and increased costs. While a turbo-charged 5.7 litre engine would gain benefits over a normally aspirated 5.7 litre engine, the cost and weight of the turbo-charged 5.7 litre engine outweighs those benefits for the average enthusiast. Produce the same or greater power as the normally aspirated 5.7 litre engine with only a 2.0 litres of displacement and now it becomes interesting to the mainstream performance population. This is from someone who grew up wrenching and blueprinting the heavy metal, but just could no longer afford to bypass the cheap, easy and addictive power of turbo-charged boost. The concepts behind the improved volumetric efficiency of high displacement engines is very well established, but turbo-charging is still taking over the mainstream performance enthusiast.

How Turbo-Charging Works:
This section contains nothing new, it is primarily meant to provide a smooth transition to other sections and to provide a basis, primer or little reminder for their discussion.
(Editorial note: the law of the conservation of energy of course holds that energy can neither be created nor destroyed, it merely changes form [kinetic, potential, heat, light, etc.]. My use of terms that suggest otherwise is not to contest this physical law but rather a limitation of conveying the ideas in a form readable to all of us who either forgot most of the physics we knew or who are not concerned with purest technical and scientific explanation. Likewise I may use terms for types of energy that are not entirely accurate, again trying to convey an idea in as non-technical a terminology as I can.)
The mechanical power output of an internal combustion engine is transferred from the energy stored in the fuel during the combustion stroke. Mixed fuel and oxygen (contained in the air) combust releasing the energy, stored in the fuel, in the form of primarily heat. The heated mixture transfers some of that heat energy into mechanical power by driving the piston down, which turns the crankshaft, etc. Internal combustion engines are not terribly efficient at transferring that heat energy into mechanical power and much of the energy released by combustion leaves through the exhaust and contributes to exhaust velocity. The velocity of the exhaust gases drive the turbine in a turbo-charger which is connected by a shaft to a compressor in the air intake. Turbo-charging works by compressing the air from the intake to force (hence the reason turbo-charging is a type of forced induction) more air into the cylinder during the intake stroke. Compressed air takes up less space allowing more of the particles in the air (including oxygen) to be present in the cylinder than there would be if the air were not compressed. The additional oxygen provides the engine with the ability to burn more fuel during the combustion stroke. The additional fuel, along with the additional oxygen to burn it, allows more power to be released during the combustion process. Greater amounts of fuel and oxygen releases more heat energy and despite all the the inefficiencies of the engine results in additional heat energy being transferred to mechanical power and higher output of the engine.
Okay, so we have all this energy being released from the fuel, some less than desirable amount of the energy is generating mechanical power and with the assistance of a turbo-charger we are redirecting some of the energy escaping down the exhaust pipe and putting it back to work to add more air and fuel to release more energy and increase the power output of the engine... So now we have a turbo-charged engine and all is happiness, right? If you are an OEM of mass produced affordable cars, yes. For the enthusiast, not even close.
The enthusiast is going to take the conservative OEM application and extend the theory. They are going to replace intake and exhaust components to allow their heat generating air pump of an engine to pump even more air, more fuel to the limits. They are going to advance timing. They are going to push right to the very fatal (in engine terms) limit that turbo-chargers can reach all to easily - knock, ping, pre-detonation.
Knock is not a turbo-charger specific problem, it is a limitation of internal combustion engines that have existed since Nikolaus Otto cranked the first one up in 1867.

Knock "Limits":
I put limits in quotations to focus on the fact that they are limiting factors, we know what they are and more importantly there are methods of managing and even extending those limits even if we cannot eliminate them.
The phenomena of knock has been existence since the beginning of internal combustion engines and has been the subject of many studies since then. Among enthusiasts it may well be one of the most misunderstood topics in existence. There are very few outside the scientific community who ever get the topic exactly right, I will try not to be off by too much (as I and many others have been when trying to explain it quickly) and will link to other resources on the topic.
Knocking in an internal combustion engine is a common term of which there are many, it is also referred to as detonation, auto-ignition, pre-ignition and pinging. The primary symptom is a sudden spike in cylinder pressure due to an irregularity in the combustion of the air/fuel mixture. The process of combustion takes time to occur. The speed with which it occurs varies based on the environmental conditions in which it occurs in and the readiness of the air/fuel mixture to combust. For illustration purposes let us pretend that it occurs at a constant rate within the cylinder and that the pressure generated by combustion increases at a constant rate as a result. When the air/fuel mixture is properly ignited by the spark plug, the flame front should move from that point outward smoothly until reaching the combustion chamber, cylinder walls, and piston top the heated gasses from the combustion process creating pressure. The pressure generated by the combustion process will grow steadily and should peak at an ideal point ATDC for the particular motor. If any portion of the air/fuel mixture, intermediary gases or end gases ignites on their own separately from the flame front started by the spark plug a sudden spike in pressure will occur. The spike in pressure causes the audible symptom of knock or ping.

