Tech Corner With Levi: Standard Transmissions

Standard transmissions on the very first driven vehicles consisted of one forward gear and one reverse gear. Since their beginnings over 100 years ago, they have come a long way to say the least. In terms of machinery and vehicles, I don’t think Henry Ford ever thought one day the modern automobile could have so many advancements. DVD players, navigation systems, electronic suspension, and blazing top speeds are a stark contrast to the 20 mph top-speed of their predecessor over 100 years prior.

The axle and differential assembly, which we discussed in prior issues, had a lot to do with the gearing and top speed of a vehicle, but the transmission is the main contributing factor. In this column, transmissions for light-duty diesel pickups will be discussed and more particularly manual transmissions. First we will review the advances which have been made in standard transmissions over the years and along the way we will dissect all of a manual transmission’s intricate parts and compare gear ratios.

The Start

In the early 80s, Ford’s 6.9L diesel was equipped with the tried and true t-19 Borg Warner transmission. It was a four-speed and each forward gear was equipped with a synchronizer. The synchronizers aid in the synchronization or mesh of two gears to reduce or completely stop gear grinding when transitioning from one gear to another. This Borg Warner unit utilized a single countershaft and an idler shaft with the reverse idler gear. It had 10 square splines on the input shaft and measured 1 1/8-inch diameter. The t-19 was used by Ford and International in various applications and was the bigger brother to the t-18 transmission. The T-19 provided a low first gear ratio of 4.02:1. Fourth gear was direct or 1:1 ratio. Chevy equipped its light-duty diesel, the 6.2L, with the proven SM465 made by Muncie. The 465 had similar features to the T-19. The 465 had a 1 1/8-inch diameter input as well with 10 splines and featured a 19/32-inch end for the pilot bearing. They are both cast iron units, both used synchronizers between the gears, and both were top-load transmissions. This last feature made them very desirable when it came time to rebuild them. The SM465 decided to not synchronize low gear or reverse in their transmission. Low gear for the SM465 received a very low ratio of 6.55:1. As the years progressed, Dodge decided to place a diesel engine in its 1989 model pickup trucks. The Cummins engine was the power plant of choice by Chrysler and the German made Getrag G360 five-speed transmission was the manual transmission coupled to it. It had very similar features to that of the SM465 and t-19 transmissions aside from one special characteristic, an overdriven fifth gear.

New Venture Gear

Soon Chevrolet also followed suit with its 6.2L and 6.5L engines in the early 90s and replaced the SM465 unit with a transmission made by new venture gear. The NV4500 transmission also was equipped with an overdrive fifth gear and was still a top-load transmission. It had a cast iron case and featured carbon composite synchronizers. This particular model of transmission was also adopted by Dodge in 1994 and replaced the Getrag five-speed. Chrysler had a great relationship with new venture and offered both the NV4500 five-speed and NV5600 six-speed as an option behind its 24-valve Cummins engines from 1998 until 2004. In 2005, Dodge decided to utilize a transmission designed in 2002 for medium-duty trucks. This transmission was commonly known as the G56 and was manufactured by Mercedes. It featured an all-aluminum housing and a very peculiar double “H” gear pattern with reverse to the left instead of to the right of the transmission. Chrysler has used this transmission ever since.

ZF5 Transmission

If we rewind the clock, Ford motor company knew they would have to also compete with Chrysler and General Motors in the late 80s in the light-duty diesel transmission game and the t-19 four-speed transmission was replaced in 1987. The t-19 was replaced by the ZF five-speed made by the ZF Friedrichshafen group. This transmission was an industry-changer in ways. It sported an all aluminum case, five speeds with overdrive, sprung synchronizers and was a back-load transmission. Ford used the ZF5 transmission with both its 7.3L indirect injected diesel and 7.3L direct injected engine (Power Stroke). Ford utilized the ZF5 behind its diesel engines until the ZF6 transmission became an option in 1999. The ZF6 was a six-speed transmission, which was everything the ZF5 trans was and more. It featured one more forward gear and also had an all-aluminum housing. One extra feature the ZF6 had that no other light-duty manual transmission had was that it was equipped with an internal oil pump. This was to help dissipate heat from the friction of several constant mesh main shaft gears. It circulated oil to an external oil cooler in the front of the vehicle.

Improving The ZF6

Chevrolet later adopted the ZF6 for use behind its Duramax 6.6L diesel as an option to the Allison automatic transmission. Chevy asked that certain enhancements be made to the ZF6 used behind its diesel. Enhancements included a different bell-housing design, no external oil pump, steeper cut gears and quieter operating noise. This transmission was the last standard transmission which GM decided to use in its light-duty diesel before abandoning all manual transmissions behind its Duramax diesels in the 2008 model year. Ironically it was also the last transmission which Ford used behind its 6.4L diesel. In 2011 Ford also ditched its manual transmission option for all diesel-equipped pickup trucks. Currently the 6.7L Dodge Cummins diesel is the only pickup truck to offer a manual transmission option.

Gear Ratio

It is important to note what the gear ratio is for the gears in each one of the transmissions. But before we do, let’s talk about what gear ratios actually mean. For instance, the Muncie SM465 has an effective first gear ratio of 6.55:1. Let’s break this down.

If the engine is idling at 800 rotations per minute then we can safely assume the flywheel, clutch driven disc and pressure plate are also spinning at 800 rpms. Because the input shaft of the transmission is coupled to the clutch driven disc through a set of splines, then the input shaft must also be turning 800 rpms at idle when the clutch is engaged. If we put this transmission in first gear then the input shaft will turn 6.55 times for every one time the output shaft turns. So simple math tells us if the input shaft is turning at 800 rpms the output shaft will be only turning at about 122 rpms because we divided 800 by 6.55. So the output shaft turns slower than the input shaft dependent upon the level of the gear reduction.

The other thing that happens with gear reductions is that we slow down the speed of the output shaft, but we also compound the torque. If we slowed down the output shaft by 6.55 times then we also compounded the input torque by 6.55 times. If the particular engine mated to the SM465 transmission had 300 ft/lbs. of torque at idle into the input shaft of the transmission, then the output shaft would have about 1900 ft/lbs. or more because of the gear reduction. Similar inverse concepts can be held true for overdrive gears.

The G56 transmission has an overdrive ratio of .79 or a 79 percent overdrive. This means that for every time the input shaft turns one revolution, the output shaft turns 1.26 times. Overdrive gears in transmissions help to keep engine rpms low when traveling at highway speeds. It is important to remember although overdrive gears reduce engine rpms, they also reduce engine torque instead of multiplying torque like a gear reduction does.

By The Numbers

Let’s compare the gear ratios for each one of the particular transmissions found throughout the years behind light-duty diesel engines found in pickup trucks in the corresponding chart.

Several variations of the standard transmission have been made over the past two decades in the diesel pickup industry. Some have been very positive and some have not, but one thing is for certain, the manual transmission had a great part of the diesel industry until lately.

Technological advancements and part innovation has led to more automatic transmission advancements and what used to be a sore subject in the diesel industry, the automatic transmission, is now the only subject in terms of transmissions in pickup trucks with diesel engines aside from Chrysler. The next Tech column in the August issue of Diesel Tech will discuss the ins and outs of automatic transmissions and their rise from the dust over the years.

About The Author

Levi Perkins is the head instructor for the diesel program at the College of Southern Idaho located in Twin Falls, Idaho. The diesel program has had 100 percent placement in the last three years and provides future diesel mechanics and engineers a bright future in the diesel industry. For more information visit www.csi.edu or call 208-733-9554.