Vintage Air University

Basic Auto Air Conditioning Compressor Facts

The next most critical part of an air conditioning system in terms of providing optimum performance and trouble-free service is matching the compressor to the other system components.

The type of compressor most commonly used now in aftermarket systems is the axial type by Sanden (See our line of SANDEN COMPRESSORS). We do see a few engine swaps that retain the original General Motors/Frigidaire (DAG and R4), or the Ford Nippondenso compressors. And yes, a very few York/Tecumsehs are still on some restored classics. The Sanden compressor is a compact cylindrical unit 04670-mta_lrabout 8-1/2 inches long and about 5 inches in diameter. It is easy to spot with its barrel shape, aluminum color and small overall size. It has five or seven cylinders running lengthwise, thus the name axial compressor. Even though it is commonly refered to as a rotary, it is not. The Sandens are so popular because they operate smoothly, with a minimum amount of torque required to operate, because the load is distributed over multiple short-stroke cylinders. The Sanden compressor has another characteristic which we like, especially for street rods: its short stroke, low-displacement-per-cylinder configuration tends to make it less efficient at low RPM ranges (1200 and below), the same speed at which our engine-driven radiator fans are least efficient. This puts less heat load on the condenser and consequently on the radiator at idle speeds. On the average classic car requiring a smaller sized condenser, that is a blessing. Above 1200 RPM, efficiency rapidly increases; at operating speeds of 1800 to 2400 RPM it is about the most efficient pump on the market. It can be operated at continuous crank speeds of 6000 RPM. These characteristics make the Sanden an almost ideal performance compressor.

Displacement sizes for Sanden units are identified by the numbers on the label on the case. The numbering system is interpreted as follows: SD 508 = 5 cylinder/8.4 cubic inch, SD 7 = 7 cylinder/ 6.1 cubic inch.

Other Compressor Types: York
The York compressor was OEM equipment on most American cars in the fifties, sixties, and early seventies. The York is a rarity these days, but can be identified by its large square shape. The York compressor is much like a lawn mower engine, being a two cylinder, reciprocating pump. The reciprocating motion causes it to vibrate more than modern Sanden compressors. The York also has higher torque requirements for peak pumping, and they don’t like higher RPM operating speeds.

Other Compressor Types: GM/Frigidaire
The GM/Frigidaire A-6 (long type) compressor shares the same basic design as the Sanden, with one additional cylinder. It has the same characteristics, except that it is very large, both in capacity and external dimensions. It is about 12.5 inches long and 5 inches in diameter, with over 12 cubic inches of displacement per revolution. We mention this compressor only lightly because the only place we would recommend using it would be in a high-value pure restoration project or in an extremely large vehicle with a high-capacity evaporator, or even two evaporators. Using the GM/A-6 compressor has been a painful and costly experience for many restorers and rodders.

The GM/Frigidaire R-4 is the short (7 1/2-inch) and fat (7-inch dia.) compressor found on later GM vehicles. It has just under 10 cubic inches of displacement per revolution, with four cylinders radiating from its crankshaft. It has basically the same characteristics as the A-6 compressor, except it seems to fit in smaller spaces and is much lighter due to its mostly aluminum construction. The R-4 is larger in capacity than we like in an average classic car installation. However, it would be a better choice than its bigger brother, the A-6. Rebuilt GM R-4 compressors are not recommended.

The GM/Frigidaire DA-6 compressor is slightly larger than a Sanden compressor and will work okay on many engine swap installations where the OEM engine drive is being retained. It is an axial type compressor with just under ten cubic inches of displacement. Custom mounting for this compressor is more difficult on non- OEM applications and new replacement parts costs are high.

Variable Displacement Compressors
These types of compressors are not recommended for A/C systems that use a receiver/drier and a thermal expansion valve (TXV). Variable compressors increase and decrease piston stroke (and consequently, refrigeration flow) based on A/C system demand to modulate the performance of the evaporator. That makes the compressor the primary control point in a variable displacement air conditioning system.

In a cycling-clutch, expansion valve system, the thermal expansion valve increases and decreases refrigerant flow based on A/A system demand to modulate the performance of the evaporator, making the TXV the primary control point for the system. All A/C systems require a primary control point. And if using a secondary control, it must be subservient to the primary control.

Using a variable displacement compressor in a clutch-cycling thermal expansion valve system results in two primary control points, and is like a business with two presidents: it is doomed to failure. The variable compressor and the expansion valve’s control activities contradict and “chase” each other, resulting in a system that never works optimally. Plus, this combination may lead to oil starvation and compressor failure.

There are some OEM systems which use a combination of modulation devices, however they are the result of long-term development programs and, as they say, “don’t try this at home!”

Other Compressor Types: Ford Nippondenso
The Ford Nippondenso compressor is an axial compressor with about 9.5 cubic inches of displacement. It’s fine on applications where the OEM engine drive is retained on your engine swap. But mounting is more difficult on non-OEM applications, and new replacement parts costs are high. We don’t recommend rebuilt compressors of this model.
Compressor Capacity Is Critical
Capacity is critical in selecting a compressor. Why? The weak link in most aftermarket air conditioning systems is the condenser’s ability to handle the demands of the other air conditioning system components. These demands are to condense the refrigerant enough to keep the compressor head pressure and corresponding refrigerant temperature within acceptable operating limits (approx. twice the ambient temperature of the day, plus fifteen percent) and to supply the evaporator with adequate refrigerant. Generally speaking, if a compressor has too much capacity, the result will be excessive high-side pressure and temperature, compressor damage and excessive load on the engine. If a compressor has too little capacity, the system will suffer inadequate evaporator performance. Our basic minimum given in the condenser part of this article is larger than most aftermarket systems are using: However, we will use that as our standard size condenser. A compressor of 8 or 8.5 cubic inches of displacement per revolution is ideal for a classic car or truck with our standard size condenser and an average size aftermarket evaporator (approx. 200 cubic inches of coil mass). We would always lean toward a smaller compressor before going to a larger one.

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