General Motors Turbo-Hydramatic 200-4R
The Turbo-Hydramatic 200-4R was introduced in 1982 General Motors rear wheel drive, overdrive vehicles. Used primarily in Buick, Olds, Pontiac and Cadillac models, they also appeared in some Chevrolet models. This was made possible by a multiple bell housing bolt pattern found on some of the cases that allowed the 200-4R to be bolted to Chevrolet's specific bolt pattern. The picture above shows such a case.
The 200-4Rs basic design follows the THM 200 which is the 3 speed version, introduced in 1976. The 200 was conceived as a way to cut some of the cost of manufacturing and was designed to absorb less horsepower so that more would be available to the rear wheels. That, of course, improved gas mileage which became a major importance in the 1970s. The 200-4R is a 200 with an overdrive section added to the front section of the trans, behind the front pump.
Though the 200 gained a reputation as an undependable transmission early on in its life, most of this attributable to the early lock-up torque converter designs, the 200-4R is being used by and adapted to cars that demand a reliable overdrive, rear wheel drive transmission for high performance use. This transmission has surprised many transmission technicians that were used to seeing the many failing 200s. Many vehicles that are pumping out in excess of 700 horsepower are still using the 200-4R. The next step up for racing purposes being the mainstay THM 400.
A quick observation I would like to add here. Since around the mid-80s I have been rebuilding transmissions. A small percentage being high performance applications. Although, it is not the majority of my business, I have been successful at putting together some transmissions that perform very well behind some unbelievable powerplants. With enough experience you start to identify all the inherent weaknesses that a particular transmission model has. Some models having more weaknesses than others. The other overdrive transmission that is commonly used for high performance and custom street rod applications is the 4L60 (700R-4) and the 4L60E. The 4L60E is just a much more computer controlled version of the 4L60. There was a point in my career where I did build 4L60s for high performance use. After 8 years of personal experience, and the observations of what other transmission builders were going through with these units, I came to the conclusion long ago that these units would never make it as a high performance transmission. They're OK for street rod cruisers, but put some horsepower to them and you'll be lucky to get a couple years of service. If you compare the 4L60(E) to the THM200-4R, the later is a much better choice if you plan to put some power through it. I know this will make some true blue Chevy fans upset, but I have to tell it like I see it. I've never been known to sugar coat anything that didn't deserve it to make a sale. I much rather would like to see a customer with a superior product, than just sell him on anything. I still have customers that insist I build them a 700R-4 for racing purposes. When I build a racing 700R-4, I put all the latest goodies in it, along with an exotic treatment, to make sure the transmission has a fighting chance. More importantly, I do my best to inform the customer, before work begins, on the limitations of the 700R-4.
Gear ratios of the 200-4R are: 2.74 first, 1.57 second, 1.00 third, .67 fourth.
There are a number of modifications that have been done by the manufacturer to increase the durability and drivability of this transmission. The next few paragraphs cover some of these.
Beginning late in the 1982 model year, the THM 200-4R transmission control valve assembly was changed to eliminate the Torque Converter Clutch Valve Train. The computer command control unit controls the converter clutch making the torque converter clutch valve train unnecessary. A new solid plug replaced the control valve assembly to direct oil flow.
In 1984 production, all 200-4R OZ model transmissions were built with a dual pump slide spring to help with a 3-4 shift busyness problem at wide open throttle.
Beginning January 1984, all 200-C, 200-4R and 325-4L transmissions were produced using rubber lip seals on a new design intermediate servo piston in place of the scarf cut Teflon seal rings used in previous years. This change was done to correct soft 1-2 and 2-3 shifts.
In April 25, 1986 a new design "feed/bleed" system for the forward clutch was introduced to give more cushioning during forward clutch apply, giving a softer feel when shifting from park or neutral into drive. This change was not used in the BRF models.
Sometime around 1989 a new pump body was made available to help prevent fluid leakage from the front seal caused by a front pump bushing that has moved forward in the bushing bore. A step was designed into the bushing bore that blocked the bushing, preventing it from being able to move forward and interfering with the front seal.
What is available through the aftermarket to further increase the durability of this transmission? We'll start at the front of the transmission and work back.
