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Old 01-26-2014, 10:14 PM   #1
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Default Need transmission info/help 78 Bonneville w/96 LT1

So ive only got 1st, 2nd and reverse now..

The car is a 78 Bonneville with a 96 LT1 and 4L60E. I believe the car is geared wrong for the trans.. I can address that but i was wondering if I could use something other than a 4l60e trans in this car with the lt1 motor..

Its currently set up to factory specs. It has all the 1996 emission junk, cats, etc.

If i can use a different trans, what trans? What would i need to do... have the computer flashed?

New to this so please be gentle.. Thanks
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Old 01-26-2014, 10:19 PM   #2
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Welcome to the forum Eric!
No doubt it will be easier and less cost/headaches to rebuild the currently 4L60E
I understand your question, what other transmission can I swap in but... Sorry I don't have the answer.
I have messaged a few Gearheads, so I hope others may chime in, to be able to add input re your questions.

Last edited by Soft Ride; 01-27-2014 at 12:38 AM. Reason: input added
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Old 01-27-2014, 07:30 AM   #3
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Well you could always go 6 speed manual with the T56. It would require reprogramming.

4L60E to T56 conversion instructions for 1994-1997 LT-1

As far as an automatic goes, you would be best sticking with a 4l60e since it'* already programmed for such.
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Old 01-27-2014, 08:09 AM   #4
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You could physically bolt up a 4L80-E, its just the 4L60E'* bigger brother, software is different, so you would need to reprogram.

Otherwise, I'm with Brian, T56 is a better option for you. Still, software would need changed. A GM dealership should be able to with a Tech2 because its not emission related.
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Old 01-27-2014, 10:18 AM   #5
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i am guessing the 78 has gears in the neighborhood of 2.70, since most of those cars were intended for the highway and had no overdrive. the engine and trans were meant to have a "sportier" gear because it has a lockup tq and OD. so all you have to do is put a more aggressive gear ratio like 3.73 or 3.42. the stock trans is plenty strong enough for the lt1, just get some updates when rebuilt if its bad. this is the easiest thing to do by far. if you do want a t56 you are talking a whole bunch of mods, programing, and you have to go to an even steeper gear like 4.10, because its .5 od and the 4l60 is .7ish
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Old 01-27-2014, 01:01 PM   #6
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The 4l60 is a great transmission and somewhat cheep to rebuild or replace and I'm sure you just smoked the 3rd gear clutches. As far as the rear diff I agree with the post above, you need to pull the diff cover and count the teeth on the gears and determine what ratio you have. The 78 Bonnie is a full sized frame that actually GM keep using through the early 90'* on the caprice, Buick roadmaster, and the caddilac fleetwood witch all came with with the LT-1 and 4l60e. Personally I would fix the transmission and look for a rear end from one of the cars I just listed.
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Old 01-27-2014, 05:24 PM   #7
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sometimes its stamped on the ring gear like 41:10 would be 4.10 gears


as far as those other cars they are geared pretty tame too as long as you are changing the gears i would buy 3.42 or 3.73, they arent that expensive
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Old 01-27-2014, 06:27 PM   #8
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Thanks for the input!! I should probably just have the 4l60e rebuilt..

As far as the gear ratio, I have a rear disc axle from a 95 caprice cop car. Cant remember what the gear ratio is in it tho.. I'll check on that..
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Old 01-27-2014, 06:45 PM   #9
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GM8 - 2.56
GU2 - 2.73
GU3 - 2.93
GU4 - 3.08
GU5 - 3.23
GU6 - 3.42

G80= Positraction








1991 - L03 Caprice(5.0) 2.56Gears

1991 - L03 Caprice Wagon(5.0) 2.73Gears

1991 - L05 Caprice 9c1(5.7) 3.42Gears

1992 - L03 Caprice(5.0) 2.56Gears or (0ptional 3.0 or (LTZ 3.23)

1992 - L05 Caprice Wagon(5.7) 3.23Gears

1992 - L05 Caprice 9c1(5.7) 3.42Gears

1993 - L03 Caprice(5.0) 2.56Gears or (0ptional 3.0 or (LTZ 3.23)

1993 - L05 Caprice(5.7) 3.23Gears

1993 - L05 Caprice 9c1(5.7) 3.42Gears

1994 - L99 Caprice(4.3) 2.73Gears

1994 - L99 Caprice 9c1(4.3) 3.23Gears

1994 - L99 Caprice Wagon(4.3) 2.93Gears

1994 - Lt1 Caprice Wagon(5.7) 2.56Gears

1994 - Lt1 Caprice Wagon Tow Package(5.7) 2.93Gears

1994 - Lt1 Caprice 9c1(5.7) 3.08Gears

1994 - Lt1 Caprice B4U Tow Package(5.7) 2.93Gears

1994 - Lt1 Impala Ss(5.7) 3.08Gears

1995 - L99 Caprice(4.3) 2.93Gears (revised from the early 2.73)

1995 - L99 Caprice 9c1(4.3) 3.23Gears

1995 - Lt1 Caprice Wagon(5.7) 2.56Gears

1995 - Lt1 Caprice Wagon Tow Package(5.7) 2.93Gears

1995 - Lt1 Caprice (c1(5.7) 3.08Gears

1995 - Lt1 Caprice B4U Tow Package(5.7) 2.93Gears

1995 - Lt1 Impala Ss(5.7) 3.08Gears

1996 - L99 Caprice(4.3) 2.93Gears (Revised from the Early 2.73)

1996 - L99 Caprice 9c1(4.3) 3.23Gears

1996 - Lt1 Caprice Wagon(5.7) 2.56Gears

1996 - Lt1 Caprice Wagon Tow Package(5.7) 2.93Gears

1996 - Lt1 Caprice 9c1(5.7) 3.08Gears

1996 - Lt1 Caprice B4U Tow Package(5.7) 2.93Gears

1996 - Lt1 Impala Ss(5.7) 3.08Gears

So if u want some good cheap gears find any one of these vehicles like 1991-1993 9c1(5.7) for the 3.42s or a 1994-1996 9c1(4.3) for the 3.23 *[/QUOTE]
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Old 01-28-2014, 04:54 PM   #10
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Wow thanks for the info!
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