It’s a great time to be into cars. The bar has been raised in every aspect of automotive enjoyment. Horsepower is up, emissions are down, and drivability, the one thing that typically took a beating when those first two were the focus, is at an all-time high.
Diehards can now enjoy some pretty stout gear-bangin’, and still find fuel mileage at the end of the shift pattern. For those who enjoy auto-shifting, they can keep up with the best of them in speed, power, and yes, even fuel flow. The trick is, making sure that you get the right torque converter to make it all come together. We contacted the folks at TCI Automotive, Neal Chance Racing Converters and B&M Racing to get the scoop on how enthusiasts can make use of all that power and drivability from today’s technology.
During a recent discussion about torque converters, we mentioned how today’s horsepower levels have soared, but so have enthusiasts’ expectations about driving their cars. While just about everyone would enjoy a ride akin to the “Two-Lane Blacktop” ’55, many would admit that such a trip might get a little old somewhere across the flat plains on their way to the Atlantic shore. And yes, we know that the transmission (M22 to be exact) was a manual, but choose the wrong torque converter, and you will definitely make concessions to drivability and reliability in your automatic-equipped ride as well.
You don’t want too much stall, especially in a street car. – Trent Goodwin, Comp Performance Group
Trent Goodwin of Comp Performance Group, the parent company of TCI, explained it this way, “Many of today’s big-power “street cars” are using what was once defined as a race-only converter. Meaning, the converter utilizes the same technology in steel stator designs, as well as billet front covers for the strength needed to withstand the application it is in.”
How do you build-in reliability and still be able to put some miles on the odometer? The answer is lock-up. In an overdrive-style transmission that use a lock-up converter, there is a clutch inside the converter that when engaged, creates a near direct drive effect. The slippage then becomes almost non-existent, regardless of the stall speed. In much the same way that the OEs were looking for ways to meet fuel standards, lock-up also helps enthusiasts by giving them better drivability, increased fuel mileage, and reduces a transmission’s deadly heat issues due to excessive slippage from a higher-stall converter. You don’t want too much stall, especially in a street car. This means the converter will be slipping the whole time, which creates heat. That is the number one killer of transmissions.
That doesn’t make choosing the right converter amid the broad scope of offerings into one simple yes/no question. There are a few variables that must be known to get the right stall: engine size, camshaft specs, rearend gear and tire size are the biggest factors. Also, the torque of the engine will dictate how much stall a given converter will exhibit, as no converter is explicitly a 3,000 stall. The engine torque will take any rated stall to different levels i.e. a 300 lb/ft. engine will stall a rated 3,000 stall at about 2,800 rpm, whereas a 600 lb/ft. engine will make the same converter stall at approximately 3,500-3,600.
Four Steps To Long Converter Life:
1) Service the unit with quality fluids and filters!
2) Make sure you have an adequate cooling system for your transmission.
3) Get yourself a transmission gauge so you can accurately see what’s happening.
4) Provide the technician that you are working with as much information about your vehicle as possible when choosing a converter.
In terms of torque and its effect on converter choice, as mentioned above, when the torque curve of the engine is raised, you need more stall speed in the converter to allow the vehicle to accelerate at lower RPM.
Trent understands camshafts, and helps explain the connection with the converter, “The camshaft plays one of the most critical roles in converter choice, as it is one of, if not the biggest factor in the powerband of the given engine combination. You must have a torque converter with the right amount of stall to allow the engine to get into that powerband for optimum performance, even in a daily-driver street car. If you have a bigger than stock camshaft, you must get the right stall to allow the engine to also idle in gear, and make the car function as it should.” As Trent said earlier, too much stall can create heat, and you don’t want that either.
When To Stall
So what is this mysterious stall speed, and how do you know if you’ve got the right amount of it? If you already have a torque converter that doesn’t have the right amount of stall, you know it. Clunking when you put the car in gear, slipping when you accelerate on the highway, or fighting excessive heat after prolonged driving are all indicators of improper stall speed. But let’s get ahead of that and make the right choice the first time.
Chances are good that you already have a torque converter company in mind. Whether doing research in the pits or online, you may have an idea of what you want. But don’t start punching part numbers just yet. You see, one of the biggest mistakes that many enthusiasts make is assuming that they need exactly the same converter as their buddy, since their cars are similar. Nothing could be further from the truth.
Kehoe gives some of the best information to begin the process of purchasing a converter. “Most aftermarket companies have tech lines that you can call for advice. Use them. You can never have enough information when building a project. When possible, get the answers straight from the source and then make your own educated decisions.
Diameter is king in designing an efficient power transfer, but there is a great deal more to stall speed than just diameter. – Marty Chance, NCRC
Can choosing a converter be that specific to each vehicle? “There is no such thing as one size fits perfectly for all,” he said. Converter stall speed is not absolute, it is relative. Therefore, everything that effects the power curve effects the stall speed. The converter needs to be designed for the way it will be used and the personal preferences of the driver.