There are a few things that can be done to the pump body. A teflon bushing to reduce wear to the bushing and converter hub. Any wear (clearance) at this location causes an excessive amount of converter oil to bypass its normal route to the transmission cooler and pressurize the front seal. Hence, front seal blowouts. The drain back passage for the front seal should be drilled out to a larger size (1/4") to help prevent this front seal blowout and a front seal retainer should be installed for added insurance. Other factors that can cause excessive wear to the front pump bushing are a bent or broken flexplate, excessive converter hub runout (should not exceed .010"), worn or missing trans to engine alignment dowel pins or bores. The pump body and pump cover mating surfaces should be flat sanded on a granite surface block to ensure that no crossleaking occurs between the hydraulic passages in the pump assembly.
On the subject of pump rotors, there are 7 and 10 vane rotors available. Either can be used satisfactorily. The trick is to make sure you use the upgraded, hardened pump vane rings. The use of these rings is a must. Broken up pump rings is not a pretty site and completely ruins the pump assembly. The misconception of the 10 vane rotor is that it increases pump capacity. Not true. What it does is change the surge pulses coming from the pump outlet and changes the harmonics that are present in the pump cavity. This change was done in an attempt to reduce pressure regulator valve buzz and decrease pump ring breakage occurrences. As stated earlier, just make sure you are using the hardened pump rings and you'll be OK.
The pump cover contains part of the front seal drain back passage and the increase in size discussed previously should be continued all the way through to its termination. The stator support, which is lightly pressed into the pump cover, should be upgraded to a hardened spline support that is available aftermarket. The pressure regulator assembly is located in the pump cover and if the trans is to be considered for high performance use, the pressure regulator spring and boost valves should be upgraded to increase the line pressure (working pressure) of the unit. If a non-lock-up converter is going to be used, the lockup apply valve in the pump cover should be swapped out for a valve that is available through the aftermarket that's designed to provide a normal oil flow pattern for a non-lock-up type torque converter.
The next item of discussion is the overdrive clutch pack. Machining down the height of the overdrive clutch piston will allow the addition of one extra friction plate. This will increase the torque load capacity of that clutch pack.
After the overdrive clutch pack is the overdrive planetary gearset. This area has not been presenting any problems, so there has not been a need to upgrade any components here. One common wear point is the overdrive sun gear, particularly on high mileage units. The sun gear should be replaced if there are any signs of pitting on the face of the teeth. Only in extreme cases (700+ HP or repeated high loading) has the spline areas of the overdrive ring gear and overdrive planetary carrier become a problem. Fatigue will eventually cause the splines to split.
The overrun clutch housing bushing acts as a sealing element for the apply pressure circuit for that clutch pack. If the fit of this bushing is sloppy, the clutch pack will experience low apply pressure and wear out.
The next component is the overdrive clutch housing and support. The bushings in this housing are critical to supporting the forward clutch drum which in turn supports the back end of the input shaft. When changing the bushings, be very careful not to disturb the forward clutch sealing ring bore. Even the slightest nick on the end of the bore can affect how well the sealing rings for the forward clutch do their job.
The high/reverse clutch housing is one of the components that usually need to be replaced when it comes time for a rebuild. The intermediate band clamps around the outside of the drum to give you 2nd gear. If the band has been slipping and burned, the band surface of the drum will become warped. When the drum warps, only the outer portions of the band will be able to clamp the drum, effectively reducing the torque capacity of second gear. If a new band is put in without replacing a warped drum, the second gear shift will not be as firm as it could be and the band will prematurely fail. A tell tale sign of a warped drum is the band having burned, black stripes along the outside edges and fresh virgin colored lining down the middle of the band. Another area to check on the drum for wear is the large lugs that engage the sun gear shell. Check for wear at the location that the sun gear shell lugs bang back and forth. Wear here will increase internal backlash and can cause undo stress on the geartrain during shifts. The sealing ring bore of the drum must be free of contact markings. It is a good idea to take a square set of brake hones and put a crosshatch pattern in the bore to make sure it is round. Metal sealing rings must have a perfectly round bore or they will not seal properly and the high(3rd)/reverse clutch plates will suffer for it. In certain extreme duty applications where high line pressures must be used, the metal sealing rings for the direct clutch should not be used and special teflon rings used instead. In very high line pressure situations the metal type sealing rings will prematurely wear causing the high clutch to burn out.
The clutch plate count should be increased to add torque load capacity to the 3rd/reverse clutch pack. Up to 9 friction plates is possible. A dual feed modification can also be employed to increase the high/rev piston apply area for high gear.
