4L60E Fortification - Extended Service Program

The GM overdrive transmissions have gone through several different phases as the powertrain experts upgraded and updated the components to handle power and reliability to go with ever-evolving GM small-block engine. The General's soldiers introduced the 700R4 in the early '80s and stuffed it into numerous rear-wheel-drive cars and trucks in the Chevrolet and GMC lineup. By the '90s the powertrain team made updates and provisions leading to a new nickname--4L60 and by 1992 GM added electronic controls and dubbed the transmission as the 4L60E. For the GMHTP crowd the 4L60E was put behind the Gen II LT1 engine in both F-body V-8 platforms in 1994. The transmission's service life continued through the Gen III LS1 reign of terror from 1998 through the end of the fourth-gen F-body run in 2002. The transmission also sits in the C5 Corvette from 1997 through 2004 models as well as the '04 Pontiac GTO. Its service through all those vehicles makes it one of the more popular transmissions running around the LS and LT performance circles.

Each passing year more mileage is accumulated and more power added, which brings most 4L60E transmissions to the brink of destruction--that's if it hasn't broke yet. The 4L60E is certainly a workhorse, but eventually the OEM clutches slip, the input shaft breaks, and/or the planetaries fail. CK Performance walked us through the guts of the 4L60E and the many upgrades it adds, including a numerically lower First-gear ratio for better performance. According to Chris Kokonis of CK Performance, there are inherent problems with the 4L60E and its replacement, 4L65E. The design flaws restrict power--even with the toughest built units--to the 600hp range, according to Kokonis. The major problem lies in the internals as the 4L60E uses a single planetary gearset for First gear while most GM transmissions, like the stronger 4L80E, utilize two planetary gearsets in First.

"Although the trans uses two gearsets, it has its own unique methods of power transmission in comparison to what most of us are familiar with seeing. To create a numerically lower 3.06:1 First gear for better acceleration with highway-oriented rear-end gears, the engineers reversed the typical input and output members of the front planetary gearset. This results in huge First gear torque multiplication through the use of only one gearset. The 4L60E also makes unique use of its rear, or Overdrive, planetary gearset," stated Kokonis. He continued, "it [the Overdrive gear] comes on twice, once in Second gear and again in Fourth (Overdrive). First gear uses the front planetary set only, Second gear is the front gearset overdriven by the rear planetary gearset, Third gear is the mass all locked together (direct-drive with a 1:1 ratio), and Fourth is the mass minus the rear gearset but overdriven. This is an efficient means of overdrive, but high speeds are reached in the rear gearset during Overdrive operation. This really puts a beating on the rear planetary, or as some dub the Overdrive planetary."

Another problem with the 4L60E is that the factory passenger-car shift calibrations cause the transmission to slip and/or flare on heavy throttle 2-3 upshifts. With proper shift timing the slip is the result of reduced 3-4 clutch holding capacity. The flare is caused by an improperly timed band knock-off and 3-4 clutch application. As the transmission starts a ratio change from Second to Third, the band disengages and the clutches start to apply simultaneously. When not timed perfectly, there is a delay in clutch apply and the result is a spike or flare in engine RPM. Kokonis explained that under this condition the clutches aren't slipping. It's just the transmission is mechanically defaulting to First gear as it attempts to lock the entire mass together. This overloads the 3-4 clutch because the mechanical default results in interruptions in power flow. This problem will eventually cause clutch slippage.

Like we mentioned earlier, the transmission is good to around 600 hp and after that one should really consider a step up to a 4L80E or a TH400, which both are extremely similar internally save for the Overdrive gear and electronics in the 4L80E transmission. The clutches or bands never disengage in those two transmissions as each gear stacks on top of the previous one, creating a lot of holding power. For applications above 600 hp, CK Performance offers two 4L80E setups, both feature a trans-brake and can be manually shifted or auto-shift. The company also has several TH400 style transmissions in its catalog of products.

The clutch and band issues can be fixed with some oil flow changes, but that isn't the 4L60E's only inherent issue. The unit's gear ratios are not the best choice for cars making big power. CK Performance offers a version of the 4L60E transmission with a numerically lower First gear. This is accomplished by utilizing a revised ratio six pinion gearset rather than a four or five pinion gearset from GM. The revised ratio brings First gear down from 3.06:1 to 2.84:1 and Second is reduced from 1.63:1 to 1.56:1. The move away from a steep first gear might sound like the car will run slower, but that is the opposite. Most of the cars that run a 4L60E transmission feature stock suspension and true 10.5-inch slicks or radial tires. The softer First gear calms the launch down and the gear drop off between First and Second allows for better acceleration, which is commonly referred to as shift-recovery as the engine remains in its power band after hitting Second gear. The six pinion unit is also stronger because the work is spread out amongst six pinions rather than four or five. Kokonis says that some of his customers report as much as a three-tenth of a second improvement on the dragstrip because the car is easier to get out of the gate with the numerically lower First gear.

The rest of the transmission is upgraded like other units with quality clutches, a better band, stronger input and output shafts, and CK Performance offers an optional SFI-certified bellhousing for those wishing to be NHRA legal. For input shafts, there are two choices, either a cryo-treated stock piece that is good up to 450 or so horsepower depending on calibration (soft or hard shifts), vehicle weight, and intended use. The other input shaft is a custom CK Performance shaft that is made from billet steel--simply put, if you thought twice about using the modified stock one in your car, then you probably need the billet one. In all, a full-boogie 4L60E with all the bells and whistles, including an SFI bellhousing and shorter First gear will set you back approximately $2,800 from CK Performance.