quote:Originally posted by David_Nunn:
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."
If you are replacing a 10.5 cubic inch compressor (York) in a old system you should replace it with another compressor of the same or very similar capacity, not one that is 20% smaller (SD508).
You would have to run the SD508 a lot faster than the York to make up the capacity shortfall and that just unnecessarily introduces more heat to the system from heat of compression and internal compressor friction that must the be dumped by the condenser in addition to the condensers normal load.
If also you intend to change refrigerants from R-12 to R-134a you the place a further requirement on the compressor as you will need to move a greater amount of R-134a through the system than you did R-12 because the properties of the two refrigerants are slightly different.
R-134a boils at a slightly higher temperature than R-12 (so it doesn't get as "cold") for any given evaporator pressure and also R-134a does not transfer the heat as well as R-12, so to move the same amount of heat as a given amount of R-12 more R-134a needs to be moved.
Think of A/C as a conveyor belt for heat.
You put heat on the conveyor in the cabin of your car, the conveyor takes it outside, then the heat is dumped to the passing air. The conveyor belt continues back to the cabin with the heat removed where you can put more heat on it to be moved outside.
If you have a big wide conveyor belt you can fit more heat on it, thus move more heat in any given amount of time, if it is small and narrow only a small amount of heat can fit on it, so you only move a small amount of heat in any given time.
Refrigeration is all about system capacity.
R-12 boils at -29.8 degrees C and R-134a boils at -26.1 degrees C (at 14.7psi absolute pressure), so it looks as if doesn't get as "cold". That's true it doesn't but -26.1 would be "cold" in anyones language so why doesn't it feel as cold in your car ?
Think of it this way, if we could get your car's interior down to 0 degrees C (32F) most people would say "gee wiz it's jolly cold".
A ice cube from your freezer is at 0 degrees C, but putting one ice cube in your car would have no effect on the temperature in your car you could possibly feel. But what about 1000 ice cubes ? Or 2000 ? Or 5000 ?
At some point if you continued to place ice cubes inside your car the interior temperature would fall to 0 degrees C.
This is capacity.
No ice cube is colder than 0 C, one ice cube does nothing to make you feel cool, but 5000 will. The temperature of the refrigerant hasn't changed but the amount of refrigeration has.
I have heard people say they are disappointed with a dashboard vent temperature of 15 degrees C and they want it "colder" say 2 degrees C. The reality is they don't really need it "colder", if all the air in the car could be maintained at 15 C they would feel cold.
Imagine if the A/C in your house or office was set to 15 C you would have to wear a coat or jumper.
What the car A/C needs is capacity.
As the article extract in the previous post said "Compressor Capacity Is Critical", it must be a match for other components in the system. Not too big , not too small. So if your current system has 10.5 cubic inches of compressor capacity replace it with as close to 10.5 cubic inches as you can. Not 20% smaller.
In the article extract they talking about new systems they are building up from scratch.
They have the luxury to choose the refrigerant from the word go, not try to get a refrigerant to work in a system designed for a refrigerant with different properties.
They can also choose condenser and evaporator designs and capacities at will, not be stuck with using what is already in the car or what could be reasonably expected to fit and in those circumstances with the components they use the SD508 may very well be suitable.
They may also have a stockpile of SD508's they want to get rid of.........Don't be fooled, financial incentives are more powerful than engineering ones when it concerns this "black art" of refrigeration. Many many customers have been made to purchase things they don't need or things not up to the task by car A/C mechanics preying on the average customers ignorance.
The Sanden compressors of any capacity run much nicer than the clanky old York. As i have said the small sacrifice of 0.5 cubic inches when replacing a York with a SD7H15 is worth it to get the SD7H15's smoothness.
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