The consequences of knock are the damage abnormally high pressure spikes can cause including damage to the spark plug, piston top, ring lands and rod journals. Additionally it has been noted that these surfaces often have a layer of protective end gases that become disturbed during this spike in pressure and as a result subject to the surfaces to higher temperatures from the combustion than they would have been otherwise. This exposure to increased temperatures puts additional stress on the cooling mechanisms of the motor as well as creating unfavorable conditions for the next stroke - increasing the likelihood of continued knock.
Auto-ignition away from the flame front can be categorized into two categories. The first is that the spark from the spark plug ignites too soon, causing combustion to occur too soon before TDC while compression is still occurring. The excessive pressure will cause auto-ignition away from the flame front. This is a matter of ignition timing and generally is easily dealt with through tuning, retard the ignition timing to occur later or closer to TDC. The second category actually is a grouping of similar issues - the air-fuel mixture ignites from a source other than the timing of the combustion. This can happen for a variety of reasons, but the causes are more common, more easily reached and more severe with turbo-charged engines (this also occurs in high compression normally aspirated engines and other types of forced induction like super-charging).

Remember that the releasing of heat is the initial change of energy that generates the engine's power and that more engine power is created by burning more air-fuel and generating even more heat still. While more heat is transferred into engine power, due to the engine's inefficiencies even more heat is left unutilized in these high compression and forced induction applications. With turbo-charged engines it is often desirable to increase the heat in the exhaust even further since that energy will be utilized by the turbine at the turbo-charger and due to inefficiencies in the turbo-charger some of that heat energy also finds its way into the turbo-charged air further aggravating the problem.
While the release of energy in the form of heat is desirable to generate engine power, it is desirable in a very controlled manner. Since all of the heat is not utilized by the engine, the remaining heat in uncontrolled conditions will cause combustion to be induced by something other than the ignition of the spark plug or the before the advance of the flame front from the spark plug. Combustion started from residual sources of heat from the last combustion stroke that remain in the cylinder walls, on the combustion surface of the heads, from the heat of the surface of the spark plug itself or even from the heat within the turbo-charged air causes combustion to occur in an uncontrolled manner prior to the time that it is ideal or even desired. At the same time a certain amount of residual heat is desirable within in the cylinder to maintain the fuel in a vaporized state, if there is not enough heat the fuel will condense and only the surface of the droplets will have access to the oxygen in the air and will burn what can best be described as too slowly. It is the maintenance of the heat balance that is most critical in turbo-charged applications, is the hardest to maintain and will be the focus of the rest of these papers.
The papers will start with the forms of heat maintenance that are most easily addressed and advance to the more complicated aspects. So on to our store of power itself, that is where the problem starts.

The Characteristics of Gasoline:
Gasoline is actually a group of hydrocarbons that are refined from crude oil. The four hydrocarbons in this group are very close to each other in chemical structure and as a result in the process of refining crude oil condense at approximately the same temperature. There are also other processes at the refinery that can break down larger hydrocarbon chains into gasoline and build up smaller chains into gasoline to yield more gasoline from each barrel of crude. This similarity means that the four chains that make up the group gasoline end up together initially in an oil refinery, which is important to understand when we find out some undesirable differences in their nature in a little bit.
Of the four, the second length chain in the group of chains is the really desirable structure. It is called octane and is made up of eight carbon atoms in a string surrounded by eighteen hydrogen atoms. It remains stable (will not spontaneously combust) even under high levels of compression - very desirable in internal combustion engines in general and high compression and forced-induction engines in particular. Besides being the name of one of the hydrocarbon chains in gasoline octane is also the measure of a gasoline's resistance to spontaneous combustion under compression.
Unfortunately, the first chain in the gasoline group heptane (seven carbon atoms and sixteen hydrogen atoms) will spontaneously combust in the presence of oxygen and the slightest amount of compression something no internal combustion engine would find favorable. Going back to the refining process, heptane is recovered from the refinery mixed with octane leaving mixtures that will spontaneously combust (one cause of knocking) at different level of compression. The process of reducing the levels of heptane in gasoline is very expensive, hence the price relationship at the pump of different octane levels.
Over the years different additives have been found to increase the octane rating of gasoline without actually having to change the amount of octane to heptane in the mix. The resulting octane rating of these gasolines is based on the point where the gasoline spontaneously combusts as if it were a specific mixture of octane to heptane. Lead was the first significant and affordable (actually cheap) additive to increase a gasoline's octane rating. Unfortunately, the byproduct is additional and severe pollutants in the exhaust and it is no longer permitted on most public roads. Today the most common additives to increase octane rating without actually resorting to splitting heptane from the gasoline mix are a form of methanol and ethanol.
So the first way to deal with knocking is the oldest - use a grade of gasoline with an octane rating sufficient to handle the levels of compression being generated by the engine either through its own natural compression or through forced induction. 91 octane is the highest grade available nation-wide. If you are lucky you have a performance vehicle that can adjust to the octane rating of its gasoline and benefit from higher grades available in certain regions of the country. 94 octane is the highest unleaded gasoline I have seen available across a wide region, but I have heard of individual stations carrying as high as 96 octane. If you aren't as lucky, and have an automobile that was designed to run the same whether it is using 91 or 96 octane and will not adjust to take advantage of higher rated gasolines, do not fret this paper will cover other ways to get more performance than the factory settings out of the level of gasoline available on your corner. The message here is do not skimp on your gasoline - it is the single easiest way to prevent the knock tendencies of turbo-charged engines.
Continue on to the following parts that will cover how even with the right octane you could still experience knock unnecessarily and how to address those issues starting with the cooling system.