With the advent of 800-1,500 horsepower street drivers, has the broader spectrum of power blurred the line between race and street converters? According to Marty Chance, not really. The overdrive transmissions that use lock up converters primarily for street cars help separate the two, plus, there are several other issues that need considered as well. Street cars are heavier (usually 3,000-6,000 pounds), street converters need to be tuned for satisfactory part-throttle driving as well as full-throttle blasts, and the fuel-saving lock-up application of the converter has a limited capacity for street cars.
Kehoe adds, “When building any type of vehicle, remember that the drivetrain components need to complement each other. If you build a 500-horsepower engine with a radical cam, then you will need a higher stall converter to allow the vehicle to idle. If you have a high stall, do you have the cooling capability? Does the rearend have the proper gears to complement the engine and transmission combination? We all like race cars, but unfortunately race car parts don’t always function well for a daily driver. Think your build all the way through and it will save you time and money in the long run.”
Does Size Matter?
“I have more horsepower, I must need a smaller torque converter, right?” The definitive answer is – maybe. According to Marty, “Diameter is king in designing efficient power transfer, but there is a great deal more to stall speed than just diameter.” The torque converter is a hydraulic transfer of power to the rear wheels, unless using lock up, which then makes it a mechanical transfer. Converter diameter is most important when it is connecting engine and transmission via its hydraulic coupling characteristics. The vehicle’s mass, final drive ratio, and torque output of the engine will all dictate the appropriate diameter of torque converter needed. The more mass and torque that a vehicle has, the larger the diameter needed be for a proper torque converter.
So, does the converter tune the entire package? Marty explains that, “90-percent of converter design is for the power curve of the engine. The correct converter will have a rate of efficiency that matches power curve of the engine. The other 10-percent may be tailored to the vehicle.” Limitations such as tire size, suspension setup, power management, and even the surface where the vehicle is being used can all factor into that 10-percent window, according to Marty.
The advent of turbos, blowers, nitrous, and other performance-enhancing add-ons has been a boon for power junkies, but how it changes the torque converter horizon is dependent on the type of power adder. Each power-adder has a unique torque curve, and tailoring the converter to that torque curve is critical. If you ask Marty what information he needs to build you the right converter, he’ll reply, “All of it!” When designing a converter, you can’t have too much information.
Torque Converters Exploded
1) Cover – The outside half of the housing toward the engine. Attaches the converter to the flexplate and contains the fluid. While not actively involved in performance, the cover must remain rigid under stress.
2) Turbine – The turbine rides within the cover and is attached to the input shaft of the transmission. When the turbine moves, the car moves.
3) Stator – Described as the “brain” of the torque converter. By changing fluid flow between the turbine and pump, it makes a torque converter also a torque multiplier and not strictly a fluid coupler. With the stator removed, a converter will retain none of its torque multiplying effect.
4) Impeller Pump – The outside half of the converter on the transmission side. Inside is a series of longitudinal fins that drive fluid around the outside diameter and into the turbine. The size of the torque converter and the number and shape of the fins all affect the characteristics of the converter.
You can’t lug a high-revving, normally-aspirated engine with a tight converter, and a loose converter in a low-RPM, boost-building turbo car will slip excessively, losing performance. The perfect converter will allow the engine to accelerate the mass of the vehicle at the fastest rate possible, and that is dependent on the design. If you’re planning on upgrading your engine with a power-adder, let your converter supplier know ahead of time.
Kehoe adds, “If the correct converter has been selected for the vehicle, there really is no reason to be concerned. If you are running a race-style converter (3000 + stall) you will want to take into consideration the fact that more heat will be generated when cruising around town. This is because you will not be engaging the converter 100-percent at lower RPM. The fix for this is to run a really good transmission cooler in-line with your OE cooler in the radiator.”
Can lock-up be a “fix” for a torque convert that is too loose? Marty has a word for you, “No!” Replace the word stall with slippage, and you can see some of the down sides of having too much. Besides the heat issue, you will also be leaving some performance on the table. Relying on lock-up to keep your torque converter and transmission safe is a bad idea. As the torque converter locks up to keep all that unnecessary slippage and heat at bay, it now has to overcome even more slippage than usual, which taxes the lock-up clutch even more, and can significantly shorten the life of the converter.
The Bottom Line
With the broad spectrum of choices available when choosing a converter, there should be no problem finding that “just-right” torque multiplier for your application. Upon doing that, we’ll leave you with some knowledgeable words from Goodwin, “The main thing with any automatic transmission, regardless of the actual stall, is keeping an eye on temperature. Heat is the number-one killer of an automatic transmission. Keep temps in check by running a good cooling system for it and with regular fluid/filter changes. Your torque converter will thank you for it with many years of service.