Transmission Conversions


Transmission swaps have become one of the most frequently performed performance modifications in our community. This FAQ is a culmination of past experiences from many of's members experiences. Feel free to contact any of the moderators if you wish to see something else included or corrected.

The techniques described in this article are purely informational only and should be performed by individuals confident in their abilities to do this. offers no expressed, implied or any other form of guarantee or warranty with the information contained within this FAQ or your property. We also warrant this type of activity as a use/perform at your own risk and will not take any responsibility for any damage to person or property that may occur at any time.  By following these instructions you agree to release all liability for harm or damage done in the event of any issues related to this information. Additionally, if you believe your warranty will be honored after doing this you are sorely mistaken, so please take this into consideration.


The LS1Tech community has a large and vast experience in using a wide variety of transmissions for varying applications.  In most cases people swap from the 4L60E or T56 commonly found in f-body based cars for something more race worthy like a TH350, TH400 or a Powerglide.  Some members have opted for the 700R4 transmission or even a TH400 with a Gear Vendors overdrive unit. The essential differences between the transmissions are fairly easy to understand.  A Powerglide is a simple two speed transmission. They are offered in forward and reverse valve body patterns depending on who builds the transmission.  And they are offered in a wide variety of first gear selections (1.76, 1.82, 1.89, 1.96, 2.03, 2.11) while second gear remains a 1.00 ratio. 1.82 Is the most common first gear selection for most drag racers followed by a 1.76 ratio.  It is often recommended that a Powerglide transmission be used in a vehicle that weighs 3200 pounds or less.  They can be built with a transbrake as well.

A TH350 and a TH400 are very similar types of transmissions.  Both are three speed transmissions with a final drive ratio of 1.00.  They essentially vary (in stock form) by their overall size and the components that are used inside of them.  The rotating components of a TH350 are lighter then those used in a TH400, thus making it a more efficient transmission, maximizing horsepower to the ground.  Like the Powerglide the TH350 is also recommended for cars that weigh 3200 pounds or less.  A TH350 is typically offered with a variety of gear choices depending on who builds the transmission.  First gear ratios are typically 2.24, 2.52 or 2.75, a 2.52 is the most common first gear.  Second gear selections are typically 1.40 or 1.52, 1.52 being the most common.  A TH400 first gear is offered in 2.10, 2.48 and 2.75 ratios, where 2.48 is the most common.  Second gear selections are 1.48 or 1.57 with 1.48 as the most common.  TH400's are rated for weight in excess of 3200 pounds, after all they were originally designed for dump trucks! 

The most common complaint/difference with the TH350 is in the rotating assembly.  The internals of the TH400 are significantly larger and ultimately absorb more horsepower, giving it a lower efficiency in stock form.  Several transmission builders have made this difference negligible by replacing the TH400 iron drum with an aluminum drum, greatly reducing the rotating mass and drastically improving overall efficiency.  Street driven vehicles that opt for a TH400 and an aluminum drum should consider having it hubbed with a steel liner to improve it's wear characteristics.  A 2nd gear band may also be useful for street driven vehicles.  This prevents the transmission from free spinning when downshifting from 3rd to 2nd under load.  This is purely optional and should be discussed when you order a transmission. 

The most significant disadvantage to the stock transmissions is that all of these options do not have provisions for overdrive, they all result in a 1.00 final drive ratio.  However, there is an aftermarket company called Gear Vendors that provides a bolt on under/over drive unit.  Essentially this replaces the tailshaft of a TH350/400 with a gear reduction box.  Unfortunately there is no alternative for the Powerglide.  The reduction can be applied to all gears, turning a 3 speed into a 6 speed.  Here's the breakdown of a TH400 gear ratio with a Gear Vendor Overdrive unit.  Overall they add approximately $2500 to the cost of the transmission.  These units may be used in some drag racing, but their added weight can be a deterrent.

Axle Ratio
    4.88 4.56 4.10 3.73 3.55 3.42 3.08
Trans Ratio
Final Drive Ratio
1st 2.48 12.10 11.31 10.17 9.25 8.95 8.48 7.64
Over 1.93 9.44 8.82 7.93 7.22 6.98 6.60 5.96
2nd 1.48 7.22 6.75 6.07 5.52 5.40 5.06 4.56
Over 1.15 5.63 5.27 4.73 4.31 4.20 3.95 3.56
3rd 1.00 4.88 4.56 4.10 3.73 3.55 3.42 3.08
Over 0.78 3.81 3.56 3.20 2.91 2.77 2.67 2.40


Finally there are the 200-4R, 700-R4, 4L65E and 4L80E  conversions.  These are the least common conversions that typically performed.  In comparison to the previously discussed transmissions, these transmissions  offer a 4th overdrive gear.  The 700-R4 is claimed to be a stronger version of a 4L60E in general, with similar gear ratios.