Engine Cooling System:
As previously mentioned gasoline when burned in combustion with oxygen releases a great deal of heat. As also previously mentioned internal combustion engines are not very efficient at fully utilizing that heat energy. If all that excess heat energy were released through the exhaust this would not be that big an issue even the heat generated by friction in the mechanical process would be easily managed. Unfortunately, much of that heat does not exit with the exhaust but is absorbed into the cylinder.
Even with the appropriate grade of gasoline, knock can occur not from spontaneous combustion through compression but rather through pre-ignition from the heat of the cylinders, the pistons, the heads and even the spark plug tip. Something is needed to absorb a good deal of that heat away from these areas (I say a good deal rather than as much as possible because a certain level of heat increases the efficiency of the engine by keeping the gasoline in an easier to burn vaporized state rather than allowing it to condense).
Water will be a recurring solution in these papers. For cooling purposes water in a liquid state is an excellent coolant. It has a very high specific heat, which means to increase water by one degree it takes more energy than most other liquids that adapt themselves well to an engine's environment. This high specific heat means that for a given volume it will absorb (and will absorb quickly) a lot of the excess heat remaining in the engine components from the combustion process and friction. This is why most engines use a water mix in their cooling systems.
Three of water's characteristics are also its limits. It freezes at a relatively high temperature when compared to the environments many vehicles operate in during at least some part of the year. It boils into a gas at a relatively low temperature when compared to both other liquids (even though they may have lower specific heat) and the desirable operating temperature of an internal combustion engine. And lastly it readily either dissolves or suspends many undesirable other materials.
The problem with water freezing at such a high temperature is that water expands when it freezes and cooling systems by necessity are closed, this expansion will damage the system. Secondly, the cooling system relies on a pump which will not move solids through the system. The solution is anti-freeze. At varying levels of mixture with anti-freeze the coolant mix's freezing temperature can be lowered to levels below all but the most extreme climates that it would be called on to operate under. Electric block heaters are required when the engine is not running in those more severe circumstances.
The problem with water boiling into a gaseous state at a low temperature is that once its specific heat has absorbed a given level of heat it vaporizes into steam. Once in a gaseous state most compounds have lost their ability to absorb high levels of heat quickly. Gases have low specific heat and as a result tend to increase in temperature even further while absorbing very little heat. This is not a desirable condition in an engine. Vapor on the cylinder wall is especially a problem for turbo-charged engines, since turbo-charged engines produce so much heat vapor along the cylinder wall allows hot spots on the cylinders that will ignite the fuel mixture and is the primary cause of coolant failure related knock. Water also expands when it boils into steam but can be contained by most engine components at water's boiling point. The first solution to water's low boiling point is that this ability for the components to handle the pressure of gases is that under pressure water's boiling point is raised. Under pressure and without room to expand water will remain a liquid and continue to absorb heat at a high rate. For a given system volume, closed systems systems that can handle pressure will be more efficient with heat management than a system that cannot be pressurized. This reference to volume provides the next solution to better heat management. A larger cooling system can circulate more coolant and absorb (and ultimately transfer to the air through the radiator) more heat than a smaller one. There is a limit to just how large a cooling system can be though both in space and weight limitations. However, if you are experiencing problems with overheating there is likely a larger radiator available for your car that will help. Lastly most anti-freeze mixes will have the secondary benefit of also increasing the boiling point of water.
The last issue is the ease with with water dissolves or suspends other materials. In an engine many of these materials, especially minerals, have harmful side effects. They corrode engine internals, leave deposits and provide electrical conductivity within the coolant. Again anti-freeze provides a solution containing anti-corrosive and deposit cleaning additives (but does not help with electrical conductivity) to provide a level of protection. However, the additives are limited in their ability to deal with these problems and the real solution here is frequent changes in the coolant mixture.
In summary, the best balanced solution for dealing with heat management in a turbo-charged engine is:
• Proper coolant mixture
• Proper maintaining of the system's ability to hold pressure
• Appropriate sized system - more power will generate more heat and more heat at some point will require a larger system
• Change the mixture regularly
For lack of a better place to discuss the topic, motor oil also plays a role in heat management. However, its role largely relates to preventing heat through lubrication. As a lubricant the function of motor oil is to reduce friction between moving parts and the heat that would otherwise result. (Energy within an engine seems to always be trying to transfer back into heat.) Once combustion has occurred the objective is to use all the energy that was transferred into mechanical power in that form. To the extent that oil reduces energy transfer into heat the engine will have a higher mechanical power output and run cooler. Despite how well motor oil performs its function, energy is still transferred by the friction. To the extent that the motor oil absorbs that heat it needs to go through the same cycle as engine coolant to manage that load of heat. Generally, motor oils do not perform their lubrication function once they reach a temperature of 250 degrees Fahrenheit. With the exception of pressure, oil heat management is the same as for water - proper mixture (viscosity), appropriate sized system (sump and when necessary cooler size) and change the motor oil regularly (it does wear out).
Next we will cover in-cylinder heat management solutions.

