Help with Transmission Selection for LS6 in Rolls Royce
Thread Starter
Teching In
Joined: Aug 2017
Posts: 2
Likes: 0
From: Houston, TX
Help with Transmission Selection for LS6 in Rolls Royce Hi All,
We are planning to drop an LS6 engine into an old Rolls Royce Silver Shadow. The car, from factory, came with a 3-speed automatic GM TH400 tranny with column mounted shifter. RR made some custom modifications to this transmission. The old transmission isn't any good and needs to be replaced anyway.
I have read on this forum that its better to go with a 4l80e or 4l60e as opposed to going with a replacement TH400, but I wanted to know which would be the path of least resistance. I've attached pictures of the old transmission we took out. It looks slightly different than your typical TH400, but which option would be closest in size and shape? I think the mounting bolts may also be in different locations.
Next, what's the best way to go from a 3 speed to 4 speed in terms of the column shifter and display?
The car will never be used for tracking or racing...but I imagine it will spend most of its time on highways.
Thanks in advance for your input and help.
We are planning to drop an LS6 engine into an old Rolls Royce Silver Shadow. The car, from factory, came with a 3-speed automatic GM TH400 tranny with column mounted shifter. RR made some custom modifications to this transmission. The old transmission isn't any good and needs to be replaced anyway.
I have read on this forum that its better to go with a 4l80e or 4l60e as opposed to going with a replacement TH400, but I wanted to know which would be the path of least resistance. I've attached pictures of the old transmission we took out. It looks slightly different than your typical TH400, but which option would be closest in size and shape? I think the mounting bolts may also be in different locations.
Next, what's the best way to go from a 3 speed to 4 speed in terms of the column shifter and display?
The car will never be used for tracking or racing...but I imagine it will spend most of its time on highways.
Thanks in advance for your input and help.
TECH Fanatic
Joined: Jan 2015
Posts: 1,114
Likes: 33
From: Fort Worth, Texas
The TH400 was built by Hydramatic for RR to thier specifications, Looks almost identical to the TH400 that Jaguar used behind their V12's.
What rear gear ratio are you going to run? I'm pretty sure the stock ratio is in the mid to high 2's, If your going to run such a tall ratio?.......Stay with a TH400, Just a Chevy version. You can install the Output Shaft & Tail Housing in/on a Chevy Case
From experience......LS6 5.7L's don't fare to well in heavy vehicles. The first gen CTS-V is a dog down low & I bet it doesn't weigh near what your Roll's weighs.
What rear gear ratio are you going to run? I'm pretty sure the stock ratio is in the mid to high 2's, If your going to run such a tall ratio?.......Stay with a TH400, Just a Chevy version. You can install the Output Shaft & Tail Housing in/on a Chevy Case
From experience......LS6 5.7L's don't fare to well in heavy vehicles. The first gen CTS-V is a dog down low & I bet it doesn't weigh near what your Roll's weighs.
Moderator
Joined: Aug 2010
Posts: 7,323
Likes: 529
From: Ann Arbor, MI
Agreed, if your rear end ratio is mid to high 2's, the overdrive of a 4L80E isn't going to work well with a heavy car and only 5.7L of low-end torque.
Another TH400 is definitely the path of least resistance.
However, if you did go 4L80E I would setup the column shifter to only select between 2nd and 4th gears. Unless you are climbing rocks, who the heck ever shifts into D1?
Another TH400 is definitely the path of least resistance.
However, if you did go 4L80E I would setup the column shifter to only select between 2nd and 4th gears. Unless you are climbing rocks, who the heck ever shifts into D1?
FormerVendor
Joined: Jun 2017
Posts: 62
Likes: 0
If you wanted some help getting that back in shape shoot me a message. We could rebuild that trans for a quick easy option to get you back on the road. We could also custom build you a converter that would pep it up. You could slap a gear vendors behind it if you really wanted the OD. We love doing cool projects like this!
