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Old 07-09-2019, 01:20 PM   #1
Wowcars
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1972 K20: TH350 or 4L80?

Hey all, I am contemplating installing a 6.0/4L80 (2wd) that I currently have into my 1972 K20 pickup. I thought about using OD trans for on the highway, but the adapter, wiring, etc, prices are making me rethink it a bit. The truck currently has a TH350 that works just fine.
Should I save my money now and use more fuel down the road, or go all out and run the 4L80?
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Old 07-09-2019, 01:24 PM   #2
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Re: 1972 K20: TH350 or 4L80?

You will love the 4L80E. It cuts the RPM in overdrive by 25% plus the rpm drop from converter lockup. Mileage will go up accordingly.
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Old 07-09-2019, 02:19 PM   #3
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Re: 1972 K20: TH350 or 4L80?

If you are going through the trouble of a LS swap, I would aim to use the 4L80.

It really depends on what you want to use it for. If you can live with 3 gears, go for it. 4 speed is a lot nicer though. Lower first and overdrive. The biggest factor is going to be cost as you mentioned.
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Old 07-09-2019, 02:55 PM   #4
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Re: 1972 K20: TH350 or 4L80?

First gear in a TH350 is 2.52. First gear in a 4L80E is 2.48. Basically the same.
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Old 07-09-2019, 08:17 PM   #5
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Re: 1972 K20: TH350 or 4L80?

You're going to need another transfer case for the 4L80e, correct? I thought the NP205 behind the TH350 doesn't work. You need one from a later truck using a TH400.
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Old 07-09-2019, 08:46 PM   #6
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Re: 1972 K20: TH350 or 4L80?

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Originally Posted by Bigdav160 View Post
You're going to need another transfer case for the 4L80e, correct? I thought the NP205 behind the TH350 doesn't work. You need one from a later truck using a TH400.
I posted a response and then realized what you are referring to--there are two things I think the OP will need to be mindful of:

1) The transfer case adapter may need to be swapped (or maybe just the internal sleeve?).
2) The NP205's originally behind the TH400 had a 90mm input bearing, whereas the NP205's behind the TH350 had a 80mm input bearing. Since the 4L80 is essentially the TH400 with overdrive, the OP's existing NP205 would need the input bearing bore bored out to 90mm diameter and the bearing and input shaft would need replaced...I think this is the part you were referring to.

Also, Advanced Adapters and Offroad Design are good sources of info on this subject.
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Old 07-09-2019, 09:16 PM   #7
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Re: 1972 K20: TH350 or 4L80?

Save the money and keep the th350.
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Old 07-09-2019, 09:23 PM   #8
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Re: 1972 K20: TH350 or 4L80?

Here is a thread with a payback calculator that will allow you to see how many miles it will take to pay for your conversion through gas savings. It of course won't calculate any change in the vehicle's resale value. Which may or may not enter into your decision.

https://www.67-72chevytrucks.com/vbo....php?p=8376137
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Old 07-09-2019, 09:37 PM   #9
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Re: 1972 K20: TH350 or 4L80?

I've been driving the K/20s since '82. I didn't buy them for hiway driving, but I have always hiway driven them with no problem. I had a 4L80E/NP241 with stand alone computer in a '91 Suburban I parted and junked after I was done using it. I decided I'm good with the truck the way it was built
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Old 07-09-2019, 10:42 PM   #10
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Re: 1972 K20: TH350 or 4L80?

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I've been driving the K/20s since '82. I didn't buy them for hiway driving, but I have always hiway driven them with no problem. I had a 4L80E/NP241 with stand alone computer in a '91 Suburban I parted and junked after I was done using it. I decided I'm good with the truck the way it was built
Dang Tim that 80/241 combo is pretty uncommon, almost as much as the factory 80/205. That would have been a hard one to let go of.
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Old 07-09-2019, 10:51 PM   #11
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Re: 1972 K20: TH350 or 4L80?

Another vote for keeping it stock. Will take a long time to recoup what you’d invest. And, while nice to drive, the 4L80E not help resale value except for a limited audience. If it ain’t broke don’t fix it.
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Old 07-10-2019, 06:39 AM   #12
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Re: 1972 K20: TH350 or 4L80?

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Dang Tim that 80/241 combo is pretty uncommon, almost as much as the factory 80/205. That would have been a hard one to let go of.
It was hard. I had that combo in mind for my '72 GMC when I bought that Suburban, once I was through with it. I have a friend who helps me a lot without question. Things were tough for me getting through the great recession. The NV4500 in my old '92 crapped out, 3rd gear ring came apart. I had another from my other truck I wrecked with a broken case. He picked the truck up from where it was stranded. swapped guts, bought needed parts, and never charged a dime. He deserved that stuff more than me. I prefer 2-lanes and back roads anyway . PLUS! The truck is a genuine nicely optioned all intact from the factory highly sought after gem of a classic in original form. I also have 1t axles. If I build anything using all my tough stuff it will be a ground up creation built from scratch. I wouldn't put an LS in a SS454 Chevelle either. That's just stupid from where I stand.
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Old 09-26-2019, 09:06 AM   #13
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Re: 1972 K20: TH350 or 4L80?

Thanks for the input everyone.
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Old 09-26-2019, 11:09 AM   #14
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Re: 1972 K20: TH350 or 4L80?

