by John Comeskey of SPS and James Walker of scR motorsports

While most enthusiasts clearly understand the need to increase airflow in order to get more power, few understand the role of fuel and how fuel delivery is controlled in a modern motor. This article will focus on fuel delivery with a specific look at proper ratios, the meaning of "stoichiometric," how fuel flow is controlled and measured, and ways that an enthusiast can modify fuel delivery to match other motor enhancements.

Why Does Fuel Mixture Matter?

As you know, energy is only released when atoms combine to create new molecules. The fuel mixture – or combination of input ingredients – are important factors determining the efficiency of the chemical reaction. Poor mixtures result in excess atoms of certain elements that are unable to mate to other atoms to create new molecules. As a result, the excess atoms take up valuable space within the combustion chamber and interfere with the reaction process of other atoms. This is costly in terms of efficiency and lost power.

In theory, one would want every bit of the combustion space to be filled only with atoms that will eventually be used to create energy from the exothermic reaction. And one should wish to avoid any "wasted" atoms - and hence "wasted" space - within the combustion chamber.

What are Fuel Ratios?

Fuel ratios are simply the relationship between the number of oxygen molecules relative to the number of hydrocarbon molecules. This is traditionally measured in terms of oxygen molecules per one hydrocarbon molecule – or "X" parts of air per "1" part of fuel. For example, "13.0:1" (read as, "thirteen to one") reflects 13 parts of air per one part of fuel.

What is "Stoichiometric?"

The term "stoichiometric" describes the mathematically "correct" combination of hydrocarbons and oxygen within a theoretical mixture for the internal combustion process. A stoichiometric mixture combines "just enough" oxygen with "just enough" carbon and hydrogen to ensure that all atoms have a mate. This "perfect" ratio has been calculated as 14.7 parts air to one part fuel – or 14.7:1.

In theory, no leftovers result from the reaction if combustion conditions are efficient and if the input mixtures are stoichiometric. In fact, water and carbon dioxide would be the only emissions produced from a stoichiometric fuel ratio in a completely efficient reaction environment.

However, even though a stoichiometric ratio is theoretically the best combination of oxygen and hydrocarbons for a perfect combustion process, in practice, it is not always the best for a specific purpose. One reason for this is that the calculation of the stoichiometric ratio assumes a "perfect" combustion environment in which every available atom is able to find every available mate. Because combustion environments are not completely efficient, there are cases in which fuel ratios will vary from stoichiometric in order to compensate for the inefficiencies.

Fuel Efficiency Versus Power

Even though stoichiometric is "mathematically" correct, it is neither the most fuel-efficient nor the most powerful mixture.

For better fuel efficiency, it is possible - and desirable - to "lean" the mixture, or reduce the amount of fuel relative to the amount of air. In fact, maximum thermal efficiency occurs at ratios between 16-18:1. And some experimental high-efficiency motors will run in "lean" mode while cruising. Such mixtures on these motors may become as lean as 20.0:1

This obviously contributes to excellent fuel efficiency. However, such lean ratios also result in very hot temperatures and relatively unstable mixtures. This can lead to detonation under load and is not best for producing power.

For more power, it is actually best to use ratios that are "richer" than stoichiometric – or ratios that use more fuel relative to a given volume of air. It is important to remember that because combustion environments are not perfectly efficient, it is sometimes difficult for every available atom to match up to a mate. This is especially true under high loads. To help ensure that every oxygen atom is used in the combustion process – and thus results in the release of energy – slight amounts of extra fuel should be added into the mixture. This increases the chance of creating the desired exothermic reaction. And it reduces temperatures, which may prevent detonation.

For these reasons, it is generally accepted that ratios for producing optimal power in any motor should be between 12-13:1. And based upon data that SPS and scR motorsports have collected on the dyno, it appears that maximum power from the DOHC Saturn motor occurs between 12.7:1 and 13.0:1.

How Does a Saturn Motor Control Fuel Delivery?

In the "Old Days," when enthusiasts tweaked on classic American V8s, motors used carburetors to combine fuel and air and to regulate the flow of the mixture into the motor. A carburetor is essentially a mechanical throttle control device that connects to the throttle pedal. It uses throttle plates to control airflow into the motor and it relies upon mechanical "jets" to control fuel flow. Larger jets deliver more fuel while smaller jets deliver less fuel. Although many older enthusiasts may tend to romanticize the effectiveness of the carburetor, in truth, by today’s standards, they were relatively inefficient and unreliable.

