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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!