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Street Race Truck More... The Toolbox

Choosing a Torque Converter

How do torque converters work?

Manual transmissions use a clutch to connect and disconnect the engine power to the transmission, but in an automatic transmission, it’s the job of the torque converter to transmit and multiply the engine power to the ground. If we were to dissect a converter, inside it contains three main components - the impeller, stator, and turbine.

The impeller or impeller pump is located on the transmission side and rotates at engine speed. As rpm increases, fluid flow increases inside the converter. That power is transferred using vanes and fins that direct the flow towards the turbine, which is bolted to the flexplate. In between the impeller and turbine is the stator. It functions as a sort of one way clutch that redirects the fluid and multiplies the torque output. When the fluid flow is powerful enough, it begins to spin the turbine, which is also splined onto the input shaft of the transmission. The characteristics of each converter are controlled by the diameter, the number and shape of fins, and stator design.

What is stall speed?

Stall speed is the amount of rpm the engine can reach with the brakes locked and transmission in gear before the drive wheels turn. The stall speed assigned is simply a way to rate the converter’s performance level. The range listed will tell you what to expect from the converter. If a 2700-3000 is selected, you should be able to footbrake stall the converter to about 2700 rpm depending on the vehicle’s setup. Compared to a stock converter, this should provide anywhere from 500 to 1500 more rpm to launch the vehicle from a dead stop, giving you a noticable improvement in the vehicle’s acceleration capability.

How to choose the correct torque converter

Selecting the correct converter and stall speed for your particular vehicle is critical – not only on the street, but also on the race track. The right choice can make the difference between winning and losing; between a car that performs to its potential or falls flat on performance. Choosing the best torque converter for your application can be tricky, but there are some basic guidelines you can follow to help out in the process.

Your power curve and torque output are two of the main things to consider. If we look at a stall speed rated at 2,200 – 2,700-rpm, the range is meant to cover a variety of engine characteristics with different power curves. Ideally you want the stall to lock up somewhere close to the peak torque of the engine. Typically engines that produce more low-end torque will increase the stall speed. The same converter will stall at a lower rpm behind a less torquey - higher rpm engine.

Also consider the resistance you will have against the engine. A heavy car with large diameter tires will create more to fight against. Generally speaking, the more resistance you have, the higher the stall rpm. The same principles apply to gear ratios. A heavy car with taller gears (low numerical value) will have a higher stall rpm compared to a light car with shorter gears and smaller tires.

The details will make all the difference, so to wrap up, the specifics you'll want to consider are: your vehicle weight; engine displacement; compression ratio; cam profile; rearend gear ratio; tire diameter; the type of fuel system, and the transmission model and year.

For example, let’s look at a 1932 Ford street rod. Say it weighs in at around 2800lbs with a 350 cubic-inch engine at 9:1 compression, running a relatively mild street cam like a Comp 260H and 3.50:1 rearend gears. With a stock-type fuel pump and a 1970’s TH350 transmission, a great match here would be our 2000-2200 stall converter like PN 109-602.

Or a 1955 Chevy 2 door hardtop which weighs about 3,350 lbs. It’s running a 383 engine with 10.5:1 compression, a Comp 292 cam (rough idling hot rod style cam), 4.11 gear ratio, high performance fuel pump, and a 1970’s style TH350 transmission. It would likely use a 3200-3500 stall converter, like PN109-603, to get the best performance.

What to expect from a performance torque converter

All of the performance torque converters offered by Speedway Motors are engineered to provide improved strength and durability. Designed with heavy-duty heat treated internals and high strength bearings, you can expect longer life along with a substantial increase in performance over a factory torque converter.

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