Thread Starter
Teching In
Joined: Aug 2017
Posts: 2
Likes: 0
From: Houston, TX
The TH400 was built by Hydramatic for RR to thier specifications, Looks almost identical to the TH400 that Jaguar used behind their V12's.
What rear gear ratio are you going to run? I'm pretty sure the stock ratio is in the mid to high 2's, If your going to run such a tall ratio?.......Stay with a TH400, Just a Chevy version. You can install the Output Shaft & Tail Housing in/on a Chevy Case
From experience......LS6 5.7L's don't fare to well in heavy vehicles. The first gen CTS-V is a dog down low & I bet it doesn't weigh near what your Roll's weighs.
What rear gear ratio are you going to run? I'm pretty sure the stock ratio is in the mid to high 2's, If your going to run such a tall ratio?.......Stay with a TH400, Just a Chevy version. You can install the Output Shaft & Tail Housing in/on a Chevy Case
From experience......LS6 5.7L's don't fare to well in heavy vehicles. The first gen CTS-V is a dog down low & I bet it doesn't weigh near what your Roll's weighs.
Agreed, if your rear end ratio is mid to high 2's, the overdrive of a 4L80E isn't going to work well with a heavy car and only 5.7L of low-end torque.
Another TH400 is definitely the path of least resistance.
However, if you did go 4L80E I would setup the column shifter to only select between 2nd and 4th gears. Unless you are climbing rocks, who the heck ever shifts into D1?
Another TH400 is definitely the path of least resistance.
However, if you did go 4L80E I would setup the column shifter to only select between 2nd and 4th gears. Unless you are climbing rocks, who the heck ever shifts into D1?
If you wanted some help getting that back in shape shoot me a message. We could rebuild that trans for a quick easy option to get you back on the road. We could also custom build you a converter that would pep it up. You could slap a gear vendors behind it if you really wanted the OD. We love doing cool projects like this!
Will do if we stick with the TH400.
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Moderator
Joined: Aug 2010
Posts: 7,323
Likes: 529
From: Ann Arbor, MI
We are talking about the rear end ratio in the differential which has nothing to do with the TH400. Very heavy vehicles which came with a TH400 probably had a read end ratio in the mid 3's. Some muscle cars in the late 60s and early 70s even had a ratio of 4.11.
However to keep the engine noise down, a luxury car using a TH400 probably had a rear end ratio in the 2s.
With a rear end ratio in the 2s, as your Rolls probably had, you would likely want a larger displacement engine in the 6.2 - 7.0L range to give your the low end torque needed for smooth acceleration.
A higher stall converter will help with acceleration and I would recommend something in the 2800 range regardless of whether you go TH400 or 4L80E.
However to keep the engine noise down, a luxury car using a TH400 probably had a rear end ratio in the 2s.
With a rear end ratio in the 2s, as your Rolls probably had, you would likely want a larger displacement engine in the 6.2 - 7.0L range to give your the low end torque needed for smooth acceleration.
A higher stall converter will help with acceleration and I would recommend something in the 2800 range regardless of whether you go TH400 or 4L80E.
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Using the 4l80e or 60e will still use all 4 forward gears when placed in drive. You just wont be able to manually select first on the sifter. Since the car is already set up with a TH400 variant, I'd stick with that. Is the bell housing the standard chevy spec. If not pick up a th400 and have that output and tail housing swapped over and you should be able to bolt right in and go. Less wiring, mounts are already there, driveshaft is the same, etc.
I would use the 60e, it has a much lower 1st gear to help move a fairly heavy car away from stoplights much better. It has a good OD ratio and has more than adequate capacity for a stock ls6 in a car that is just intended to cruise around. Since it is a cruiser anyway, I would get the shifter adjusted so that P R D 3 2 for the transmission are correct, and just leave manual low unavailable.
Just my 2 cents.
Just my 2 cents.