Manual transmissions are crude, simple devices. Most of the internal parts are big, heavy and made from steel alloy, which makes them tough. The basic technology—helical gears and sleeve synchronizers—has been around for more than 100 years. This is good, because we enthusiasts tend to ignore our gearboxes until something goes wrong—like when nasty noises appear or the shifting becomes hard.
Step 1: Learn to Shift
The No. 1 thing you can do to keep your transmission alive—and the one thing that people commonly ignore—is to treat your gearbox with respect. Learn to shift with some mechanical sympathy, whether you’re driving on the street or at the track.
With a modern, production car gearbox, the easier you make the synchronizers’ jobs, the better. To extend the life of your synchronizers, push in the clutch all the way with each shift and change gears firmly but gently—don’t bang into the stops. The synchronization process takes a fraction of a second to complete, and forcing a shift before the synchronizers are finished is a recipe for hard shifting down the line.
It isn’t necessary to double-clutch with every downshift, but a quick tap on the throttle pedal while the clutch is in can go a long way toward reducing the speed differential between gears. This matching of revs takes some practice, but the payoff in transmission life is worth it.
The subject of downshifting to use engine braking on the street is somewhat controversial: Modern cars have plenty of braking power, so it isn’t really needed. Also, would you rather place wear and tear on your clutch or the brake pads?
Drag racing and standing start launches, of course, are hard on transmissions. This is especially true for rear-drive cars fitted with lightweight transmissions. The shock loads from a dumped clutch can be high enough to rip the teeth right off the gears. Even a transmission strong enough to survive thousands of miles of road racing (assuming flying starts) will be much too light for drag racing with any kind of reliability.
Some modern cars—the Lancer Evo comes to mind—actually have transmission protection built into the clutch hydraulic system. A small-diameter restrictor in the clutch line slows the rate that hydraulic fluid returns to the master cylinder, delaying the release of the slave cylinder and clutch fork.
Since the clutch cannot release quickly, it slips slightly and acts as a fuse, dissipating some of the power that might otherwise shock the transmission. Of course this reduces clutch service life somewhat, but clutch plates are cheaper than transmission gears and driveshafts. Take a lesson from the Mitsubishi engineers: A slipped-clutch start is less dramatic, but it will keep your transmission alive much longer.
Step 2: Use the Right Lube
The most important internal part of a manual transmission—the lubricating oil—is probably the least considered and certainly the least understood component. Lubricant not only keeps the gears wear-free and cool, but it also impacts noise, temperature, transmission life, synchronizer performance and power-handling ability.
Although at one time it was almost universal, these days not all manual transmissions are filled with gear oil. Factory fill in a modern gearbox can be anything, from automatic transmission fluid to gear oil, engine oil or a specially formulated lubricant. Unless you have a good reason for doing so, don’t stray from the manufacturer’s recommended lubricant type. Lubrication needs vary depending on the unit’s internal design, synchronizer material and intended use—delivery truck transmissions require stronger lubricant than car transmissions, for example.
Choosing an oil for a street application is fairly simple. OEM gears have fine, small teeth that have lots of load sharing and low contact pressures, and most street driving will never heat the transmission enough to stress the oil significantly. For most stock applications, the best lubricant will be a high-quality synthetic oil in the manufacturer’s recommended viscosity and API (American Petroleum Institute) rating.
The EHL (elastohydrodynamic lubricant) film thickness of synthetic oil tends to be more stable than that of mineral oil. That makes synthetic oil a very good choice for any high-performance application. A thinner oil can be used with less power loss and fewer detrimental effects on the gears, or a thicker oil can be run to provide a thick EHL film even at high temperatures in a heavily stressed transmission.
If you want to experiment with different oils in a stock transmission, make sure you understand the alternatives and choose based on sound principles. If any problems develop, however, be ready to swap back to the recommended fill as soon as possible.
If the transmission specifies automatic transmission fluid, stick with it since the synchronizer blocking rings may be damaged by anything else. Unless the transmission specifies API GL-5 lubricants only, GL-4 oil will most likely be a better choice since it has a lower percentage of EP (extreme pressure) additives that reduce synchronizer efficiency.
Step 3: Adjust the Clutch
A clutch that does not release completely is one of the most frequent causes of excessive wear and bad transmission performance. If the clutch doesn’t release, the synchronizers have to fight the drag and will wear much faster, if they can work at all. Therefore, always check clutch adjustment when you have the transmission out of the car, or when you suspect a problem.
If everything looks okay, you can perform a simple test to check for a dragging clutch. While the engine is running and the parking brake is set, disengage the clutch and put the transmission into any gear. Keeping the clutch pedal down, pull the lever out of gear, and then immediately push it back into the same gear. If it grinds on the second attempt to shift into gear, the clutch is probably not completely releasing.
This test works for transmissions both with and without synchronizers. A clutch that fails the drag test could have one of several problems, but the most likely are a worn or badly adjusted linkage, sticking input shaft splines or a failed pilot bearing.
Incidentally, give your motor mounts a once-over at the same time you check the clutch. A broken or sagging mount can sometimes cause the shift or clutch linkage to bind up.
Step 4: Tighten the Shifter
If your car has vague, notchy shifting even after the clutch, fluid and driver are in perfect condition, first make sure the factory linkage has new, tight bushings and is adjusted properly.
