After determining your engine package the next logical step is to choose what type of transmission you’re going to use. The transmission is the next component in the flow of power from the engine and, combined with the rear axle ratio, it determines your overall drive ratio (ODR). It’s also appropriate to note the ODR changes based on transmission gear.
This Tech Tip is From the Full Book, FORD MUSTANG 1964 1/2 – 1973: HOW TO BUILD & MODIFY. For a comprehensive guide on this entire subject you can visit this link:
LEARN MORE ABOUT THIS BOOK HERE
SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. You can copy and paste this link to share: https://www.diyford.com/transmission-performance-upgrades-mustangs/
This brings up the issues of how many gears (“speeds”) the transmission has and what the maximum ratio spread is between them. In general, more gears means a wider ratio spread. This is usually a good thing for a streetdriven vehicle because it allows better acceleration in the lower gears while reducing the RPM at a given speed in the higher gears. This reduces wear, noise, and (normally) fuel consumption.
A high-performance Mustang benefits from a close-ratio transmission where the ratio differences between gears is reduced. This can be helpful in keeping the engine in a more beneficial RPM range and in reducing the effect of gear changes on the handling of the vehicle. A wide-ratio transmission does the opposite and provides a wider ratio spread, which can provide some of the benefit of going to more gears without the transmission becoming bigger and/or heavier.
Converting from an automatic transmission to a manual takes many steps and component installs. It’s almost always easier to convert from a manual transmission to an automatic because you don’t have to install a clutch pedal or entire pedal box. It’s a lot easier to ignore or remove than it is to add. It’s also more difficult to route and anchor linkages and cables.
From Automatic to Manual
At a minimum, when you convert from an automatic transmission to a manual you need to install a pilot bearing in the end of the crankshaft, get a new block plate, choose a transmission and bellhousing, check to see if you need to change the transmission crossmember and/or driveshaft, choose a pressure plate and clutch disc, install a third pedal and make it functional (with either a linkage, a cable, or via hydraulics), choose and install an appropriate shifter, and figure out how to get the speedometer and any other connections (backup lights, clutch safety switch, etc.) to work. Whew! It sounds somewhat intimidating but it really isn’t because these conversions are common and the necessary components are readily available.
Modern Driveline (MDL), for example, offers many conversion kits. In fact, the company specializes in 5- and 6-speed conversions for Ford vehicles, Mustangs in particular. Unlike some companies that just assemble parts made by others into a kit, MDL (in most cases) designs and manufactures their own components or has it done to their unique specifications. Examples of the former include cable clutch conversion kits, hydraulic clutch conversion kits, and conversion crossmembers. The parts are engineered to complement one another, and you need proven parts to complete the conversion to a modern overdrive (OD) transmission.
In the case of MDL you also benefit from their extensive experience with Ford. If you buy a complete conversion kit, the instructions include all of the detailed steps to complete the conversion for your specific car.
The following photos are highlights of installing an OD manual transmission into a car that originally had an automatic. This is not a step-by-step guide because there are too many differences among cars. It is an overview of some of the more critical things you need to address.
From Manual to Automatic
Converting a manual transmission car to an automatic is considerably simpler and less labor intensive. As mentioned, you don’t need that third pedal anymore. You can either remove it (and its linkage) or just ignore it once it’s been disconnected. It’s better to take it all out to free up space, especially around the exhaust.
You may also need to rewire the clutch safety switch to work with the neutral safety switch on your automatic transmission or just bypass it. You can also wire it to a hidden/disguised switch to act as an anti-theft “kill switch.”
You must remove your flywheel, pressure plate, clutch, etc., and replace them with the proper (in terms of size, number of ring gear teeth, balance, etc.) flexplate and torque converter.
Because most commonly used automatics have an integral bellhousing you need to remove the old manual transmission housing and likely install a different block plate. Depending on the specific combination you may have to modify or replace the crossmember and/or driveshaft. If you’ve increased the power significantly these changes become much more likely.
You need to install a new shifter, boot, and linkage (or cable), possibly along with some other changes. Depending on the new transmission you also have to adapt additional control cables (kickdown/TV cable), possibly hook up a vacuum line (for older transmissions), or use an electronic controller (for newer, electronicshift transmissions).
If there were no transmission coolant lines (most likely) you must add sufficient cooling capacity via the transmission cooler in your radiator (if so equipped) or to a separate cooler that you install.
Finally, you must adapt the speedometer cable/gear as needed and hook up all electrical connections (backup lights, neutral safety switch, etc.).
The following photos show highlights of this process. This is not a step-by-step guide because there are too many differences among cars.
Manual Transmission Upgrades
If you have a manual transmission in your daily driver there really isn’t that much you need to do other than replace worn components and/ or do some upgrades if you increase power significantly. If gas mileage, reduced wear, and noise reduction are priorities then you may want to install an OD manual transmission. The cost, however, usually isn’t practical for a limited-budget daily driver unless you drive enough miles for the fuel savings to make up for it.
