View Full Version : transmission change

11-15-2005, 02:54 PM
I have a '74 Javelin with a 360 and 998 3sp automatic transmission. I'm interested in upgrading the transmission to have either an overdrive or to put a new one in with 4 speeds. The reasons for doing so include better gas mileage and less engine noise/stress. In my younger years I had a '72 AL trooper Javelin with a 401, and it had a fourth speed, but I don't remember what kind of transmission it had. Unfortunately, it's long gond now. Does anyone have any recommendations on the transmission upgrade?

Big Bad AMX
11-19-2005, 01:50 PM
GM's Turbo 700R is a good choice for a four-speed auto.


11-19-2005, 04:33 PM
GM's Turbo 700R is a good choice for a four-speed auto.

.thanks big bad. i'll do some research on the 700r.

11-19-2005, 05:22 PM
Hi, if you want a 700r4 your going to pay at least $2,000 for a good rebuilt one. If you bother to look at some GM boards on the net, they are the worst auto from GM in stock form.

I did a simple rear end swap (69 Polora 2.74 gears @ a total cost of $100.00) on my 304 way back and was able to get an average 22MPG out of my 998 tranny. Did some wild things like cruised on the freeway in 1st at 55MPH at 4900RPM, and had a top speed of 140MPH. The best thing you can do on a budget.

If you have a 401 the only tranny on a budget is the TH400. Get one from a good performace tranny company. They will put the good stuff in it and will hold up well. Too many Torqflights fail under abuse unless you spend some money on upgrades.

If you have some money get a Gear Vendors overdrive. Basically you can have a better 4 speed auto than any new GM version that won't cost you as much and is less in weight than the L80E. Plus all the 4 speeds are metric and require an electronics module. You can still get Jeep TH400's which will bolt right in. But you must have it rebuilt... since the output shaft is for the Jeep transfer case.

11-20-2005, 03:55 PM
thanks for the reply 304, i just wanted summarize what i believe to be your recommendations:
- gear vendors is preferred solution
- could try a rear end swap
- since i don't have a 401, the th400 is out of the question (i have a 360)
- 700r4 is an expensive solution; stock not readily available

i did not understand the comment about the jeep th400 that you said would bolt right in. you had qualified that with having a 401 in the previous paragraph. would the th400 also fit the 360?

11-20-2005, 10:55 PM
Yes they do, in fact you can find a lot of 78 - 79 Wagoneer's with 360's and TH400.

Even if you have the 401 fly wheel you can have it re-balanced.

The retro fit to adapt the TH400 should not cost more than $50.00 (output shaft, housing, and slip yoke) from the tranny guy... but you will need to add it to the total cost of the rebuild.

A second gotcha is the drive shaft. With any transmission swap you must have the proper length drive shaft.

You have my order wrong.

1) Rear end ($100.00)... since the 90's 68 and 69 B-bodys are hard to come by; but it is worth the effort if you want a quick and easy setup. Note you will need to grab the shock plates and it is best to get new replacment U-bolts. PS don't trash your brake lines... they will fit on the MOPAR axle just fine if you don't mind a little work. AMC had done right with aluminum brake lines... old steel lines are a bit rusty now these days.

Another thing 71-73 Mustang rears are close but need the purches moved 1/2"... a lot of AMCrs have been able to bolt them in (Mostly because of old rubber in the spring eye bushing), but will make the springs point inward.

2) TH400 ($100.00 core) $600.00 on up on a good rebuild plus additional costs for Tranny mount and the drive shaft length adjustment.

3) Gear Vendors overdrive over ($1950.00 installed) + cost for drive shaft length adjustment.

Big Bad AMX
11-20-2005, 11:55 PM
Here's a couple of links on the 700R you may find interesting:

GM TH700R4 / 4L60 & Early 4L60-E Automatic Transmission

Adapting AMC Engines to GM Automatic Transmissions:

GM TH700R4 Transmissions:

If you'd like a higher rear gear ratio AMC had a 2.87 ratio available for the AMX and Javelin. The AMC V-8 cars (Spirit, Concord, Pacer) in 78-80 had a 2.56 gear available which drops right in, no mods needed.

There's plenty of options available to acheive the results you're looking for. As always some are less expensive and easier than others.


11-21-2005, 06:11 AM
big bad & 304, thanks a bunch for the info sharing. i have some research to do and a decision to make, but at least I have some good options.