Drivetrain Design
The drivetrain starts downstream from the engine’s crank flange and goes to the driven wheels and tires. For this article, I’ll limit the details to rear-wheel drive (RWD) applications, but front-wheel drive (FWD) and all-wheel drive (AWD) have similar design concerns and face similar problems but use completely different parts.
Words: Larry Shepard
The drivetrain starts downstream from the engine’s crank flange and goes to the driven wheels and tires. For this article, I’ll limit the details to rear-wheel drive (RWD) applications, but front-wheel drive (FWD) and all-wheel drive (AWD) have similar design concerns and face similar problems but use completely different parts.
At first, changes came slowly to the drivetrain system with the first major step being the upgrade to 4-speed manual transmissions from the standard 3-speeds used in the late 1950s and early 1960s. The engine development had gotten ahead of the drivetrain hardware so heavy duty, strength and durability was every bit as important as extra ratios. The next major step was the introduction of the 3-speed TorqueFlite automatic transmission in 1962. This allowed the torque converter to be a part that could be modified for performance purposes and opened up perhaps the largest market for performance parts in the drivetrain category. The next major steps were the introduction of the lock-up converter in the mid-1970s and the addition of overdrive ratios or a 4th speed to the basic 3-speed automatic transmission which occurred in the late 1970s. Add to this the introduction of electronic controls and electric shifting to the automatic transmission in the late 1980s and the early 1990s. On the surface, it would appear that not much has changed in the drivetrain area from 1962 to the mid-1990s. Slow or not, in the last few years, things are changing very fast including 5- and 6-speed automatics, 8-speed automatics, 5- and 6-speed manuals. I’ll focus on the basic hardware and try to catch some of the new technology as we discuss the specific topics.
The drivetrain has to be looked at as a complete system—change one part and you may have to change others. This interconnection of the parts makes it hard to discuss the complete drivetrain in a short, concise article, so I plan to take some short-cuts. Therefore I strongly recommend that you use the latest Mopar® Performance catalog for the extra technical information that relates the use of the parts mentioned.
The drivetrain starts with the crank flange and there are two: the 6-bolt flange used on most V-8s and the 8-bolt flange as used on the 426 HEMI® Gen II and many race/billet cranks. From here on, I’ll divide the discussion into the manual transmission hardware and the automatic transmission hardware.
The manual transmission is joined to the block by the bell housing like P5153602 SFI housing (safety version) for the small block (LA-engine), Magnum® and HEMI Gen III engines. The B-engine (383 and 440) and the 426 HEMI Gen II use a different bolt-pattern at the rear of the block. The front of the transmission has to pilot in the rear of the bell housing but there are several different drive pinion retainers (front piece of the transmission) that can help mate the trans to the housing. The typical package uses a gear-reduction starter, so the starter has to be considered but isn’t really part of the drivetrain.
The flywheel bolts to the crank flange like P4529110—a 10.50" unit for the 6-bolt crank flange and is made of cast iron. Flywheels must match the crank flange and line-up with the starter—ring gear location and the number of teeth. The flywheel bolt pattern must match the diameter of the clutch being used—such as the 10.5". There is also a lightweight 18.5 lb. steel flywheel P4876047 for use with the 10.5" clutch and 6-bolt cranks. Flywheel inertia (weight) is an important aspect of flywheels for both street and race performance. Production and street flywheels are around 30 lb. units.
The clutch bolts to the flywheel and consists of two parts: the disc and the pressure plate. The pressure plate has a size—like 10.5" unit P4529140—to match up to the bolt pattern on the flywheel. Pressure plates are also rated by load or application. The disc must mate to the pressure plate and it must have splines that match the splines on the input shaft of the transmission—like P4529138 for an 18-spline A833. Note #1: There are also 23-spline A833 transmissions.
The performance 4-speed manual transmission is the A833—in production through the mid-1970s. In the 1970s, ‘80s and early ‘90s, the manual transmission wasn’t very popular in racing or on the street. Parts became hard to find. Complete assemblies weren’t serviced. However, today A833 parts are much more readily available.
The transmission shifter bolts to the side of the transmission. Many shifter levers—like P4510948, Pistol Grip for 1970 B-Body with Console—are available from Mopar Performance.
There are several aftermarket options for 5- and 6-speed manual transmissions. One of the more common options is the Tremec 5-speed options offered by Keisler.
There are two basic sizes of 3-speed TorqueFlite automatic transmissions: the A904 and the A727. The newer 4-speed overdrive versions are called the A500 and the A518. Automatic transmissions attach to the crank flange through the flexplate like the SFI (safety) 6-bolt version P4529751AB. One of the trick flexplates is P5007378 because it is an 8-bolt plate with 5/16" converter attaching screws. Production 426 HEMI Gen II and most race converters use 7/16" while most econo-street converters use 5/16" bolts.
