1975 Chevrolet Laguna Torque Converter - Torqued Off!
Proper Torque Converter Selection Is The Key To Maximizing Performance On The Street And At The Track.
From the February, 2010 issue of Popular Hot Rodding
By Johnny Hunkins
Photography by Greg Ducato, Johnny Hunkins
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.
Brazed Vs. Non-Brazed Bowls...
Brazed Vs. Non-Brazed Bowls
On non-brazed bowls (left), fins are pressed in and bent over and can work loose over time, making bad stuff happen really quick and usually really messy. The furnace-brazed bowl has outer fins that are TIG brazed after they have been bent to the desired angle. Brazing and TIG welding the fins as Phoenix does creates a much stronger assembly, and keeps fins from working loose or bending under extreme load and rpm.
According to Phoenix, this converter has a flash speed of up to 3,200 rpm, but drives like a much tighter converter. The idea is that we wouldn't even notice we had a high-stall converter until we nailed the throttle. It sounded perfect for us, but would it deliver the goods? We threw it in the trans, and hit the road for some road testing before our date with the chassis dyno. Ducato had warned us that if it was too tight, our relatively big-cammed small-block might want to stall at idle when placed into gear, but we had no problem keeping the 408 lit. That was a good sign. Next, we did some around-town driving, trying to see if the revs would still flash way up while pulling away from the light modestly. We quickly discovered that unless provoked, our new converter was very well mannered in traffic, a bonus considering our obnoxiously loud side-exit exhaust. Next, we accelerated hard on the highway on-ramp, pushing the engine toward redline and digging deep into the converter's meat. Even with Nitto Drag Radials, the posi rearend broke loose with ease, proving that the bigger converter still had the torque multiplication we'd need for dragstrip duty. On the freeway, with the converter locked up in Fourth, the high-strung small-block settled back into a deep freight train rumble. So far, the larger 258mm converter had performed as advertised.
Back on Westech's Superflow chassis dyno, we elected to do an unaltered baseline test, realizing that should there be a dramatic difference, we'd probably need to reconfigure some of the jetting to the carb. On the very first pull, the dyno registered 434.9 hp (5,900 rpm) for a gain of 69.3 hp. The bigger converter was clearly showing more efficiency, so Westech's Ernie Mena began a tuning loop using the facility's wide-band O2 monitoring equipment. With the converter's flash point occurring another 500 rpm lower, the engine was now demanding more fuel earlier, and after a few changes to the primary and secondary jetting (pulling fuel out of the secondaries and adding it to the primaries), and upping the size of the squirters, the Laguna's small-block pushed the pony count north to 441.9 hp (5,900 rpm) with 431.3 lb-ft of torque (4,800 rpm). The increase in output over the smaller, less-efficient converter was 76 hp and 23 lb-ft of torque. We were stunned.
These are two stators from...
These are two stators from the 9.5-inch 245mm GM converter series. This is the converter on which so many aftermarket converters are based in the over-2,800-rpm category. You can easily see the difference in fin angle and opening between the fins. All other components being equal in the converter, simply changing from one of these to the other can make a difference of several hundred rpm in stall speed. With the 245mm converter, there are dozens of possible stator and fin-angle combinations to produce stall speeds from 2,600 to over 4,500 rpm. The trick is to know which combination to use.
That's a lot of increase, but it remains to be seen if that translates into real performance at the dragstrip. It's really important to note that a chassis dyno is not the best tool for evaluating converter performance, simply because it treats all cars as if they weighed the same. (The dyno's inertial roller drum acts as a proxy for the mass of the car, and is usually much less.) We do not want to convey the idea that a larger converter is always going to be better. It may be for some cars, but it really is an individual thing. In our case, our initial chassis dyno numbers were so far off from our engine dyno numbers, that most likely any tightening would've shown an improvement at the wheels, and that's why we did it before hitting the track. Another point we want to make is that our Laguna is on the extremely heavy side at 3,900 pounds (and that's a conservative estimate). At race weight with driver, it's looking more like 4,100 pounds. Our looser, 245mm converter would be right at home in something like our 3,660-pound '68 Chevelle. Said another way, the same converter when put into two different cars with two different engines will behave in completely different ways.
We hope to get out to the dragstrip soon to see what the real result is. We think a realistic goal would be to go 12s with drag radials, and eventually low 12s with slicks. Once we fix a few things, we'll hit the track.
After we swapped from Phoenix's...
After we swapped from Phoenix's smaller 9.5-inch (245mm shell) converter to their 10.15-inch (258mm shell) converter, we took the Laguna back to Westech Performance for another session on the Superflow chassis dyno. Here, Westech's Ernie Mena makes an adjustment to the carb squirters. We found that the tighter converter turned on sooner, demanding more fuel at a lower rpm. For that, Mena added some jet to the primaries. The bigger squirters helped eliminate a dead spot when rolling into the throttle off idle.
How Not To Build A High-Stall...
How Not To Build A High-Stall Converter
Notice how the "builder" of this 298mm converter was going for "maximum" stall speed here. Unfortunately, in creating the highest stall speed, he also created a giant heat pump incapable of good hydraulic lockup, and wasting engine horsepower that would have never made it to the rear wheels. Note how the center fins have been bent over with a chisel and the outside fins are bent at the most negative fin angle possible. If the builder had selected a smaller cover and pump to begin with, the high stall could have been achieved without sacrificing the efficiency of the converter. These kinds of decisions are usually the result of someone who needs lots of stall, but has very little money to spend.
The dyno shows the huge improvement...
The dyno shows the huge improvement in torque converter efficiency, with the red traces showing the first converter, and the black traces showing the larger, tighter converter. Max torque with the first converter was 407.8 lb-ft (4,400 rpm), and max power was 365.6 hp (5,700 rpm). The tighter converter maxed out at 431.3 lb-ft (4,800) rpm, and 441.9 hp (5,900 rpm).