The forward clutch drum with its integral shaft (intermediate shaft) has come to be a problem spot for applications that are running over 400 horsepower. When put under repeated heavy stress, eventually metal fatigue gets the best of the shaft at the base of the splines and the shaft snaps at that point. The solution is to install a custom forward drum where a special billet intermediate shaft is grafted to the forward drum. The shaft was developed by Bruce Toelle and we have installed quite a few without one single failure to date.
The next area is the gear train. All gears should be checked for pitting and wear. Planetary gears should be checked for proper thrust travel and wobble. All bushing journals should be checked for signs of wear and, if OK, polished. Of course all bushings must be replaced to assure that the gear train runs a true centerline. Any wear on the rollers of the low roller clutch assembly means it's time to replace it. That goes for the low roller inner race also. Rear unit endplay must be adjusted to the tight end of the allowable range to prevent excessive clunking when dropping into gear.
The stock filter and pan is inadequate for high performance use. The G-force encountered under heavy acceleration causes the ATF to slosh to the rear of the pan and allows the filter to uncover. The filter opening, being on the top side of the filter, does not help either. The result is a loss of hydraulic pressure. I have measured losses as much as 70 psi. It happens like this. You have some tires that hook up good and floor it from a stand still. A second or two before the 1-2 shift, the line pressure of the transmission starts to fluctuate wildly from its optimum 200-210 (stage I level) psi reading. By the way, optimum line pressure may be higher depending on the horsepower level of the application. The fluctuating pressure gauge needle is caused by the air and fluid that is being sucked in by the filter. The 1-2 shift occurs while the pressure is still fluctuating and because of the insufficient pressure being supplied to the intermediate band, the shift is softer than one would expect. A couple more seconds and the pressure starts to stabilize as the G-force and fluid level start to relax. A few of these kinds of shifts and the band is permanently burned. A burned band is a dead band and will only get worse. They never get better by themselves.
How much the filter is uncovered is going to depend on the performance of the vehicle. Every 200-4R considered for high performance use should be running the fluid level at one quart over the full mark. Any vehicle that is capable of mid 12 second quarter mile times or quicker should be installing a 4L60 filter and deep pan with the level still one quart over the full mark. When we were doing the pressure testing with a Buick Grand National, we started out with a stock pan, stock filter and stock fluid level. We started to increase the fluid overfill, first by 1/2 quart. The pressure still fluctuated, but was better. We added another 1/2 quart and the pressure fluctuated less, but still was not stable. We installed a deep pan and 4L60 filter, leaving the fluid level 1 quart overfilled. The pressure was as stable as a rock.
The next question. Can we install a 4L60 filter using the stock pan? No. The 4L60 filter does not hug the valve body like the stock filter and must be allowed to hang down to keep the plastic portion of the filter from being crushed between the valve body and pan. The stock pan depth is 2 1/2 inches. An aftermarket deep pan, after having the filter stand off pads milled to make clearance for the new filter is 3.2 inches. As the following pictures show, the 4L60 hangs down with the deepest point measuring 3.2" from the pan rail of the case.
The math says go with an aftermarket deep pan with specially modified stand offs. The filter spring clip should be moved forward to the next valve body bolt as the pictures show. This will keep the filter from contacting the 4th clutch pressure switch terminal.
We can provide you with a high quality deep pan for the 200-4R that we modify and ship with all the necessary mounting hardware, gasket, updated filter and detailed instruction sheet.
Below are some pictures of the identification markings of a BRF valve body. The purple paint blotch and the ink stamped code BR3. I have also found BR2 ink stamped on BRF valve bodies. I also took some pictures of some cast in numbers, but I don't have any idea what they mean.
Below are some pictures of a BRF governor. Notice that the weights are much lighter than your average application 200-4R governor. The one governor spring under the light secondary weight is blue in color. Plastic gear color, orange.
A common problem that occurs with some 200-4Rs is the governor spring getting shot out of position after a sudden stop of the wheels. This typically occurs after a burnout. As of January 17th 2001 we will be modifying all governors for 200-4Rs destined for high performance use so that this problem will not be a worry.
We also carry the spring for the governor. So, if you lose your spring, please give us a call.
DRW Transmission Specialties, Inc. 10728 Kenney St. Ste. B, Santee, CA 92071
Office phone: (619) 258-4240 Fax: (619) 258-9105
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