The most cost-effective thing you can do is replace the bushings in the equalizer/Z-bar. There are replaceable parts on each end, which usually wear out. Installing new ones improves the action of your clutch significantly.
Similarly, the bushings on which the pedal assembly rotates are also prone to wear. These can be replaced with new bushings or, for a relatively minimal extra expense, roller bearings, which reduce play even more while making the clutch pedal movement smoother. They last a lot longer too. Even when properly maintained, these parts wear from movement and exposure to the elements.
This setup is adequate for many street cars but is usually not up to high-performance use. The pilot bearing in the crankshaft and the throwout bearing should be replaced if either is making noise, allows excessive movement of the input shaft, or is otherwise worn or damaged. Realistically, it’s best to just replace them unless they’re fairly new. They’re relatively inexpensive parts but fairly labor intensive.
While you have access to the inside of the bellhousing you should ensure that all of the lubrication points (pivot ball, throwout bearing at the clips, etc.) are coated with a small amount of suitable grease. Do not put any lubricant on the input shaft splines. This causes clutch chatter from grease “slinging” onto the friction surfaces, and requires the clutch disc be replaced, again.
If the clutch disc is worn out you should replace it with one better than a direct-replacement version. An aftermarket disc may not cost any more but should last longer and be able to take more power. Make sure the output of the engine matches the clutch disc set when assembling a high-performance engine package. If you upgrade the clutch for this reason you should also upgrade the pressure plate. If your pressure plate is heat checked from a worn-out disc you may need to replace it anyway.
If you upgrade the pressure plate to one with significantly higher clamping pressure use an aftermarket Barillo Z-bar or similar unit that has roller bearings instead of bushings at its ends. This provides smoother clutch action and greatly improved life over the stock parts while being much less prone to bending with the higher load.
Likewise, if the flywheel is damaged it can usually be ground smooth enough to be reused. You probably don’t need to spend the extra bucks for a different flywheel; just make sure yours is in good shape. If you change the flywheel and/or pressure plate you may need to rebalance them as an assembly. The cost of doing this, however, can often tilt the decision in favor of simply buying new parts.
If you plan to keep your transmission you might want to upgrade the shifter. You should also consider changing the engine and transmission mounts if they’re worn out. They can cause shifting problems because the engine and transmission are moving all over the place. This causes the Z-bar to go out of position and not properly transmit the motion of the clutch pedal. In severe cases the clutch may not fully disengage.
For relatively unmodified vehicles new, direct-replacement rubber mounts are the cost-effective way to go. If you plan to make significantly more power and/or you like to beat on the car (i.e., “powershift” it) often, you may want to spend a bit extra now and upgrade to stronger polyurethane mounts such as those offered by Energy Suspension. These greatly reduce movement of the engine and transmission plus they also last much, much longer. The only real downside for a daily driver is they may transmit a bit more noise and vibration into the car. That’s a small price to pay for making sure you don’t have a sudden failure.
For this performanc
Energy Suspension’s mounts also feature an integral safety interlock design so even if the polyurethane should fail, interlocking metal components prevent excessive movement.
You should upgrade the transmission fluid whenever you’ve had any significant increase in engine power. The forces on the internal parts increase, possibly beyond the point that a standard lubricant can provide adequate protection. This is especially true if the fluid is old or of unknown origin. If you’re upgrading the clutch and/or you haven’t changed the transmission fluid in a while it’s best to do so.
There are proven benefits to using a synthetic product such as Synchromax from Royal Purple. It lasts much longer than regular fluid and it reduces internal friction so more power gets to the wheels while lowering transmission temperatures. It costs a bit more than regular fluids but the performance benefits far outweigh the cost, especially because it lasts so much longer.
A very popular upgrade for relatively unmodified, daily-driver vehicles with a manual transmission is to install a newer OD manual transmission to reduce engine speed on the highway. This reduces wear and noise while also improving fuel mileage. This can be practical, especially if you drive your car a lot.
For most daily drivers it’s hard to beat the Borg-Warner/Tremec T-5 transmission. It comes in several variations but used ones from later Mustangs (1979–1993 Fox-Bodies in particular) are plentiful and not very expensive. These are 5-speed transmissions, which perform similarly to (yet better than) the older manual transmission through the first four gears with fourth still being 1:1. The benefit is that the T-5 gives you an OD fifth gear, usually with a .63:1 ratio, which reduces engine speed by 37 percent!
The T-5 is also pretty light (about 75 pounds), as is the OEM Fox-Body aluminum bellhousing that goes with it. Adapter plates allow you to keep your existing bellhousing and linkage. It’s generally better to swap over to the newer stuff to save weight, avoid issues with the driveshaft and/or crossmember, and have smoother shifting.
