Each Lycoming reciprocating engine has a model designation. The designation is made up of a prefix which is a series of letters, a three-digit number, and a suffix which combines letters and numbers. The letters and numbers in this model code have meaning. Most people who fly or work on general aviation aircraft are curious about the meaning of the code, but only a small number thoroughly understand it. Perhaps the explanation and examples provided here will promote a better understanding of what the engine model designations mean. Below are some examples: 
TO 360 C1A6D
IO 540 AA1A5
IO 360 A3B6D
L—Left-Hand Rotation Crankshaft Cubic Inches* A- or AA-Power Section & Rating
T—Turbocharged (exhaust gas driven)   3-Nose Section
I—Fuel Injected   B-Accessory Section
G—Geared (reduction gear)   6-Counterweight Application
S—Supercharged (mechanical)   D-Dual Magneto
V—Vertical Helicopter   (Subsequent changes to models are reflected in the suffix.)
H—Horizontal Helicopter    
AE—Aerobatic Engine    
O—Opposed Cylinders    

*Note (541)-A  displacement  ending in "1"  indicates a   specific engine  model which  incorporates integral accessory drive.

With the information above and a few explanatory details, the Lycoming engine code is not difficult to understand. Starting with the prefix section, an O will be found in the engine designation of all flat opposed cylinder engines. In addition to the O, a combination of the other letters may be used to further describe the engine. The O alone indicates a carbureted engine, but an IO will show that the engine is fuel injected. A further example is the TIGO prefix. Broken down, this says that the engine is (T) turbocharged, (I) fuel injected, (G) geared (which means the prop will run at a lower speed than the crankshaft) and, finally, the (O) for opposed cylinders. The three-digit number always provides an indication of engine size in terms of approximate cubic inches of displacement. Engines currently in production at Lycoming Williamsport have displacement values of 235, 320, 360, 435, 480, 540, and 720 cubic inches.

The suffix of the reciprocating engine code is a little more complex and the differences signified by each letter or number are not readily apparent. The first characters of the suffix will always apply to the parts of the engine indicated in the examples; in some cases, such as the IO-540-AA1A5, two characters are used to designate one section of the engine. The fourth place in the suffix will usually be a number to indicate a specific counterweight application. Depending upon the need for a counterweight number, a D may be used as either the 4th or 5th character. The D indicates that the engine uses a dual magneto contained in a single housing.

To determine the minor differences in an engine model which are reflected in the model code suffix, it is necessary to consult the engine specification. Most aircraft owners or pilots will have no need for this type of detail. Those who are curious about an engine can get a good idea of its size and character by simply applying the model code information which has been presented in this brief outline.