How 2 Fix Videos
We are going to use a 24 valve 5.9 Dodge cummins as a example but all cars and trucks will work the same way just different springs. Second generation cummins comes with springs that are good for about 35 pounds of boost or about 3800 RPM. What this means is if you run over 35 pounds of boost there is a chance that the valves could do what is called float. If the piston is under compression the valves are pushed shut and can not open. If the piston is not under compression the valves can open and let boost in where it does not belong and hurt your horsepower. The other thing is if you want to run higher then stock RPM the spring will not react fast enough for the valves to shut when they should, so you will lose horsepower.

Understanding valve springs
The valve springs are part of the valve train. The valve train is the mechanical system responsible for operation of the valves. Poppet valves typically require small coil springs, appropriately named valve springs. Formula 1 engines have a pneumatic valve springs in which pneumatic pressure closes the valves so they have no spring. Understanding Formula 1 will help understand what spring you need.

So when do you need to up grade you valve springs? This depends on how much you want to do with boost or RPM. I had stock springs on my 01 cummins with compound turbos and they worked fine. I rarely went over 40 pound of boost with it and maybe the valves did float a little but it was a big horsepower gain from stock. When I rebuilt my engine I put 110 pound springs in. Not to say springs are not a very important piece of power production they are. Correct valve spring pressure is a good thing if you want you truck running it's best.
Diesel injectors worked off a a injector pump. Older diesel had mechanically driven pump. It was timed with gears or a belt off of the crankshaft and camshaft. Just putting bigger injectors would increase horsepower because the amount of time the injectors opens was not controlled a computer. Most of the time the timing has to be advanced to work better. If you go big it will create a lot of smoke, smoke is unburned fuel.

If you put a new cam, like a tow or performance cam you could benefit from heavier springs it is best to talk to the place you are buying the cam from and ask what they suggest.
The other thing you don't here people talk about is what is called Nose pressure this is when the spring is compressed  on the nose or high point of the cam. The pressure will be higher because  the spring is compressed. If this is to much it will ware the cam faster, so just getting high pound springs is not a good thing and will not benefit you. The best thing to do is talk to a knowledgeable distributor and tell them your plans and go form there. If you just want boost and not high RPM it might be a different spring you need. I always tell people be honest if you tell them you are planning to build a 1000 horsepower truck they will give you springs for a 1000 horsepower. If you are only going for 500 horsepower they will give you a different spring
Can you benefit from just upgrading your springs? Maybe, if you put 80-90 pound springs in you will not notice a horsepower increase but it might make you engine run a little more efficient, And it wont hurt anything. You don't have to worry about cam wear till you get to very heavy springs.