Converting to a hydraulic clutch setup is the preferred way to go although even the cable conversion kits outperform mechanical linkages. The hydraulic setup is especially handy if you need headers and steering to get around. A stock T-5 out of a Fox-Body is usually good for about 300 ft-lbs or so, especially in a lighter first-generation Mustang that’s going to be on relatively narrow street tires and hasn’t been modified too much. In many cases, the T-5 goes in without any need to change the driveshaft, and the transmission crossmember can often be drilled to mate up with the T-5 mount (it’s about an inch farther back).
To summarize, switching from your existing manual transmission to a newer T-5 involves:
- Replacing the transmission and bellhousing with the newer parts (including the clutch, etc.)
- tossing the mechanical clutch linkage for a pull-type cable or push-type hydraulic system that’s smoother and less affected by transmission movement
- getting a new transmission crossmember and/or driveshaft (though not usually)
- hooking up the various electrical and speedometer connections (at most, an adapter cable and perhaps a new electrical connector or two along with a new speedometer gear)
- Installing a new shifter (usually comes with the transmission though you likely want to replace the OEM shifter with a short-throw aftermarket version) with a new handle.
The high-performance street version gets up to about 400 hp or so; something of a domino effect occurs where modifications beget more modifications. This means going to a stronger version of the T-5 or using a Tremec TKO or T-56 instead. To some extent, the decision of which of these to use is determined by the year of your Mustang.
The 19641 ⁄2–1966 models have a smaller transmission tunnel and thus the T-5 remains the best choice. However, if you need a stronger transmission with more torque capacity and/ or you’re willing to cut/modify the transmission tunnel, you can install a transmission with a larger case. These cars are also pretty limited in terms of tire size unless you enlarge the wheelwells; you probably don’t need more than the T-5 anyway. You can’t usually get the extra power to the ground.
The FRPP World Class Z transmission (PN M-7003-Z) transmits up to about 500 hp and more than 400 ft-lbs of torque. It’s conservatively rated at 330 but that’s based on an extremely rigorous test procedure; many T-5s have successfully been used at more. It should be fine if you’re not running slicks and doing a lot of clutch drop/drag race starts.
Modified T-5s and rebuild or upgrade kits are available from companies such as G-Force Transmissions and Astro Performance Warehouse, which claim to raise the capacity of a T-5 to as much as 600 hp and 500 ft-lbs in a 3,300-pound vehicle with the standard T-5 housing. You even can buy specially built T-5s with a proprietary housing and internal upgrades for significantly more torque capacity. Using a TKO is a better option unless you need a physically smaller T-5.
The TKO is not as easy to install as the T-5 because it requires a different bellhousing, clutch disc (usually must go from 10 to 26 splines), driveshaft yoke (31 versus 28 splines), and other parts.
Using a different bellhousing necessitates installing a new block plate as well, unless you’re upgrading from a T-5. The stamped, lightweight OEM version should be sufficient. The overall length is also different and the shifter can be in a different place, so you need to resolve those issues. In addition, you may have to install a different crossmember, driveshaft (possibly), shift handle (the TKO has three possible shifter locations), and wiring adaptations.
These are common swaps and various companies have stepped up with components and/or kits to make it easier. The TKO is rated at either 500 or 600 ft-lbs of torque and, in reality, can take more depending on vehicle weight, suspension design, tire choice, etc. It is a bigger transmission and it also weighs more (about 99 versus about 75 pounds for an unmodified T-5) but its shift quality isn’t as smooth. It doesn’t cost much more than the Z transmission and certainly is less than the modified T-5s.
There’s not that much difference in the ratios between the T-5 and the TKO in that they both have first-gear ratios in the 2.87:1 to 3.27:1 range and fifth-gear ratios of .62:1 to .68:1. The only exception to this is a special close-ratio version of the 600- ft-lb TKO that has a ratio spread of 2.87:1 to .82:1 for potentially better performance when road racing. It’s not very likely that a T-56 6-speed transmission would be considered for this type of vehicle due to its packaging, installation, and cost issues. (It’s more justifiable for a highperformance street car.)
Clutch Disc Material
The transmission needs to provide smooth and precise shifting on the street for the high-performance street car but some smoothness may get sacrificed for higher torque capacity. A higher-pressure plate-clamping force and different clutch disc materials provide a firmer and sometimes more noticeable shift.
In some cases you can increase the clutch disc diameter to gain more capacity but this isn’t always possible and it can slow engine response due to the higher rotational inertia of the larger disc. The more likely means of increasing capacity is through a change in the facing material. The three most commonly used facing materials are organic, Kevlar, and ceramic/metallic. Each may be used alone or in combination with another.
Organic facings are used on the vast majority of OEM clutches and on lower-cost lower-power aftermarket units. They’re generally very smooth (minimal chatter) and wear reasonably well but have limited torque capacity. Other aspects of the clutch disc design (hub/springs, backing/ marcel, etc.) can raise torque capacity but the “Achilles heal” of organic facings is they can’t take higher heat.
