Text and Photography by Chirag Asaravala

Let's face it, if you want quarter mile performance straight off the dealers lot an automatic is not a good option. Automatic transmissions, in stock form, give up a significant amount of power compared to their manual counterparts -the hydraulic coupling is less efficient, the gear ratios aren't as low or as abundant and, to add insult to injury, they weigh more. So does this mean if you've got an automatic equipped car you're destined to run several tenths slower than a comparable stick car? Fortunately the answer is no. A few modifications can get your slushbox running right along side a four or five speed, with all of the benefits that an automatic has to offer. In fact when we bought our AOD equipped '88 Mustang, the first mod we had planned was to swap in a T5. But the AOD grows on you quickly -not having to use your left foot or right hand in heavy traffic, being able to eat a burger and adjust the radio simultaneously, never missing a shift...hey automatics definitely have their advantages. So we decided to hold off on the five speed, and see what we could achieve with the AOD. The results have been surprising. A set of 3.55 gears and a TransGo shift kit has put the stock 5.0 car into the 13.90's on radials. Heck, that is better than many five speed Mustangs. We started to wonder what a higher stall torque converter would get us. Higher than stock stall converters are the key to achieving maximum performance out of an automatic car. Many people make the mistake of modifying the engine with better induction components without changing the stall converter to match the power band. This usually leads to disappointment in the vehicles performance, and eventually the automatic is ditched for a manual.

Torque Converters Explained
Torque converters are somewhat of a mystery, and their function can be difficult to understand. We'll try to make some sense of their function here without getting into the physics of fluid dynamics.

Simply stated, a torque converter functions to transfer and multiply engine torque to the transmission. This is accomplished via the precise movement of hydraulic fluid, under centrifugal force, within the converter. To understand this concept, think of two fans facing each other. If one fan is on, and the other is off, the air flow from the running fan will turn the blades of the other fan. A torque converter works the same way, but uses fluid rather than air, and several other components to actually multiply the energy and create a mechanical advantage.

A torque converter consists of four major components, a rear cover (impeller), a turbine, a stator, and a front cover. The rear cover is typically the image that comes to mind when you think of a torque converter -the dimpled side, that slips into the transmission. The dimples are actually impressions of the impeller vanes. The front cover is the end which connects to the flywheel, it is welded to the rear cover to form a leak free housing. Inside the converter sits the turbine, a vaned element facing opposite the impeller, like the two fans facing each other. The stator sits in the center of the turbine and also has vanes with specific pitch angles. The turbine and the stator are the only components that connect to the transmission input shaft.

As the engine spins it drives the impeller, which forces fluid out the impeller vanes and against the vanes of the turbine, causing it to rotate and drive the transmission input shaft. The stator is the middle element and is held stationary (hence the name) against fluid flow by a one-way clutch bearing, and redirects fluid flow from the turbine to boost impeller action and thus multiply engine torque.

When the fluid flow within the converter approaches maximum fluid flow (known as Vortex flow) the stator begins to freewheel in the same direction and same rate as the turbine and impeller. At this point the torque multiplication stops and the converter essentially becomes a fluid coupling.

What is stall speed?
Stall speed is the point when a converter has reaches it's maximum fluid flow and torque multiplication has peaked.

Stall speed is determined by the number, shape and angle of the vanes in the impeller, turbine, and stator. Other factors, such as the physical size of the unit, clearances between the impeller and turbine, and strength of the vanes also determine the stall speed. Stock converters have a multiplication ratio of around 2:1, while a higher stall converter (due to the changes mentioned above) could be as high as 2.8:1.

The point of selecting a higher stall converter is to find one which stalls close to the peak power range of the engine. If an engine makes peak torque between 3000-3500 rpm, a converter which stalls at 3000 would give the car a much quicker acceleration and launch, compared to the same engine and a stock converter which started the car off at 1800 rpm. Therefore the stall speed of a converter is not only determined by its design, but also by the power band of the engine and camshaft.

Another way to look at this is to think of a manual transmission and clutch. At the race track you could rev the car to 3500 rpm and dump the clutch, and assuming the clutch did not slip the engine and driveline would couple and the car would launch hard. A torque converter offers the same advantage, but it is constant and not as hard on the driveline -how often can you dump the clutch on the street?

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