The typical automatic transmission does not have a separate bell housing. The bell housing is cast as the front part of the main transmission case. Therefore there are two main styles of cases: big block (383, 440 and 426 HEMI Gen II) and small block (LA-engine 318/340/360, Magnum 5.2L and 5.9L, and HEMI Gen III 5.7L, 6.1L and 392). Complete A904 and A727 transmissions are serviced by the aftermarket.
The flexplate connects to the automatic transmission through the torque converter. These converters must be matched to the transmission like P4876947AB which is designed for the A727 lockup transmission and has 1900 to 2100 rpm stall. Generally converters are rated by stall-speed rpm. Most production converters are in the 1200 to 1500 rpm area.
One of the most confusing issues relating to torque converters is the engine’s balance condition. Most forged crank engines are internally balanced. However, most cast crank engines are externally balanced engines. This means that the flexplate/torque converter has weight(s) added that balance the engine and converter assembly. These weights are added to the engine side of the converter, typically next to the oil drain plug. If these weights are required and your performance converter does not have them, the weight kit is P4120241 (for the 340 and 360 cast crank LA-engine, the 400-440 cask crank big blocks, and the 440-6BBL). The 5.9L Magnum weight kit is P5249843. Note #1: The exception to the cast crank rule is the 440-6BBL engines which have a forged crank but are externally-balanced because of the heavy rods used in these engines. Note #2: Engines that have the ring gear attached to the torque converter itself have external weights added to converter as described above. Engines that have the ring gear attached to the flexplate, add weight to the flexplate, which requires special weights.
There are many performance parts for the automatic transmission ranging from shift improver packages like P4349469AB designed for the A727 up to 1986, to the full manual shift (control) valve body P4007291 for the A904 and A727 which eliminates the 1-2 shift bump. There are various transmission rebuild or overhaul kits like overhaul kit P5007172 for the A727 and P5007568 for the A904. There are chrome-plated, deep transmission pans like P4120161AB for the A727 which increases the oil capacity which can help durability and performance. Plus there are cast aluminum pans like P5007793AB for the A727. All automatic transmissions use a cooler in the bottom of the radiator. If you use a hi-stall converter, you should add an oil cooler in front of the radiator and hook them up in series.
From the transmission, the engine torque is transferred to the driveshaft. The driveshaft consists of the front slip-spline, the driveshaft tube, the rear yoke and the front and rear U-joints. A driveshaft can be shortened but should not be lengthened. A longer driveshaft requires a new driveshaft tube. The proper length of driveshaft is related to the distance from the end of the transmission to the center of the rear yoke. There are several front slip yokes like P5007409 for the 7260 U-joint and A904 trans. For the 8 ¾" rear axle, there are two axle yokes: P4876804 for the 10-spline pinion and 7290 U-joint and P4529483AB for the 29-spline, 7290 U-joint. There are also billet versions. U-joint strap and bolt kit P4120469 for the 7290 U-joint can also be very helpful.
The driveshaft attaches to the rear axle and transfers the engine torque to the ring and pinion gears. There are several sizes of rear axles like 7 ¼", 8 ¼", 8 ¾", 9 ¼" and 9 ¾". With all of these axles (except the 8 ¾"), the ring and pinion gears are removed toward the rear. The 8 ¾" axle center carrier removes toward the front of the vehicle. There is a stronger and light 8 ¾" carrier housing P4876445 which is made of aluminum. For performance purposes you should use a Sure Grip differential like P4876118 for the 8 ¾" axle. If you have a production 8 ¾" Sure Grip, you can use clutch kit P4529484 to rebuild the Sure Grip. They should be rebuilt every few years. Axle ratios are commonly changed. Production ratios or street ratios tend to run from the 2.5/2.75 area down to the 3.2/3.5 area. Drag race ratios range from 3.9 to 5.0 or higher in some cases.
The rear wheels can be made of steel or aluminum which relates to weight. Wheels have a diameter to match the tire being used and a width that is related to the amount of tire tread. The wheel has a bolt circle that allows it to be bolted to the axle assembly. The typical bolt circles are 5-on-4" and 5-on-4.5" but there are others. Tires are chosen based on the use or application. They have a width and tread pattern and a diameter, both for mounting on the wheel and overall. They also have an aspect-ratio—height to width. Tire clearances are important in any application.
There are so many aspects to the drivetrain system—and one element to consider is related to another component in the drivetrain—so it is very difficult to cover all the angles. If you have any questions, you may contact the Mopar Performance Tech Line at (888) 528-HEMI (4364), refer to the latest Mopar Performance catalog or visit www.mopar.com
Larry “Shep” Shepard is a retired Mopar Performance engineer, author of numerous Mopar Performance books and a Michigan Motor Sports Hall of Fame inductee.
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