I see this question asked often, so I took this from one of our site sponsors website:
http://www.fuddleracing.com/WhatConvertersDo.html
1. Higher RPM Launches:
Largely similar to "dumping" a clutch at a higher RPM, a higher stall converter allows your car to leave the line with more power hitting the wheels as you launch.
Here are a couple of examples of how the launches change when you step up to a performance converter. Variables such as different brakes, tires, and gear ratios effect how the converter stalls, but these will give you a general idea.
Example 1- Assume your stock stall is around 1600 RPM. Get into the car and hold your foot on the brake. Now at the same time you let off of the brake, stab the throttle like you are at the track. Keep a close eye on the tach. It will jump to about 1600 RPM and start climbing from there. If you moved to a 3200 stall, it would jump to about 3200 and start climbing from there. It gives you a huge punch by launching at a higher RPM and at an RPM that is more in the meat of your powerband.
Example 2- Again, assume a stock stall of 1600 RPM. Now, in drive, hold your left foot on the brake as hard as you can. Now slowly rev the engine with your other foot. Notice that about 1200-1300 RPM the tires start to break loose. Now imagine that you have a 3200 stall. You could now spin the motor closer to 3k before the tires started to brake loose.
2. Torque Multiplication:
When an automatic transmission is launched, the torque converter actually multiplies the input torque. That multiplication is called the Stall Torque Ratio or STR. If you raise the STR, the amount of torque you have on launch also becomes greater. With a sticky tire, launches with a high STR can be down right brutal. STR
3. Shift Extension:
Most automatic cars experience a dead spot immediately after the 1-2 and 2-3 shifts. It will feel really strong all through first gear (with LS1's particularly at the top RPM because that is where LS1's like to be, high RPM's). Notice immediately after the 1-2 shift (and 2-3, if you get that high) that it seems to fall flat on its face for a couple of seconds. That is the dead spot and can feel like more than a couple seconds at the track. It feels weak until the RPM's get to where they should be, and the power feels like it comes back. With a stall converter, the RPM's do not fall like they do with a stock converter. Expect the RPM's to stay considerably higher after the shifts with a stall converter, completely getting rid of the dead zone.
4. Reduced Weight: (Does not apply to stock style converters)
A stock converter on an LS1 Camaro weighs 53 lbs. Our basic 245mm conversion weighs 30 lbs. While 23 lbs. of weight reduction does not sound like much, it is entirely rotational weight. The engine is now required to spin 23 less lbs. It now revs faster and considerably more freely.
Example- Take a short pole with a 53 lb. weight at its end and one with a 30 lb. weight. Try turning the one with the 53 lbs. and turn it by hand. Now try the same thing with the other pole. The lighter pole is considerably easier to spin. The same is true for the engine. The lighter one is easier not only to spin faster but to start spinning.