 

In-Cylinder Heat Management:
In most engines the cooling and lubricating system is sufficient to manage heat to the point where most function well with available gasoline octanes. Forced induction engines on the other hand introduce a whole new set of complications in the form of heat that cooling systems can not be designed efficiently to handle. First heat is introduced directly into the intake through heat transfer from the exhaust and friction within the turbo itself. Second the resulting higher level of relative compression from the increased pressure of the intake air generates more heat (the mechanical action of compressing air heats it through a transfer of the mechanical force into heat). These heat levels occur too quickly and instantaneously prior to combustion for the cooling system to efficiently transfer the heat away from the cylinder space and surfaces. This in cylinder heat is the largest hurdle for designers and tuners in the prevention of knock.
The first source of heating in the intake from the turbo process is generally handled via charge intercoolers. Most charge intercoolers are air to air, in that the charged air passes through a core with fins similar to a radiator. The passing of ambient air through the fins absorbs the heat in the charged air. With sufficient and efficient heat transfer the charged air can be cooled to within a few degrees of ambient. Heat transfer efficiency is accomplished through designs appropriate for air (rather than the designs for a liquid in a radiator) and the volume of the charge intercoolers. The primary limits of charge intercoolers are their volumes and air passages will lead to an undesirable loss of boost pressure. Still to a large extent charge intercoolers perform their function well.
There are also water to air intercoolers that perform exactly the same function (remember high specific heat). They can transfer much more heat with smaller intercooler volumes than air to air intercoolers with less to no pressure drop. However, water to air intercoolers are less common because they add the complexity of moving parts (pump), water cooling (routing hoses and locating a radiator), and volume storage. In total they take up more space and are more likely to fail than air to air intercoolers.
The purpose of forced induction is to push more air into the cylinder for the combustion stroke, the heat from compression can not be dealt with as easily - we want the compression to occur. OEM engineers and tuners are fortunate because they already have a ready source of coolant to deal with this issue. Gasoline is injected during the intake stroke and it just so happens has a decent level of specific heat. If the level of heat being generated during compression is too high and causing knock, just inject more fuel. The additional fuel will not burn during combustion since there is not sufficient oxygen for it to all burn, but it will absorb sufficient levels of heat to prevent knock.
Isn't such a rich air-fuel mixture inefficient both from a fuel consumption standpoint and a power generation standpoint? Yes. However, this is where the physics gets away from me. In net terms the increase in volumetric efficiency and power output from forced induction, overcomes the inefficient fuel mixture. We can observe this on every production turbo vehicle on the street. Managing in-cylinder heat is the only reason for utilizing these overly rich fuel mixtures in forced induction engines. The excess fuel has no other function in power generation other than to permit the benefits sought from forced induction.
So what does this mean to the tuner and what can be done about it?

