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Manual Transmissions Explained

Whether you are dealing with sprocket teeth on a bike or gear teeth on a transmission gear, the simple formula for calculating a gear ratio is:

 

Ratio = Driven ÷ Drive.

 


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So if the drive gear has 10 teeth and the driven gear 20 teeth, the ratio equals 2. This is expressed as a 2:1 or a 2.00 ratio. The drive gear must make two turns to make the driven gear turn once. Any ratio with a number greater than 1 is an underdrive ratio. A ratio less than 1 becomes an overdrive ratio. If the drive gear has 40 teeth and the driven has 30, the ratio becomes .75. This is expressed as.75:1. Sometimes overdrive ratios are expressed as a percentage. If the ratio is .75, the difference between .75 and 1.0 is .25 or 25 percent. Therefore, a .75 overdrive ratio is often called a 25 percent overdrive.

Because the average manual-shift transmission contains more than one pair of gears, the same formula holds true for each drive and driven set. The ratio of each drive and driven set is multiplied by each other to give the final ratio. The formula is:

 

Ratio = (Driven ÷ Drive) x (Driven ÷ Drive)

 

When looking at the nearby power-flow pictures for first-gear mode, you see that the power comes into the input shaft (drive), down to the countergear (driven), then from the countergear (drive) to the firstspeed gear (driven). You can now figure your overall gear ratio. Here is an example: Your input shaft has 21 teeth. The mating driven section of the countergear has 25 teeth. The first-gear section of the countergear has 17 teeth, and the first-speed gear has 36 teeth. Using the formula:

 

Ratio = (25 ÷ 21) x (36 ÷ 17) = 1.19 x 2.12 = 2.52

 

Your first-gear ratio is 2.52:1. Gear ratios are a very important aspect of transmission selection and transmission design. Ratios can help determine proper application as well as the torque capacity of a transmission. Two areas often overlooked when selecting a transmission are gear ratio and center-tocenter distance.

The center-to-center distance is the distance between the centerlines of the upper and lower geartrains. To help visualize why a center-to-center distance is important, here is an extreme example: You can have two gearsets. Both sets have a 20-tooth driven gear and a 10-tooth drive gear; however, one set has a center-to-center of 1 inch while the other has a center-to-center of 31⁄2 inches. The set with the larger center-to-center obviously has larger teeth and bigger gears. This yields a stronger transmission, but with a heavy geartrain. The transmission with the small center-tocenter may shift easier, because of the lighter mass of the geartrain, but it will be weaker.

 

Power flow in neutral.

 

Power flow in first gear.

 

Power flow in second gear.

 

Power flow in third gear.

 

Power flow in fourth or direct gear.

 

Power flow in fifth gear.

 

Power flow in reverse.

 

This chart shows different ratios of popular manual gearboxes. The Muncie M21 and M22 close-ratio provide the narrowest range of gear ratios while the T56 provides the widest range of ratios through the gears.

 

The proper selection of a transmission for a particular application is based on this idea. Thus, a car with a 150-hp engine that weighs 1,800 pounds would be robbed of performance if the engine had to turn a transmission designed for a 600-hp 3,000-lb vehicle. Torque capacity seems to be the latest buzz word. Published torque ratings of transmissions are often misleading as well as misunderstood. As mentioned above, longer centerto- center distances improve capacity. Gear ratios affect capacity in several ways. A 3:1 ratio compared to a 2:1 ratio along the same center-to-center distance is usually weaker, by the nature of gear design. Fewer teeth are needed to obtain a larger reduction. A smaller-diameter gear with fewer teeth is weaker than a larger-diameter gear with more teeth.

There are some trade offs. The more teeth you put on a gear within a fixed diameter, the finer the pitch of that gear. This finer pitch results in a gear-tooth profile with a thinner cross section in contrast to a gear of the same diameter with fewer teeth. You can actually improve the strength of the gear by giving it a bigger diameter with fewer teeth. In mass-produced transmissions, the manufacturer very rarely makes a separate profile for each gearset. That is what the specialty performance shops do. So what does a published rating of 300 ft-lbs of torque really mean? A torque rating is not a static rating.

