Unless you have a lot of experience with torque converters, it is nearly impossible to choose the correct one for any given application and predict with certainty how it will work in the car. When looking at all the options available, how can you narrow all the choices down to the right one for your ride? Before we can get into choosing a torque converter, you should at least understand how they work so let us start with torque converter basics.
The Torque Converter Basics
When using an automatic transmission, it is the torque converter’s job to transmit and multiply the engine torque and deliver it to the transmission. If you cut open a race torque converter, inside you will find: the turbine, stator with sprag, and impeller pump. The converter’s turbine is attached to the drivetrain via the input shaft of the transmission. When the turbine moves, the car moves.
The stator is the fluid director and changes fluid flow between the turbine and pump. It is what makes a torque converter a torque multiplier, and not strictly a fluid coupler. Remove the stator, and a torque converter will retain none of its torque multiplying capabilities. For the stator to properly function, the sprag (a mechanical, one-way clutch mounted on bearing races) must hold the stator in place while the converter is in stall mode (at engine idle) and then allow the stator to spin with the rest of the converter after the turbine speed approaches the pump speed (as engine RPM increase).
The impeller pump is the outside half of the converter, on the transmission side of the weld line. Inside the impeller pump are a series of longitudinal fins that push the fluid around its outside diameter, and into the turbine. This component is welded to the cover, which is then bolted to the flywheel, and spins with engine RPM. The size of the torque converter, impeller pump, and the number and shape of the fins, all affect how the converter works.
Torque Converter Basics on Stall Speed
When someone talks about stall speed, they are talking about how many RPM the engine can attain with the transmission in gear and the car’s brakes applied — before the drive wheel(s) spin. When discussing street-use torque converters, stall speed is typically listed as a range (example: 2,500 to 3,500 rpm). The range listed will tell you what you can expect from the converter. If a 2,500 to 3,500 stall speed is selected, you should be able to hold the brakes (footbrake), and the converter should allow the engine to rev to around 2,700 to 3,200 rpm. Of course, this is dependent on the vehicle’s setup.
There are many factors that determine what RPM the converter will flash stall after it is installed in your car. When discussing converters that are race car specific, they are not built with such a wide range of stall speeds.
A torque converter will never be able to footbrake stall up to where it is actually flash stalling.
Torque Converter Size
To an extent, the overall diameter and the internal construction of a converter can cause a substantial change in footbrake stall. Typically, the 8-inch converter (depending on internal fin angle and the style of stator), will most likely flash stall at a much higher RPM than the 10-inch competition unit.
Tire Diameter
The tires on a vehicle play a large part in footbrake-stall capabilities as well. For instance, a car with a set of wider tires at every corner will hold the car better because of the large contact patch of the tires holding the vehicle. This will allow you to get higher in the RPM range than with the same or equivalent vehicle equipped with a set of big and little tires.
Vehicle Weight
The vehicle’s weight affects footbrake stall, by giving the converter more load to move. The heavier the vehicle, the higher it will push the stall of the converter. Of course, the opposite is true of lighter-weight vehicles. Since they are lighter, they are easier to move and cause the converter to react with less stall, because it does not have to work as hard to move the vehicle. Finding how much your car weighs is as easy as driving to a local truck stop.
Rear End Gear Ratio
This one is straightforward, the lower numerical-value gears, such as a 2.73 or 3.08 gear, the higher the converter will stall because it is taking more force to move the vehicle.
For instance, a heavy car with numerically low (highway) gears is more resistant to forward motion, than a lighter car with numerically higher gears. When used in the heavier car, the converter will stall at a higher RPM than it will in the lighter car. The easier the engine can accelerate the vehicle, the lower the converter will need to stall to get the car moving. This is as having more mechanical advantage, which makes it easier to move the weight of the vehicle. Therefore, the converter must do less work. The opposite is true as well.
Of course, the power and torque curves of your engine will have a huge effect on stall speed. Typically, engines that produce more low-end torque can increase the stall speed of a torque converter to a higher RPM. Conversely, the same converter will stall at a lower rpm behind an engine developing less torque. If you buy a converter that is rated at 2,000 to 2,500-rpm stall, that rating is meant to span a variety of engines with different power curves.
When discussing the stall of a converter, you need to be able to distinguish between flash stall and footbrake stall. A torque converter’s flash stall is typically the most accurate rating, as a footbrake stall is dependent upon too many variables. (i.e., type of brakes — disc or drum — how well adjusted the brakes are, reared gear ratios, etc.).
Torque Converter Choice Results
How do you know which converter is right for your application? Since we know that no torque converter will react the same way when installed in different situations, does that mean that it is almost impossible to buy an off-the-shelf converter? Not necessarily, the best advice we can give you is to contact the manufacturer and get their suggestions. As knowledgeable as your friend might be, we still recommend getting the advice of professionals. The number one suggestion I can recommend to people in the market for a stall converter, is to do some research on their setup. The more knowledgeable a person is about their specific application, and the more information that they can provide to us about their specific setup, the better we can get them dialed in with the perfect stall for their vehicle.
When relaying the required information to the manufacturer, there are several things they will need to know.
Information You Should Know Before You Call About A Torque Converter
- Vehicle weight
- Rear end gear ratio
- Tire size (diameter)
- Transmission and its low gear
- Trans brake (Y/N)
- Engine size (horsepower/torque if known)
- Are you using a power adder?
- Exhaust
- Camshaft duration at .050-inch
- Desired use of the car
Torque Converter Application Example
When choosing the stall speed that is right for your application, a rule of thumb is that the advertised stall speed will need to be at least 500 rpm higher than the beginning of the camshaft’s powerband. All aftermarket camshafts are delivered with a recommended RPM operating range. If your camshaft has an operating range of 1,500 through 6,500, you will select a torque with a minimum of 2,000 rpm stall. If choosing a torque converter for use in a streetcar, you might want to select a torque converter with a stall speed that is below the engine RPM at 70 mph, since this is where your engine will spend a lot of its time. An exception to this theory is if the transmission/converter is equipped with a lock-up feature.
For example, let us look at a 1969 Camaro. Let us say it weighs in at around 3,200 pounds and is equipped with a 350 cubic-inch engine that has a 9.0:1 compression ratio, and is running a relatively-mild street camshaft like a Comp Cams 260H, and has a set 3.50 rear-end gears. With a stock-type fuel pump and a Turbo 350 transmission, a good converter here would be a 2,000 to 2,200 stall converter. Let us use that same Camaro but swap the 350 cubic-inch engine for a 383 cubic-inch small-block, with a 10.5:1 compression ratio, a Comp Cams 292 camshaft, a 4.11 gear ratio, a high-performance fuel pump, and the same Turbo 350 transmission. It would use a 3,200 to 3,500 stall converter to get the best performance.
Finally
When it comes to choosing a torque converter, one size certainly does not fit all as you can see from the torque converter basics. All the major torque converter companies employ skilled personnel that have a wealth of information available. But that information is useless if you do not provide accurate information about your car and its components. If you work with the manufacturer, you should be able to get a torque converter that is the perfect fit for your car. You will not regret the time spent, and the cost of the telephone call(s) is minimal when you consider how poorly your car will run with the wrong torque converter.
