A Mustang’s valve body is the organ that brings life to the transmission. First a valve body detects how hard your Pony is working and at what speed you’re travelling. With these two readings it can then decide whether it needs to upshift, downshift, or hold the current gear.

Unfortunately it doesn’t do this by taking a reading of your tachometer and speedometer. Instead it relies on hydraulic pressure which varies with engine load. Different fluid pressures cause different circuits and valves to open, allowing for different transmission operations.

The pressure is provided via a main pump which in turn is operated by the torque converter. The fluid pressure generated from the pump is then regulated and used to control a certain function, depending on what circuit has opened. Recall that there are many different circuits, layers, and valves for the fluid to flow, and each one may have an independent task.

To fully understand the exact mechanics and functionality of a valve body, you would probably need a PhD in engineering (heck, I would need at least 2 PhD’s!). Because of their complexity, we’ll be examining a more simplified version. If you want a really complicated and in-depth explanation, see Valve Body Circuits by Toyota Technical Training (be warned, it is 20 pages of highly technical jargon).

To extrapolate on what was said previously, the valve body can sense how hard the engine is working via 2 methods (depending on what year your Mustang is):

1.    Cable linkage (AOD, 1979-95)

This type of setup is found on older Foxbody and SN95 era Mustangs. These generations use a mechanical cable linkage. The further the throttle pedal is depressed, the greater pressure is put on the throttle valve.

2.    Vacuum modulator (4R70W - 1996-04, 5R55 – 2005-2010, 6R80 – 2011-2014)

A vacuum modulator is the method of choice for later model automatic-equipped Mustangs. The vacuum modulator runs off manifold pressure. It reads the manifold pressure (which increases under engine load) and consequently increases pressure on the throttle valve

Consider this simplified circuit: a throttle valve, a shift valve, and a governor. All of these are connected in a loop, in that order. The circuit is provided fluid via the main pump. The throttle valve provides pressure when the engine is under load whereas the governor provides pressure based on vehicle speed.

Take a situation of hard acceleration from a stop in 1st gear. When you stomp the gas in your Mustang for maximum acceleration the engine will be under a lot of load. At this moment the throttle pressure is much greater than the pressure given from the governor, therefore it keeps the shift valve closed and holds 1st gear.

As your Pony gains speed, the governor will place increasing pressure on the shift valve until it overcomes the throttle side, moving the shift valve and opening a different fluid circuit. At this point your Mustang will be at or close to redline, and the shift valve will open, allowing for a shift to 2nd gear. In this case, the 1st gear was held for as long as possible because the throttle pressure (engine workload) was greater than the governor supplied fluid pressure up until maximum 1st gear velocity.

Now, reverse the situation where instead of burning rubber you’re just cruising along. In this instance the engine is not under a tremendous work load, therefore throttle pressure won't be as high. Conversely, it is much easier for the governor pressure to overcome throttle pressure and cause a gear change. This is what happens when you are not accelerating hard and you see early changes (often every 800-1000 RPM). 

Yes, you most certainly can. In fact, upgrading the valve body is a very common route for those seeking to wake up their automatic equipped Mustangs. Changes in the circuit length, paths, diameter, valve sizes etc can all have a major impact on the overall performance of your transmission. Typically, aftermarket valve bodies are designed with a more aggressive nature in mind, such that gear changes are done faster, crisper, and with less overlap in between.

Mustangs (old ones, in particular) are usually equipped with automatic transmissions, sometimes referred to as “slushboxes” because of their low response time to driver input. Need to pass someone on the freeway? Smash the accelerator, and it can take a few seconds for the transmission to downshift and get you into the power band.

Why?

Well, it’s actually built to do that. Factory automatics are engineered for comfort first, then performance. They are designed to shift smoothly such that the vehicle’s occupants are not overly disturbed (but who doesn’t like being throw in their seat??). They accomplish this by actually using two gears at once.

During a shift, an automatic transmission will have what's called gear overlap where one gear is switching out and one is switching in. During this transition, there will be a point where both gears overlap (gear one is halfway in, gear two is halfway out).

By slipping the two gears like so an automatic transmission is able to make a smooth shift. Unfortunately, this smoothness sacrifices performance as this shift overlap process takes longer to complete.

Furthermore because of the added length of the shift and the slip present, more heat is generated which in turn adds more wear. As such this is the way Ford factory automatics are programmed. Lucky for us we don’t need to leave it that way.

The last sentence in the above paragraph pretty much sums it up. There are different types of shift kits, but the objective of each is the same; to modify/re-engineer the fluid circuit such that transmission shift characteristics are modified as well.

Depending on the manufacturer and transmission in use, a shift kit could simply contain a few modified accumulator springs (replace the stock springs) and valves, or could contain entirely new plate separators.

It is entirely reliant on the kit manufacturer, as there are multiple ways to achieve the same effect of crisping up the shifts. Frequently, shift kits up the fluid pressure at all times, so that all actions are performed faster.

To reiterate, a properly installed shift kit will:

1 - Reduce/eliminate gear overlap

2 - Reduce internal heat

3 - Increase shift speed and feel (crisper)

Number 1 and 2 are actually pretty important. Eliminating the gear overlap will reduce the wear on the gear clutches (they no longer have to slip against each other) and in turn reduce the amount of heat produced.

This is great news because it is essentially less wear and tear on the transmission, and ideally improve longevity (heat is THE major automatic transmission killer). Overall the installation of a shift kit will lose the stock slushbox feeling and drop some time off your quarter mile run. Who isn’t interested in this?!

However not all transmission are created equal, and shift kits can have an adverse affect on such slushboxed. For example, aftermarket results indicate that installing a shift kit on a 4R70W automatic transmission (1996-2004 Mustangs) can reduce lifespan and cause premature failure due to the higher line pressures.

For this reason, it is important to research compatibility for any shift kit you may wish to install. If in doubt, ask the manufacturer. If you have a Mustang with a 4R70W, look into doing a Jmod, replacement valve body or even an electronic tuner. Reliability is said to be much better with any of those. Mustang owners of the Fox/SN95 era (1979-1995) needn’t worry. Their AOD transmissions work fine with an aftermarket shift kit.

A shift kit vs a tuner has some small differences, but many similarities. One specific difference is a shift kit changes the transmission in a physical manner while a tuner changes a transmission in the computerization manner. A shift kit will be more difficult to change vs a tuner as they have physical parts. A tuner can simply change the tune for a change in performance. On the flip side, a tuner can be harder on a transmission if not done properly. A shift kit changes physical properties which can allow the transmission to shift differently, but with proper hardware. If the tune is done correctly, it will not decrease the life of the transmission.

The installation of a shift kit is a great and cheap way to firm up the factory slushbox. They aren’t very hard to install (just follow the instructions to the tee) and do not necessitate dropping the transmission. They can be installed in a few hours by the average Joe, no problem.

On the other hand, they aren’t the be-all-end all of automatic transmission modifications. Keep in mind, a shift kit is only modifying a factory design. The aftermarket has come up with a lot of better ways than the stock valve body and components. For this reason, before making a decision, you should definitely read up on aftermarket valve bodies, completing a Jmod or making use of an electronic tuner.