Gear 200-4R 700-R4 / 4L60E 4L80E
1 2.78 3.06 2.48
2 1.57 1.63 1.48
3 1.00 1.00 1.00
4 0.67 0.70 0.75

Finally there are all different types of valve bodies that you can choose from. Some of the transmission builders actually make or modify their own, so no two are necessarily identical inside, but the concepts are the same.  The TH350/400 and Powerglides are also offered with forward and reverse manual valvebodies. Think of a forward valvebody as something you would find in a passenger car (P R N D 3 2 1). A reverse manual valve body is typically shifted by hand and the gears are reversed so that it runs through gears in the following order P R N 1 2 3.  When using a manual valvebody you can often get a transbrake installed.  A transbrake is a great way to stage a car during a race, lock the transmission so that the car does not move, step on the gas and build RPM's.  During the build up of RPM's the engine will continue to climb until it reaches the stall speed of the torque converter.  With a boosted car the stall speed will increase slightly as a function of torque, so this needs to be taken into account when you order your converter.  A transbrake effectively works in first gear by applying second gear at the same time.  This creates a functional equivalent to putting the car in park, thus preventing the car from moving.  When you let go of the button, the transbrake solenoid is pushed open releasing the car into first gear.  For some Powerglide cars the valvebodies are modified so that they do not have a normal reverse position.  Rather, the driver simply puts the car into reverse and holds the transbrake button to engage reverse.  Under all other conditions you should never engage the transbrake otherwise severe damage may occur.  It is also recommended to never engage the transbrake on the street and without sticky tires, otherwise serious transmission damage may occur. 



There are several important safety factors that need to be considered if your transmission swap is being done for use with drag racing. Ideally these things should also be considered for even vehicles with factory transmissions in accordance with either IHRA or NHRA rules and guidelines.  It is recommended that you use a SFI certified flexplate with your new transmission.  There are numerous advantages to doing this.  The first is that the SFI approved flexplates are more durable and less prone to breaking like the factory flexplates.  Companies like TCI and Reactor make LS1 flexplates that are certified already.  TCI Sells theirs as a small bolt pattern 153 tooth flexplate, part number 399753.  Reactor sells them as a 153 tool small bolt pattern as well.  Common failure modes with the factory flexplates is that they often break or crack where the convert bolts up to it.  Also, if you are going to use the stock flexplate it is not recommended to use the ARP flywheel bolts.  The unthreaded  shoulders on the bolts are longer then the thickness of the flexplate, which will not allow you to tighten it properly.  If you are using aftermarket flexplate, the use of the ARP bolts is highly recommended. Here you can see the difference between the bolts.

Flexplate Scattershield

The NHRA and IHRA require cars to be equipped with a flexplate scatter shield and a transmission shield. There are several ways to accomodate this requirement. The first is that CSI produces a composite shield that covers the entire bellhousing and the transmission as one piece. The second is that you may use a SFI approved bellhousing/scattershield like a Ultra Bell. You may find that space constraints a little tight to use the CSI shield, but it is possible.  Most aftermarket transmissions can be modified to allow the use of a JW Ultra Bell.  Unlike a stock bellhousing on the transmissions, the Ultra Bell is rated as a SFI approved scatter shield.  The Ultra Bell does require two minor modifications to fit the LSx blocks.  The first is that the ears that come on the ultrabell for other block applications may be cut off.  The second is that the bellhousing face that mates to the block is considerably thicker then the stock bellhousing.  This ultimately makes the bellhousing interfere with four bolts located on there rear cover of the engine.  This can cause some significant misalignment problems and ultimately damage the transmission and/or converter.  Here you can see the rear cover and the four upper bolts, followed by the modifications required in the bellhousing.

Five bolts are used to mount the bellhousing to the block.  The four holes adjacent to the locating pins and the next upper left hole by the driver side cylinder head.  The top center hole in the block will not be used and the block also has no provision for the upper right hole in the bellhousing.  The lower two holes in the oil pan that the factory bellhousing bolted to are not used as well.

Transmission Shield

Finally you need to consider a transmission shield over the drum area.  TCI Makes a aluminum SFI approved shield for this purpose.  It's a simple bolt on shroud that covers the drum area.  One of the most dangerous failures in a transmission, besides a flexplate failure is the drum letting loose.  Many racers each year suffer a severe transmission failure like this that ultimately rips through the floorpan of the car when unchecked.  A secondary result of this is that hot transmission oil is scattered everywhere.  Of the racers that have experienced this they now often run an Ultra Bell, a transmission shield and a transmission blanket.  The thinking behind the use of the blanket is to contain any remaining parts that ricochet off the shields as well as the oil.  This is purely an optional step, but it is completely dependent on how safe and protected you want to be in the event of a serious failure.

