Selecting the right torque converter is somewhat of a dark art-in fact, the typical hot rodder is more likely to get it wrong on the first try than right. Finding the right torque converter is like trying to hit a moving target; big variables like vehicle weight, the engine's power curve, and final gearing all have to be considered. Making things more difficult is the convention of identifying converters by their stall speed, a patently impossibly task, given that the stall speed is far more a function of the car's weight, power, and gearing than the converter's design. Identifying converters by their physical size (diameter) is a little easier, but not by much. In the end, there is no reliable formula for selecting a converter; it's largely a matter of finding the right manufacturer, and developing a rapport with them.
If you're getting across your message about how you plan to use the car, the vehicle's mass, and especially the power and torque curves of the engine, you'll end up in a good place. If you guess about things, or you're too optimistic, you may need to revisit your converter on more than one occasion. And even if you do everything right, you can still get it wrong. It sounds like a big pain, and it is, but if you stick it out and make the right adjustments, it will pay off with a big grin on your face.
Our project cars serve as real-world test beds, and our '75 Chevy Laguna has proven to be a bit elusive on the power front. Back in the January issue, we took the Laguna to Westech Performance Group in Mira Loma, California, to do a standard chassis dyno tuning loop. At that time, we were able to eke out 365.6 hp at the rear wheels-a far cry from the 560 flywheel horsepower (5,800 rpm) of our initial engine test. To recap, our 408ci small-block Chevy sits at 10.5:1 compression for using 91-octane pump gas, and breathes through AFR Comp Eliminator 210 heads with a mild .600-lift solid-roller cam from COMP. A Holley 750 HP carb sits on top of an Edelbrock Performer RPM Air-Gap intake, and it's all fired by a Performance Distributors HEI ignition. We posited that our torque converter-from Phoenix Transmission Products-might be on the loose side for our needs, so we vowed to return with something more appropriate for a follow-on dyno test.
We then called Greg Ducato, owner of Phoenix Transmission Products in Weatherford, Texas, to discuss our dyno results. And while there was nothing wrong with our previous converter (other than its stall speed), Ducato agreed to swap our 245mm shell converter for something tighter. Phoenix had also built our 700-R4 overdrive, and we were extremely happy with its build quality. We saw no reason to work with another manufacturer, and having supplied the initial torque converter, we felt Phoenix was in the perfect position to nail the right converter for us. Ducato suggested that we try a new, larger-diameter lockup converter he had just developed, one with a 258mm shell design, but that otherwise had the same bulletproof construction of our first converter, with a fully furnace-brazed bowl and turbine, hardened splines, hardened drive hub, and anti-ballooning technology. Phoenix carries this larger converter as PN PTGM18/258HS ($589.92) which fits all 700-R4 and '93-98 4L60Es without removable bellhousing. It also has a fullsized 298mm heavy-duty clutch assembly and damper for solid, reliable lockup durability-just what we needed for our heavy highway hauler.
Both of these 7004R converters...
Both of these 7004R converters could be rated at a 3,000-rpm stall speed. The difference is that the small 9.5-inch unit (245mm shell) would go into a very light car with an engine that produces peak torque high up in the rpm range. The larger 10.15-inch unit (258mm shell) would go into a 3,800-pound car with a healthy big-block, making lots of torque way down low. By choosing an application-specific converter, you are assured that it will deliver the performance and driveability you expect. Both of these converters were used in our '75 Laguna project, illustrating the difficulty of empirically rating a converter by its stall speed alone.
In this picture, we see two...
In this picture, we see two of the same stator, but the one on the right has been machined to provide greater clearance between it and the turbine cover. It also redirects the fluid flow. Both of these attributes will create a greater stall speed than on the stator at left, but will also cause a loss of efficiency to some degree, especially in a vehicle. Phoenix Transmission Products can tailor the stall speed without sacrificing efficiency in this key area.
Fin Angle & Stall Speed
Fin Angle & Stall Speed
The converter on the left has a fin angle that is what is referred to as "negative," meaning negative as viewed from center. The converter to the right has a bowl that has a "positive" fin angle. The positive fin angle produces a lower stall speed. Converter fins are measured in degrees of fin angle for proper identification. Some are stamped with a number from the factory, and these can be used as identification of a bowl design or fin angle. The more negative a fin angle is, the less efficient a converter will be, especially at lower rpm. While it is possible to solely use fin angle to tune the stall speed, it's preferable to get things into the ballpark by selecting the proper converter shell diameter first, then fine-tune stall speed through fin angle, as Phoenix does.