One solution is to upgrade to Kevlar, which is superior in terms of resistance to heat plus it has much better wear characteristics. When used properly at relatively moderate power levels a disc with Kevlar on the sides can last indefinitely. Discs with an organic facing on one side and Kevlar on the other are also available as a lower-cost option that can be a bit smoother but the organic facing is still the weak link. This option may wear longer but the difference in torque capacity is significant.
It’s generally better to use all Kevlar. Some companies advertise their discs as Kevlar when they only have a relatively small percentage (10 to 20) of Kevlar fiber in them. Reputable companies, such as Modern Driveline, however, offer discs that are almost 100-percent Kevlar and thus deliver its full benefits.
The choice of ceramic/metallic is relatively extreme and usually reserved for race vehicles. This is because these materials tend to have very abrupt engagement characteristics and they can cause greater wear on the flywheel and pressure plate surfaces.
These materials also are affected by temperature so there are very few situations where they may be suitable for street use, particularly in traffic and/or in colder weather. For the vast majority of cases in this performance category an organic and/ or Kevlar disc with a sprung hub and a marcel (the wavy spring-like metal between the two clutch faces) along with the appropriate pressure plate is more than sufficient for the typical power level.
High-Performance Street Car
It’s usually best to source individual components from a single supplier to ensure their compatibility. Most suppliers offer complete kits that include the clutch disc, pressure plate, throwout bearing, alignment tool, and sometimes even the pilot bearing. Different combinations are offered to suit various torque capacities, budgets, and intended purposes. Some can be supplied with a matching flywheel as well.
For this performance level there usually isn’t much need to replace the flywheel unless the original can’t be reused. If this is the case an option to consider is a lightened flywheel to gain better engine response. A reduction of about 20 to 40 percent in total weight is significant by itself but when most of the weight is reduced at the outer edge of the flywheel the effect on reducing rotational inertia can be much more significant.
For the small-block Ford, Advanced Clutch Technology (ACT) removes the maximum feasible amount of material from the outer edge of the flywheel without compromising support for the starter ring gear or the overall strength of the flywheel. The steel flywheel has enough weight to reduce engine pulsations and help prevent stalling without the need for special finesse in your footwork. This can be desirable on the street, especially in a heavier car.
The rest of the necessary modifications for the high-performance street car category mirror the “optional” modifications for the daily driver category. Polyurethane engine and transmission mounts, cable or hydraulic linkages, a short-throw shifter, a stronger/compatible crossmember, and so forth almost always need to be installed because of the increased power and the more aggressive use.
Unless you do a lot of drag racing you can probably get away with the OEM-style aluminum bellhousing and stock driveshaft/U-joints. If you’re going to be seeing very high RPM and more than 400 hp you may want something safer, such as a QuickTime (QT) steel bellhousing and/or a stronger driveshaft.
Streetable Track-Day Car
The streetable track-day car sacrifices refinement during street use for durability and reliability under more-severe track conditions. With a lighter vehicle (such as a 1965/1966) a stronger T-5 may still be feasible if the power level is about 500 hp or so and the primary use is road courses, autocrosses, or open road races. In such situations the ease of packaging the T-5 and the limitations of the chassis in terms of wheel/tire size may make it a better choice than the TKO/T-56.
If the power level is higher and/ or the primary use is drag racing, the T-5 may not be up to it. In such situations, it may be necessary to step up to the TKO and cut the transmission tunnel. The TKO fits the 1967–1973 cars and in my estimation is one of the most cost-effective options. It’s also good for 600 ft-lbs or more depending on vehicle weight, tires, gear ratios, etc., which covers the vast majority of vehicles.
If even more torque capacity is needed you need to beef up the TKO or go with the T-56 Magnum. The latter is an improved version of the original T-56 and is good for more than 700 ft-lbs. It has been upgraded in terms of strength and shift feel to where it’s not only much stronger than the TKO but also has smoother shifting. Unfortunately, it’s also a much bigger transmission and thus requires quite a few changes for it to go into an early Mustang. These include all of the items mentioned previously for the TKO (the crossmember/mount, yoke/driveshaft, bellhousing, clutch mechanism, shifter, clutch disc) plus the new speedometer and electrical connections. Modern Driveline and others offer complete kits to avoid any guesswork.
Because there’s a much greater likelihood of higher RPM and power levels with the streetable track-day car it’s prudent to use a steel “scattershield” for safety. In some forms of racing, particularly drag racing, this is a requirement anyway because compliance with SFI or similar specifications is mandatory once a certain elapsed time or trap/maximum speed is reached. Even if it’s not a requirement it helps ensure you keep all of your body parts in the event of a clutch explosion.
The solution is readily available in the form of a QT steel bellhousing and engine plate to replace the OEM aluminum parts. These are available for common transmission/engine combinations for a reasonable cost. Unlike older scattershields, which were large, heavy, and not very well made (huge flanges and loose bolts), these units are much lighter (about 17 pounds), made from high-grade steel, and manufactured within very accurate tolerances for the best fit and quality. They’re SFI compliant yet are totally suitable for street use in all but the harshest weather. They also use factory hardware (linkage, etc.).