Modern motors, including the Saturn motor, use an engine management computer and electronic injectors to control fuel. As a result, the Saturn Powertrain Control Module – or PCM – is able to execute more precise fuel delivery and it is able to adapt (within limits) to varying conditions and fuel needs without making mechanical changes. Compared to the carburetor, modern fuel injection is much more efficient and reliable.

The process of Saturn fuel management becomes more complex, but the following explanation should provide a suitable basis of understanding:

One should first understand that the PCM is programmed to target specific fuel ratios, based upon the operating conditions of the car. (For full-throttle acceleration, it will seek richer mixtures. For highway cruising, it will seek leaner mixtures.)

The important tool for controlling these mixtures is the "pulse width," or the amount of time that the electronic injector is allowed to open. Since the size of the injector and the operating fuel pressure are known constants (or so the assumption goes), the time that the injector is open will determine the amount of fuel delivered. And thus, controlled time intervals will result in controlled amounts of fuel. Specifically, longer pulse widths will result in more fuel, while shorter pulse widths will result in less fuel.

The fuel pressure is governed by a "fuel pressure regulator." This device ensures that fuel pressure at the injector opening is regulated to a known value. If fuel pressure were to vary, then calculated pulse widths would no longer result in known amounts of fuel delivery, since higher fuel pressure results in more fuel for a given pulse width, and less fuel pressure results in less fuel for a given pulse width. (Note that the Saturn fuel pressure regulators are slightly variable as a function of manifold vacuum. More vacuum (less throttle) results in lower fuel pressure, while less vacuum (more throttle) results in greater fuel pressure.)

The PCM starts its fuel delivery attempts by considering sensor inputs that measure coolant temperature, incoming air temperature, throttle position, manifold pressure, and other vital operating information. It uses this information to formulate an "educated guess" for a mixture - which it executes by holding the injectors open for the "appropriate" corresponding pulse width.

The PCM will monitor the outcome of its attempts after the motor fires by evaluating oxygen content within the exhaust gases (through the use of O2 sensors.) Since "ideal" exhaust gas oxygen content can be mathematically calculated, the PCM can use O2 sensor input to make an evaluation as to whether current mixtures are "too rich" or "too lean."

To adjust fuel control to correct levels, the PCM can adjust the pulse width – or add or reduce fuel by increasing or reducing the amount of time that the fuel injectors are held open – during the next injection cycle.

So, in a nutshell, the PCM will make an estimation about fuel delivery prior to each engine cycle. It will execute by controlling the length of the injector pulse width. It will monitor its success through the O2 sensor, and then it will make a new calculation prior to the next cycle.

Open Loop and Closed Loop

The operation described above is known as "closed loop" operation. It determines the Saturn’s fuel management under almost all conditions – the notable exceptions being initial startup and full-throttle acceleration. The name "closed loop" is derived because it describes the complete-circle path of the decision-making process. During closed-loop operation, the PCM will work to maintain a near-stoichiometric (14.7:1) fuel ratio.

Under certain circumstances – such as wide-open throttle – it may not be appropriate or necessary to calculate and perform such exact adjustments. So instead, the PCM uses "open-loop" operation to control fuel delivery. During open-loop operation, the PCM does not consider the results of each cycle in the fuel decision for the next cycle. Instead, it resorts to a pre-programmed pulse-width that results in a pre-determined quantity of fuel. The PCM’s decision-making process during open-loop can be illustrated with this simplified example: if throttle position = 100%, then pulse width = "X."

Again, the name "open-loop" is derived from the graphical representation of the process. Since the results of the previous mixture are not factored into the calculations for the next pulse width, the loop remains "open."

In most cases, open-loop operation will deliver unusually large amounts of fuel – often resulting in mixtures richer than 12.0:1 on a stock motor. This not only ensures adequate fuel supplies, but it keeps piston and cylinder temperatures lower which will lessen the likelihood of detonation and engine damage.

Throttle-Body vs. Multi-Port vs. Sequential Fuel Injection

Since 1991, Saturn has used three types of electronic fuel injection systems. The 91-94 SOHC (single overhead camshaft 8-valve) motors used a throttle body injection – or TBI. The 91-95 DOHC (double overhead camshaft 16v) and 95 SOHC motors used the multi-point injection – or MPI. And the 96-99 SOHC and DOHC motors used the sequential fuel injection – or SFI.

Each of these three systems works in the way that this article has previously described – altering the pulse width of the electronic injector to deliver the proper amount of fuel. However, the three systems vary in terms of where and when the fuel is added to the mixture.