 

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This means you don’t stick a breaker bar and lock the transmission in two gears and apply 300 ft-lbs of torque to the input shaft and watch something snap when you exceed the 300 pounds. Consider a torque rating as a life factor of a transmission based on a particular application.

After speaking in depth to various engineers from several manufacturers regarding this issue, I’ve come to the conclusion that you can think of a torque rating as being a dynamic rating based on a theoretical life factor. For example, let’s say we feel a transmission should yield good service for 100,000 miles in a car that has an engine that produces 300 ft-lbs of peak torque. Obviously, the transmission is never seeing that engine’s peak torque all the time. Actually, unless you have no rear axle, the torque coming out of the engine is absorbed by both transmission and rear axle. In fact, a dead rear-axle ratio, such as a 3.08 rear, will cause the transmission to load more than a 4.11 rear.

You can exceed the rating of the transmission but you will shorten the transmission’s life factor. The rating is actually a safe benchmark. Different manufacturers arrive at published ratings using different methods of calculations, making it really impossible to compare one transmission to another.

The Tremec T5 5-speed originally designed by BorgWarner had its published ratings actually change when Tremec took over the line. The T5s have their published ratings exceeded every day. The ones that survive probably have better rearaxle gearing, such as a 4.11. Transaxles designed for the 24 Hours of LeMans race are loaded on a dyno to simulate real-life loads against engines producing more than 1,000 hp. They are designed to last for that one race. I prefer real-world applications and factual data as opposed to hypothetical data any day. So keep an open mind when asking about or reading published ratings.

Today, more and more people are swapping various engines and transmissions into custom projects. Few take the time to understand these basic principles. Much energy, time, and money are wasted trying to adapt Toyota Supra transmissions to Jaguars or 4-cylinder S-10 Truck 5-speeds to a 1968 Camaro. I am asked these “Will it work?” questions every week. In some parts of the world, certain transmissions are more readily available than others. Australia has an over abundance of Toyota 5-speeds, thus people are always trying to stick them into everything in sight. The point is to save time and money by selecting the proper application for a project.

 

Wide- and Close-Ratios Explained

 

What does having a wide- or close-ratio manual-shift transmission mean? First of all, it has nothing to do with how close or wide your shifter moves within its pattern. That is called having a “long- or short-throw” shifter. I’ll discuss here a simple method that will show what close and wide ratio is.

There is no particular formula that determines whether you have a close- or wide-ratio gearbox. I say this because what was considered wide ratio in the 1960s is considered close ratio today. Confused? Since you now know what a gear ratio is, the difference between two gear ratios is called a ratio spread or drop. That drop is a percentage of the previous ratio. Here is a formula to calculate that change:

 

Percentage Drop = (Ratio 1 – Ratio 2) ÷ Ratio 1

 

For example, let’s say we have a close-ratio 4-speed Super T10. The ratios are: 2.64 first, 1.61 second, 1.23 third, and 1.00 fourth.

 

(2.64–1.61) ÷ 2.64 = .390

(1.61–1.23) ÷ 1.61 = .236

(1.23–1.00) ÷ 1.23 = .186

 

Rounding up a bit, you can see that the ratio change between the gears is: 39 percent, 24 percent, and 19 percent.

BorgWarner’s extra-low-ratio Super T10 had a 2.88 first, 1.75 second, 1.33 third and 1:00 fourth. If you plug these gear ratios into the above formula, you end up with the following drop: 39 percent, 24 percent, and 25 percent.

Think of gear ratios as a distance between two points. In a typical muscle car, 4-speed application fourth gear is direct, or 1:1 ratio. Again, thinking in terms of distance, consider fourth gear or direct as your final destination. The further you get away from 1:1 (direct) the wider the ratio.

The Muncie M21 and M22 closeratio 4-speeds were the closest ratio 4-speeds ever put into a production car. Their ratios were 2.20 first, 1.64 second, 1.28 third, 1:00 fourth. If you plug those ratios into the formula, you end up with 25 percent, 22 percent, and 22 percent. Compare these percentage drops to the previous drops found using the Super T10. Notice that although the 2-3 and 3-4 drops of the Super T10 and Muncie are pretty close percentage- wise, but the 1-2 drops vary quite a bit. The reason is that the Super T10s have a lower (higher numerical) first gear. The 2.88 and 2.64 ratios are further away from direct than the Muncie’s 2.20 first-gear ratio.

