Hello again everyone! It’s time for the final installment of our three part series called “Air/Fuel Ratio Management For Racers”!
In our last issue, we discussed how the engine reacts if we tune it on a dyno or at a track in one location and then take the engine to a totally different altitude or location. We found a government study put out in the 1920’s shows that the same air to fuel ratio would be required of the engine at any reasonable altitude. The engine made less power overall due to the lack of air density, but the ratio of air to fuel did not need to be changed because of this. Knowing this, the only thing a racer needs to do is make sure to maintain the same air fuel ratio at the track that they found to work when they were on the dyno!
In this final article of the series, we wanted to ask the question: “What happens to my engine at various inlet air temperatures, and how does this affect my choice of air fuel ratios?”
Let's take one last look at our favorite document, "NACA report 189" to see if we can find the answer!
PART III
When I find the right Air Fuel Ratio for maximum power, will that number change when I race at tracks with different temperatures?
If you’ll remember back to the last article, we used a mathematical formula to calculate the mass of air that went something like this:
Mass = V*D
Where:
V = the CFM of air the engine was breathing,
and
D = the density, (or weight) of one cubic foot of air.
There are primarily two things that affect the density of air. One is the air pressure, and the other is the air temperature.
We can use the following formula to determine how much one cubic foot of air weighs:
Density = 2.7 P/T
Where:
P = PSI (absolute)
And
T = Temperature in degrees Rankine (Degrees F + 460)
If we use the standard temperatures and pressures at sea level, we will find that one cubic foot of air weighs around .076 Lbs.
Ex: 2.7 [14.7/(60 + 460)] = .076
Now, if we simply plug in different values for various altitudes or temperatures, we can find out how much change in air density we have and then add or subtract fuel from the engine accordingly to maintain the same air to fuel ratio.
Take a look:
Lets say we are up in the mountains, and the barometric pressure is down to around 12 psi absolute, (which is around 24.4 inches of mercury, or about 82 Kpa), and the outside temperatures are about 40 degrees F.
Using the above formula, we see that:
D = 2.7 P/T
D = 2.7 [12/(40 + 460)]
D = .0648 Lbs per cubic foot
So, .0648 / .076 = .85 or about 85% of the original air density at sea level! That means in order to keep the same air to fuel ratios, we would need to subtract about 15% of the fuel we were previously giving the engine!
We can very easily program a table into the engine computer to automatically measure the intake air temperatures, and then add or subtract fuel to maintain a constant air fuel ratio at all temperatures.
The question is though, do we need a different air fuel ratio when the air gets very hot, or very cold?
Well, to find the answer, we must once again visit “NACA Report 189”.
On pages 111 and 112 we see this following statements, (which are paraphrased here):
“An analysis of a large number of tests covering an inlet temperature range of –20 C to +40 C has shown maximum power to be obtained with approximately the same air fuel ratios at each temperature.”
This would indicate that one would always want the same air fuel ratio, regardless of the inlet temperatures. However, the report goes on to state the following:
“The volatility of the fuel is in reality the determining factor in this question. A constant fuel air ratio is desirable only so long as a change in air temperatures does not appreciably change the relative quality of the mixtures supplied to the various cylinders or the amount of fuel that has been vaporized at the time the compression stroke is completed.”
Essentially, what they are saying is that if the intake temperatures are so hot or cold that they cause the fuel to be ignited prematurely, causing detonation, or cause the fuel to remain in a more liquefied, un-vaporized state, which would make it not ignite so easily then the need for a richer or leaner air fuel ratio might exist.
Overall, what we learned from this is that if the fuel being used is fairly stable, and the temperatures encountered while racing are not extreme, then a constant air fuel ratio is desirable across a wide range of air temperatures. If however, the temperatures your engine will see are extreme, then there is a possibility that a change in air fuel ratios might be warranted.
However, most ECU manufacturers have understood this for some time, and nearly all give you one or more tables to create a method for adding or subtracting fuel as the inlet temperatures increase or decrease.
Hopefully, this series of articles has given you a small amount of insight into understanding the engine’s requirements when it comes to selecting, and maintaining a given air fuel ratio.
Only thorough testing and some trial and error will tell you what is exactly right for your engine, but perhaps with a better understanding of the factors involved we can shorten the time spent tuning, and increase the time spent racing and enjoying your vehicles!
See you at the Track!
Written By: Ben Strader
EFI-101 at Horsepower Connection in Lacey, WA & Forged Performance in Marietta, GA!