PM temp confusion...All Grainers help!

I've done about 8 partial mash batches and I've mashed at 155 for 30 minutes in 2gal then sparged twice at 170 with 3qt each. I've recently read lower temps bring more Sugar conversion. This being said, how come most recipes I see have the mash at 154-158. Does higher break down all the starches too far? How low is too low for a complete conversion? I guess while I'm here, I should ask how my volumes look. I'm not one to try to hit gravities without topping off at the end, but I want everything I can get from the grains. Any simple explanations and tips that can help would be greatly appreciated.

Mash temperature impacts how fermentable your wort is - higher mash temperatures yield more unfermentable sugars, lower mash temperatures will leave you with a wort that has more fermentable sugars.

What that means for your beer is that a higher mash temperature will leave you with a beer that has more sweetness, fuller mouthfeel. A lower mash temperature will leave you with a beer that has a drier finish and a crisper mouthfeel.

About the lowest I have ever mashed is 148 and the highest is 158. Most beers, I mash around 154.

I don't understand your question about volumes, maybe you could say a little more about that.

It's not that the higher temperature is getting you less conversion. It changes the type of sugar being created. At higher temperatures a different enzyme is more active that creates longer chain sugars that are less fermentable by the yeast. So at a higher temperature you will generally leave more sugar and body in the finished beer. This is desireable in many styles like porters, stouts, and beers that should have a big malty backbone.

For lighter style beers you mash lower to create more sugars that are fermentable and create a drier beer. The generally accepted happy medium for an average beer is in the 152-154 range, and then brewers move up or down from that number based on how much body and residual sweetness they want in their beer. John Palmer explains this in a bit more depth in his book "How to Brew" which can be found free on line.

Great, I get it now, thanks. One more fun thing to play with. As for volumes, I didn't know if the effectiveness/efficiency changes with your mash volume or your sparge volume. I should also note my pm grains are usually 2-3 lb and my sparge is basically a steep for a minute or so then I let it drain, and repeat once more with new water.