 

What Makes a Close-Ratio Close?

As you have learned, you can’t gain distance without losing closeness. The lower the first gear, the wider the 4-speed will be. If one takes a historical look at the close-ratio gearbox, it usually had 25 percent or lower drops across all gear spreads. The problem is: the Muncie closeratio box was designed in the 1960s when we really didn’t worry much about gas mileage. In order for you to get a “close ratio,” a Muncieequipped car had at least a 3.70 rearend gear. You needed at least that low of a final drive to get your car moving. The “wide ratio” M20 had a 2.52 first gear. You could get a 3.31 or 3.55 rear with that gear and gain a little more economy, but it wasn’t cool to have what was considered a wide-ratio box in your Corvette.

By late 1974, fuel economy and air pollution became considerations, and catalytic converters were mandatory by 1975. Axle ratios had to drop to get the improved gas mileage the Environmental Protection Agency (EPA) demanded. So in order to get cars moving with 3.08 or 2.88 rear-end gears, cars that had engines with decent amounts of lowend torque were equipped with “close-ratio” transmissions. Larger engines carried transmissions with 2.64 first gears while smallerdisplacement engines’ first-gear ratios were in the 3.0 range. So the newer close-ratio transmissions are actually wider than the older wide- and closeratio transmissions!

 

 This chart shows different ratios of popular manual gearboxes. The Muncie M21 and M22 close-ratio provide the narrowest range of gear ratios while the T56 provides the widest range of ratios through the gears.

 

Doug Nash Corporation had an interesting solution to this dilemma. It developed a 5-speed box with a close-ratio spread like a Muncie M21 with a fifth-gear direct. Again, think about “distance between two points.” The more stops you make in your travel to get to your final destination, the closer the distance between your stops. By adding a lower first-speed gear and keeping a direct fifth, you gain more distance, but add an extra stop. The Doug Nash Street 5-speed came with a 3.27 first gear. This allowed drag enthusiasts to use a 3.08 final drive and still get good close-ratio acceleration. It worked great.

Today, cars produce a great deal of low-end torque. Most peak power is made in the 4,500-rpm range. The average 5-speed, such as a T5, is an extremely wide ratio by 1960s standards. The close-ratio T5s used by Ford Motorsport have a 2.95 first, 1.94 second, 1.34 third, 1.00 fourth, and .80 fifth. You do the math with the formula. These new 5- and 6-speeds, in a sense, are really wide-ratio 4-speeds with additional overdrive gears for fifth and sixth speed. Cars can now cruise at 1,800 rpm at 70 mph because the engine’s torque curve can handle the load. If you are still thinking in terms of distance, not only are you getting further away from our final destination of direct drive, but you are now going past your final destination into two levels of overdrive.

Chrysler’s PT Cruiser has a 2.4L 150-hp 4-cylinder engine. The engine produces 162 ft-pounds at 4,000 rpm, and the 5-speed transaxle has a final drive of 3.94! The first ratio is 3.50, second is 1.96, third is 1.36, fourth is .97, and fifth is .81. This is really a 3-speed with two overdrive ratios. The engine doesn’t produce power until it hits 4,000 rpm. Thus, the gearing sort of works, but the car can be boggy at lower RPM.

 

Goodbye Wide- and Close- Ratios

The chart on page 14 shows shift points of some popular transmissions. For the most part, cars are no longer offered with close- or wideratio transmission options. If you look at the chart, you see that by plotting stops of a distance traveled, you realize what close or wide ratio may mean. In one sense, if you compare stops, some transmissions have similar distances between one or two stops while others are drastically different. What is close or wide ratio is no longer an issue. Rather, how much percentage drop your engine can handle is the issue.

Road racing events, such as NASCAR- or SCCA-type races, still require transmissions with a closeratio spread. It is common for some of these cars to have 1.86 first-gear ratios. Plot a gearbox on the chart on page 14 with a 1.86 first, 1.59 second, 1.17 third, and 1.00 fourth. How do those results look in comparison to the others? What percentage drops does that same box create using the formula? In our example, a low first gear is not needed because road race cars tend to operate at the higher range of RPM and speed with no stopping other than for a pit stop. An ultraclose- ratio gearbox has other advantages as well. Since the load changes are not as severe, drivetrain parts tend to live longer. The high-shock loads of wide-ratio transmissions usually cause gearbox failure and rear-axle failure.