Our First Tuning Opportunity:
This entire paper to this point has been laid out to prepare someone to tune their forced induction engine for more power.
OEMs design their vehicles including their engine management to the masses. They design them to have a very low failure rate for the average driver. They build in huge safety factors to prevent knock no matter who the driver is even if they use poor quality gasoline or do not maintain the engine properly. With forced induction engines these safety factors are to dump even more fuel to prevent knock than would be optimally necessary.
Why does this matter? Because all engines have an air-fuel mixture where it will experience maximum fuel efficiency, minimum pollutants and maximum power production.
Minimum pollutants is pretty constant on all vehicles and are at or near the stoichiometric air/fuel ratio or the point where theoretically all oxygen and fuel in the mixture is burned. The actual setting is adjusted (and in practice has been redefined) for use of catalytic converters since minimizing the three major pollutants are conflicting and need the help of a catalyst. In a catalytic equipped vehicle the stoichiometric ratio is around 14.6:1 since this is the level where the three pollutants cross at approximately their lowest possible levels. Most engines are most fuel efficient at air/fuel ratios leaner than stoichiometric at around 15.5:1 and most produce their most power at air/fuel ratios of around 12.5:1.
The exact ratio for maximum power output is different for each engine and different even for the same engine across the rpm and load bands. However, substantially all engines at all operating ranges produce their most power somewhere between 12:1 and 13:1 air/fuel ratios. This is an undeniable fact that all tuners rely on heavily. Forced induction engines from the factory run air/fuel ratios as rich as or richer than 10:1 to achieve the cylinder cooling discussed in the last section. To gain additional power from the factory settings all a tuner needs is a method to lean the OEM mixtures. Any move towards 12:1 AFR will produce additional power. While the exact amount of power gained may not be linear its slope is continuous - any movement from below 12:1 towards 12:1 will increase power on any engine. What prevents tuners from going all the way to 12:1 and even beyond to the exact levels of maximum output for the engine they are tuning on is eventually they will move through the margin of safety the OEM engineers programmed and into the knock limit. Almost all forced induction engines will experience knock before 12:1 AFR without the assistance of fuel as a cylinder coolant. Octane comes back to play a role here. Higher octane fuels will allow tuners to move closer to 12:1 than lower octane fuels. Which is why tuners offer off the shelf maps for 91, 93 and racing fuel octanes.
There it is the "secret" of 90% of the tuners out there. It never was really a secret and many will readily admit it and some even provide AFR charts to show exactly where they tuned to get additional power. The real art of tuning is the touch of tuner to lean to just the right point, where the user will get maximum power and small chances of experiencing knock. After they are comfortable with their fuel settings, tuners will adjust the timing of the ignition optimize the fuel settings they have arrived at - again pulling back some from the knock limit.

The actual order of methodology for most tuners is as follows:
• Build the desired boost profile - invariably increasing boost.
• Build the desired fuel profile - invariably leaning the fuel mixture.
• Build the desired timing profile - invariably advancing timing.
A good tuner will likely do this in many iterations to get the tune to their level of satisfaction, since the variables each do have a correlated impact on each other, especially fuel and timing.
For some vehicles there are some other limits that the OEM may have designed in such as boost limits and rev limits - these types of ancillary limits are generally also increased if it is reasonable to do so. These adjustments are not actually tuning though and occur prior to the actual tuning work begins.
So is that it? Is this as far as we can go? Not even close and this paper would not offer much if this was all there was to it, the information is available throughout the web and on almost any performance forum. But this is where a lot of street tuners will stop, either because their customers are satisfied with the results or because they don't realize or accept how much they are leaving on the table. As was pointed out until you are at the actual maximum performance AFR for your engine any move towards it will result in an increase in power.

