Holley COMMANDER 950 - Manual (Page 50)

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Afterstart enrichment is found in “Fuel” and then “Startup Enrichment”. There are three parts of this area that deal
specifically with afterstart enrichment. They are:
Ø Afterstart Enrichment – Works the same way as the coolant temperature enrichment. If the number is 140, then 40%
more fuel is added from the base fuel map when the vehicle starts. Unlike the coolant temperature enrichment that
decreases as the engine temperature increases, afterstart enrichment decreases based on the “Afterstart Decay
Rate”, defined below.
This is by far the most critical value of the three mentioned here. For moderate performance engines, 130-140 is a
good value for temperatures between 40 and 60° F. The values will need to be increased as the temperature gets
colder and decreased as the temperature gets warmer.
Ø Afterstart Holdoff – Number of engine revolutions that the engine turns before adding the afterstart enrichment. This
number is usually between 15 and 5. It allows the engine a few revolutions before adding the fuel.
Ø Afterstart Decay Rate – How quickly the Afterstart Enrichment decreases. The number entered is how many
revolutions of the engine between each 1% decrease of the afterstart enrichment. A higher number slows the
decrease and a lower number increases the decay. This number usually needs to be between about 20 and 5.
You can watch how much afterstart enrichment is added on the data monitor screen when the engine starts and is
running. You can also view the coolant temperature enrichment there. When you do a cold start watch these numbers
and listen and feel how the engine is running. Make modifications to them until the engine starts and will not stall or run
roughly.
NOTE: Since the base fuel map was already tuned when the engine is warm, DO NOT change it to help cold start
performance. If you do, you will ruin the hot performance. Change the parameters above only to help cold
start performance.
10.0 ALPHA-N TUNING
NOTE: Alpha-N is never used for supercharged or turbocharged engines.
The information in this section translates into some basic fundamentals for Alpha-N systems:
1) Only use Alpha-N if you absolutely have to. This means that your engine has roughly less than 8.5” (Hg) of vacuum at
idle.
2) On street driven cars, select a camshaft that will provide more than approximately 8.5” (Hg) of vacuum. Ideally at least
12” (Hg).
3) If you must use Alpha-N on a street car, use a throttle body that delivers a linear increase in airflow.
4) Alpha-N can be used on drag-only cars successfully no matter what the camshaft or throttle body design is.
Alpha-N Overview – Speed density and Alpha-N are the two methods that the Commander 950 can use to determine fueling
requirements. Most all aftermarket EFI systems allow the user to use one of these methods. To determine how much fuel to
inject, the ECU uses two base inputs. For speed density it uses the RPM and manifold pressure (from the MAP sensor).
Alpha-N also uses RPM but instead of manifold pressure, it uses the throttle position sensor (TPS) to calculate fuel
pulsewidth. You should always use speed density with engines running a blower or a turbo as they need the MAP sensor to
sense boost and add the proper and safe amount of fuel.
Alpha-N should only be used when there is not enough manifold vacuum at idle which is typical of race engines with
camshafts with large duration and overlap. There are no set-in-stone rules when to run Alpha-N. The following reviews when
you most likely need to use it and the problems and limitations of Alpha-N.
The advantage of speed density is that it truly senses changes in engine load by monitoring manifold pressure (or vacuum),
which directly relates to fuel requirements. Alpha-N can not truly sense engine load, just throttle position, which in some
cases can not come close to accurately sensing large changes in engine load. Why is this true? The following example will
explain this:
A good example of an engine that will not work well running an Alpha-N strategy is a hot street/strip car with a single,
monoblade throttle body, such a naturally aspirated 5.0L style Ford engine. A vehicle like this, especially if it is
running big diameter throttle body, will have driveability problems running Alpha-N. When tuning an engine like this
you would first adjust the idle fuel. This would be no problem to accomplish. However, because of the design of the
throttle body, a small change in the throttle position just after idle creates a large change in airflow into the engine.
The area that you are on the fuel map may only change a little, but because of the large increase in airflow, you need