The clutch disc and pressure plate choice generally comes down to the torque capacity needed. If you can get by with a single-disc clutch you may want it to be as light as possible so the engine can rev more freely. An exception to this strategy may be in drag racing where you still need some flywheel weight to help launch the (usually heavier) car. Once your power level gets above 600 ft-lbs or so you probably want to use a twin-disc clutch to get the torque capacity you need without the extra rotational inertia that comes with a much larger/heavier disc.
A twin-disc clutch, by design, has four clutch facings instead of two so the clutch disc diameter can be A twin-disc clutch, by design, has four clutch facings instead of two so the clutch disc diameter can be reduced while still maintaining the surface area of the facing material needed for the torque level. Alternatively, it’s also possible to use ceramic/metallic facings with a single disc but the result is usually a very abrupt engagement that would be a real handful in street use. Besides, the twin-disc design provides the greater torque capacity without this drastic deterioration in driveability. In fact, a twindisc clutch can often be smoother than many single-disc clutches that don’t even match its torque capacity.
The main drawback of the twin disc is its higher cost and small penalty in overall weight.
Once you get above 600 ft-lbs or so you’re pretty much looking at the twin-disc setup if you still want to drive the car on the street without major adjustments to your driving style. In a lighter car (19641 ⁄2–1966) where you can’t use that much power anyway (unless you’ve made some relatively radical modifications elsewhere) a lighter single Kevlar disc setup likely suffices.
The remainder of the related components can generally follow the lead of the street-performance car with the possible exceptions of the need to further beef up the crossmember, yokes, U-joints, and driveshaft based on the torque level. You may also want to consider using some form of torque strap, chain, or link between the engine and the frame to supplement the polyurethane mounts and further limit excess engine/transmission movement. This is particularly relevant in drag racing where the shock of shifting is more severe and can damage the polyurethane.
At this level you must ensure all of the small parts (such as fasteners) are up to the task as well. It doesn’t do much good to have a strong transmission, clutch, and so forth if they’re being held on by weak hardware. Upgrading to stronger, higher-quality fasteners such as those made by ARP ensure you have the strength you need and will, in many cases, improve the appearance of your ride and make subsequent removal easier.
Automatic Transmission Upgrades
In general, owners do not mate an automatic transmission to street or race engines with more than 700 hp. Most automatics are inherently weaker than the manual transmission that was available for the same vehicle. They tend to not have as high a torque rating and their gear ratios are optimized for general street driving and fuel mileage.
Automatics can be an advantage in drag racing where their greater shifting consistency and torque multiplication are assets. On the street their primary appeal is convenience, particularly in heavy traffic.
In this section I discuss the common automatic transmission modifications for the three vehicle types. (I don’t cover transmission brakes because they aren’t for the street.) I give greater priority to driveability on the street over maximum torque capacity. These modifications are generic and apply mostly to strip use.
Replacing any worn mounts, properly adjusting all control cables and links, verifying the vacuum modulator (if so equipped) is functioning properly, etc., are all necessary steps before doing upgrades. A new transmission filter and the proper automatic transmission fluid should also be installed. The latter is particularly important to make sure the type of fluid (Type F, Mercon, etc.) is correct for the transmission. Failure to do so could cause improper shift behavior and expensive damage. Just use OEM-spec fluid unless you’re making more power.
While the pan’s off to change the filter/fluid, inspect inside the pan and the other transmission internals to make sure there aren’t any signs of excessive wear of the friction materials (fine, dark sediment). If there are metal particles and/or the fluid has a burned odor you may be in for more work; a more in-depth inspection is needed.
If some freshening is required it is better to install a performanceoriented overhaul kit from a company such as TCI or B&M Racing rather than a standard rebuild kit. The aftermarket kits may cost a bit more but they give you better materials, which should hold up better under hard use. Most also allow you to tune your shift quality to some extent.
If the transmission is okay, a desirable modification for a daily driver is the installation of a shift kit. This firms up the shifts for a performance feel and also helps prevent slippage under load.
As an example, the B&M Racing Shift Improver Kit does a good job. It is inexpensive, require only hand tools, and can be installed in an afternoon.
Their Transpak adds full manual control to most applications. This allows you to downshift at any speed and hold the transmission in first gear to whatever RPM you want without the transmission automatically upshifting. As you modify your vehicle more this allows you to realize the extra power you make at the top end.
Their TransKit is better suited to vehicle types other than a daily driver. B&M calls it “a transmission in a box” because it essentially allows you to rebuild your transmission/core to the same specs as their fully assembled transmissions.
B&M and others offer full rebuild kits that can rehabilitate and improve just about any automatic transmission that hasn’t been “ventilated” or otherwise suffered a catastrophic failure. These kits come in various levels, from street/strip to full racing, with the most robust materials.
