Many on here want to put down the auto transmission in the Js as useless for racing. Nothing could be further from the truth. The Autos that came in the Js were designed more for fuel economy, than performance, especially the 3 speed. With the right combination of gearing and the proper torque converter, some unbelievable(to the uninformed) gains can be made.
If you've ever read through a
Crane Cams catalog, you should have read how important the correct gearing is to matching the power band of the cam(and motor) is to performance. If you look into their catalog they show you hoe to determine the correct gearing for the cam you are using, if it's a Crane Cam. Talk to your cam manufacturer or engine builder for the best gearing for you car.
Yank Converters also has a Converter Selector, that can aid you in the selection of a torque converter that matches you motor. For best results though, talk to your high performance/race torque converter builder.
The following charts come from the
DACCO Transmission Parts Torque Converter Catalog. These charts are good for someone looking for the gear ratios in their own car or what car has a particular gear ratio that someone may be looking for. Although these charts don't show what car they came in, if you find a car of a particular year and what motor it has and/or the transmission ID Code, you can see what gearing it has.
For example: My car (a '96) has code AKC(on the metal tag on drivers side, located under the brake master cylinder). Under that number is a stamped set of numbers, 96_6046. The first two numbers are the year, the next are the serial number. in the serial number, the third digit is the prefix number(if my memory serves correctly, someone correct me if I'm wrong), so my code would be 4AKC. Then look under the '92-up section for the 2.2L motor and code 4AKC. There you'll find I
had converter code FZCB, with a stall of 2375rpm, 2.84 final drive and sprockets 33-37. These will be explained later.
You can also look up you RPOs(under the spare tire cover in the trunk, if you still have it) and look them up on some of the may RPO Decoders that can be found on many sites.
Final Drives, Sprockets and Effective ratios:
If you look at the charts, you'll see that the 3speed TH125C/3T40 transmission came with three different Final Drives(Differentials), they are: 2.84:1, 3.06:1 and 3.33:1. When Combined with the four different Drive Chain Sprockets (38-32, 37-33,35-35,33-37, fist number is the
drive sprocket tooth count, second the
driven sprocket tooth count), you have a wide range of
Effective Drive gear ratios. These range from a measly 2.39:1(High geared, good fuel economy, better have a ton of torque) all the way up to a 3.73:1(Low geared, great acceleration, high cruse rpms/poorer gas mileage). To determine your effective ratio, take the
driven sprocket tooth count and divide it by the
drive sprocket tooth count. take that number and multiply it with the gear ratio. This will give you the effective ratio.
Example: Using my transmission information in the above example, divide the driven number by the drive number:37(driven)/33(drive)=1.1212121 for the multiplication factor. Now multiply that by the gear ratio, 2.84*1.1212121=3.1842423 or 3.18:1
effective drive ratio.
I know there are multiple Final drives for the 4T40E, but sorry, I don't have that information. For those who do, please fell free to add to this thread.
Torque Converter Codes:
These codes apply to the torque converters(TC) used in the 3 speed automatic(TH125C/3T40) and the 4 speed automatic(4T40E), as well as other transmissions not used in the J-Bodies.
(Remember these are STOCK torque converters!) These trannies used a 245mm TC. These TCs used a four digit code of numbers and/or letters to identify them, such as my example listed above of FZCB. The definitions for these codes can be found on the last image, below. The first digit is for the trans type, in the case of the J-Bodies they all used code
F, for FWD 125C(not listed, but the 4T40E uses the same). The second digit is for the stall speed. In my case it was the letter
Z, for a 2375 rpm stall. The third digit is for the torque converter clutch(TCC) style. Mine was
C, for Carbon Filled. The third digit, which is common for all stock J-Bodies is for the bolt circle. Ours being
B, for 237mm bolt circle for the 125C(and 4T40E).
Most, but not all, of the 3rd Gen J's came with with 2375 rpm stall TC. In my experience, the 2560 rpm stall TC would be a good choice for a stock to very mildly modded (bolt-ons only) J, but not worth the time and/or money to just change it out. If you got to have the transmission or engine out for any reason, then it would be a good time to install it. If you've done more than the basic bolt-ons, then you should use one of the 2760/2795 rpm stall TCs or an after market performance TC. When you have a TCC type that has a number and pound sign never use a lower number. This is the clutch pressure and if you use a lower pressure slippage will result. My recommendation is to stick with the Carbon Filled and Woven Graphite TCCs, which are what come stock in the 4T40Es.
Now for the reason for this post:
Click on the image for larger view.
A few additional notes:
When changing the final drives, don't worry about getting the speedometer re-calibrated. The VSS signal is picked up from the FD's outer housing and all housings use the same gear tooth count to signal the VSS. You should only need to recalibrate if you change tire diameter.
The TCs listed above are only stock replacements, they will not have the tight tolerances as a true performance TC, don't expect to run a serious amount of boost or nitrous with these, but lower power levels, they should be fine.
While you have the transmission out, you might want to consider a rebuild, especially if you have a lot of miles (100K+) on the it and/or if you drive really hard. If you do drive hard on the TH125C/3T40, look into the
Alto-Red Eagle rebuild kits. They make about the best frictions and steels, used in racing this transmission. The Master Kit is a complete rebuild kit(click on the Hi-Performance catalog, under the catalog button).
There are three Shift kits, that I know of, for the TH125C/3T40. The
Superior Shift Correction Package, which is more like it's name implies, a shift correction. I would consider this more of a Stage I type kit. There is also the
TransGo Shift kit, which, from what I understand, has three levels to chose from. Finally there is the
J-Body Performance Shift Kit, which I hear is the same as the TransGo kit, just costs more.
For the 4T40E, there are two electronic shift modifiers, The AutoTrans Interceptor, who are out of business and the
B&M ShiftPlus Electronic Shift Improver. I don't have any experience with them, so I'm just going by what I've heard about them. The AutoTrans Interceptor had, I believe, 10 settings of firmness. It had a lot of good reports. The B&M ShiftPlus, reportedly can cause problems if used all the time, so save it for the track.
I hope this helps those with some questions about the automatic transmissions. Good Luck!
