Nearly all of the GM automatic transmissions have a few things in common. They are all made of aluminum. The Turbo 350 is arguably GM’s most popular automatic transmission with the aftermarket even today. This is espescially true for the people who are still using the generation I and II small-block Chevy.

• Chevy 350 Transmission For Sale
• Chevy 350 Transmission Pan

Although the TH350 has been phased out for more than 20 years, you can still find one in the salvage yard. You certainly should not depend on the tech working at the yard for confirmation of what you have. You’ll certainly find them at swap meets and craigslist. Do not depend on someone’s word when they tell you that they have something. I am not saying that people are liars. Most of the time they are not. It’s often the case that the part you are looking at has been sitting around someone’s garage for a long time and they don’t perfectly recall what they have.

There are a few things that all of the longitudinal gm automatic transmissions have in common. They all have basically the same bolt pattern on the bellhousing. Although if you use an older transmission with a newer “LS” engine, you’ll have an extra bolt. But, they do not all bolt into the crossmember in the same space. They are also not the same length. Therefore, they are not all naturally compatible with one another.

The TH350 is a popular exchange transmission for early Jeep four-wheel-drive enthusiasts, because of their short profile and their ability to easily accept multiple four-wheel-drive transfer cases. Studebaker, Hudson and International engine adapters allow a TH350 to be mounted behind them as well. This Speedway exclusive 3-speed Chevy Turbo 350 Street Rod crate transmission package comes dyno tested to ensure quality and performance 2200-2300 stall torque converter is included and will accommodate performance camshafts Heavy duty Raybestos clutches, new bushings, seals, and rebuilt pump ensure long trouble free operation. The 700R4 was used widely on both truck platforms and rear-wheel drive car platforms. Some of the popular trucks it was used in were the Chevrolet Blazer and Suburban and the Oldsmobile 350 diesel. This transmission was also used in the Chevrolet Camaro, Corvette, Impala, and many others.

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Identifying the TH350 the Easy Way

Certainly the easiest way to identify a GM automatic transmission is by the bolt pattern. The TH350 has a 13 bolt pan gasket. This is a fine way to whittle what you have in front of you. If you counted 13, you are either looking at the Turbo 350 Transmission or the Turbo 400.

You’ll also notice that the pan on the turbo 400 has quite a few more angles than the turbo 350. So, if your pan doesn’t look like a square with a corner taken out than you may want to move along.

Although they aren’t very common, there is a transmission based on the TH35o known as the TH250. It’s visually similar to it’s bigger sibling, but lacks the capability to handle a lot of torque. GM often bolted it behind it’s more anemic six cylinder engines.

It’s gosh darn tough to identify one visually. You’ll need to look above the coolant line fittings to see if there is a threaded screw there. It’s for band adjustment. Most of the info on this site I just know. In this particular case I found that right here:

Using the ID Tag for Identification

Really it is a lot easier to ID a transmission visually compared to the alternative of decoding it, but it is reasonable to try and understand exactly what you have. Now, for the the purpose of this guide there are two different ways that GM printed the the serial label. They changed the way that they labeled their transmissions for the 1967 model year. The TH350 began production in the 1969 model year. That would put it squarely in the second era, and this is how it went.

You’ll need to find the ID tag before you get anywhere. This can be extremely difficult on the TH35o particularly because the ID tag can be found on several different places on the transmission. It can be found on:

• Near where the shift linkage attaches to the body
• Just above the housing on the right side of the pan
• On the passenger side on a flange by the bellhousing

So, you if you are trying to wiggle under a car to read this thing it can be very problematic. You’ll not only need to hunt for this thing, but it needs to be free of enough grime to be legible.

You’ll be looking for these numbers on the ID tag in order to properly tell exactly what kind of transmission that you are dealing with. Most gm transmissions use the M RPO code. Here is a pretty complete list of all of them from the folks over at NastyZ28. The more common ones are:

• MV4- This code identifies your turbo 350 as a “C” model, which had a locking torque converter. This code was used for ten years from 1976 to 1986.
• MX2- Another 350C code. Found on 76-84 models
• MX3- Another Turbo 350C code. 1976-1981 model years.
• MX5- Turbo 350C, 1982 and 1983 only.
• M33- This standard Turbo 350 code identifies it as a standard THM350. This version did not have a locking converter. This one was manufactured from 76-81.
• M38- Standard Turbo 350 manufactured from 1976 to 1981.

What transmission is right for me?

Your transmission and engine combination is very important. Your vehicles weight and how you drive it are also a major factor. Transmissions come in two basic types, Automatic and Manual:

Automatic

TH350

Best transmission choice, hands down.