If you’ve got the skills, experience, and a factory shop manual it can often be less costly to start with your own transmission and use the appropriate kit to rebuild it to the level you want. This is normally not the way to go for a daily driver with a tight budget.
A rebuilt or known, good used transmission is a less costly alternative if your existing transmission is shot but it can be the best choice if you plan on keeping it as you further modify your vehicle. It’s possible to rebuild a stock transmission to where it is suitable for a street-performance vehicle and possibly even for some street/strip/track-day cars.
A common upgrade for cars equipped with older automatics is to upgrade to a later OD transmission. The automatic overdrive (AOD) is the best choice for a daily driver based on its easy availability and low cost when purchased used or as a rebuild.
They’re easy to modify and, best of all, they can often go right in where a C4 came out with little or no modification. They bolt right up to all Windsor small-blocks and some other engines. They can often use the same crossmember (the holes for the transmission mount may need redrilling) and, surprisingly, even the driveshaft. You may have to cut and slightly chamfer the driveshaft transmission yoke to prevent it from bottoming out in the AOD.
With a shift kit AODs are strong enough to be usable well into the street-performance category, at least until your modifications and torque level become fairly high. The biggest hassle with installing an AOD is the need to hook up the TV cable to the carburetor. You need a special adapter, available from many sources including Lokar. They also have flexible dipsticks and cable or tail-mount mechanical shifters (with boots, etc.) to further simplify your installation.
The AOD is a high-performance street transmission so there isn’t much reason to keep your older C4 or C6 automatic transmission nor is there any need, yet, to go with something more robust.
The AOD provides a huge reduction in RPM at higher speeds. Improvements in shift quality and (usually) lighter weight make up for the expense of switching. It certainly makes sense to do so if your older transmission needs an overhaul. The AOD with a shift kit suffices up to about 400 hp and 350 ft-lbs of torque, again depending on vehicle weight, power level, rear gear, tires, and so forth. The weak point of the standard AOD is the dual-input shaft; that’s why going above these levels can be risky.
Upgraded AODs from companies such as TCI, B&M, and others are capable of much higher power levels (more than 600 hp) due to substantial design changes in the input shaft, valve body, and friction materials.
These transmissions are significantly more costly but they provide a way to stay with the AOD as your vehicle performance level increases well beyond what a standard AOD can handle. If you plan on going to higher power levels and want to stay with the AOD there’s no real downside to going for one of these stronger transmissions from the start. There isn’t any real difference in driveability if you use the same type of shift calibration; they are fine on the street yet can handle the strip. You won’t have the expense of upgrading later.
AODs for higher outputs often omit the lockup feature of the converter because of changes to the valve body, etc. The loss of lockup may cost you a few MPG, mainly on the highway. Some say it’s worth it because modified AODs with lockup can be a bit jerky at times. This isn’t always the case, however, as it depends on the way the valve body and torque converter have been matched and your feel for such things. The difference in mileage is fairly small in any case.
Most other items (crossmember, mounts, driveshaft, control cables, etc.) are basically the same as in upgrading to an AOD in a daily driver unless, of course, you’re at the higher power levels. Then you need to beef up these parts accordingly. Generally, you don’t need to change the driveshaft or crossmember. You can also use the same control cables and other external connections. You want polyurethane mounts, a stronger/higher stall-speed torque converter, a better shifter, and high-performance transmission fluid with a supplementary fluid cooler. In the case of the latter, unless you live in a fairly cold climate it’s better to cap off the cooler in the radiator and use an external cooler. It doesn’t see the heat from the engine coolant and works better.
The main things to consider when choosing an AOD are the stall speed of the torque converter, whether or not you want lockup, and the type of shifter you want. The stall speed of the converter you choose depends on the torque curve of the engine, the weight of the car, the gears and tires used, and whether or not you’re using any power adders (nitrous, supercharger, etc.). Transmission and converter manufacturers can help you make a final choice.
In general, the lower your engine’s torque peak, the lower you want the stall speed of the converter to be. On a stock vehicle with a stock transmission and converter the stall speed is usually less than 2,000 rpm and is often in the 1,500-rpm range. This is mainly for fuel efficiency purposes; a “tighter” converter slips less and thus gives better MPG with less wear on the engine. A “looser” converter with a higher stall speed allows the engine to reach a higher RPM at launch, which (hopefully) puts it closer to its torque peak and thus makes for a stronger launch.
This is another reason automatic transmissions aren’t normally used for track duty other than at a drag strip. The loose converter you need to get the most out of the engine would be problematic over the course of a longer race. Why? The inherent slippage of a higher stall speed converter is less efficient and creates more heat, which likely leads to durability issues in anything more than a fairly brief run.
It’s necessary to distinguish between types of stall speed: “flash” (momentary) and “brake pedal” (continuous). Newer converter designs allow a brief, transient spike to a higher engine RPM before settling at the RPM the engine maintains while it is revved in gear with the brakes on. This can allow a better launch with less sacrifice in MPG and street driveability because the converter has something of a dual personality.
