Building an overdrive automatic the Gearstar way

Building an overdrive automatic the Gearstar way

How times have changed. When our Chevy was new, there were two choices when it came to transmissions: a three-speed manual or a two-speed Powerglide automatic. Interestingly, when the automatic was ordered it came with a larger displacement engine (235 versus 216 ci) with more ponies (105 compared to 92 hp).

In most cases when the running gear of an early car is being updated an automatic transmission is the gearbox of choice. They are easy to install and hook up, they’re reliable, user-friendly, and there are a variety of designs to choose from. While the array of offerings may seem confusing, like most choices that have to be made, the more informed you are the easier it is to make a decision. To that end let’s look at the basic parts of an automatic, what’s available, as well as the modifications Gearstar makes to enhance performance and longevity.

Our Choice—the Gearstar 4L60

For our purposes we decided on a Gearstar Level 3 4L60. Basically an improved version of the 700-R4, this transmission doesn’t require a computer like any of the later transmissions with an “E” suffix (4L60E as an example). Thanks to a variety of modifications and the Yank lockup converter this transmission is tough enough to endure aggressive launches from a dead stop yet the overdrive gear and lockup feature make it perfect for highway cruising. This package is recommended for 355/383/396/400/454 naturally aspirated Chevrolet engines producing 450hp.
Modifications include:

  • Yank 10-inch Lockup Racing with stall speeds ranging from 2,600-4,500 rpm for a stronger launch from a standing start while retaining the advantages provided by the lockup feature.
  • Improved reactor holding capacity from ALTO Redline racing frictions and band, hardened steels, expanded capacity direct clutch drum, expanded capacity forward clutch pack, and four-clutch overdrive pack.
  • A combination of TransGo and Gearstar reprogramming shift kits, Corvette servo and Sonnax “Superhold” 3-4 Servo to provide firm shifts that are timed appropriately.
  • A13-vane, high-output pump and 0.500-inch boost valve for higher and more consistent hydraulic pressure.
  • For increased strength a 300M hardened steel shaft and hardened expanded capacity drum, monster sun gear shell, and extreme duty springs and sprags.

Zack Farah, owner of Gearstar, is justifiably proud of the group he calls, "The best staff in the business." His crew is proud of what they do as well, Patrick has 30 years of experience, is an ASE Master Transmission Tech, and is GM certified and says with a grin, "I get to build with the best parts, take all the time in the world, and know that it’s going into a really cool car. What more could I ask for?"

Gearstar doesn’t crank out rebuilt transmissions, they specialize in automatics for performance applications, and consequently many of the internal parts are replaced with improved versions. In some cases it could be considered overkill. Zack considers building tough transmissions good business sense.

Gearstar substitutes stronger, hardened, and stress-relieved 4340 steel shafts in Level 3 and 4 transmissions for reliability. Level 2,3, and 4 transmissions carry a 24-month, 24,000-mile warranty.

Shown here are the front (larger pair, left) and rear planetary (smaller pair, right) gear sets. Both are upgraded from a cast housing with four pinion gears to five gears and a hardened steel housing.

This is a great example of Gearstar's attention to detail. Thanks to a sharp bend made during the manufacturing process, the factory sun gear shell tends to fracture adjacent to the splines. Gearstar’s replacement is made of heavier material and is gusseted around the splines for increased strength.

This is the clutch pack for the rear planetary set that provides low and reverse gear - note that steal and friction discs are used alternately. Gearstar uses Alto frictions and hardened steels exclusively.

A critical component, this is a sprag, or one-way clutch. In most cases more elements "or peanuts" that rock between the inner and outer races are considered to be beneficial. Gearstar has found that by using one less element and making them bigger, strength is increased dramatically - sometimes less is more.

Then again, sometimes more really is more - increasing the number of frictions and steels in a clutch pack increases torque capacity. This is the 3-4 clutch set.

Another Gearstar upgrade is the 13-vane, high-output pump. It supplies the hydraulic pressure necessary to shift gears and lubricate the moving parts. Pump pressure is increased for firmer shifts and increased torque capacity.

Several variations of shift kits are used in the valve body to tailor the transmission's performance to the customer's needs. It does, however, take a lot more than a shift kit to make a performance transmission.

The governor mounts in a cavity at the rear of the case. Something that is often overlooked or modified incorrectly, the governor influences when shifts occur - Gearstar goes to great effort to match the governor weights and springs to the modifications made to the transmission.

Gearstar has proved that something doesn't have to be complicated to work well - case in point is their lockup converter control. There are none that are simpler, easier to install, or work better.

Our finished transmission, ready for it's new home in the '90 AMSOIL / STREET RODDER Road Tour Chevy.

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