How To: Understand and Identify Automatic Transmissions
Vintage Mustangs used a variety of C4, C6, and FMX automatic transmissions
When Ford was developing the C4 light-duty three-speed automatic, powertrain engineers were also working on an FX/MX replacement for heavy-duty applications. Although the C6 is more heavy-duty than a C4, function is basically the same. Unlike the C4, a C6 bellhousing and main case are one large casting. Expect to see at least four different casting types for four different engine families. Castings across these engine types will vary a lot. Some cases will be finned while others will be smooth.
FE big-blocks: Round six-bolt bellhousing
385-series 429/460/351M/400: Angular six-bolt bellhousing with closed bottom
6.9L/7.3L Power Stroke Diesels: Angular six-bolt bellhousing with open bottom
302/351W/351C: Six-bolt small-block bell
Ford's C6 consists of a torque converter and front pump like the C4 with Simpson compound planetaries like a C4 controlled by a single friction band, three multi-disc clutches, a one-way roller clutch, and a low-reverse clutch pack. The C4's second friction band controls the low-reverse drum function. The C6 uses friction clutches splined into the case instead of a band and drum, which are more durable.
Like the C4, the C6 was a "Dual Range" green dot transmission prior to 1967. Because the Mustang was never fitted with a Dual Range C6, there isn't much point in discussing them at length here. However, if you're shopping for a core, it is wise to make sure you're not buying a Dual Range for your'69 428 Cobra Jet. The best means of identification are Ford casting numbers in the case and valve body. Anything prior to 1967 (C7OP or C7AP) is a Dual Range.
The C6 didn't change much over its long production life. Even after C6 transmissions were no longer available in new Ford vehicles, they were popular with heavy equipment vehicle manufacturers. Ultimately, the C6 was replaced with the E4OD, which was little more than C6 internals with overdrive in a new and rugged case. In fact, you can improve your C6 with E4OD planetaries, Torrington bearings, and hard parts for less friction and improved acceleration with components from TCI Automotive. When you build a C6 using close-ratio E4OD parts, you're building a more durable C6 offering improved acceleration and longer life.
The larger "R" intermediate servo or TCI 2.465-inch servo will increase pressure apply area by 25-30 percent for incredible drum gripping power. Important to remember is which intermediate servo apply lever to use. Use the "E" or "F" apply lever with the "R" size servos, never anything more aggressive. As with the C4, you want as many intermediate drum frictions and steels as possible. You also want the widest intermediate band available.
Because the C6 is a rugged transmission from the factory, very little needs to be done to make it stronger. For street and occasional strip, all the C6 needs is the larger intermediate servo and band. Where the C6 falls short is internal friction. When you upgrade to E4OD parts from TCI, you reduce friction with Torrington roller bearings. You also improve gear ratio.
If you have a '69-'73 Mustang with the cast-iron FMX transmission, you have durability in a very rugged three-speed automatic box. However, the FMX is old transmission technology with little, if anything, available to improve performance and efficiency. At best, you can purchase clutches, bands, and some hard parts. The FMX was in no way a fresh design when Ford began installing it behind 351W and 351C engines. Prior to 1966, Ford used the MX Ford-O-Matic three-speed transmission in most heavy-duty applications. The FMX is basically MX components in the lighter-duty FX case. Like the C4 and C6, the FMX is hydromechnical with a vacuum control throttle valve and kick-down linkage. A tailshaft governor contributes to shift control.
The C4, C6, and FMX transmissions employed throttle valves, or vacuum modulators, to control shift points. Based on intake manifold vacuum, throttle valves adjust transmission control pressure as you drive. In light throttle driving, manifold vacuum is high, which causes control pressure to be low for a soft upshift. By the same token, downshifts as you slow down are soft so you can barely feel them. At wide open throttle, vacuum is low and control pressure is high, causing a hard upshift and firm clutch/band engagement.
There are two basic types of throttle valves—screw-in with a gasket or press-in with an O-ring. C4, C6, and FMX had screw-in prior to '72. From '72-up is a press-in valve with a stamped steel clamp.
Some throttle valves are adjustable. For those that are adjustable, turn the adjustment screw clockwise to increase control pressure (firmer upshift) or counterclockwise to reduce control pressure (softer upshift). A firm upshift means longer clutch and band life. Make your adjustments carefully with one-quarter and one-half turns. One full turn increases or decreases control pressure by 2-3 psi. Ideally, you will have a pressure gauge connected to the transmission's pressure port for an accurate indication and proper adjustment. If you don't have a pressure gauge, try this. Make your adjustment clockwise one-quarter or one-half turn and take a drive. At wide-open throttle, observe upshifts. You want firm, but not hard. Firm means solid clutch and band engagement. Mushy upshifts mean burned clutches and bands if you don't make proper adjustments. Bone crushing upshifts mean too much control pressure and potential transmission failure. You want firm upshifts and shouldn't be able to feel downshifts coming to a stop.