If this doesn’t help things, try using a weighted shift knob. Weighted knobs improve shift feel because they add inertia to the shift lever—the initial push to get the lever into gear requires a bit more force, and the inertia of the knob helps snap the lever more positively into gear on each shift. Due to the differences between shift linkages and transmission detent designs, there is no one knob that will work for all transmissions. Plan on trying a few before you find one that gives the results you’re after.
While not a “must-have” part, short-throw shifters top the list of aftermarket transmission modifications. A short-throw shifter—often simply called a short shifter—is a linkage modification that reduces the distance the shift knob must travel to engage each gear, allowing for faster shifts (at least in theory). They are inexpensive, generally easy to install, and can be effective.
Unfortunately, the shortened throw of an aftermarket shifter on a synchromesh transmission does not really reduce the time it takes to make each shift. The synchronizer assemblies determine the delay, as they must take time to accelerate the next gear on an upshift, and decelerate it on a downshift. In fact, forcing the shift before the synchronizer allows it can result in a bent shift fork or damaged synchronizers.
That’s not to say that short shifters are worthless. On a non-synchronized transmission, precise, quick shifts are an absolute necessity, and the shortest, tightest shifter available should be used. Any slop or delay in the shifting process can severely damage the dogs and sliders.
For synchronized transmissions, the biggest advantage to a short shifter is an improvement in shift feel that comes from replacing the factory rubber bushings and reducing play in the linkage. This can improve transmission life by allowing more precise, accurate shifts. Some short-throw shifters (like many of those for the Tremec T-5) are an improvement because they incorporate adjustable stops into the shifter mechanism, preventing shift linkage over-travel and damage.
Short-throw shifters vary greatly in quality, and some of the cheaper ones cannot stand up to even normal usage. A broken shifter can result in bent linkage, tweaked shift forks and synchronizer damage, so it pays to investigate your options carefully.
Step 5: Time to Rebuild
If you still have problems with the transmission despite meticulous care, you can either replace it with a different transmission or rebuild your existing box to handle more power. The first choice varies in difficulty, but it might give you a stronger gearbox with a smaller cash outlay.
The second choice is usually the only one, and it’s not a bad way to go if your current gearbox isn’t drastically weak. The first stage of transmission modification is simple: a careful rebuild, using quality bearings and new synchro rings. This will solve failures caused by running worn-out parts, but it won’t do much to strengthen the transmission.
If you spend a bit more time and money during the rebuild, you can end up with a much stronger gearbox. Start by deburring and shot-peening all the internal parts. Then do your homework to find out what updating and backdating options are available. For example, manufacturers often update to better synchronizers and bearings as a transmission’s production life progresses.
During a rebuild, choose only the best parts you can find, including bearings and seals from a reputable manufacturer. As you assemble the shafts, set them up on the tight end of the manufacturer’s specifications. Most bearings should be assembled “tight,” with a bit more than recommended preload. The bearing outer races are held in the aluminum case, and therefore move further apart, while the inner races (mounted to the steel shaft) stay relatively stable. This causes the initial preload to become looser as the transmission gets hotter.
Step 6: Shed Your Synchros
If you’re building a true race car, don’t bother with a synchronized transmission if you can avoid it. If you are stuck with a production-based transmission and are allowed by your rules and budget, eliminate the synchronizers and convert it to a dog ring box.
The conversion can be performed in one of three ways: The existing gears and synchronizers can be replaced with gears and sliders machined as a unit with the dog ring; the existing gears can be modified by welding and machine work and mated to new sliders; or the existing gears and sliders can be modified to create a kind of pseudo dog ring transmission with some of the benefits of a full conversion.
The easiest and best way to convert a transmission to dog ring shifting is to replace the entire gear and synchronizer stack. If you have one of the dozens of transmissions with strong aftermarket support, this should be no more difficult than calling your favorite speed shop and ordering the gear set. The gears in these conversion sets are usually wider and stronger than the original pieces to take advantage of the space gained by removal of the synchronizer cones and blocking rings.
Replacement gear sets with dog rings are designed from the start with racing in mind, so they also have the benefit of closer, higher ratios and better gear design for high-performance use. Most are cut with lower helical tooth angles (or even straight teeth) to reduce heat and friction buildup, which also improves bearings and case life.
If your transmission does not have a good selection of alternative gear sets with dog ring shifting, the existing gears can be modified to provide the same benefits. To turn them into dog ring gears, several companies can machine off the original synchronizer cones and dog rings. They will then weld on new dog rings. The gear hubs are then honed to the proper size, and the sliders are modified by removing teeth until the spaces between the internal splines are the same width as the gear dogs.
Liberty’s Gears in Michigan is the best-known shop for this kind of work, and their Pro-Shift conversions are very popular in some drag racing classes. They are surprisingly inexpensive—about $75 per gear—and a good way to get fast shifting on a budget. Of course, all of these modifications are for racing use only—if you need more strength in your street car, look into a transmission swap or start driving gingerly.
When it comes to transmission life, it’s all about respect. Respect means developing your shifting technique and avoiding standing starts. Don’t neglect gearbox maintenance, either, if you want to avoid the fun of an unplanned rebuild.
And when the time comes for a rebuild, don’t forget that a few little tricks can increase both performance and transmission life.