Less heat is generated in a closeratio box. A car having a final drive of 2.98 and a direct fourth gear has the same overall ratio as a car with a .80 overdrive fifth and 3.73 final drive. The car with the overdrive uses more horsepower and generates more heat through the transmission than the direct-drive box. However, the 3.73 rear may offer more lowspeed punch on turns.

 

The How and Why of Synchros

Historically, manual transmissions were defined as either progressive or selective. When you shift a transmission, you are disengaging one gear from the mainshaft (also called the output shaft) and engaging another. A progressive transmission has one massive gearset with one shifting mechanism. When you shift you mate this single one-piece set to various gears on the countergear assembly. Thus, it has one lever. A selective transmission has individual gears that are locked to the output. A typical 4-speed transmission has three levers, one lever each for 1-2, 3-4, and reverse. How you make the transition from one gear to another is accomplished by sliding one gear into another by use of a synchronizer.

“Sliding gear technology,” as I call it, is often misunderstood. Today, very few transmissions use sliding gears, but any old 4-speed still uses a sliding non-synchro reverse gear. The later T5 5-speeds used a sliding reverse and then a synchronized reverse brake. I sell more reverse gearsets because people see chipped teeth and think they need to be replaced. In many cases, they probably don’t need to be replaced and will be chipped within a few months anyway. A sliding gear has no way of stopping the gear it is being mated to, so it will usually grind and chip the leading edges. What most people don’t realize is that the gear, when fully engaged, over-hangs the mating gear. The edge that initially contacts the mating gear slides past it and overhangs on the back side, while the mating gear overhangs on the front side. Consequently, this area is designed to be chipped! Usually, if the gear is chipped no more than one-quarter of the way, the gear will be fine.

  • Synchronizers make sure the selected gear is locked to the output shaft at the same speed so no grinding occurs. Every performance manual-shift transmission that uses synchronizers follows these rules:
  • All gears are in constant mesh; this means that they don’t need to slide into one another. They are, in fact, already spinning as a matched set.
  • Although the gears are in constant mesh, they are floating on the output shaft. • The synchronizer is always locked to the output shaft.
  • The synchronizer’s job is to lock a particular gear to the output shaft, so power can now flow through the gear and out of the shaft.

Understanding theory, components, and operation of the manual transmission is the key to successfully diagnosing problems. Many people think they know how synchros work, but usually misdiagnose because of a poor understanding of the theory. Let’s see if we can resolve this mystery once and for all.

Synchronize, as defined by Webster’s Dictionary is “to cause to move, operate or work at the same rate and exactly together.” In fact, you are trying to get a speed gear to operate at the same RPM as the mainshaft so you can lock it to the shaft without any hint of a grind in the split-second called a “shift.” The synchronizer (or synchro) in a common performance transmission is made up of a hub, slider (also called a clutch), strut keys, springs, and synchro rings (also called blocker rings). The hub is splined to the output shaft. The slider is the sliding component that physically mates the speed gear to the hub, thus locking the speed gear to the mainshaft. In the process of moving the slider to engage the selected gear, strut keys track with the slider and exert pressure on the synchro ring. In turn, the gear slows down to allow the slider and gear to couple.

 

 Leading edges of gears chip, but then overhang once the gears are engaged.

 

In the downshift mode, the slider is powered. In order for the slider to mate with the gear, it must index behind the gear. Whether you are upshifting or downshifting, a “blockout” condition can occur in which wear of the synchro assembly allows the ring and slider to meet point-topoint. The arrow shows the direction the ring must take in order for the slider to continue on its path to locking on the gear.

 

In the upshift mode, the gear is powered. In order for the slider to mate with the gear it must index ahead of the gear.

 

 The back cut is referred to as a torque-locking spline, which forces the gear to stay engaged.

 

The synchro pictures on pages 10 to 12 show several important aspects of synchro design. They show how a synchro ring relates to the gear on the upshift in contrast to the downshift.

During the upshift, as the slider moves toward the selected gear, the slider’s keys exert pressure on the ring, which tries to lock onto that gear. The drag of the ring against the cone of the gear forces the ring to wind up in the same direction the gear is moving.