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I've been driving the K/20s since '82. I didn't buy them for hiway driving, but I have always hiway driven them with no problem. I had a 4L80E/NP241 with stand alone computer in a '91 Suburban I parted and junked after I was done using it. I decided I'm good with the truck the way it was built
I also had a '91 K2500 suburban with the same combo and I really miss that rig.

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Old 11-23-2019, 08:57 PM   #15
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Re: 1972 K20: TH350 or 4L80?

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I wouldn't put an LS in a SS454 Chevelle either. That's just stupid from where I stand.
I know I couldn't make myself do it.
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Old 11-23-2019, 09:17 PM   #16
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Re: 1972 K20: TH350 or 4L80?

Davischevy, you're not stupid afterall
-------------------------------------------------------------------------------------------
I have the '72 K2500 Pickup with original 350/TH350/NP205/4.10s & 285/75-16s.

I have the '95 K2500 Suburban w/original 350/4L80E/NP241/4.10s & 285/75-16s.

both get the same MPG
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Old 11-24-2019, 10:19 PM   #17
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Re: 1972 K20: TH350 or 4L80?

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Originally Posted by DeadheadNM View Post
Save the money and keep the th350.
Pete, did you put an overdrive transmission in your red and white truck?
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Old 11-25-2019, 10:07 PM   #18
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Re: 1972 K20: TH350 or 4L80?

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Pete, did you put an overdrive transmission in your red and white truck?
Hi Larry - yes a 700r4. Clearance is tight at the front yoke but no banging in 4wd (similar to Gary’s here: http://67-72chevytrucks.com/vboard/s...5&postcount=11 ). It rides very nice at 70 mph with the 4.10s. I didn’t bother with a transmission swap for the blazer I restored later. It had its original th350 and that was a strong incentive to keep it. That said, I doubt I’d have swapped in a 700r4.
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Old 11-25-2019, 11:36 PM   #19
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Re: 1972 K20: TH350 or 4L80?

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Hi Larry - yes a 700r4. Clearance is tight at the front yoke but no banging in 4wd (similar to Garys here: http://67-72chevytrucks.com/vboard/s...5&postcount=11 ). It rides very nice at 70 mph with the 4.10s. I didnt bother with a transmission swap for the blazer I restored later. It had its original th350 and that was a strong incentive to keep it. That said, I doubt Id have swapped in a 700r4.
We have some 700R's in 2 wheel drives and they work great for carbureted old school rigs. The K10 I am building is all original, and I intend to keep the four speed. I have a new Gear Vendors OD that I am contemplating putting in it. That way if for some reason somebody wanted to put it back, no harm.
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Old 11-26-2019, 12:45 AM   #20
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Re: 1972 K20: TH350 or 4L80?

I briefly looked into that route. If I recall at that time there was an issue like no available adapter for the overdrive to the t-case or some such. That would be a fun way to go.
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Old 11-26-2019, 03:18 PM   #21
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Re: 1972 K20: TH350 or 4L80?

I'm digging that 58 Chevy! I have two 72 C/10's I'd trade for that baby! "Mikey Likes It!"
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Old 11-26-2019, 04:07 PM   #22
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Re: 1972 K20: TH350 or 4L80?

A TH400 adapter bolts up to the 4L80. The index ring on the adapter just needs to be machined down to fit into the transmission. The 205 needs the short female 32 spline input gear. Your male input NP205 will need the input bearing bore machined to accept the larger female shaft input bearing/gear. Output shaft on transmission needs to be change to a long full splined shaft then cut to correct length. Was typically used in RV's. You can source all the parts or get this from Advance Adapters.
https://www.advanceadapters.com/prod...h350-or-sm465/
Includes everything except the transmission output shaft. Adapter is already machined, your NP205 will need the machine work. I have one of the 4L80 output shafts that I am not going to use as I went the 6L90 route. May fit your transmission.
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Old 11-26-2019, 11:43 PM   #23
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Re: 1972 K20: TH350 or 4L80?

Quote:
Originally Posted by DeadheadNM View Post
I briefly looked into that route. If I recall at that time there was an issue like no available adapter for the overdrive to the t-case or some such. That would be a fun way to go.
It never occurred to me to check on the adapter. I bought this one for a 400 turbo in another project and switched directions.
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Old 01-19-2021, 10:34 PM   #24
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Re: 1972 K20: TH350 or 4L80?

This thread has some payback calculators you can download so you can see how long it takes to make an overdrive conversion pay for itself.

https://www.67-72chevytrucks.com/vbo....php?p=8376137
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Old 01-20-2021, 12:47 AM   #25
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Re: 1972 K20: TH350 or 4L80?

If your putting in a 6 liter ls use the 4l80e that goes with it i am contemplating building a 68 k30 truck it will be a 4l80e with Ord adapter and convert one of my np205s or find a t400 np205 to go with it
A t350 is a weaker tranny then a 4l80e
The 4l80 also has the benefits of a 25 % overdrive gear

Are you going to tow with it work it hard drive it hard ?
Use it like a real work truck or will it be more a patina mall crawler truck ?

Using a rpm calculator 4.11 gear 33 inch tires shows 3 speed auto at 70 mph 3095 rpm compared to a 4 speed auto at 2051 rpm

https://www.differentials.com/techni...io-calculator/

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