The TBI system is characterized by one single injector that is used to deliver fuel to all four cylinders. This single injector is mounted just above the throttle body. This makes the TBI system unique since both air and fuel travel through the throttle body and intake manifold while en-route to the cylinder head. (This is similar to the carburetor – except for the fact that the carburetor introduces fuel through mechanical jets, while the TBI system uses an electronic injector.) Of the three systems, the TBI is the least precise. Since fuel is mixed with air before entering the intake manifold, there is time for the fuel and air to separate before it reaches the motor. This problem is known as poor "atomization," or the tendency for the mixture to settle from its atomized – or vaporized state – into separate pockets of fuel and air. And it is one of the reasons that the 91-94 SOHC motors are rated at only 85HP.

The MPI system advances beyond the ability of the TBI system by employing four separate injectors – one for each cylinder. And, unlike the TBI system - which adds fuel at the throttle body - the MPI system mounts each injector directly into the intake port of the cylinder head. This ensures that only air travels through the throttle body and intake manifold - delaying the introduction of fuel until it actually reaches the cylinder head – and avoids many of the TBI system’s problems with atomization. However, even though atomization is improved as a result of the injector positioning, potential problems still exist with regard to injector timing – since, like the TBI, all four injectors are controlled simultaneously. This allows time for the mixture to settle while waiting for certain intake valves to open.

The SFI system is yet another evolution beyond the MPI system. Like the MPI system, four separate injectors are positioned directly into the cylinder head. However, unlike the MPI, in which all injectors are controlled simultaneously, the sequential system fires each injector at its own optimized instant – allowing more precise injector timing and reducing the chance of poor atomization as a result of settled mixtures.

Ways to Increase Open-Loop Fuel Delivery in the Saturn Motor

There are only three factors that determine the amount of fuel delivered during open-loop operation, so getting more fuel is as "simple" as adjusting one or more of these three factors.

  1. Adjust the Pulse Width. The pulse width is, of course, the length of time that the injector is held open. And it is the PCM’s primary tool for controlling fuel delivery during both closed-loop and open-loop operation. As long as fuel pressure and the size of the injector opening are constants, then a longer pulse width will obviously result in more fuel being delivered. The big advantage of adjusting the pulse width is that very precise amounts of adjustment are possible. However, since the Saturn PCM is not re-programmable, the only way to make these changes is to purchase an aftermarket engine controller and start calibrating!
  2. Use Larger Injectors. If the fuel pressure at the injector and the pulse width are constant, then the size of the injector will determine fuel flow. Obviously, a larger injector orifice will result in greater fuel flow, while a smaller injector orifice will result in reduced fuel flow. Finding the appropriate orifice size and the labor involved with replacing injectors is relatively difficult. And the cost of purchasing electronic injectors is steep, so we do not necessarily expect many enthusiasts to be quick to replace injectors. Nevertheless, SPS is currently working with OE suppliers to develop bolt-on performance injectors with an enlarged orifice.
  3. Increase Fuel Pressure. Increasing the fuel pressure is the obvious solution for most enthusiasts. If the size of the injector is constant, then adding more pressure to the injector will result in more flow for each and every pulse width. (And vice-versa, reducing pressure will reduce flow for each and every pulse width.) Best of all, manipulating fuel pressure is easy and inexpensive, as parts are readily available from reputable tuners such as SPS.

Evaluating Fuel Needs on Modified Saturn Motors

So far, so good, but if you have done some power-producing tweaks to your Saturn, then you may be asking, "How do I know specifically if or when fuel delivery needs to be addressed?"

Unfortunately, there is no one formula that guarantees optimized open-loop fuel ratio conditions. It is important to understand that the internal combustion motor operates in a very dynamic environment – meaning that many conditions and factors are subject to change simultaneously. And there are far too many variables affecting one another for anyone to enjoy an exact understanding and control of the fuel mixture at any given moment.

However, it is possible to gain an overall understanding of the motor’s behavior by evaluating actual data and to use this understanding to establish a framework that can guide tuning procedures with a very generalized course of action.

First of all, understand that closed-loop ratios are irrelevant to the issue for three reasons:

  1. Closed-loop only applies during partial-throttle driving. If you wish to go faster during partial-throttle, just press harder on the pedal. Eventually, you will reach a full-throttle condition which will lead to open-loop operation.
  2. Any changes that you make to fuel pressure or injector size during closed-loop operation will be negated by an altered pulse width, since the PCM will actively pursue a 14.7:1 ratio.
  3. Stoichiometric ratios are preferred to richer ratios during closed-loop (partial throttle) operation, since it leads to better fuel economy and reduced emissions.

However, once the throttle position becomes completely open (100%), one can now assume that speed and power become the primary goal. And it is logical to accept reduced fuel economy in the pursuit of this goal. (Since wide-open throttle operation leads to open-loop fuel management, it is reasonable that an enthusiast may take action to ensure an ideal open-loop ratio between 12.7 –13.0:1.)

