Horsepower has a way of finding hiding places throughout the drivetrain. You might be sacrificing a few horsepower here and there due to normal parasitic loss or you might be dealing with a much more serious situation. In the case of high-horsepower street cars that use automatic transmissions we can assure you that the torque converter is one of the best hiding places for horsepower. An automatic transmission in general has a way of soaking up horsepower, but the torque converter can take a potent combination and turn it into a pig when the rubber meets the road.
A common mistake is choosing a torque converter based solely on stall speed or diameter. We hear people refer to their converter as a 10-inch or a 3,500-stall, but it’s always important to gather more information before you go through the trouble of swapping torque converters. And while stall speed is important for a good launch, it’s also a place for horsepower to swirl down the drain. We recently ran into this issue at a dyno session with Kyle Shadden’s 1970 Chevy Nova. You might remember it from our budget turbo build and Holley EFI install articles. The car made impressive horsepower (562 hp), but it was leaving a lot on the table due to a torque converter that just couldn’t keep up.
The car featured a mystery transmission and torque converter combination. The TH350 had been modified, but we’re not sure who did the work or the extent of the modifications. As for the torque converter, it was a no-name piece that clearly wasn’t ready for a turbo car. It offered a roughly 3,500-stall speed but it couldn’t hold the power at high rpm where it needed to be much tighter. Shortly after our dyno session the mystery TH350 expired, so it was time for a whole new combination. We called up TCI Automotive to get our mess straightened out.
Turbo cars are picky about torque converters because you’re faced with needing stall speed to build boost off the line, but also needing the converter to fully engage and lock up on the big end. Specialized lockup torque converters are available but they’re expensive, so to get the best bang for our buck we went with an off-the-shelf torque converter. We spoke with Will Vance at TCI and he gave us a suggestion on a torque converter that could handle the abuse. In fact, our TCI Ultimate StreetFighter torque converter is rated for 1,200 horsepower. And while the converter doesn’t offer a tremendous amount of stall speed (3,000 rpm on the foot brake with our combination), we opted for TCI’s full-tilt TH350 “Bracket Race” transmission, which has a trans brake to allow for a higher launch rpm. Even though we can only foot brake it to around 3,000 rpm, the trans brake will allow the torque converter to reach its maximum “flash” stall speed, which should be well north of 3,500 rpm. Stall speed varies greatly depending on your combination. For our small-displacement engine, stall speed was much lower than if we had this converter behind a torque-happy big-block.
The most important data that we received during the previous dyno session was the wheel speed. We were experiencing 39 percent converter slip, which is way outside of normal specifications. To be fair, we’re dealing with a high stress combination—a turbocharged car with a relatively high rearend gear (3.27:1)—so the load on the converter is substantial. With the TCI Ultimate StreetFighter converter, we experienced less than 10 percent converter slip (9.612 percent to be exact, according to TCI’s online converter slip calculator). This increased our high gear mile per hour from 120 to 146, a drastic difference.
As a result of the tighter torque converter, the turbocharged Nova gained 128 horsepower and 131 lb-ft of torque without making any engine modifications. We noticed the car acted differently with the additional load placed on the engine, so it was necessary to adjust the tune accordingly. James Rowlett from Lebanon, Tennessee, tuned the Holley HP EFI.
Even if you’re making more or less than our 690-horsepower boosted combination, a loose torque converter could be robbing power before it makes it to the rear wheels. The guys at TCI Automotive helped us take the guesswork out of the selection process and unlocked a bunch of hidden horsepower without turning wrenches on the engine. Now, it’s time to take this sneaky Nova to the track and see how it likes the newfound horsepower!
1. Our project car is a 1970 Chevy Nova featuring a boosted LS combination backed by a TH350 transmission. As you can see, the car has a homebuilt transmission crossmember and very few distractions, making the transmission swap fairly easy.
2. The first steps are to remove the clip that holds the shifter cable and then disconnect the cable from the shift lever on the transmission. Then, the cable can be fed back into the driver’s compartment and removed. Next, we disconnected the transmission cooler lines and unbolted the transmission from the engine.
3. It is important to note that when using an old-style transmission (Powerglide, TH350, TH400, and so on) with an LS engine you must install a flexplate spacer, like the one pictured here.
4. The TCI flexplate (PN 399753) slides into place and gives us peace of mind thanks to its heavy-duty construction and SFI approval. TCI flexplates are 0.035-inch thicker than stock and have the correct torque converter bolt pattern.
5. Even though the heart of this experiment is the TCI torque converter, it was a good time to upgrade the transmission from a barely modified TH350 to a race-ready TCI “Bracket Race” TH350 transmission. This three-speed automatic is built to handle over 750 horsepower and it comes with a trans brake installed.
6. The TCI Ultimate StreetFighter (PN 241004) torque converter is just what the doctor ordered for our turbocharged LS engine. It is a 10-inch converter that is considered a “tight” converter, which is what we need as the boost ramps in and horsepower levels increase.
7. In this particular car, we’re using -AN lines on the transmission side of the cooler lines, so we install the correct fittings before the transmission is under the car.
8. Before we install the transmission, we add a few safety items. First is a TCI Trans-Shield (PN 975000), which is built from 6061-T6 aluminum and protects the driver in the event of a drum explosion.
9. Removing and installing a transmission on the garage floor isn’t easy, but sometimes it’s necessary. In this case, we use a floor jack and a 2x4-inch wood block to support the transmission while we get it situated and lined up with the engine.
10. After the transmission is in place and securely fastened, we can feed the new TCI shifter cable into place and thread it into the cable bracket. We’ll adjust the cable once the new TCI shifter is installed.
11. With the starter removed, we have easy access to the torque converter bolts. It’s best to use new fasteners when dealing with driveline components.
12. Now, the driveshaft can be installed to complete the bottom side of our installation.
13. TCI sent one of its external transmission coolers, so we mounted it front and center for maximum airflow. Heat is one of the biggest enemies of an automatic transmission.
14. We also called upon TCI for fluids, and they advised us to use their Max Shift Break-In fluid for our initial usage. The plan is to first put some street miles on the car before changing out the fluid and filter.
15. With the fluids topped off, we move on to the shifter install. Another piece from the TCI catalog, the Outlaw shifter will be the perfect fit for our reverse manual valvebody. We install the optional side cover with trans brake button.
16. The TCI Outlaw shifter looks great and offers great functionality for our high-powered street car. We fed the wires from the trans brake button through the handle and down into the driver’s compartment.
17. One of the downfalls of a “tight” torque converter is that it doesn’t provide a lot of stall speed for high-rpm launches. The remedy is a trans brake, which allows the torque converter to reach its maximum “flash” stall speed quickly. This usually results in a significant launch rpm gain compared to the foot brake.
18. After about 100 miles on the street, it’s time to drain out the break-in fluid and change the filter. In order to drop the pan we had to remove the lower straps from the TCI Trans-Shield.
19. The real test was on the Dynojet. The previous chassis dyno numbers were hindered by a slipping torque converter, but the new setup shows great potential with only 9.612 percent converter slip. Our high gear mph went from 120 to 146. The car stalls to 3,000 rpm on the foot brake and we’re expecting at least 3,500 rpm with the trans brake.
20. Without any engine modifications, the car picked up more than 125 horsepower and 130 lb-ft of torque with the new TCI torque converter and transmission. We ended the dyno session with 690 horsepower and 566 lb-ft of torque, with a 4.8-liter boosted LS engine on pump gas. We’re going to call that a success!