The Second Tuning Opportunity:
This is not really a second tuning opportunity but more a continuation of the methodology of extracting the most reliable power from you engine. Tuning to the levels described in the last session leave the most opportunity for improvement in the realm of fuel tuning. Knock avoidance dictates that without some additional form of help, under the boost pressures most turbo-charged engines utilize today, your engine will be operating under air-fuel ratios less than optimum for maximum power output.
The exact air-fuel ratio that maximizes power output for any given engine in any given rpm range is specific to the individual engine. It is determined by that engine's volumetric efficiency at that rpm range and by the efficiency of the cam, valve, head and piston design to encourage a flame front that transfers as much of the heat release from the fuel as possible into mechanical energy. Volumetric efficiency quite simply is the relative ability of the cylinder to fill with air during the intake stroke, utilize the heat during combusion and expel the exhaust on the exhaust stroke. On most engines the point where they are most efficient is the point where maximum torque is achieved. This is where the most exhaust has been expelled from the previous combustion process and the most air is allowed to enter on the intake stroke which permits the most fuel to be consumed in the combustion process releasing and transferring the most energy to drive the engine.
This paper assumes that volumetric efficiency is fixed for any given engine. Improving volumetric efficiency involves every aspect of air and exhaust flow from the point where the air enters the intake prior to the air filter to the tip of your exhaust pipe. Generally for any given set up there is one point that is most restrictive and is limiting to the ability of the rest of the system to move gases - however that point can be anyplace in a particular design and once that restriction is eliminated another one develops elsewhere. Improving volumetric efficiency is very vehicle specific and requires considerable flow testing of all components and development of replacements that work well with the parts not being replaced. So we will just take volumetric efficiency as a given and see what can be done to maximize power through fueling with what we have.
Again volumetric efficiency at a given rpm point or range determines the exact air-fuel ratio where an engine will produce its maximum power, but in general most people would agree that performance designs produce their maximum power somewhere between 12:1 and 13:1 air-fuel. The following quotes should reinforce that idea:
Bosch state (sic) that most spark ignition engines develop their maximum power at air/fuel ratios of 12.5:1 - 14:1, maximum fuel economy at 16.2:1 - 17.6:1, and good load transitions from about 11:1 - 12.5:1. However, in practical applications, engine air/fuel ratios at maximum power are often richer than the quoted 12.5:1, especially in forced induction engines where the excess fuel is used to cool combustion and so prevent detonation.
Source: http://www.autospeed.co.nz/cms/A_1595/article.html
Typically cars make best power at about 12.5:1 air/fuel ratio
Source: http://www.sficc.net/features/feature4.html
Usually the air/fuel ratio which produces maximum power is from 12.5:1 - 13.5:1, but this varies with engine type.
Source: http://hondata.com/techlambda.html
The chemical minimum for complete fuel combustion is a 14.7:1 air-to-fuel ratio, termed the "stoichiometric" ratio. Generally, peak horsepower is achieved when an engine (any engine) is run on about 5% to 15% less air than stoichiometric.
Source: http://www.autotech.com/powermod.htm
Maximum power is found when the ratio is about 12.6:1 (Lambda 0.86)
Source: http://www.ffp-motorsport.com/tuning/o2meter.php
Maximum Power is achieved with a slightly rich mixture of approx. 12.8:1 or .9 Lambda.
Source: http://www.datsuns.com/Tech/oxygen_sensors.htm
As a general rule, maximum power is achieved at slightly rich, whereas maximum fuel economy is achieved at slightly lean. (relative to chemical stoichiometric combustion)
Source: http://www.uvi.edu/Physics/SCI3xxWeb...solineFAQ.html
for any given quantity of air inside the combustion chamber there is an ideal quantity of vaporized fuel (ideally about 12 or 13 to 1 for maximum power under acceleration).
Source: http://www.svrider.com/tips/jetting.htm
So how do we get to these power maximizing levels? One common way is to increase the octane of the fuel we are using. On moderately boosted engines some can achieve these levels safely just by using race gas which has octane ratings of 100 and more. Another way is to fall back onto a very reliable method of increasing the effective octane of fuel which is to add lead - this is frequently employed in racing fuel that has an octane ratings of 103-110 (this approximately means that the net result of the fuel is to have even more resistance to detonation than pure octane). There is another way to further increase the resistance to detonation in the cylinder that has been around since the 1930's - water injection.


Water Injection:
Water injection has been around for decades. Its sole purpose is to suppress knock. Keeping in mind that most knock is caused by uncontrolled and/or unintended combustion of the fuel. Water with it high specific and latent heat is an excellent tool for bringing combustion back into control when charge and cylinder temperatures or compression get too high for the fuel being used.

Effect of water in the induction system:
Water is generally introduced to the induction system after the turbo-charger and before the throttle body (unless multi-port water injection is being used, which is more complicated than this paper contemplates). It may be injected pre- or post-intercooler or both.

