air conditioning

After an unknown period of disuse(years), I recently recharged my A/C using a diy r134a conversion kit. Miracle of miracles, it blows cold air, and has for a month now. I know I should have changed the dryer, but since the A/C had been abandonded for so long, I figured I had nothing to lose. by charging it up. My question is this: although it works, my mini york compressor is REALLY LOUD around 2500 rpm. At all other speeds it is quiet, but it booms badly at 2500, almost as loud as the engine itself. The motor does not overheat with the A/C on but it does lug. About 150 rpm idle drop with A/C on, and if I turn it on while on the freeway, it feels like I've hit a strong wind gust. Am I putting my engine at risk running this old A/C? What is the best compressor upgrade? I've heard good things about the seiko true- rotary upgrade, anyone tried this? Whatabout the Sanden? Prices? By the way, my car is a 78 911SC with the "mini york" compressor.

Serge- I had a similar experience with the York. I subsequently installed a Sanden and have been happy with it's performance. It's installation wasn't a pancea, it still takes HP to run(a smaller drop in idle rpm) and the AC is still not icy cold, but it's better.

In my 78 Sc with the a/c on there is a noticable drag and my gas milage loses about 4mpg but no loud knocking or strange noises
At idle its a little rougher though, sounds like a piston is going out in your compressor
Your a/c belt needs to be real tight or it'll vibrate and make a lot of noise, did you have the compressor oil checked when serviced?There is a bolt on top of the compressor you loosen and check with a dipstick but you can only do it when the system is empty.When I had my a/c charged a month ago the shop just put it on the charging-leak test machine and let it cycle, they did not check my oil which I told them to do.