Willoughby Z said:

The Shrader valve holds pressure (if it's any good). The cap just prevents dirt from gumming up the valve. I've removed a cap and heard a little "pssst", indicating a valve leak. The cap, if snug, would hold a little pressure but not much.

I've actually had caps with good o rings hold the pressure that would have otherwise escaped through a leaky valve. ZCurves, how did you arrive at the 10 to 1 ratio? Generally, low side should be about 40°+/- 5, and 10 times that for the high side would be way too high.

SBlake01,

I agree that 400lbs Highside would be crazy. When I got my certification, we were taught that a 10:1 Compression Ratio is a good "rule of thumb" - but then the FSM contradicts that with the 40lbs Lowside instruction. The lower the CR, the more efficient a compressor will be. The '78 FSM provides an excellent chart of ratios on AC-56 - none are exact 10:1 CR's. Looking at my notes for my Z, I was actually running 235/35 or 7:1.

I believe that would be the 'Performance Chart' on AC-57. The reason I say that is beacuse I have that page printed out and on the wall above my workbench. It along with the chart for the 810 I used to own weren't in the database I use. Anyway, pressures are only a part of the story, however, and should only be used as a general guideline. That chart also takes humidity and ambient temperature into consideration; factors which are as important if not more so than suction/discharge pressures. I use a charging machine which has a thermometer and a hygrometer on it and I charge by the recommmended system refrigerant weight rather than pressure. When I do put gauges on the system after charging it with the machine, the pressures are within the figures shown on that chart or the chart for the particular vehicle's sytem I'm working on. Using the machine it's 'one and done' while charging with cans or even refrigerant cylinders and gauges can be somewhat trial and error or hit and miss if you don't have a way to factor in temperature and humidity.

Edited July 12, 201212 yr by sblake01

That is good information, now if I only had a charging machine....