The struts have a two-fold purpose. The first is to apply pressure to the ring so that it can lock on the gear and the second is to “index” the ring. Proper synchro indexing is the ticket to a clean and effortless shift.

Note that the ring must move in the opposite direction in order for the slider to pass through. When you shift, you press down on the clutch pedal, which temporarily uncouples the transmission from the engine. Once uncoupled, the gear can slightly “blip” in the opposite direction and complete the shift.

It’s important to realize that the complete gearset is in constant mesh. As a result, the inertia of all the gears not any one particular gear is overcome in order to make a shift. During a downshift, the same series of events occurs. The difference is that the power flow starts at the output shaft. Since power flow switches, the synchro rings use the opposite side of the teeth on a downshift. Sometimes a bad synchro will work on an upshift but grind on a downshift because the ring uses different aspects of itself.

One common misconception is that synchro rings cause jumping out of gear. In reality, synchros prevent grinding in gear. Therefore, if you make a clean shift and cannot keep the transmission in gear, you usually have worn “hard parts,” such as a gear, shift fork, or slider. It can be one item or a combination of any of them.

The tiny clutch teeth transmit the power. If they get worn or twisted from lugging a transmission, the spline of the slider applies uneven pressure on the worn tooth and allows the two parts to slip away. A worn shift fork may not allow the slider and gear to engage fully, and a worn fork groove causes that same problem.

Torque locking sliders have the ends of their splines back cut. This, in effect, causes the clutch tooth of the gear to sort of fit in a pocket within the slider’s spline, or it forces the gear to ramp in toward the center of the slider, whether you’re on or off the gas. This design is common in most new transmissions. Some aftermarket gears for older transmissions are adding these features as well.

 

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A synchro ring is nothing more than a cone clutch. The ring has a tapered inside diameter, which sits on a tapered cone of the gear. The ring has tiny threads, which lock onto the cone of the gear. In order for the ring to grab the gear properly, the ring’s inside diameter has to be concentric with the mating surface of the gear, and it also has to be smaller. A good ring won’t wobble or sit low on the gear. When a transmission grinds going into gear, the synchro ring has lost its ability to stop the gear. The ring is either warped or worn so much that no interference fit is left. I stress this issue because synchro rings are often blamed for jumping-out-of-gear problems or block-out problems.

 

Gear Design Basics

Today’s transmission gears are computer designed to yield the maximum capacity in the smallest package. Fuel efficiency and preventing frictional losses are paramount. Often, people insist on purchasing NOS (new old stock) gears from the 1960s rather than new products offered by current aftermarket vendors. When it comes to old transmission gears, some people can’t comprehend that in 40 years better methods of manufacturing have developed. The gearsets today can be made with more finely pitched helix angles to reduce noise. In addition, exotic alloys and profiles, which were next to impossible to mass-produce in the 1960s, are now being produced. Heat treating has also reached new levels of accuracy and control.

 

A synchro ring that sits correctly on the cone of the gear usually has a clearance of 0.050 inch.

 

The same bearing different raceways designed for 5/16- and 1/4-inchdiameter balls. The bearing that accepts the larger ball has a higher load rating.

 

Early spur-gear designs were very strong but noisy and inefficient. Most 3- and 4-speed transmissions of the muscle car era used helical gears for quiet, efficient operation. General Motors’ M22 Muncie 4-speed was the first semi-helical gearbox offered in a passenger car. The gears were at half the normal helix angle, creating a stronger but noisier transmission. This gear noise earned it the “Rockcrusher” nickname. Helical gears, because of their climbing nature, produce more heat and thrust loads. A spur-gear transmission hardly produces any thrust loads. Most transmissions destined for endurance racing use spur-gear designs because less heat is generated and little thrust loads are put on the bearings.

When BorgWarner released the T5 5-speed in 1982, die-hard manualshift enthusiasts laughed at the whole design. The T5 was the first American transmission that was truly computer designed. It was never intended for drag racing or road racing. It sported a smaller center-tocenter distance than older muscle car 4-speeds and was designed for lightto medium-duty use. Now, nearly 30 years later, the OEM is still using it, and countless applications have been developed based on that radical design. This transmission started to change people’s perception of transmission design, especially when it came to bearings.

