And there is another reason why overdrive makes sense; less engine revs means less wear and tear, less heat, and less strain on that 40-50 year-old engine. So when you think about it, overdrive makes sense. (surprisingly, Detroit offered overdrive on manual transmission cars back in the day for much of the same reasons).

So let’s say you decide that overdrive is the way to go – the next question is, of course, which transmission should I consider – what would be the best choice for me? Well, there are a few issues to consider, but first, let’s mention the transmissions that you could select from, and then we’ll talk about your vehicle and which one might work best for you. There are three common overdrive automatics that should be considered the TH-200-4R; the TH-700R4 (also called the 4L60, 4L60E and 4L65E) and the TH-4L80.

The 200-4R grew out of a hybrid TH200/350; the TH-700R4 was a reengineered TH-350, and the 4L80 is pretty much a TH-400 with an extra gear. Last month we discussed these transmissions in detail, and if you’d like to review what we said, click here. All GM transmissions after 1986 used a simple nomenclature to designate them – with the first letter indicating the number of speeds in the tranny and the letter indicating the orientation, and the last two numbers indicating the strength. Thus a 4L80 is a 4 speed longitudinal (fore and aft) high strength trans.

A few things should be considered before you select a transmission. The first is, of course, will it fit? And first in that is will it bolt up? Assuming it will bolt up, most GM overdrives came with a selection of tail shaft lengths and cross member mounting setups that will allow a little juggling and at worst a driveshaft alteration, at most to fit it in your vehicle. The electronic versions (computer controlled – they have an “E” at the end of their ID. Like “4L60E”) can even be modified to shift w/o the computer. Let’s talk about the various potential transmissions and which ones might work for you.

TH200-4R

This guy is very popular for early, pre overdrive G-Body and all X-Body cars, since it  will bolt up to any GM bellhousing as it has a universal bolt pattern. The major problem with the 200-4R is that the trans mount to cross member location is the same distance back as the TH400, which requires a reward positioning of the cross member on early G-Bodies. And all X-Bodies. Of course, if you have a 1964-77 A-Body, you will find that your frame has positioning holes to move the cross member back because of the potential for a TH 400 in these cars.  Further, there are a  lot of TH200-4Rs out there in scrap yards as many were produced. That’s the good news. 

The bad news is that this trans has the tail shaft  integral to the case, which means that there is only one length tranny – 28.25″. Again, this is close to the TH-400’s length with the short tail shaft (29″), and dead on with the TH-200, but too long for the TH-350 (unless you have a B-Body car) and for any Jetaway users.  The other issue is that the 200-4R is the most fragile of the units and is marginal for torque ratings over 350 ft. lbs stock, and even when modified, 450 is about max.

TH-700R4

Next up in the strength department is the 700-R4.  Based upon the popular TH-350, the 700-R4 has the guts to handle torque outputs in the 400-500 ft. lbs. range in stock form, and up to 650 when modified properly.  These transmissions are quite popular and in their early iteration were only setup for the Chevy style bellhousing. Later versions came with either a universal bellhousing or a removable bellhousing which could be swapped out for BOP or Chevy style. During its life the nomenclature was changed to 4L60 or 4L65E.

The 700-R4 is very popular for most GM cars because it is similar in size and length to the TH-350.  The trans mounting point on the 700-R4 is 23′ from the engine and that is only 0.75″ longer than the 350, meaning a little juggling should allow the cross member to bolt up. The 700-R4 shifts by throttle position, not by vacuum, so there is an extra cable (TTV cable system) that attaches to the carburetor that must be properly adjusted or the trans can be ruined. Click here to see for info on this cable and kits to adapt to your non 700-R4 equipped car, as well as more about the transmission.

TH4L80

The 4L80 is in fact, a TH400 with overdrive added.  Sounds simple, huh? Well not so fast, in order to make that extra set of gears fit, the 4L80 is about 3″ longer than the TH400 and the trans mount is about  2.5″ further back.  Next, the bell housing is set up for the Chevy, not the BOP, though an adapter could be made.  If you have a big HP engine with lots of torque, you should look into this transmission.

Bits and Pieces

Many of the manufacturers listed below sell the components necessary for conversion.  Bowtie Overdrive has an extensive list of what you may need to both convert and upgrade your car from a Powerglide, Jetaway or any of the 3-speed units in the TH family, including adjustable mounts, cables, bell housings etc.

Be sure you have everything you need before you start, and we suggest that if you have the coin, you buy a complete transmission and conversion kit from one of the suppliers listed below. Be sure to tell them your current application and your engine’s vital statistics.

Most also sell kits to rewire your trans to “lock up” if the converter has that feature.

Shiftworks sells linkage to convert your non-overdrive transmission linkage and indicator to show th eoverdirve position – and has the extra detent.

VENDORS

The following vendors sell modified TH2004R, TH700-R4 and 4L80 transmissions (this is just a short list):

Art Carr Transmissions: http://www.cpttransmission.com/
Bowtie Overdrive: http://www.bowtieoverdrives.com/index.shtml
DRW Transmissions: http://www.drwtransmission.com/thm_200-4r.htm
Performance Automotive: http://www.transmissioncenter.net/200-4r.htm
TCI Racing Transmissions: http://www.tciauto.com/tc/transmission-packages-1.html?main=2 

Shiftworks: http://www.shiftworks.com/kits.htm