If your car has an automatic trans, chances are good that you're giving up some quarter-mile performance through that stock torque converter. Most factory-spec converters are designed to stall at a fairly low rpm, usually between 1,500 and 1,800 rpm. This is done to maximize the efficiency by reducing the slippage for a street-driven engine. But the negative effect is that the engine must begin to accelerate the vehicle at well below its torque peak. Aftermarket torque converters are tuned to offer higher-rpm stall speeds, starting just above stock and continuing right on up the tach. This allows the engine to reach its power peak before the drivetrain is loaded and makes for more dramatic launching, which should hopefully translate into quicker 60-foot times, and therefore, quicker e.t.'s.
Stock converters for V-8-powered cars usually measure around 12 inches in diameter, and the mildest aftermarket converters are often based on the same units. However, the more competition-oriented converters are generally smaller, measuring in at 9 or 10 inches. The smaller diameter means less surface area on the internal vanes of the converter, which in turn means that more rpm is required to "lock up" the unit. The added bonus is less rotating mass, which allows the engine to accelerate more quickly. A full-race 9-inch converter with a stall rpm of around 4,500 can launch a properly setup drag car as violently as a side-stepped clutch in a manual trans car, but the converter actually offers an additional benefit of a more cushioned "hit," meaning that the drivetrain is coupled a little less harshly, and that can enhance traction at the drive wheels.
Of course, we once again have to caution against reaching for the full-race stuff for the street. A small-diameter, high-stall converter is always very inefficient at low rpm, which means that when you're cruising down the street, or even on the highway, your converter is slipping significantly. This requires higher engine rpm to maintain cruise speed along with building lots of additional heat in the transmission. Since race converters aren't intended for prolonged use, particularly if it involves low-rpm, the manufacturers aren't typically concerned with this heat buildup, but it will kill your trans in short order if not properly managed on the street. Keep in mind also that the 60-foot capability of the 300hp engine in the car you drive to work, or even the car you occasionally take out from cruising, shouldn't be your primary concern. Street cars can benefit at the strip from converters that stall around 2,200-2,800 rpm without creating too much negative side effect, but an external trans cooler should be considered mandatory.
An added benefit of many late-model vehicles is a lock-up torque converter, which can provide direct drive, much like a manual transmission's clutch, when activated. Transmissions that use this feature, like the GM TH700-R4 and TH200-4R, can be fitted with aftermarket performance lock-up converters that will provide higher stall while maintaining the lock-up feature to enhance the strip performance of street cars without killing fuel economy.
Conversations regarding improved braking used to be an afterthought when discussing classic musclecars. Fortunately, the popularity of Pro Touring-style cars has brought brake upgrades to the forefront for many car crafters. What's more, these upgrades are no longer limited to assembling factory-style front discs on '60s-era cars. Instead, those same cars can often be retrofitted with modern high-performance brakes at all corners. In fact, even late-model vehicles can often be treated to upgraded braking thanks to a hearty aftermarket for stopping hardware.