 

Bearing Selection

Some form of a bearing supports upper and lower geartrains in any transmission design. The obvious reason for bearing use is to reduce friction. A properly selected bearing for an application withstands several different loads. The amount of thrust load, radial load, and RPM required for a particular application demands a certain style of bearing design. Helical gears produce both thrust and radial loads. Tapered bearings as well as ball bearings can handle both of these loads simultaneously. That is why they are frequently used in transmissions. Needle bearings are often found supporting countergears, but they cannot handle any thrust loading. That is why countergears employ the use of thrust washers.

Space can be a deciding factor for bearing selection as well. Tapered bearings (often called cupand- cone bearings) can handle the same loads as a ball bearing but in a smaller format. Since tapered bearings are of a two-piece design, they require a preload or some means of keeping the cup and cone together. This is accomplished using a design that requires shims to set the preload. Usually a transmission that strictly uses tapered bearings is more expensive to produce than one using ball bearings.

There are always gains followed by losses when choosing a bearing. A big bearing may be able to handle a certain capacity issue, but the larger size may require a bigger case or longer center-to-center distance. There has been a debate about whether you can increase the capacity of a ball bearing by fitting the maximum allowable number of balls into the raceways. You can, up to a point. A standard ball bearing is called a “Conrad”-style bearing, and the bearing with the maximum number of balls is called a “Max Capacity” or “Filling Slot” bearing. Conrad-style bearings can have high raceway walls allowing for highthrust capacities. The filling-slot bearing literally has slots in both the outer and inner raceways. The maximum number of balls can be dropped in the slots before the bearing cage is riveted into place. The problem is that the filling slot reduces thrust capacity because the raceway walls now have an opening or may not be as high as a Conradstyle. However, this added number of balls increases radial load or up-anddown load.

 

 A reduction in rated thrust capacity decreases the thrust loading of the bearing, which is not desirable when used with helical-cut gears because the drive angle of the gears naturally produces a thrust load. The filling-slotstyle bearing is not recommended to use with helical gears.

 

Excessive heat in a filling-slot bearing caused the bearing cage to spread a drop on the inner raceway. Sometimes when bearings have too tight of fit tolerance heat will destroy them. To avoid this problem, I don’t recommend using max-capacity bearing. Instead, stick to bearings that have a C3 fit rating.

 

Bearing design and ball size present some trade-offs as well. If you have a fixed diameter, you can fit more 1/4-inch-diameter balls than 5/16-inch-diameter balls in the same complement. One bearing may handle loads in one direction better than the other, and one may have more stability than the other. Ball bearings designed for industrial applications may have tighter clearances than the same-size bearing used in automotive applications. General automotive grade bearings have what is called a C3 fit. This looser fit enables dirt to pass through the raceways better. Part numbers of bearings are usually etched on the raceways.

All race and street transmissions I build use regular Conrad bearings. Max-capacity bearings develop more heat, which also kills the bearing’s life. I’ve actually seen more failures with max-capacity bearings and generally avoid using them. The fact that a Conrad bearing is less expensive is an added bonus.

By the early 1980s we saw a trend in which transmission manufacturers placed needle bearings under each individual speed gear. Before that, gears rode directly on the shafts, with oil grooves or machined valleys for oil to gather in. It was an inexpensive process that made assembly very easy. Another alternative was sintered iron or bronze bushings. Almost all of the transmissions produced today use needle bearings under all the gears. This produces less drag (peristaltic drag) and allows the transmissions to shift easier. It also aids in achieving better fuel economy. Use of needle bearings under individual gears also prevents seizures at high speeds. Some early T10 and Muncie 4-speeds had rollerized first-speed gears. When used in endurance race situations, the first gear would get hot and weld itself to the mainshaft. Since first gear can be spinning more than two times faster than the output shaft it rides on, a car running at 7,000 rpm can come to a pretty nasty stop if first gear overheats.

When I think about the huge number of changes in the past few years in manufacturing, one thing sticks in my head. Advances in machine design and gear design software are allowing more and more people to create stronger, more efficient and easier-to-shift transmissions. The world is truly a better place to shift in.

 

Written by Paul Cangialosi and Posted with Permission of CarTechBooks

 

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