"But," you may ask, "when is the proper time to take such action?" Again, there is no "correct" answer to this question, but reference the chart below to gain a general understanding of how open-loop mixtures are affected by basic modifications.

Note that this data is NOT meant to represent the actual modifications made to the scR ITA or SSC race cars – this was dyno data generated using experimental hardware on a dedicated engine dyno in order to provide product information for SPS associates.

Set-upStock Baseline Intake, Exhaust
Intake, Exhaust, Header
Intake, Exhaust, Header,
WC Exhaust,
WC Exhaust,
WC Exhaust,
52mm TB
HP +/-

The first column presents data collected during the baseline run during dynamometer testing performed by scR engine builder Mark Womack. The only modification performed during this test was the installation of a K&N drop-in air cleaner. Average open-loop ratios averaged 12.66:1. This clearly gives room to lean the mixture further before reaching the suggested limit of 13.0:1.

The second column presents data on the same motor with the installation of the ceramic SPS Powerstack and Kayne exhaust with a Sport muffler. Average open-loop ratios here were 12.72:1. As expected, open-loop ratios became leaner as more air was allowed to flow into the motor as a result of modifications. Again, the average ratio is shy of the 13.0:1 suggested limit.

The third column presents data on the same motor with the addition of a ceramic-coated try-Y exhaust header. The data here may seem surprising – since the average ratio fell to 12.40:1 while one would expect a leaner ratio (perhaps around 12.8:1.) However, remember that the motor is a dynamic set of conditions. Not only did airflow change with the header, but manifold temperatures, position of the O2 sensor, the number of cylinders being measured by the PCM, and many other factors also changed. You may not see this same effect on your own car on any given day – but such is the result of our particular test.

The fourth column finally gives us an idea about fuel ratios – and what ratio is optimal for best power. In the fourth column, Mark altered open-loop pulse width parameters (using a dyno-specific engine computer) to create a leaner mixture with an average of 12.72:1, all other factors being the same as the previous test. The result was a 2.5HP gain. Clearly, a leaner 12.7:1 ratio is superior to the richer 12.4:1 ratio. So adding fuel at this point – or prior to this point – is not recommended.

In the fifth test, the street-legal exhaust setup was replaced by Mark’s race-only World-Challenge exhaust which will be used on the IT racecar. This system uses no catalytic converter and a side-exit exhaust for additional flow capability. As would be expected, average ratios became leaner still at 13.09:1.

In the sixth test, Mark tested the limits of the lean mixture by again tweaking the dyno PCM to result in an average 13.17:1 ratio. A slight loss of power was experienced.

This can now allow us to draw two conclusions:

  1. Ratios should be between 12.7:1 and 13.0:1 for maximum power.
  2. Adding fuel to a vehicle using open-loop ratios already richer than 13.0:1 is expected to reduce power.

But what series of modifications will create a leaner-than-13.0:1 ratio which may require additional fuel? This is another one of those "crystal-ball-required" questions. But while no-one can answer this question with exact certainty, evaluating the data can again lead to some very general conclusions.

Notice that the Powerstack, header, and exhaust combination resulted in a slightly rich mixture of 12.40:1. Obviously, this configuration does not warrant additional fuel. However, notice the result upon the average ratio using Mark’s racing exhaust without a catalytic converter. The average ratio in this configuration rose to 13.09:1. This suggests that any enthusiast who has removed or gutted the catalytic converter and enjoys a modified intake and exhaust system may be at the lean threshold – and on the verge of requiring additional fuel.

Now consider the results of the test in the seventh column. This test shows the World-Challenge setup with the stock PCM and the addition of the SPS 52mm throttle body. A very modest power gain was experienced with this setup, but notice the dramatic effect on the fuel ratio- a very lean average of 14.09:1! This dramatic effect on fuel ratios suggests two additional conclusions:

  1. The throttle body would add considerable power if combined with an ideal 13.0:1 fuel mixture.
  2. The throttle body creates considerable additional airflow that is likely to create the need for additional fuel.

So, Do I Need More Fuel?

While no one person can determine exact fuel needs on every modified motor in every situation, most motors will need more fuel if they fall into any of these categories:

  1. The motor includes all elements of the 21HP package including a larger throttle body.
  2. The motor no longer uses a catalytic converter.
  3. The motor uses any combination of "advanced" modifications including extrude-honed intake manifold and/or ported and polished heads.
  4. The motor has been bored to a displacement greater than 1.9 liters.
  5. The motor uses any form of forced induction.

See you at the finish line!