Automatic vs Manual Transmission

Comparison of Automatic and Manual Transmissions in Diesel Pickups

 

If you're in the market for a new Ford Super Duty, Ram HD, GMC Sierra HD, or Chevrolet Silverado HD and contemplating what type of transmission to opt for, you're out of luck. General Motors and Ford have long retired manual transmission options for their diesel powered pickups. Meanwhile, Ram continues to offer a 6 speed manual transmission behind the 6.7L Cummins, but the option is only available on a severely derated engine. The auto vs manual dilemma is therefore going to be primarily faced by those looking to purchase a used pickup. Automatic transmission technology has made huge strides in the last decade and they are widely favored to manual transmissions. However, each transmission type displays inherent advantages and disadvantages.

Service Cost & Complexity - An automatic transmission tends to be more expensive and time intensive to service than a manual transmission. The typical manual transmission service consists of simply draining and refilling the gearbox with fresh fluid. In addition to having a significantly larger fluid capacity, an automatic transmission service can include replacing multiple filters, removal of the transmission pan, and flushing transmission cooler lines. Additionally, automatic transmission performance can suffer from dirty or broken down fluid, making routine service much more important for reliability and longevity than in a manual transmission. In terms of service cost and ease, a manual transmission holds the clear advantage.

 

Automatic Transmission

Manual Transmission

Large fluid capacity

Low fluid capacity and therefore lower service cost

At least one transmission filter that requires periodic replacement

Typically no transmission filter

Transmission cooler and lines may require periodic flushing

Relatively simple and straight forward service process for anyone with basic mechanical aptitude

 

Time consuming service process for the average DIYer

 

Repair Costs and Complexity - Entire gearsets rarely fail in either an automatic or a manual transmission. Rebuilding or repairing an automatic transmission is typically required due to clutch pack wear. A typical rebuild, granted there is no need to replace any "hard parts", consists of a new/remanufactured valve body, shift solenoids (where applicable), torque converter, and clutches. The rebuild process is time consuming and requires specific expertise in addition to special service tools in order for repairs to be performed correctly. As a result, automatic transmission repairs can run several thousand dollars.

A manual transmission is a simpler machine; less moving parts and only a single clutch. When a manual transmission requires a rebuild, it typically includes replacing bearings, seals, and synchronizers. Having a manual transmission rebuilt or replacing a manual transmission with a remanufactured unit is therefore less costly in most instances. However, the clutch life on a manual transmission is much less than the expected life of an automatic transmission clutch pack. When factoring in periodic clutch replacements and clutch master cylinders/slave cylinders, the overall cost of ownership may level out between a manual and an automatic. The service complexity, however, tends to be lower for a manual transmission.

Transmission Weight - One would expect a manual transmission to weigh significantly less than a comparable automatic transmission, but this is not necessarily always true, especially in the diesel category where relatively high torque and payload capacities necessitates the need for large, heavy gearsets in manual transmissions. It's true that many manual transmissions weigh less than their automatic transmission alternatives. However, the New Venture NV5600 for example, weighs in at roughly 360 lbs - that's equivalent to the weight of some medium duty automatic transmissions. Because transmission weight varies considerably, neither transmission type holds a definitive advantage in this category. In instances where a manual transmission option is lighter than the automatic, the weight difference is negligible in the grand scheme of things.

Comfort, Convenience, and Control - Modern automatic transmissions often include a manual shift mode, sophisticated engine braking shift schedules, and integrated exhaust braking technologies. As a result, it is difficult to declare whether a manual or automatic transmission is more advantageous in this category. Prior to sophisticated advancements in automatic transmission technology, a manual transmission was advantageous in exhibiting the ability to use gear selection as a means of slowing down and/or maintaining a safe decent speed by progressively selecting a lower gear. Since many automatic transmissions shift schedules include downshifting and torque converter lockup techniques to provide the same characteristics, neither transmission type has a significant advantage.

In terms of convenience and comfort, any argument that lends the advantage to a manual transmission is clearly flawed. An automatic transmission does all the shifting for you, and therefore a vehicle can be operated with one hand and one foot. Meanwhile, operating a manual transmission equipped vehicle requires two hands and both feet. The automatic transmission is therefore much more convenient and comfortable for the operator; one may also argue that the automatic has a slight advantage in terms of safety.

Gear Spread and Progression - An automatic transmission contains a series of planetary gearsets, while a fully synchronized manual transmission consists of a series of constant mesh helical cut gears. In a true truck manual transmission, the 1st gear ratio will always be considerably lower (numerical higher) than that of an automatic transmission. Take the Allison 1000's 3.10 to 1 compared to the ZF 6 speed's deep 5.79 to 1 ratio; the manual is geared much lower, which presents obvious advantages in terms of gear multiplication. A lower gear ratio (numerically higher) yields greater torque multiplication and therefore a higher tendency to overcome load with less effort.