The Turbo Hydra-matic 350 was first used in 1969 model cars. It was developed jointly by Buick and Chevrolet to replace the two-speed Super Turbine 300 and aluminum case Powerglide transmissions. So, although it carries the Turbo Hydra-matic name, the Hydra-matic Division of General Motors had little, if anything, to do with its design. The 350 and its 250, 250c, 350c and 375b derivatives have been manufactured by Buick in its Flint, Michigan, plant and by Chevrolet in Toledo and Parma, Ohio, and Windsor, Ontario. Both Chevrolet and Buick divisions produced the TH350. For the 1981 model year, a lock-up torque converter was introduced which coincided with the new EMC control of most GM cars; This version is the THM350-C, this transmission was phased out in 1984 in GM passenger cars for the 700R4. Chevrolet/GMC trucks and vans used the THM350-C until 1986. The lock-up torque converter was unpopular with transmission builders B&M Racing once marketed a conversion kit for THM350-Cs during the early 1980s until the advent of high stall lock-up torque converters when its overdrive counterpart (THM700R4/4L60) were modified.

Pros

• Excellent gear spread between all three gears.
• Very low cost to build up to various levels of power handling.
• Can be built up to extreme power handling capabilities.
• Most all transmission shops are capable of building these to at least level 1 or 2.
• No TV cable to deal with.
• Stall converters are low in cost, and the selection is broad.
• Very reliable and simple to repair if needed.

Cons

• Select a final gear that will allow you the best overall performance and one that allows a decent off the line acceleration, and one that will be in a tolerable rpm range at highway speed. You must have the correct stall converter for optimal performance. This can make a huge difference in the way your vehicle accelerates.
• One THM350 weak point was excessive end-play between the pump and center support and resulting wobble of the direct clutch drum due to both the end play and use of a relatively narrow bushing in the drum. Add an extra thrust washer between the planetary gear and direct clutch to remove the end play and using a wider aftermarket bushing in the direct clutch drum.
• The relatively thin center support and lightweight matching splines in the case cause some people to beef up the case with an aftermarket case saver kit.
• These are becoming scarce, but there are still plenty of them around, and high performance parts are easily obtained.
• You may have to go through this long story to your less than knowledgeable friends as to why you didn't choose a 700R4, and when you explain it to them, they may still think you don't know what you’re talking about.

TH400

Better than a stock TH350, more expensive to build compared to TH350.

Pros

• Excellent gear spread. Very similar to a TH350.
• Strong internal parts.
• Reliable.
• Simple to build or repair to stock buildup levels.

Cons

• Very expensive to build up to higher power handling levels, as compared to a TH350. The TH350 has far more high performance parts available at reasonable costs. The 400 trans. is not a practical choice.
• Internal rotating parts (cast iron drum) are very heavy and create a huge drag compared to most any other trans., and this is not a subtle amount of drag. The TH400 is well known to be a heavier duty trans. than a TH350 but this heavy duty factor was designed more for heavy vehicles that may encounter pulling heavy loads. Only the largest of the GM cars weighing around 5000 lbs had these trans. in them, along with heavy duty pickups, usually 3/4 ton or larger trucks. Even the half ton Chevy trucks didn't come with these in them.
• They will certainly hold up better than a stock built TH350, but they are not a desirable trans. for a street rod. You can build a TH350 to level two that would be superior to any stock TH400 in every way for about the same cost. Building a TH400 to a high hp handling level will cost a great deal of money.
• You will never see this trans in a true Pro-Street car.

Powerglide

Only two gears, best suited for very light vehicles, under 1800 lbs. The Powerglide is a two-speed automatic transmission designed by General Motors. It was available primarily on Chevrolet from January 1950 through 1973, although some Pontiac models also used this automatic transmission after the fire at the Hydra-Matic factory in 1953.

Pros

• Excellent transmission to build up to just about any power handling level.
• Most trans. shops are able to build this trans with ease.
• Very reliable, and simple to work on. Not much to go wrong.
• Decent first to second gear spread.
• Rotational drag is very low with this trans., moreso than any automatic on the market. Very efficient trans.
• Stall converters are not expensive and many choices available.

Cons

• Two gears, that's the problem. This trans. can only be run in cars that have a very high hp to weight ratio. It would be fair to say a vehicle weighing approx. 3400 lbs would require an engine output of 1000 hp or more to make it a rational choice, and even then, you would still need a decent final gear to get things moving off the line. The first gear in these is somewhat tall. Those running a T-Bucket roadster weighing around 1800 lbs. with a 500 hp or higher engine could make great use of a Powerglide trans.
• Having only two gears limits having the best of both worlds when it comes to off the line acceleration and cruising at highway speed.

700R4

Worst possible transmission choice.

Pros

• None, other than there are plenty of them around for a cheap price, and for good reason.
• Fuel injection does make this trans. more tolerable.
• It will get you from point A to point B period!

