I use a pressure cooker quite often and its all about the cut of the meat. Much like slow cooking, using a often cheaper, fatty cut with lots of connective tissue - a chuck roast for example, yields much, much better results. Lean cuts of meat are make for fast searing and that's about it. If you try to coax them to be fall apart fork tender, you'll fail - those are the cheap, fatty cuts that go that way.
Pressure cookers and pressure canners are the same thing; with the canners being larger, and often having a pressure dial. Both can reach the same pressures and therefore temperatures if designed and manufactured correctly
The pressure canners dial gauge is more accurate for adjusting for food types and altitude, as you can get exact numbers not just 10 psi and 15 psi as with typical weight regulated pressure cookers. But for low acid foods you need 15 psi anyway
Pressure preserving is done because of the higher temperatures reached. This saves time and energy, and some food react better to a short 250°F (120°C) cook that an long 210°F (100°C) cook
Pressure preserving high acid food is worth doing as long as the food does not deteriorate quicker due to the high temperature (some soft fruits will)
For preserving low acid foods you need a known good pressure cooker that can reach 15 psi (103 Pa) and therefore 250°F (120°C)
With a commercially published recipe that has been acid level tested, you will have a published time that will be safe. For a home-made recipe you have to take the worst case scenario for the acid level of the main content, and cook for that. Your local government health department will most likely publish tables for this that you should be using. Remember to use altitude adjustments for timings too
Just relying on adding a teaspoon of vinegar or lemon juice per jar is not a safe and scientific method for raising the acid level, though many people do it, and have not come to any trouble with it?
You pressure cooker should come with a manual that confirms what it can or cannot do. Download a new copy if this has been lost
Also see What kinds of pressure cookers are there, and what are they good for?
Best Answer
There is a complicated formula for calculating cooking time based on energy input, surface area, thermal conductivity of food etc. Smaller pieces with a larger surface area will cook slightly quicker than large single pieces
In general for roasts (large slabs) just add the weight of the pieces and cook for the minimum time recommend by your pressure cooker for that (or from your experience). Then test using an accurate thermometer and cook some more as required. In my experience most people overcook meat anyway, so try it a little more undercooked for a change. Remember to let it stand at least 5 minutes per 500 g too
There are too many variables to give a blanket answer (stove energy, pressure in pressure cooker, meat type, fat content etc.)