However, automatic transmissions feature a unique device called a torque converter. While the function of a torque converter is to transmit power between the engine crankshaft and transmission and negate the need for a manual clutch system, a torque converter also acts as a torque multiplication device. When a torque converter slips, as it is intended to do, it serves as a gear reduction unit. As an arbitrary example, take the Allison 1000's 3.10 to 1 ratio at a 1.7 to 1 torque converter slippage rate. The overall reduction at the transmission output shaft would therefore be 3.10 x 1.7 = 5.27 to 1; much closer to the manual transmissions deep 1st gear.

One could argue that slipping the clutch in a manual transmission serves the same function, and they would absolutely be correct. However, the clutch in a manual transmission is intended to be slipped as little as possible as this practice contributes to excessive wear. A torque converter, on the contrary, is designed to experience slippage through all gears and can, within reason, endure extended periods of operation in this manner. In terms of gear progression, neither transmission is inherently advantageous as the gear spreads depend on the individual transmission designs and are not necessary attributes directly related to the type of transmission.

Efficiency and Parasitic Loss - Parasitic loss through a manual transmission is the result of frictional losses (bearings, contact between gear teeth), the weight of the rotating assembly (input/output/intermediate shafts, individual gears), and any shearing force between a rotating gear and lube oil bath. These losses are typically low and a manual transmission tends to be highly efficient. Parasitic loss through an automatic transmission also includes these factors, but in addition to supplementary losses in the transmission fluid pump and torque converter. An automatic transmission must use engine power in order to pressurize fluid, and the energy required by the pump is not transmitted into usable drive power at the transmission output. Though an automatic transmission is not necessarily inefficient, a manual transmission is inherently more efficient.

Torque and Payload Capacity - With a manual transmission exhibiting inherently less complex mechanical characteristics, it is completely normal to expect a manual transmission to carry a greater load capacity. However, this is not necessarily the case, and the reasoning is interesting. The diesel pickup category reached a milestone near the 2010 model year; they outgrew all available (and more importantly, feasible) synchronized manual transmissions. The Mercedes Benz G56, offered with the 6.7L Cummins, is available only with an engine detuned to 660 lb-ft (based on 2016 model year). This seems to be the ceiling on available light and medium synchronized manual truck transmissions. That's not to say that a manual transmission suitable for a pickup application could not be engineered for a higher input torque rating. However, there has not been a great enough demand to tempt a manufacturer into designing such a gear box. Strength and tow capacity is therefore not necessarily directly related to the type of transmission, but the automatic has a slight advantage in the pickup market do to the lack of availability of suitable manual transmissions.

The ironic part is, at least for Dodge/Ram, the opposite was true in years past; for many model years, the Cummins was detuned when equipped with an automatic transmission. This stems from the fact that Chrysler's automatic transmissions were, at least in some instances, insufficiently adapted to the Cummins and lacked the strength necessary to manage the high torque characteristics of a diesel. In fact, when Dodge first tinkered with the concept of a "High Output" Cummins model, it was only available with the NV5600 manual transmission.

Manual vs Automatic Transmission Comparison

Transmission:

Ford 5R110W

Allison 1000

Chrysler 48RE

ZF 6 Speed

NV5600 6 Speed

Mercedes G56

Type:

5 speed automatic

6 speed automatic

4 speed automatic

6 speed manual

6 speed manual

6 speed manual

1st:

3.11 : 1

3.10 : 1

2.45 : 1

5.79 : 1

5.63 : 1

5.94 : 1

2nd:

2.20 : 1

1.81 : 1

1.45 : 1

3.30 : 1

3.38 : 1

3.28 : 1

3rd:

1.54 : 1

1.41 : 1

1.00 : 1

2.10 : 1

2.04 : 1

1.98 : 1

4th:

1.00 : 1

1.00 : 1

0.69 : 1

1.31 : 1

1.39 : 1

1.31 : 1

5th:

0.71 : 1

0.71 : 1

N/A

1.00 : 1

1.00 : 1

1.00 : 1

6th:

N/A

0.61 : 1

N/A

0.72 : 1

0.73 : 1

0.74 : 1

Weight:

300 - 350 lbs

330 lbs

248 lbs

230 lbs

360 lbs

~ 225 lbs

Max Input Torque:

1,100 lb-ft

775 lb-ft
(engine torque)

< 600 lb-ft
(engine torque)

650 lb-ft
(engine torque)

550 - 650 lb-ft.
(engine torque)

~ 700 lb-ft
(engine torque)

 

Manual, Automatic Transmission Advantages & Disadvantages - Summary

 

Manual Transmission

Automatic Transmission

Advantages

Less complex operational design

Convenience and versatility - no need to manually select gears

Potentially lower service/repair costs and frequency

Integrated engine and exhaust brake features (where applicable)

Highly efficient, low parasitic loss

Greater input torque capacity (based on transmission availability

Disadvantages

Gears must be selected manually, frequent driver input necessary

Higher service cost, more complex service procedures

No integration between transmission and engine with regards to shift strategy and engine/exhaust brake functions

Relatively high parasitic loss, lower efficiency than manual