Torsion springs question

Bear in mind, that the spring rate is proportional to the lenght (double the length = half as stiff) but the rate increases with the 3rd power of the diameter (double the diameter = 2*2*2 = 8-fold stiffness).
Nick

No you cannot. The springs are quite carefully balanced to the door weight and height. They get wound up many turns to provide the correct torque, and they provide a more or less constant torque as the door rises and the spring unwinds. Even a small change in the spring will mess things up.
-- Dennis

When spring preload is critical to 1/4 turn, changing size can result in unwanted action. The spring may only hold the door after you lift it or it might just snatch it open the instant you unlatch it.
If your current springs have a short life perhaps they are not sized properly.
--Andy Asberry recommends NewsGuy--

I'm not a "spring expert", but if you can't get the exact spring you need you have to get as close as you can - if you go only slightly oversized and approach the torque setting slowly it should be fine. A radically oversized spring probably will not fit the shaft hardware anyway.
If you go way too big on the springs on the 'More Is Better" plan, the door won't work right. The spring force has to be high enough to balance with the door fully down, but taper off at the proper rate as the weight of the door segments gets transferred to the horizontal track. With the door fully up, the tension needs to be just enough to hold position, or now your big trouble will be in getting the door closed again.
And way oversized springs could be adjusted fine for when the door is closed, but you'll 'run out of spring' and let the tension cables go slack and snarl approaching full open - that would be bad.
WARNING! Those springs have some serious tension on them and can cause Serious Injury or DEATH if you screw up on the tensioning or tension release procedures. And it's a stupid way to go.
Read the instructions - strike that. Read and FOLLOW the instructions, and have the proper tools on hand. You need a few adjustable safety props to support the door and take the torque off the spring shaft while pulling on the tensioning dog, don't skimp. And if you invent and build your own tools like the tension-wheel dogging bar instead of buying them, Make Sure You Make Them Strong Enough.
If you have a lot of tension on the tool and the little pin that engages the dog wheel shears off, at best it's skid-marks time - at worst it's "Call 911!" time...
-->--

Yep, that is exactly how it works. While your door's weight and lift drum diameter fixes the proper "torque rate" (inch-pounds of torque per turn of winding), there are an infinite number of springs that will produce a given torque rate. The garage door industry deliberately chooses very light springs that have a limited cycle life.
You can calculate torque rates and cycle lifetimes yourself. See the "dangerous bend" section for formulas at the end of my page:

Yes you can. Your "carefully balanced" and "constant torque" notions are mistaken.

When I broke one spring and found no ready replacement I drew-up the springs and had two made by the door service company in my area. It seems the proper spring will just barely unload when full up which makes it easy to work with the cables on the drums (as mine is equipped with). If there was much leftover torque at this point I think I'd have a problem. I also found after a short while I had to retorque after the springs had "worn in" as they had and everything worked out fine. Good luck.
dennis in nca

Talk to a *good* garage door supplier, one that does both residential and commercial. The usual springs are aimed at what folks want to pay: they're good for a few thousand cycles. Better ones are available, they just cost more. They can often tell you exactly what springs to use and how many turns each spring should be torqued just from the make and model of your door.

Longer-lived springs cost more because they're simply heavier; there's no "better" or "worse". They're all made from the same A229 alloy oil- tempered steel wire.
The cycle lifetime is simply a function of the bending stress. A heavier spring of equal torque rate undergoes less stress during the cycle than a lighter spring, and thus fatigues less and lives longer.

Think about going back in time. If you have the space, rather than fiddling with another torsion spring, put in the old-fashioned system of cables to counterweights (and use the wonderful trick of chain as a self-adjusting counterweight to compensate as the door is opened/closed). No springs to break. Once adjusted correctly, so long as nothing chafes, very little at all that's likely to break or go out of adjustment.