Park / Neutral Safety Switch

Both A4 and M6 cars have the park / neutral safety switch wired in.  You will need to wire in your shifter for use with this safety circuit in order to start your vehicle.  For M6 cars you can splice into the lower clutch sensor wires.  There is a plug that goes into the sensor at the bottom of the clutch pedal.  The wires are black with a white wire.  You can simply splice into those two wires and run them to your new Park / Neutral switch on your shifter.  You will also need to determine how you want to illuminate your reverse lights.  Some people mount a button on their steering wheel, other shifters may have a provision for it already.  For A4 cars you will need to splice into the wires from the factory shifter.  The purple and dark green wires are for the park / neutral safety switch.  The brown and light green wires are for the reverse lights.



For the Powerglide and the TH350/400 there are no provisions for a speedometer normally.  The TH400 can often times be purchased with the GM VSS Sensor installed.  All that you would have to do is splice in the connector to the factory harness.  The VSS Sensor is GM part 8655291 and it requires the reluctor ring GM part number 8655315.  The pigtail harness is GM part number 12085498.  All are available from the dealership, but may have to be ordered.  You can either order them from and pay their high shipping charges or order it from the dealership.  Expect the parts to cost close to $200.  Once you have the speedo spliced in you will need to calibrate your PCM with LS1 Edit.  For cars converting from an M6 you have to multiply the tire diameter by 2.5 to make up for the different number of pulses per mile that the PCM will see.  A4 cars should be ok.  Ideally you can calibrate it in third gear under load by taking your RPM and MPH with AutoTap (or by eye) and then running it through a gear calculator to find out where you are.  Or ask your local law enforcement for some help!  Ideally the radar speed checks are a nice way to confirm it as well. 


Depending on the transmission you can get either a Lokar flexible dipstick or order a hard tube dipstick from Madman.  The hard tube dipstick fits nicely but can be difficult to install as you have to do it as you are putting the transmission in.  The Lokar dipstick is significantly easier to install.  You can custom order a Lokar in longer lengths to increase your mounting options as well.  Lokar part number TD3350-400-FM-36 is a 36" dipstick for a TH400 that is optimal for use with an LS1.  You can also order them for TH350's and Powerglides as well.  The dipsticks are different per transmission as the pan has a different size hole that the dipstick tube plugs into.

Converter Selection

The converters typically used are standard SBC converters that have two minor modifications.  The first is that they use a LS1 Pilot diameter.  The second is that they should have the pilot extended 3/8" to make up for the difference in length between the LS1 and a traditional SBC.  If you are running a mid plate, you can either figure out how to space the converter away from the flexplate or order one with the pilot extended that much further.  You cannot offset the flexplate easily before you run into issues with the starter engaging properly.


There are a multitude of shifters available on the market today.  Almost all of them will work depending on their correct application.  Nearly all of them will require some type of fabrication and will depend more on what type of console you have.  A4 Consoles are typically suited for ratchet type shifters like a B&M, that have a more linear shifting pattern.  M6 Consoles can accommodate ratchet shifters as well as gated shifters like the TCS Cheetah shifter.  Some people have modified the factory A4 shifter to work as well.  You can find more information on the factory shifter conversion HERE.

Transmission Coolers

There area  variety of coolers that you can use.  The easiest cooler for cars converting from an A4 is to use the factory cooler.  This can be done by placing either 5/16 hose to a 1/2 or 5/16 NPT  pipe adapter in the transmission.  Using approximately 3" of 5/16 hose connect the transmission fittings to the stock cooler lines using four hose clamps.  It is a wise idea before hooking the transmission up that you blow the factory cooler out using a air line.  It also is advisable to use a vacuum pump to draw clean transmission fluid through the cooler.  This may take some time, but it is worth the effort to remove any possible garbage from the cooling lines.

For cars that are going to use an externally mounted cooler you can simply install two 1/2" NPT to -6 AN fittings into the transmission.  Using two 45 degree -6 AN fittings and several feet of braided line to the front of the car you can place a number of different size coolers in front of the radiator.  A common unit is the B&M Super Cooler for 24,000 GVW vehicles.  You can get the cooler with and without a fan.  Once you have the cooler mounted you can finish connecting the -6 AN lines to the cooler.  Another considerably larger cooler and fan that works very well is the Derale 16 pass trans cooler with a 10" fan.  You can purchase an inline 160 degree thermostat for the fan so that it turns on and off with the transmission temperature.


Driveshaft lengths will vary from car to car depending on a couple factors.  If the car is using a 12 bolt or a 9" you will see some variances.  Also, if you are using different types of motor mounts compared to others (stock, solid, poly or motorplate).  It is best to measure the driveshaft for your application.  In most cases a 12 bolt to a stock located engine with a standard tailshaft on a TH400  transmission will be approximately 43" in length.  A Powerglide will be slightly longer.

Torque Arm

If you never converted your factory torque arm over to an aftermarket torque arm, now may be the time to consider this.  Unfortunately most


Powerglide info here.


TH350/400 Info Here

4L Series

4L60E, 4L65E, 4L80E Info Here


T56 Info Here


Common LS1Tech Discussion Threads

Stock Automatic Shifter with a A3 Transmission