Under normal driving conditions it behaves as if it has a lower stall speed but at the strip it can briefly allow a higher effective stall speed at launch, especially if the engine is held at less than full throttle and full throttle is applied when the car launches. This requires some finesse in driving technique but it’s worth it if you go to the strip often and don’t want a loose converter on the street.
Some examples of stall speeds and compatible vehicle equipment are illustrated by TCI’s range of street/ strip high-performance converters. Their mildest version is the Breakaway series. These have stall speeds of about 2,200 to 2,400 rpm for small-blocks and 2,400 to 2,600 rpm for big-blocks (yes, the torque of the engine makes a difference). They are intended for vehicles with a 3.00:1 to 3.73:1 rear gear ratio and a camshaft duration of 265 to 280 degrees, which still have a relatively smooth idle and are only mildly modified. They increase stall speed about 1,000 rpm over stock while still emphasizing low-end performance and providing a harder launch and quicker acceleration.
The next step up is the StreetFighter series. These are for vehicles where the fuel system has been upgraded and the compression raised to the point where the idle is noticeably rough. This is due in large part to camshaft duration in the 280- to 300-degree range. The rear gear likely falls into the 3.55:1 to 4.56:1 range. The end result is stall speeds in the 3,000- to 3,400-rpm range for smallblocks and 3,400 to 3,600 rpm for big-blocks and roughly 1,500 more than stock. This allows even higher RPM launches with quicker reaction times and a greater emphasis on mid-range as well as low-end power.
The Super StreetFighter series is likely the most extreme level for the majority of high-performance street cars. Here you’re talking about a car with cam duration in the 280- to 310-degree range. Thus the idle is clearly rough and there is a greater emphasis on top-end power with an even higher compression ratio and additional modifications. The rear gear is in the 3.73 to 4.88 range with the higher figures really only being usable with an OD transmission such as the AOD. The stall speed is about 2,000 rpm over stock or roughly 3,500 to 3,800 rpm for small-blocks and 3,800 to 4,000 rpm for bigblocks, thus providing even higher RPM launches and lower ETs.
The Ultimate StreetFighter series really isn’t well suited for most streetdriven vehicles and is better suited to streetable track-day cars. It’s recommended for engines with more than 290 degrees of camshaft duration, a rear gear of 3.73:1 or more, and horsepower up to 750 (more than 1,000 with a power adder). The stall speed is about 2,000 rpm over stock (about 3,500 rpm for small- and big-blocks). This level of power and stall speed requires significant internal modification (hardened pre-ground pump hub, heavy-duty needle bearings, etc.) for the highest-RPM launches and greatest torque multiplication of these converters. This noticeably reduces ETs at the strip yet still allows limited use on the street, with more noise, wear, and lower MPG.
With the standard AOD either you have lockup or you don’t because it’s purely mechanical. This is a big plus when installing an AOD in an older car (no electronic controls). Because the lockup feature depends on the converter design and valve body the two must be compatible. A good rule is that lockup works better with fewer modifications, though, again, that statement depends on driver preference. The decision on lockup usually comes down to a tradeoff between driveability (perceived or not) and RPM/fuel economy.
If you have a relatively high threshold for shift quality you may make MPG with reduced engine noise and wear a priority. If you’re more about smoothness and driveability you may decide against lockup. Differences in driveability and mileage are usually pretty small.
The tradeoff is also dependent on power level and the individual component manufacturer. Some manufacturers do a better job of balancing these tradeoffs than others. They’re able to provide lockup with little, if any, effect on driveability. Most tend to delete lockup on AODs when you get into higher stall/torque capacity products due to the inherent weakness of the dual-input shaft. This is replaced by a solid, much stronger single shaft, which, by design, eliminates lockup.
Choosing a shifter usually comes down to features and appearance. The functions you choose drive the appearance to some extent. If you want to have a reverselockout feature required by some sanctioning bodies, you likely have some form of lever or trigger. There are a few “stealth” designs that incorporate the feature (such as B&M’s Quicksilver shifter) but most are more obvious about it.
Another consideration is how and where you want to mount the shifter and its compatibility with the transmission. If you have a reversepattern valve body in the transmission it somewhat limits your shifter choices. A cable-shift mechanism gives you the most freedom to choose where you want to locate the shifter but it may not be the best choice if you shift very aggressively. A mechanical-linkage shifter, by design, must be attached to the transmission, thus limiting its mounting options. It can take more abuse but it is much more dependent on the design of the specific transmission in order to get the best shift action.
Another thing to consider is whether you want a ratchet-style shifter (where there are detents, which allow you to simply move the shift lever without missing the gear you want). Also, you need to decide on having a neutral safety switch, backup light switch, and so forth.