The introduction of water into the induction charge will immediately absorb heat from the charge thereby increasing its density. Evaporation of the water at this point is unlikely or very minor. The induction charge can at a maximum reach 100% relative humidity and will stabilize at an equilibrium state that is governed by temperature of the charge exiting the turbo and the relative humidity of the ambient air. At full saturation the induction charge will have a significantly reduced temperature somewhere between the ambient temperature and the temperature of the injected water. This heat absorption is achieved primarily through the water's specific heat very little vaporization will occur at this point. The boiling point of water is higher than most charges exiting the turbo and the cooling effect of the water will reduce even the hottest charges without significant vaporization and never beyond the point of saturation and equilibrium.
The effect of water on the induction charge will be somewhat greater in drier climates where more water can be evaporated into the air. However this should not be taken as an indication that it is not useful in more humid climates. Once the induction charge gets heated it will always have room to accept the evaporated water whether the initial relative humidity was high or low - and this evaporation will always absorb heat from the induction charge.
As the water absorbs heat the droplet sizes will decrease and the surface area of the water droplets will increase. Any additional volume from this reaction will be more than made up for by the reduction of the charge temperature and its resulting increased density. Water is not displacing the volume or weight of air in the induction charge and actually working to increase the volume and weight of air that reaches the cylinder. Sizeable droplets of water will reach the cylinder with significant heat absorbing potential intact.
The cooling of the induction charge is the first way that water injection suppresses knock and permits more air-fuel mixture to enter the cylinder.

Effect of water while entering and in the cylinder:
During the intake stroke as the induction charge passes the valve the charge picks up heat from the heated surface of the intake valve. Additionally the effort of sucking the charge past the valve transfers heat to the charge as well. The presence of water in the induction charge absorbs this heat more readily with a lower increase in the temperature of the charge. Remember that it takes more heat to increase the temperature of the water present in the charge. Air and fuel alone in the charge increases in temperature much more readily and rapidly as it absorbs the heat from the valve and effort exerted to bring the charge into the cylinder.
*Thanks to hotrod on NASIOC for pointing out the starting basis for the conclusions reached here.
Once the charge enters the cylinder, water in the induction charge following the exit of the super heated exhaust gases (1500+ degrees) begins to immediately cool the surfaces within the cylinder, including the cylinder walls, piston head, combustion chamber, valves and spark plug tip. This cooling will have its most dramatic effect on any possible hot spots the coolant system has not adequately addressed. Surface temperatures will be significantly reduced by the water even more so than the extra fuel that was previously being used to cool the cylinder surfaces.
With twice the specific heat and six times the latent heat of gasoline, 1/6 as much water by weight is necessary to fully replace the heat absorption of the decreased gasoline in the induction charge. Richer than 12:1 none of that extra fuel is being burned in combustion and the extra fuel robs significant power while serving as a poor coolant. Since the amount of water being added is only a fraction of the amount of the decrease in gasoline there is actually more air and as a result more oxygen in the induction charge for additional combustion to take place.
The cooling of the cylinder area surfaces permits additional air/fuel mixture to enter the cylinder thereby increasing volumetric efficiency. The combination of decreased liquids in the induction charge combined with the increased VE from in-cylinder cooling disproves the myth that water injection will displace air in the induction charge or reduce the amount of fuel that will be burned in combustion.
During this process the continued absorption of heat will decrease the water droplets further increasing their surface area and volume but full vaporization accompanied by the resulting 1700 times increase in the volume of the water will not have occurred at this point. The water will still reach equilibrium prior to this point, albeit in a hotter state.
The absorption of the heat gain while actually entering the cylinder and the cooling of the cylinder area and volume is the second way that water suppresses knock - the cylinder and the charge is much better prepared for a controlled burn during combustion.

Effect of water during compression and combustion:
By this time water has lodged itself between the air/fuel mixture although the percent of water to the mixture is only around 1%. Though the ratio is small the water further suppresses any remaining tendency of the fuel to pre-ignite by lodging itself between the air and fuel during compression. Before actual combustion occurs the water works against auto-ignition by continuing to absorb the heat generated during compression.
Water is a byproduct of combustion it is the chemical reaction of oxygen with the hydrogen freed from the hydrocarbon chains during combustion. How could the injection of additional present water before combustion contribute further to the combustion process? This is where it gets a bit complex and I will try to do the best I can. This pieces together various pieces of my understanding from different sources.
During early combustion when the fastest reactions occur the effect of the water in the mixture is to cause a more controlled and stable flame front. The freeing of hydrogen and carbon to combine with oxygen has to work around the present water to form OH radicals and CO. By slowing this early combustion process there is further suppression of the potential for the mixture to burn too fast and contribute to knock.

Later in the combustion process slower and more complex reactions occur. The formation of OH radicals is very fast and interferes with the completion of the combustion process to form CO2 from the first step creation of CO. It is during this phase of combustion where present water helps to complete the slower reaction to complete the formation of CO2 since water is about the only way to complete the oxidation of CO. The additional present water actually speeds this reaction which also happens to be when as much of two thirds of the energy from carbon combustion is released.
* Sources: Bob Harris note on DIY_EFI thread and the reference within to Combustion, Third Edition by Glassman (reference provided by Jon [in CT] on NASIOC).
Water injection can actually help reduce maximum cylinder pressures while increasing BMEP.

