Transmission / Driveshaft FAQ

Transmission Selection:

sources are from the net, with modifications as required

Transmission conversion: Most people feel the 3 best choices are the Chevy Turbo 350, Turbo 400, and Turbo 700-R4. 

The Turbo 350 is very suitable; is abundant and most easily incorporated. 

The (short shaft) Turbo 400 is the best choice for performance considerations. It is more robust than the T350 but is harder to find and more expensive. It makes driveshaft fabrication more difficult, but it is the way to go if one has the need for speed (but not MPG). Note: be aware that "Chevy" style TH-400 with a short tailpiece are getting rare. 

The Turbo 700-R4 is a good choice* due to the higher rear-end gear ratios on older cars. On a Stude with 3.31 rear end gears, the overdrive T700-R4 will drop engine speed from 2500 RPM to 1750 RPM at 60 MPH. Highway fuel economy will increase about 30% and general wear-and-tear on the engine components will be reduced. Avoid pre-1985 versions of these transmissions, as they have known reliability problems, unless they are salvaged from Police Cars or they are remanufactured for heavy duty use . Also, a switch is required to operate the lock-up feature of the torque converter in high gear-- failure to do so will cause damage** to the transmission. B&M sells this Power Switch for about $40 and it includes wiring instructions. It is also possible to have a hydraulic version of this valve installed within the transmission. 

Note: When searching for a 700-R4 for use in a conversion, remove the torque converter, and look on the front plate inside the bell-housing for the casting number. Get one with the last three digits being 732. This ensures an 87 or newer unit with the best of GMs updates.

'*' Be wary of using a 700 behind a higher performance engine whose operating range may be beyond the rpm resulting from 4th gear and a locked up converter. ie. While an engine with a 280 duration cam will idle satisfactorily, the true operating range is around 2000 - 5000 rpm. " " IE tires = 215-65R15 - 3.31 axle (((((215 / 25.4)* .65)) * 2))) + 15))))* pi))))) = 81.6" (6.8') (circumf.) ((5280 / 6.8) * 3.31)) = 2750 revs per mile....... 2750 - .30 Overdrive = 1979 rpm - lockup (300 rpm) = 1679 rpm

As a rule of thumb - If it idles smoothly and pulls fairly strong from idle on up, then you're ok. If it 'lopes' a bit, you're getting out of range. If it has to idle fairly high to give good low end response, then you'll have problems with driveability.

'**' - If lockup does not engage, the mechanical action of the torque converter required to propel the car will generate more heat than can be naturally dissapated. Heat in excess of 260 degrees can fatally damage the clutches and electronic components.

GM Automatic Transmission Gear Ratios
Name	1st	2nd	3rd	Overdrive
Powerglide	1.76	1.00
TH-350	2.52	1.52	1.00
TH-400	2.48	1.48	1.00
700-R4	3.06	1.63	1.00	0.70
200-R4	2.74	1.57	1.00	0.67
4L60	3.06	1.63	1.00	0.70
4L80	2.48	1.48	1.00	0.75

This table list the most common GM automatic transmissions and their gear ratios

 

Rear-End Selection:

This second table should also be useful-- It shows the most common GM transmissions as well as the overall gear ratios when they are used with the factory Stude rear-end ratios. This table should help greatly with transmission/rear-end ratio selection:

GM TH-350
Ratio	1st	2nd	3rd	O/D
@1:1	2.52	1.52	1.00
3.07	7.74	4.67	3.07
3.31	8.34	5.03	3.31
3.54	8.92	5.38	3.54
3.73	9.39	5.70	3.73
4.09	10.3	6.21	4.09
GM TH-400
Ratio	1st	2nd	3rd	O/D
@1:1	2.48	1.48	1.00
3.07	7.61	4.54	3.07
3.31	8.21	4.90	3.31
3.54	8.78	5.24	3.54
3.73	9.52	5.52	3.73
4.09	10.14	6.05	4.09
GM 700-R4 / 4L60
Ratio	1st	2nd	3rd	O/D
@1:1	3.06	1.63	1.00	0.70
3.07	9.39	5.00	3.07	2.15
3.31	10.13	5.40	3.31	2.32
3.54	10.83	5.77	3.54	2.48
3.73	11.41	6.07	3.73	2.61
4.09	12.51	6.67	4.09	2.86
GM 200-R4
Ratio	1st	2nd	3rd	O/D
@1:1	2.74	1.57	1.00	0.67
3.07	8.41	4.82	3.07	2.06
3.31	9.07	5.2	3.31	2.22
3.54	9.70	5.56	3.54	2.37
3.73	10.22	5.85	3.73	2.49
4.09	11.20	6.42	4.09	2.74
GM 4L80
Ratio	1st	2nd	3rd	O/D
@1:1	2.48	1.48	1.00	0.75
3.07	7.61	4.54	3.07	2.30
3.31	8.20	4.89	3.31	2.48
3.54	8.77	5.23	3.54	2.65
3.73	9.25	5.52	3.73	2.79
4.09	10.14	6.05	4.09	3.06

This table lists selected GM transmissions and their overall gear ratios
when multiplied by the respective Stude rear-end ratios

T700-R4 Adjustment......Important

 

Torque converters and transmission coolers: 

The transmissions from high performance GM cars generally have a higher stall speed torque converters and higher shift points than stock vehicles. After market performance torque converters like the B&M Torque Master 2000 can also be used to raise the lock up point of the converter by about 500 RPM over stock. 

The benefits of these high performance torque converters-- is a better mating of the engines torque output to the engine/vehicle matchup. It allows the engine to rev a little higher and to get into the camshaft's "power-band" resulting in improved acceleration, smoother feel, and reduced "creeping" at idle. Disadvantages are reduced fuel economy and higher transmission oil temperatures. 

Transmission Coolers & Filters: 

Since heat is the number one reason for transmission failure; the Chevy/Stude conversion requires a transmission oil cooler for insurance. A heavy-duty cooler is usually already installed in the radiator and an additional external cooling system should be installed. The Hayden #403 is a good choice. Mount it in series with the internal Radiator cooler and in front of the A/C condenser.

Perma-Cool now markets an external oil filter that supplements the one in the trans pan with a general purpose engine oil filter. One of the variations of this unit comes with a temperature sending unit fitted to the filter casting and a guage for your dash.  

U-Joints: 

You'll have to have a new driveshaft made by an aftermarket service dealer, usually a truck service center. The new driveshaft measurement is from the center of the yoke saddle on the trans to the center of the pinion yoke on the rear axle. This measurement must be made after you fit the trans to the car. Take the Stude driveshaft to a aftermarket manufacturer and have the rear yoke removed and welded to the new driveshaft. Use heavy duty u-joints that can be greased, also cheap insurance and anyway, you need a few more grease fittings under that Stude

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Some technical opinions are my own from experience, other informational data is from online sources with credits when available and while care has been taken to be as accurate as possible, it is offered only as a guide and caution should be exercised in the application of it.