Tech Review—Sonnax Smart-Tech Ratio-Style Servo Kit

By the FSC staff
Photography courtesy of the manufacturer

Whenever things go wrong, racers are always “on a pass.” Like a fisherman’s tale of the one that got away, those mythical incomplete runs are favorite tales to tell around the pit when the racing action dies down for the night. While everyone enjoys a good story about what might have been, it makes for a much better story if your combination stays together and achieves its elapsed-time goal.

One of the most popular automatic transmissions in racing is the aluminum Powerglide. Designed by General Motors and first deployed during the 1962 model year, this two-speed transmission faded away from production models in the early ’70s, but it lives on to this day on quarter-mile tracks. While it is a reliable performer in a wide range of modern performance applications, the engineering that went into its components is more than a half-century old. One facet of the Powerglide’s design that can cause failures that ruin that perfect pass is the servo.

“It is an antiquated design from the late 1950s through early ’60s with a smaller release versus apply area that is not compatible with increased pressures common in racing transmissions,” Gregg Nader, High Performance Product Line Manager at Sonnax, explained. “The band cannot fully release and often burns badly or drags and has a burned stripe down the center. All GM/Ford/Chrysler transmission servos designed after this period avoid the Powerglide servo shortcomings by having a larger release versus apply area.”

The Sonnax Smart-Tech ratio-style servo kit (PN 28821-10K) imbues Powerglide transmissions with hydraulically released servo actuation like more modern transmissions. This offers a number of performance and durability enhancements.

If you have pulled your trans apart only to find the band is marred by such a racing stripe, you might have wondered what could have gone wrong. You probably upgraded the servo spring and thought this and your other internal upgrades were more than up to the task for your combo. This is where the inherent flaw with the ’Glide servo design comes into play.

“In one sense it means even when the servo is thought to be released (as in high gear), there remains residual apply force on the end of the servo apply pin (effectively a mini servo, if you will). This residual apply force can overcome most release-assist springs,” Nader said. “In these situations, the majority of band apply force is removed, and it will seem as if the transmission has shifted. However, the servo does not fully retract. The band can drag on the internal drum, burn the band, damage the drum, burn the fluid. The expanding gasses from burning fluid can cause fluid to burp out of the trans. The dragging band robs power and is literally like going down the track with your foot resting on the brake pedal. This is also worse in quarter-mile cars versus eighth-mile due to more time in high gear.”

Prevailing Powerglide wisdom says that high clutch oil pressure should release the servo on the 1-2 upshift. The truth is, that in either gear, apply force remains on the servo pin. Running in high gear, this force can overwhelm the return spring and prevent a free servo release. This allows the band to drag and burn as seen on these examples.

The servo issues don’t just rear their ugly head at the top end. The factory servo design can become an issue when your ride is on the chip at the starting line as well.

“Additionally, there can be staging issues where the unit backs out of the beams when the transbrake button is hit. During this instance there is a momentary pressure drop, sometimes longer if the unit has been hot-lapped without cool down,” Nader said. “During this moment of pressure drop, the servo release spring can push the servo off right as the transbrake (reverse clutch) applies. Rather than locking in two gears to stage, the car backs out of the beams in reverse. There are ways to deal with the pressure drop, but some will deal with this symptom by installing a weaker/lighter servo release spring in an attempt to prevent the servo from releasing.”

To cure this ailment requires matching the release spring to the line pressure to offset the pin bias force. However, that becomes challenging due to inconsistent spring pocket sizes.

“It’s not an exaggeration to say that, to make the traditional-style Powerglide servo work well, you have to do all the calculations you would with an engine valve spring, such as measure and calculate spring installed height and working height, measure the spring loads at those heights, calculate pin-bias forces for your given line pressure—all to make sure your spring is strong enough, but not too strong. Kind of like wiping out a lifter so you put in lighter valve springs, only to end up with valve float at 6,500 rpm,” Nader explained.

The low-profile design of its 6061 Anodized billet aluminum cover delivers plenty of clearance.

Much like any combination, it is the entire system that needs to be addressed to avoid all the potential pitfalls.

“Everything is connected when you distill down what is really going on. So the guy that fixes his backing out of the beams issue with a weaker/lighter servo release spring finds later on that he can’t keep a band alive for more than a few runs,” Nader elaborated. “First the staging issue, now burned bands, and eventually they want to kick that ancient old Powerglide to the curb.”

To address this situation, Sonnax Technical Sales & Training Specialist Randall Schroeder spearheaded the development of a patented solution that fits in the stock servo location and delivers all the functionality of servos in more modern automatic transmissions: the Sonnax Smart-Tech ratio-style servo kit.

“The Smart-Tech servo eliminates the old design pin bias flaws and the need for all of those formally critical calculations,” Nader explained. “As a result, it uses a significantly lighter spring that you could otherwise not get away with in a regular servo. This also gets you more net band-apply force in first gear. It’s completely compatible with all other Powerglide upgrades.”

“As a lower-cost approach for racers on a budget, Sonnax also has a race-calibrated return spring that offsets the pin bias flaws up to 275 psi,” Nader added. “It works with stock servos and is sold separately or as part of the Sonnax supported servo master kit.”

Installing the Smart-Tech servo on a Powerglide is an elixir for a number of the transmission’s ailments in a performance environment. Those improvements include eliminating piston binding; preventing input shaft failure resulting from shift tie-up; and reducing fluid loss thanks to improved sealing. The kit utilizes a lighter spring that prevents backing up when the transbrake is applied. It also applies more force than stock-size servos. As a result, racers can tune the band release/high clutch apply to prevent tire spin. In all, Sonnax reports that racers have shaved off as much as .2 seconds of elapsed time with this upgrade.

“It reconfigures the servo with a larger release versus apply area to be the functional equivalent of the later-model GM/Ford/Chrysler transmission servos. This means the Smart-Tech servo will fully retract in the bore when shifting from low to high gear and the band will not drag,” Nader said. “It also means more and more shops are doing end-of-season freshen-ups that go back together with the original band because everything is cherry…” That sort of durability enhancement helps ensure that when your ride is on a pass, it will run the number.

Source:
Sonnax Transmission Company
(800) 843-2600
Sonnax.com

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