From left to right: The A518, 727, A500, and 904.
In recent months, we've chronicled the pros, cons, weaknesses, and strengths of the various automatic transmissions offered by General Motors and Ford. This month, the third and final installment in our automatic transmission series focuses on the Chrysler Corporation's TorqueFlites. Mopar guys don't have as many choices, but while their options may be limited, the offerings are a good foundation on which to build a hot street or strip car.
Dimensions of Popular Mopar Automatic Transmissions
The Three Speeds
Mopar three-speed transmissions can be broken down into two groups: The light-duty 904 transmissions and the heavy-duty 727 series of transmissions. The 904 made its debut in 1960 and the 727 hit the scene in 1962. We've always found it interesting that Mopar was the lone OEM to make a three-speed automatic transmission standard on all cars rather than offer it as a higher priced upgrade for a two-speed. Heck, if you bought a Chevy, you couldn't even get a three-speed until 1965, and then only behind a big-block, and two-speeds were the standard automatic in most GM cars into the early '70s.
B&M builds high-quality TorqueFlites and accessories (shifters, shift-improvement kits, torque converters, aluminum transmission pans, trans coolers, flexplates, and so on) for all your street and strip needs. This is one of B&M's brutal street/strip TorqueFlite 727 transmissions.
Both TorqueFlite transmissions came standard with a ball-and-trunion output yoke through the '65 model year when a more conventional slip-yoke became standard. We'll look at the lighter duty of the two first, noting that the 904 was never offered behind anything larger than a small-block and was never considered a performance option. In the case of automatic-equipped performance small-block applications, such as 340-powered vehicles and heavy-duty applications with 360s, a small-block version of the 727 was standard.
Generally speaking, the 904 and its derivatives are considered some of the most mechanically efficient automatic transmissions offered to the American public.
Introduced in 1960, the light-duty TorqueFlite came in three basic designations--the 904, 998, and 999. The 904 was equipped with three direct friction plates, the 998 had four, and the 999 came with either four or five. They are otherwise dimensionally identical, and beginning in 1980 all were offered with a low gear set (2.74 First and 1.54 Second) as standard equipment. TorqueFlites were also used in some American Motors automobiles from 1972 until AMC's demise, and they even made an appearance in some import applications, including Mitsubishis. Due to its high mechanical efficiency and low weight, 904-based transmissions are a popular choice with drag racers because the light internals eat a minimum of horsepower compared to heavier duty offerings. But there are certain inherent design drawbacks with the light-duty TorqueFlite. A particularly weak link is the spindly, slotted front pump drive of the torque converter. The slots themselves are wide for ease of installation, but the resulting sloppy engagement makes the drive prone to cracking. Many aftermarket torque-converter manufacturers use much narrower slots to reduce the cracking problem. Later 904-based automatic overdrive transmissions (A500 series) have a flat machined pump drive that eliminates the crack-inducing feature entirely.
The 727 transmissions were offered with either a three- or four-pinion aluminum front planetary depending on application. Big-block hi-po versions receive four-pinion units. In the foreground is A-1's steel five-pinion planetary.
Turbo Action's Paul Forte tells us his company addresses this weakness by substituting a chromoly pump drive for the weak factory version on its high-performance torque converters. T/A's street and strip (S/S) 904 transmissions receive modifications to their lubrication circuits to improve flow along with a high-performance valvebody. Other than these basic mods and good clutch materials, T/A's S/S units can easily handle 500 eager horses. When moving up to a race unit, a full manual valvebody is standard, a transbrake is optional, and a Torrington bearing is used in the tailhousing instead of a fixed bushing. The cumulative effect of these mods brings the level up to the 600hp range. If the racer is looking to eke out the last 100th of a second and is not concerned with durability, Turbo Action can build an all-out lightweight unit featuring aluminum drums and a plethora of other lightweight parts. These top-dollar units are able to withstand up to the 900-horse range as they are lighter and more efficient, but they are built to go fast and not to last in a street car.
727/A518, 904/A500, Mopar automatic trans pan gaskets.
The heavy-duty 727 TorqueFlite was used by Mopar to back up virtually all of its high-horsepower mills from the 426 Max Wedge to the aforementioned 340 high-performance V-8 engines, to the brutal Street Hemis. While not nearly as efficient as its little-brother 904, the 727 can handle an obscene amount of horsepower. Early versions of the 727 ('62 through '65) used a push-button cable-operated shifting mechanism as well as a smaller-spline input shaft that required a corresponding torque converter unique to these model years. The '65 727 did not use push-buttons, but retained the cable-operated shift mechanism. From 1966 on, all linkage was mechanical.
Here are the two Mopar V-8 bellhousing bolt patterns used on TorqueFlite transmission. The big-block (B and RB) pattern is on the left and the small-block (A and LA) is on the right.
Marv Ripes of A-1 Automatic Transmission has developed many components to make the already stout 727 even stronger. The 727s were produced with both three- and four-pinion front planetary-gear sets, and all were constructed of aluminum. Heavy-duty applications, such as cars and trucks equipped with high-performance big-blocks, got the four-pinion planetary. Light-duty applications received the three-pinion unit. Ripes offers a steel planetary with five pinions for use in all high-performance 727 automatics.