American Automatics

Specialising in Classic American Transmission Rebuilding

Technical Info and Advice

How does an automatic gearbox work?
I could tell you - but then I'd have to kill you! Ha Ha. Seriously though, if your having trouble with your gearbox acting up and playing silly, then by all means call me up and describe the symptoms and we'll see if its terminal. There are far too many questions I need to ask you to list here.

Need advice on performance upgrades?
Sure no problem. A whole range of aftermarket goodies are available for most popular gearboxes. I can advise on which particular products are worth purchasing in relation to your horsepower needs.

Need advice on gear ratios for your rear end?
No problem - see the list below for most common gearbox ratios. If you dont find this helps then call me for a chat.

TH350 - 2.52 - 1.52 - 1
TH400 - 2.48 - 1.48 - 1
TH700R4/4L60 3.06 - 1.63 -1 - 0.7
Th200-4R  2.74 - 1.56 - 1 - 0.67
C4 - 2.46 - 1.46 - 1
C6 - 2.46 - 1.46 - 1
AOD - 2.40 - 1.47 - 1 - 0.67
Torqueflite - 2.45 - 1.45 - 1

How do I check my oil and how much does it take?
This such a common question I thought I'd clarify the procedure and provide a list of gearbox capacities in LITRES not darn Quarts! See below.

Oil checking process - Drive your car until the engine is hot. The hot water in your radiator also warms up the gearbox fluid as well as cools it down, so they come to a common temperature. Find a good level flat area and park up. Leave the engine idling. Stick the handbrake on. Put the gearbox into 'park'. (Neutral if you have a Torqueflite!) Let it idle a few minutes then pull the transmission dipstick, wipe it thoroughly. Stick it straight in all the way and quicky pull it back out. Hold it horizontal so it doesn't run down, and see where the oil level is at. It should be at or just below 'Full'. Do this a few times to confirm the level and look on both sides of the dipstick. Sometimes oil inside the filler tube will give you a false reading, when its actually low! so take care. Take no notice to what the levels are with the engine off or stone cold - it varies and doesnt matter a hoot. What matters is when its pumping and running.

Capacities for common boxes
I've rounded up from the internet a list of APPROX total capacities so you can buy the right amount of trans fluid. The exact amount for a dry system will vary depending on a few factors such as if you have an after market deep sump pan or a smaller or larger torque converter so common sense applies. 11 to 12in is usually stock for a converter so go by this if you dont know.

Transmission Fluid Capacity - Litres

GM TH350                                   -  3.78
GM TH400                                   -  5.67
GM Powerglide                            -  3.78
GM 700R4/4L60E                      -  5.67
GM 2004R                                    -  5.67
GM 4L80E                                    -  7.28
Chrysler Torqueflite 727          - 4.73
Ford C4                                         - 5.2
Ford C6                                         -  6.62
Ford AOD/AODE                         - 6.15

Converter Fluid Capacity - Litres

Size of Converter 
  13"     - 4.73
  12"     - 3.78
  11"     - 3.31
  10"     - 2.83
   9"     - 2.36
   8"     - 1.89
   7"     - 1.89

Why shouldn't you tow an automatic?

Good question ! Here's the answer - In a manual gearbox car, the gearbox is full of oil and the parts get lubricated all the time it turns around. So even with the engine off, when the propshaft turns the tailshaft, it is still lubricated. In an automatic gearbox, the oil level is below the moving parts. Only the residual film of oil is lubricating the parts with the engine off. This is because oil is fed under pressure to moving parts from the front pump when the engine is running. If you tow an automatic any great distance or too fast it will wreck the bushings through lack of oil. This is particularly important when an old classic has stood for years without any oil circulation. I have seen the evidence!

Torque converter stall speed

A lot of people find this a tricky subject. So lets get down to basics and keep this in plain speak.
My First Torque Converter - Imagine 2 desk fans facing each other, one is switched on and the other is not. The fan thats blowing (your engine side) makes the other one spin (your gearbox side). If you move them further apart then it doesnt work as well. Move very close and it works really well. In a nutshell, thats what goes on inside your torque converter, some fan blades, dipped in oil, wrapped in a steel doughnut doing their thing. This process creates heat which is why you need an oil cooler.
Stock stall speed - This is not anything to do with stalling the engine. Its about the speed of internal parts of the converter doing their thing and getting a good hard bite with each other. Stock stall is around 1600 rpm. In practise this is about the max rpm you can shove into the converter, with your foot on the brake, revving the engine.
High stall speed - the point of raising the stall speed is to allow your engine to get to a higher rev range before it starts to really bite and grip. This done by changing the length, width, or angle of the fan blades inside the converter housing - this is specialist stuff, thats why they cost shed loads more.
Lets look at this example -
Standard stall - engine hp at 1600rpm = 130hp. This is the power you can launch off with at the lights, regardless of how much top end power your engine produces. So your buddy beat you off the lights? Off to the shops - 'I'll have one of those tasty higher stall converters please mate' Kerching! £££
Higher stall - engine hp at 2300rpm = 200hp. Suddenly you have the abilty to launch with an extra 70hp! Woo hoo, now your cooking on gas !
But ! dont go crazy ! Resist the urge to buy a 4000rpm stall converter for a daily driver. All of this has a trade off unfortunately so its a carefull decision to make. The higher the stall, the more the slip at lower rpm, the more the fuel consumption suffers, the more you have to keep gassing it just to keep up with the traffic. Also other factors determine the stall speed. The weight of the car, the power of the engine all affect stall. Plan your purchase for its application.