Chevy 350 Transmission For Sale

Cons

• Very wide first to second gear spread. First gear is a 3.06 ratio Second is a 1.62 ratio. This nearly equates to skipping from first to third gear with a manual trans. This kills acceleration when these trans go into second gear on a carbureted engine with a healthy cam and large runner intake manifold. Compare this to a TH350. First gear at 2.52 ratio and second gear is 1.52 do the math.
• The 3.06 first gear is so low that any final gear of above 3.55 or numerically higher will have you right at the max rpm at around 35 mph, only to then have your car fall on its face when it hits second gear, feeling like it skipped a gear. Wheel spin is uncontrollable upon hard launches, unless you have a very tall final gear, and then you can for sure count on going nowhere quickly when second gear comes in. Overdrive would be totally useless at highway speed regardless of detuning with a tall final gear, unless you were cruising at around 100 mph.
• Huge cost to build up to a decent street rod level. Don't even consider a stock rebuild to handle 500 hp or more. Under hard use, the 700R4 will fail quickly.
• Limited as to what level these can be built up to. High performance transmission builders and parts suppliers don't even recognize this as a serious transmission.
• More complex to build than a TH350 and far more expensive, especially when built up to handle high hp numbers.
• Having to run a cumbersome throttle value cable to the carb. linkage. This feature actually serves a good purpose, but if not set properly, it can destroy your transmission quickly. Very critical adjustment, and often overlooked.
• The overdrive is actually useless with engines running decent size cams relative to the cubic inch, especially with large runner dual plane intakes, and single plane intakes are out of the question with this trans.
• Stall converters are double the cost of the one run in a TH350.
• This is not a reliable transmission in a street rod.
• Lowers the value of your vehicle to some degree, especially on high hp builds. These transmissions do not belong in most carbureted street rods. You must detune an engine considerably to operate the overdrive and be able to tolerate the first to second gear drop.
• Could be classified as the worst mismatch of parts known on a true street rod.
• As a side note, I would like to mention that most of us have owned a regular stock street car with a 700R4 trans, or its electronic likeness, a 4L60E, and didn't find much of an issue with the way they operated, especially if the vehicle was fuel injected. This is because stock car engines are designed to operate totally different in the very low rpm range. They have huge amounts of off idle torque compared to the average carbureted street rod that makes 400 hp or more. This off idle instant torque allows the vehicle to keep its speed up decently when the 700R4 drops into second gear, and the same low rpm also allows the overdrive gear to operate normally at very low rpm. There are other factors that also allow these trans. to operate at low rpm, especially on fuel injected computer driven engines.

GM200

Most all the cons you see for the 700R4 exist within the GM200

Pros

Chevy 350 Transmission Pan

• Has a better gear spread than the 700R4 but not as optimal as the TH350.
• Not much good to say about this trans. It certainly has a place in some special late model applications, but old school Chevy engines is not one of them.

Cons

• This transmission can cost up to three times more to build up to a decent power handling level compared to a built up TH350.
• Cannot hold up to high torque engine builds unless an enormous amount of money is spent on this trans.
• This trans is not a reliable trans. compared to any of the early GM trans.

4L60-E

Electronically controlled trans. Most of the same issues as the 700R4. Terrible choice to run in an old school carbureted engine.

Pros

• No TV cable to deal with or shifting linkage. It has pump pressure control and shifting via electronics.
• Not much good to say about this expensive trans. It is virtually a 700R4 with a modern twist.

Cons

• Very expensive to build up to a high power handling level. Can cost up to $4,000 to build this trans up to a decent level. Computer controlled engines may require this unless you bypass the car’s computer system.
• Complex. Very complex!
• Pointless to run such a terrible transmission.
• If you insist on such a trans, the 4L80-E is a better choice, as it has a normal gear spread, and is a much stronger trans. Still pointless to use with an old school carbureted engine.

Manual

Four and Five speed Manual

Don't forget to upgrade your clutch and flywheel components.

Pros

• High quality manual transmissions can be very expensive.
• Excellent acceleration at any speed due to many selective gear ratios. Finding a gear at any cruising speed to hit it hard is easily accomplished with a manual compared to an automatic.
• More efficient delivery of horsepower to the rear wheels with a manual trans. versus an automatic. In other words, less loss of power due to slippage as encountered with an automatic transmission.
• For some people, the fun factor of shifting gears is one of the high points of owning a street rod.
• Very reliable, long lasting. Low maintenance. Simple to change out a clutch if needed.

Cons

• Can be very expensive to purchase a high quality modern manual trans. and high performance clutch and flywheel, especially the Tremec Series.
• Can be somewhat complex to change over from an auto to a manual trans.
• If you think your car is going to run faster with a manual versus an auto, you will be greatly disappointed. The automatic transmission can not only shift much faster, but off line acceleration can be controlled to a much greater degree. This is why most drag cars have automatic transmissions. All things equal, a car equipped with an auto trans will outrun one equipped with a regular street rod manual with great ease.
• Missing gears when racing can cause damage to your engine, and loss of control of your vehicle should you drop into the previous gear during an aggressive run. Been there a couple times, dropped a valve in brand new Lotus Esprit shifting out of second into first rather than third, and locked up the back wheels on a Dodge Viper shifting into second rather than fourth at high speed. Nearly lost control of the car at around 80 mph.
• If you’re deciding to run a 5 or 6 speed manual trans because of the overdrive gear, be aware that most engines built with large lopey cams, and single plane intakes running a carb will usually not allow effective use of the overdrive gear.
• Tremec T56 transmissions are very popular and highly advanced. They are expensive. They tend to be notchy to shift when new. Takes many miles before they become easy to shift. They can be difficult to install in some early model classic cars, as they are huge compared to early model transmissions. They can cost a great deal of money to rebuild if needed. Still about the best manual trans out there, and can handle very high horsepower levels.