Take the GAMI Lean Test
The purpose of the GAMI Lean Test is to quantify the fuel/air ratio balance in the engine either before or after installing the GAMIjector® fuel injectors. Knowing this balance, and the value we call the GAMI Spread is important because it will tell you how much you can benefit from GAMIjectors® if your engine is not already equipped with them, and it will tell us if we need to adjust your engine balance if you are already equipped with GAMIjectors®.
The GAMI Spread is calculated by determining at what total engine fuel flow each cylinder reaches peak exhaust gas temperature (EGT) and subtracting the lowest flow from the highest flow. Most engines are considered to have good fuel/air ratio balance if the GAMI Spread is less than 0.5 gph.
There are three basic methods for performing the GAMI Lean Test which work well. The following methods assume that you have some instrumentation to measure the EGT on each cylinder and the total engine fuel flow (this may be digital or analog).
While following these test methods, GAMI highly recommends a safety pilot accompany you. We also recommend you keep the cowl flaps open, if so equipped, and you perform the test at 65% power. It may be necessary to use a lower power setting, as required, to keep the CHT under 400°F and/or the Turbine Inlet Temp (TIT) under your max continuous redline (Note: All turbochargers allow for short term exceedances of the TIT limit for leaning purposes - usually one minute or less).
If your engine monitor has integral fuel flow and downloading capabilities, this is probably the easiest method. In flight at around 65% power at your typical cruise RPM, you should lean very slowly from some point rich of peak EGT to some point where all EGTs are lean of peak. You will know that you are lean of peak EGT on all cylinders once the exhaust temperature of each cylinder continues to drop as you reduce the fuel flow. Once the flight is complete, you should download the monitor and email the file in its native format to email@example.com.
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Most engine monitors will allow you to view all of your EGTs at the same time in a graphical format as individual columns. As you slowly adjust the mixture from rich to lean those columns will rise as each individual EGT nears its own peak. The columns will then fall as each cylinder reaches peak EGT and becomes lean of peak. Some monitors make determining peak EGT easier by inverting the EGT columns, making them flash, or changing the color of the bar when that particular cylinder peaks. Once you identify that a cylinder or number of cylinders has reached peak EGT, you can write down which cylinder or cylinders has peaked and at what total engine fuel flow. Continue in this fashion until you have recorded the total engine fuel flow for the peak EGT of each cylinder in the order which they reach peak. You can email these numbers to firstname.lastname@example.org or fax them to 580-436-6622.
This method is more complete, but more time consuming than the shorthand method. With this method, you record each EGT (and each CHT optionally) at small increments of fuel flow adjustment from around 2 gph rich of the first peak EGT to some point lean of the last peak EGT. You want these lines of data to be in the smallest practical fuel flow increments - 0.2 to 0.3 gph works well. You will also want to use the smallest EGT resolution possible. For many monitors this means 1°F, though 5°F or 10°F is the smallest for some monitors. 1°C resolution works fine, also. It may be necessary to use the monitor's "lean find" or "lean assist" function to find the 2 gph rich of peak starting point. The reason for starting 2 gph rich of peak is to prevent you from either taking much more data than necessary, or starting the data collection too late to capture enough information. DO NOT lean each cylinder one at a time. Start at a fuel flow where ALL EGTs are ROP, and lean in small increments until ALL EGTs are LOP, recording the EGT of ALL cylinders at each increment.
You can download blank copies of this test form in PDF or XLS (Excel) format or even create your own. You should then email the completed form to email@example.com or fax it to 580-436-6622.
- Tips for performing the GAMI Lean Test with a GEM 602
Interpreting the results:
It is sometimes helpful to repeat the test, to improve reliability of results. Before repeating, make sure that you have gone back to the rich side of peak EGT and that all of the cylinders are under 400F.
Samples of what the data might look like can be seen below. Note the fuel flow of the first cylinder to reach peak and fuel flow of the last cylinder to reach peak. This difference in fuel flow - what we refer to as the spread - is the most important piece of information to be gained from these tests. When leaning this engine from a rich condition, leaner cylinders peak first and richer peak last.
While there is some variation from one engine type to another, typically, engines such as big bore Continental Motors engines without GAMIjector fuel injectors will have a fuel flow spread from 1.3 to 1.75 GPH, with some engines being well over 2.0 GPH.
Typically, after GAMIjector, fuel injectors have been installed, and sometimes tweaked after a further test, that same engine will have a spread of under 0.7 GPH. We have seen some engines with truly spectacular results, going from over 2.0 GPH to under 0.5 GPH in their spread.
Lycoming engines and so-called tuned induction Continental Motors engines may have an initial spread of somewhere between 0.7 and 1.3 GPH, although we have seen some truly extraordinary examples of these engines in which the spread was much larger. Experience has shown that it is worthwhile to adjust the fuel injector fuel flows for these engines until we get them below 0.5 GPH in spread from the richest to the leanest cylinders.
Once you have performed this GAMI Lean Test, email the results to firstname.lastname@example.org. Someone should respond to you within about a day or less. If you fax the data to 580-436-6622, please give us a call toll free to discuss the results on 888-359-4264.
If you have any questions about the lean test procedure or using your particular engine monitor system for the test, please feel free to call us toll free – 888-359-4264.