Will lighter springs or heavier springs improve low end?

Depends what your overall timing map is and what your engine combination likes based on your weight, gearing, and other factors. Lighter springs will bring your centrifugal in sooner, but you want to avoid detonation; lighter springs may not "improve" your low end at all.

What's your initial (base) timing?
How much centrifugal is your distributor providing?
When (rpm) does your centrifugal start?
When (rpm) is your centrifugal "all in"?
What's your "total timing" (initial plus centrifugal)?
How much vacuum advance, at what vacuum level is it fully-deployed, and is it connected to a full manifold vacuum or to a "ported" vacuum source?
Does it ping (detonate) now? Under what conditions?

:beers:

Funny thing is, I've never felt a difference with any of the springs regardless of the stiffness. What I did notice was a better idle out of the stiffer ones. You want the curve to start just above your idle speed. You dont want to idle "on the curve" or you will get an erractic idle. The consensus is to have your curve fully in by 3000. I've had them come in at 4500-5000 and can't feel any difference what so ever.

I started out with light springs, but the engine would want to stall when first started, doesn't do that since I changed one of the springs to a heavier one. Engine is a 396 punched out to 414 ci, Holley 4346 780 cfm vac. sec. carb., msd 8360 dis. with 38 degrees total advance, M20 and 4.10 gears. Car hooks well, doesn't bog but I wanted to make sure I wasn't missing something, basically just looking for opinions and other peoples experiences. Thanks.

Dan E.
69 SS396 4 speed 4.10 posi. X66 coupe