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Old 12-18-2013, 02:56 PM   #1
GRADYS Performance
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85 Suburban TH400 to 4L80E swap

I am preparing to change the TH400 in my 85 2WD C20 Suburban 454 to a 4L80E transmission. Does anyone have experience with what needs to be done for the crossmember?
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Old 12-18-2013, 06:47 PM   #2
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Re: 85 Suburban TH400 to 4L80E swap

How to here: http://67-72chevytrucks.com/vboard/s...d.php?t=574800
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Old 12-19-2013, 11:27 AM   #3
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Re: 85 Suburban TH400 to 4L80E swap

No details in that thread about the cross member modifications.
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Old 12-19-2013, 11:43 AM   #4
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Re: 85 Suburban TH400 to 4L80E swap

I think he said he used the stock cross member with the 4L80E mount.
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Old 12-20-2013, 12:39 AM   #5
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Re: 85 Suburban TH400 to 4L80E swap

when I put an 6.0l/4l80e in my 86 Suburban (originally a 350/th400) I used Dirty Dingo mounts. all I had to do was re-drill the cross member holes in the frame around 1.5" back. but the cross member its self needed no modification. The 4l80e trans bushing bolted right to the cross member.
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Old 01-14-2014, 05:17 PM   #6
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Re: 85 Suburban TH400 to 4L80E swap

I am starting this swap project. I have the transmission and torque converter complete and ready to install. Transmission is fresh overhauled with a shift kit, manual speedometer output housing with gears and deep aluminum pan.

I will try to update with progress and pictures as I work on it and make progress.
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Old 05-10-2014, 03:06 PM   #7
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Re: 85 Suburban TH400 to 4L80E swap

Moved the transmission crossmember back to line up the front crossmember bolt holes with the rear bolt holes in the frame and the mount on the transmission lines up perfectly. Drilled new rear holes in the frame using the crossmember as a template and bolted everything up. Transmission and torque converter are now installed and bolted in.
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Old 07-14-2014, 10:50 AM   #8
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Re: 85 Suburban TH400 to 4L80E swap

Driveshaft shortened 3-3/8 inches and re installed. Cooler lines from TH400 moved back 2-3 inches and re bent to fit 91-96 4L80E case.
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Old 07-26-2014, 09:26 AM   #9
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Re: 85 Suburban TH400 to 4L80E swap

Installing the TCI Transmission control unit and TPS sensor this weekend, That just leaves the shifter linkage and speedometer cable left to finish up.
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Old 09-03-2014, 02:55 PM   #10
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Re: 85 Suburban TH400 to 4L80E swap

Finished up the installation and drove it several times pulling my boat. Really satisfied with the results. Tinkered around with the controller settings to get a good shift feeling. And the Lockup converter settings based on MPH and TPS percentage. Mileage is much better what used to take a full tank is now down to about 3/4 tank for a vacation trip.

Pictures of the transmission with the TCI deep aluminum pan and the Mechanical Speedometer tail housing so I can use my original speedometer. Low stall torque converter for the 454.
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Last edited by GRADYS Performance; 09-03-2014 at 03:24 PM.
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Old 09-03-2014, 03:09 PM   #11
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Re: 85 Suburban TH400 to 4L80E swap

Good to hear it. Any idea how much the entire conversion ran? I know the controllers can get pricey.
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Old 09-03-2014, 03:19 PM   #12
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Re: 85 Suburban TH400 to 4L80E swap

Pictures of the cross member moved back 3.75" with a 4L80E mount.
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Old 09-03-2014, 03:57 PM   #13
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Re: 85 Suburban TH400 to 4L80E swap

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Originally Posted by BRUISER View Post
Good to hear it. Any idea how much the entire conversion ran? I know the controllers can get pricey.
The complete conversion can vary depending on what you do for a transmission and converter, Year of vehicle you are putting it in for Speedometer use, Cross member modifications and Controller with TPS if needed.

The Transmission controller I used was a TCI TCU unit #302820 which cost $614.00, TCI Remote TPS for the Carburetor cost $142.00 and the cable bracket to mount the remote TPS cable to the carb is TCI part #376700 and cost is $23.00.
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Old 09-03-2014, 04:02 PM   #14
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Re: 85 Suburban TH400 to 4L80E swap

Driveshaft shortened 3-3/8" the difference in case length between the TH400 and the 4L80E Transmissions with the bolt on yoke
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Old 09-03-2014, 04:18 PM   #15
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Re: 85 Suburban TH400 to 4L80E swap

Engine RPM at 65 MPH with 235/85/16 tires with the TH400 was about 2700, with the 4L80E and the converter locked the RPM is about 1950. What a difference a .75 Overdrive and a Lock Up Torque Converter make.

Computer cable plug on side of transmission.
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Old 09-03-2014, 11:22 PM   #16
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Re: 85 Suburban TH400 to 4L80E swap

Lookin good! Who sells the speedo tail housing?
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Old 09-04-2014, 08:36 AM   #17
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Re: 85 Suburban TH400 to 4L80E swap

Thanks for an excellent how-to and pics. Any plans for fuel injection?
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Old 09-04-2014, 01:32 PM   #18
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Re: 85 Suburban TH400 to 4L80E swap

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Lookin good! Who sells the speedo tail housing?
I sent a PM to you about the Speedometer conversion kit that we offer.
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Old 09-04-2014, 01:34 PM   #19
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Re: 85 Suburban TH400 to 4L80E swap

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Originally Posted by BRUISER View Post
Thanks for an excellent how-to and pics. Any plans for fuel injection?
Thanks for the compliment.

No plans for fuel injection at this time. It would be a nice update if I ever decide to do it.
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Old 09-08-2014, 01:03 PM   #20
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Re: 85 Suburban TH400 to 4L80E swap

Picture of the shifter linkage. Moved frame bracket back putting front bracket hole in rear frame hole and dropping down about 1/4". Lengthened shifter rod down from column by 2".
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Old 03-09-2015, 04:11 AM   #21
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Re: 85 Suburban TH400 to 4L80E swap

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I sent a PM to you about the Speedometer conversion kit that we offer.
Is it possible you could send me that info also?
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Old 03-09-2015, 03:23 PM   #22
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Re: 85 Suburban TH400 to 4L80E swap

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Is it possible you could send me that info also?
I sent a PM to you about the mechanical speedometer conversion for the 4L80E.
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Old 03-11-2015, 07:47 PM   #23
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Re: 85 Suburban TH400 to 4L80E swap

Nice write up, I have been thinking about the swap with my 454 swapped c10 if you could please send me that info also! Thank you!
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Old 03-11-2015, 08:18 PM   #24
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Re: 85 Suburban TH400 to 4L80E swap

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Nice write up, I have been thinking about the swap with my 454 swapped c10 if you could please send me that info also! Thank you!
I sent a PM to you about the speedometer conversion.
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Old 03-12-2015, 12:28 PM   #25
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Re: 85 Suburban TH400 to 4L80E swap

Any reason not to post the speedo conversion info here ? I am interested too. Do you sell the kit ?
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