AODE or 4R70W
For the streetable track-day car, more extensive upgrades are required. The AOD is marginally acceptable due to its torque limitations, unless it’s been extensively modified. A better choice is the AODE or the 4R70W. These are successors to the AOD and eliminate most of its limitations. They’re virtually identical in external dimensions so they fit virtually anywhere an AOD fits. They’re rated for significantly higher torque levels (more than 1,000 hp when modified, and about 500 ft-lbs stock). They are electronically controlled so you don’t have to make as many tradeoffs and you gain additional tuning capability. The main difference between the AODE and the 4R70W is that the latter has a wider ratio spread (that’s what the “W” signifies).
The AOD and AODE have a 2.40:1 first gear with a 1.47:1 second and a .67:1 fourth while the 4R70W has a 2.84:1 first, a 1.55:1 second, and a .70:1 fourth (1:1 in third). This can be significant depending on vehicle weight and other factors. In general, if you have good traction at the line the 4R70W allows you leave a bit harder, whereas the AODE is a better bet if your car is traction limited due to weight, suspension, tires, etc. The AODE also reduces engine revs a bit more on the highway and at top speed so that could be a factor for MPG, noise, and/or rear axle ratio choice.
With either version you must choose the right case because these transmissions were used mostly on 4.6L modular engines. The 5.0L and other small-block/Windsor versions are the cases you need to bolt up to an older engine. Most aftermarket suppliers can build the transmission the way you need it with the right case and internal modifications. You need a special AODE/4R70W torque converter (an AOD unit doesn’t work). They are usually offered in similar variations. You should also use a stronger, SFIcompliant flex plate.
The main advantage of the AODE or the 4R70W is, of course, the electronic control. This allows you to choose and tune your shift points and shift firmness with much greater ease, flexibility, and accuracy compared to using a shift kit on an AOD. It also allows you to decide when (or not) to lock the converter. This eliminates the tradeoff of power versus lockup; you can have higher torque capacity and lockup. You can even have lockup in some other gears besides fourth if you want. This all requires an electronic controller. These transmissions came with one from the factory but it wasn’t adjustable.
Aftermarket products provide greater functionality with compatibility. An excellent example of this is the TCM-2300 Simple Shift controller from PCS. It uses simple rotary switches so no laptop is needed to make adjustments plus your passenger can do so while the car is moving. Built-in diagnostics indicate the gear and the lockup status. Each unit comes with a wiring harness specific to the transmission, with the proper connecters, for easy installation.
PCS also offers the Simple Shift Tuner, which can remain connected and so the controller can be stored in a less-visible location. It allows you to do everything the rotary switches do in a more graphic and user-friendly format. It provides a gauge function that allows you to display live data for various parameters.
PCS also offers a software package for your laptop that allows you to view live data for the parameters and diagnostic trouble codes. It adds a datalogging function with a monitor screen to help with tuning. The D200 stand-alone datalogger can combine the signals from the controller with inputs from other sensors. It can even act as a shift light!
Once you have chosen the transmission, torque converter, and controller, another way to take advantage of the benefits of electronic control is with a paddle shifter mounted on the steering wheel. You can go F1 in your classic Mustang! The convenience of having the shift mechanism on the steering wheel lets you keep your hands on the wheel while shifting. This is safer and should also result in quicker, more consistent shifts and lower ETs. The electronic controller only works once the transmission is in gear so you need a manual shifter to go from park to drive or reverse, etc.
If you skip the paddle shifter, you should step up to a competition type shifter. This can be a bit less convenient on the street but is more consistent and reliable at the strip. Companies, such as B&M Racing and Hurst, offer many variations.
At the high-torque levels of your high-powered streetable track-day car you may need an even stronger transmission crossmember to prevent flexing and/or outright failure. Stronger steel units are available from many sources or can be fabricated.
Another option to consider is a billet aluminum X-Factor version from American Powertrain. It’s plenty strong, it’s lighter than a comparable steel unit, and it provides a lot of adjustability.
At the power level of a streetable track-day car you should never use the cooler in the radiator tank. Instead, you should fit the largest, most efficient external cooler you can. Other than a small weight penalty and maybe some tighter packaging in some areas you really can’t go too big.
Adding a small, high-flow fan is a good idea so cooling can continue after a run when the car is being driven back to the paddock or pit area. B&M Racing has a particularly efficient product in several sizes.
Depending on how quick/fast your car runs, a transmission blanket may be required by the sanctioning body. A scattershield is required for a vehicle with a manual transmission. The purpose is identical: If the transmission blows this helps keep the flying parts from taking part of you with them.
The aluminum housings on OEM automatic transmissions were never intended to contain the internal parts in the event of a catastrophic failure. With a streetable track-day car’s higher speeds, power levels, and forces it’s possible “shrapnel” could come through the floorpan and injure you. A transmission blanket reduces that possibility and should be used at this level even if it’s not a requirement.
Written by Frank Bohanan and Posted with Permission of CarTechBooks
GET A DEAL ON THIS BOOK!
If you liked this article you will LOVE the full book. Click the button below and we will send you an exclusive deal on this book.