If it had more stall speed it would run quicker. That is perhaps one of the most frequently made statements during bench racing sessions. Torque converters and stall speed largely remain a mystery to many hot rodders because so many variables can affect a converters performance.
But matching the right converter to your vehicle and engine combination isnt that tricky once youre armed with the right knowledge. To gain that information we recently spent an afternoon at B&Ms new torque converter dyno to learn more about what affects stall speed, and what to consider when selecting the proper converter for your vehicle.
What Is a Torque Converter?
A torque converter is the mechanism that couples the engines rotational force to an automatic transmission. The clutch performs this function on a manual transmission. Instead of transmitting the power via direct contact friction as a clutch does, the torque converter transmits power from the engine to the transmission by the force of oil flow. This offers some benefitsor drawbacks when using an improper torque converter.
Before choosing a torque converter, remember to be realistic when evaluating your engines power. Many hot rodders will think that their engine is making much more torque than it is. Many high-performance engines lack low-speed torque because of long-duration cam timing, which means a torque converter with higher stall speed is needed for optimal acceleration.
High-stall converters allow the motor to launch the car at a higher rpm where more torque is available. But remember that too much stall speed can be detrimental to performance. Because stall speed is slippage, excessive stall amounts can equal slower e.t.s. and add unnecessary transmission heat >> and fuel consumption. The key to choosing the right converter is like all other performance choices. Take into consideration your particular vehicle and engine combination and what you intend to use the car for before choosing a converter.
For dyno-testing, B&M installs a converter into a special housing and the test data is electronically relayed to a PC. The dyno collects data for torque multiplication and slippage while also monitoring variables such as fluid temperature and hydraulic line pressure.
In a car, true stall is the maximum engine rpm attained with the transmission in gear, the brakes locked, and the throttle opened to the highest rpm possible. But achieving this ultimate stall speed in the vehicle is often difficult because frequently the brakes are not able to hold the power transmitted through First gear and the axle ratio to the drive wheels. Transmissions equipped with a transbrake can reach a converters true stall because of the ability to lock the transmission up internally by engaging two gears simultaneously.
Flash stall is the maximum amount of stall the engine obtains from a full-throttle launch at a complete stop with no brakes applied and is higher in rpm than the stall speed created with the brakes applied.
A Formula for Success
Acceleration is all about torque, and by definition, a torque converter multiplies engine torque. To accomplish this, the torque converter relies on a component called a stator, which increases the output torque by redirecting the oil flow into the impellers inlet fins in the same rotational direction as the engine. The stator also incorporates a one-way clutch assembly to allow this process to occur when the converter is at stall, and to overrun when the converter is 90 percent coupled hydraulically.
After measuring the torque output on the dyno, we can assign a value or ratio to the torque multiplication by using the equation: output torque / input torque = torque ratio. B&Ms dyno measured 537 lb-ft of output torque, and we divide it by the input torque of 230 to find the torque ratio of 2.33 (537/230=2.33). This is the multiplication ratio produced by the torque converter. Torque ratios between 2 and 2.5 are common for most converters. Torque ratio is like having an additional lower gear in your transmission.
Monitoring the Data
To thoroughly test a converter, B&M monitors input oil pressure, input oil temperature, input torque, output oil pressure, output oil temperature, and fluid flow in gallons per minute (gpm). During stall-speed testing we watched the transmission fluid temperature quickly climb past 300 degrees. This demonstrates why a transmission cooler is imperative with a high-stall converter, and how repeated stall-speed testing can raise fluid temperatures and be damaging to any torque converter. The screen also displays input torque, output torque, input temperature, output temperature, input rpm, output rpm absorber loading percent, reaction torque, and fluid flow in gpm. Data logging is handled by the system PC and will print copies on demand.
Its All About Torque
Many engine modifications, particularly camshafts with long duration, tend to reduce low-end power and torque. For example, if a 350ci engine makes 230 lb-ft of torque at 2,500 rpm, a 2,000-stall converter will produce a 2,000 stall-speed. If you install a 268-degree (advertised duration) cam into the motor you will lower the torque at stall-speed. With less torque, the 2,000-stall converter may only produce 1,800 stall or less. Remember that when the torque curve moves up the rpm scale, you will need more stall speed in the converter to allow the vehicle to accelerate at low rpm (below 3,000 rpm).
The Numbers Game
Stall-speed numbers are listed primarily to rate a converters performance level. Typically, the number assigned (2,000, 2,500, 3,000) will let you know the ballpark stall-speed rpm to expect from the converter. If a 2,500-stall converter is selected, matched with the proper components you should be able to foot brake stall the converter up to about 2,500 rpm. This should provide anywhere from 500 to 1,000 more rpm at the launch compared to a stock converter. Lighter cars weighing 2,800 pounds and under will typically stall a little lower than the above-mentioned numbers, therefore you should choose a converter with a higher stall number for optimum performance.
Also, when choosing a converter consider the First gear ratio, final drive ratio, tire diameter, vehicle weight, vehicle brake system, and engine size. These all play a role in torque converter selection.