Using a mixture of 50/50 water/methanol will have lower specific heat and latent heat than straight water and besides having a very high 113 octane level itself, it also contributes additional fuel to the combustion process. Contrary to what their heat values would suggest some studies have even reported such a mixture can have greater knock suppression than straight water. Additionally climates that have cold winters benefit from preventing the freezing of the injection mixture.

Conclusion on the contributions of water injection:
Throughout the process water helps to manage heat, contributing to higher VE and suppressing knock. Discounting arguments that water will rob the combustion process of precious air. Then in the final combustion reactions it actually improves the reaction of CO => CO2 for a faster more effective release of the bulk of the energy during combustion. Discounting arguments that it slows and inhibits combustion.

Tuning and miscellaneous notes on water injection:
Always keep in mind that water injection is being used to suppress knock and permit tuning AFR to its maximum power levels. Water in excess of the amount needed for a safe knock suppression margin will hurt net output. Not leaning AFR enough or leaning it too much can result in decreased power levels.
The relationship that you should seek to manage with water injection is the ratio of water to fuel. Metered to exacting proportions as little as 3% water to fuel can replace the amount of heat absorption that fuel previously provided when leaning from 10:1 to 12:1 AFR. There is no system that can meter water in that exacting a relationship with fuel that does not utilize a full fuel injector driver, port installed nozzle jets, high flow pump(s) and a rising rate pressure regulator. At this point you are talking about stand alone engine management like a Motec and a duplication of the fuel system but for water. That being the case the best solution is to use a system that will get you as close as you can to mirroring the fuel injector flow and run at least 10% water to fuel for margin.
Some water injection users have used significantly higher water to fuel ratios (25%+). The technique usually involves increasing water flow until the engine bogs down or comes close to misfiring then pulling back a little bit, before leaning the AFR significantly and finding the MBT advance needed. These techniques take significant amounts of experience and are less compatible with most street setups.
Average street uses should try and target 10%-15% water to fuel. Aggressive street and moderate track use can pretty readily utilize as much as 25% water to fuel.
In general water injection not only permits advancing timing, it is required due to the slower initial flame front during combustion. This is contrary to many nitrous techniques so great care and skill must be employed to find the appropriate balance when trying to use both water and nitrous to make significant power gains.
When adding 5%-7% water to fuel without leaning rich fueling levels - a degree or two of advance will produce almost the same power result as without water injection (leaning slightly will increase the power output). This is good for people who have engines tuned for high octane or race fuel but on a daily basis want to use lower octane grades. This is purely an economic use of water injection - power use requires fuel and timing tuning.
The more boost you are running and the more fuel you are replacing with water injection the more dangerous it becomes to run a constant pressure / static flow water injection system. In order to prevent bogging down the engine at lower boost loads the flow of water is less and less likely to be sufficient for your full boost application in these static setups.
The state of your air/fuel mixture and timing after tuning for water injection will be more than sufficient to destroy your motor if the water injection fails while under load. The responsible user will take measures to ensure adequate water is in supply and flowing during high power usage. Ideally along with a fail safe that will cut boost if a fault in the system occurs. At a minimum warning lights for water level, water pressure and water flow should be employed. Ideally triggers that close all boost solenoids in automatic response to faults would be employed by high end systems.
The entire body of information in this paper was developed relatively informally by me over the last 15 odd years through research, reading barely comprehensible scientific papers and through verbal and written discussions with others about our mutual experiences with water injection. And some of the most important additions to my knowledge have been contributed by the helpful response of people who have read and reviewed this work online. I hope it has proved a beneficial addition to your research on the subject.
I will make every attempt to update and edit this paper as I receive additonal feedback from readers.

Ed.

Copyright 2003 J. Edison Haney II

 

many thanks for posting the study in our AI section. i found it valuable and worth more than one read.

there's lots of good combustion dynamics relating to AI info in the paper.

i would like to post parts of it in my AI thread in the 3rd Gen section w a link to the full paper in the AI section. all with proper attribution of course.

should you wish, feel free to do it since you found this valuable paper.

hc

 

Yeah this paper was great. Im suprised that no one else posted. Maybe it was too long to read? It might be, but it's worth it.

 

I agree. Well worth the read.

 

Great read, thank you.

 

^Yes. I've been lurking for a long time and hadn't see it. Glad it got bumped.