![]() |
Register or Log In To remove these advertisements. |
|
|
![]() |
|
Thread Tools | Display Modes |
![]() |
#1 |
Registered User
Join Date: Mar 2013
Location: St. Paul MN
Posts: 9
|
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? |
![]() |
![]() |
![]() |
#2 |
Registered User
Join Date: Dec 2013
Location: Columbus, Oh
Posts: 366
|
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.
|
![]() |
![]() |
![]() |
#3 |
Registered User
Join Date: Feb 2011
Location: Parkville, MD
Posts: 1,024
|
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.
__________________
1970 C20 Custom Camper - 350, TH350 1997 GMC Suburban 1994 Acura Integra GSR 1987 Dodge Daytona Shelby Z |
![]() |
![]() |
![]() |
#4 |
Registered User
Join Date: Dec 2013
Location: Columbus, Oh
Posts: 366
|
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.
|
![]() |
![]() |
![]() |
#5 |
Registered User
Join Date: Apr 2011
Location: Klein Texas
Posts: 3,852
|
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.
__________________
My Classics: '72 K20 Suburban + '65 Dodge Town Wagon '72 Corvette Roadster +'67 Corvette Roadster '73 Z-28 Camaro '63 Ford SWB Uni Pickup '50 Ford Coupe |
![]() |
![]() |
![]() |
#6 | |
Registered User
![]() Join Date: Sep 2003
Location: Denver, CO
Posts: 3,727
|
Re: 1972 K20: TH350 or 4L80?
Quote:
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. |
|
![]() |
![]() |
![]() |
#7 |
Carpe manana
![]() ![]() Join Date: Dec 2010
Location: Eau Claire, WI
Posts: 12,379
|
Re: 1972 K20: TH350 or 4L80?
Save the money and keep the th350.
__________________
1972 K5 CST Highlander Blazer: http://67-72chevytrucks.com/vboard/s...d.php?t=708547 1972 K20 Cheyenne Super: http://67-72chevytrucks.com/vboard/s...d.php?t=528308 ![]() |
![]() |
![]() |
![]() |
#8 |
Post Whore
![]() Join Date: Feb 2016
Location: Portland Oregon
Posts: 11,117
|
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
__________________
Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. ![]() RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377 |
![]() |
![]() |
![]() |
#9 |
Special Order
![]() ![]() Join Date: Nov 2004
Location: Mt Airy, MD
Posts: 85,859
|
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
__________________
"BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ |
![]() |
![]() |
![]() |
#10 | |
All about them K’s
![]() Join Date: Jun 2014
Location: Land of Entrapment
Posts: 6,374
|
Re: 1972 K20: TH350 or 4L80?
Quote:
__________________
Tyler '57 3100 http://67-72chevytrucks.com/vboard/s...d.php?t=813888 '72 K20 Cheyenne http://67-72chevytrucks.com/vboard/s...d.php?t=662879 ‘69 K10 SWB http://67-72chevytrucks.com/vboard/s...d.php?t=805206 '98 Silverado LT K2500HD ECLB Vortec 454/4l80E: 6" lift 315/75/16's ‘87 IROC-Z all original 50K mile survivor TPI 305 IROC Blue ‘10 Camaro 2SS/RS Aqua Blue Metallic #93 -version 2.0 ![]() |
|
![]() |
![]() |
![]() |
#11 |
Senior Member
![]() Join Date: Oct 2009
Location: Godley, TX
Posts: 17,963
|
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.
![]() |
![]() |
![]() |
![]() |
#12 | |
Special Order
![]() ![]() Join Date: Nov 2004
Location: Mt Airy, MD
Posts: 85,859
|
Re: 1972 K20: TH350 or 4L80?
Quote:
![]()
__________________
"BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ |
|
![]() |
![]() |
![]() |
#13 |
Registered User
Join Date: Mar 2013
Location: St. Paul MN
Posts: 9
|
Re: 1972 K20: TH350 or 4L80?
Thanks for the input everyone.
|
![]() |
![]() |
![]() |
#14 | |||
"I ain't nobody, dork."
Join Date: Sep 2007
Location: Whidbey Island, Washington
Posts: 8,940
|
Re: 1972 K20: TH350 or 4L80?
Quote:
![]() Gary
__________________
'cuz chicks dig scars... My 1972 GMC 1500 Super Custom (Creeping Death) "long term" build thread. The Rebuild of Creeping Death after the wreck Quote:
Quote:
|
|||
![]() |
![]() |
![]() |
#15 |
Senior Member
![]() ![]() Join Date: Sep 2008
Location: Harrison, Arkansas
Posts: 9,815
|
Re: 1972 K20: TH350 or 4L80?
I know I couldn't make myself do it.
__________________
other Larry Build thread, Arkansas K10 https://67-72chevytrucks.com/vboard/...=755797&page=5 The ability to speak several languages is an asset, but the ability to keep your mouth shut in any language is priceless. |
![]() |
![]() |
![]() |
#16 |
Special Order
![]() ![]() Join Date: Nov 2004
Location: Mt Airy, MD
Posts: 85,859
|
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
__________________
"BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ |
![]() |
![]() |
![]() |
#17 |
Senior Member
![]() ![]() Join Date: Sep 2008
Location: Harrison, Arkansas
Posts: 9,815
|
Re: 1972 K20: TH350 or 4L80?
Pete, did you put an overdrive transmission in your red and white truck?
__________________
other Larry Build thread, Arkansas K10 https://67-72chevytrucks.com/vboard/...=755797&page=5 The ability to speak several languages is an asset, but the ability to keep your mouth shut in any language is priceless. |
![]() |
![]() |
![]() |
#18 | |
Carpe manana
![]() ![]() Join Date: Dec 2010
Location: Eau Claire, WI
Posts: 12,379
|
Re: 1972 K20: TH350 or 4L80?
Quote:
__________________
1972 K5 CST Highlander Blazer: http://67-72chevytrucks.com/vboard/s...d.php?t=708547 1972 K20 Cheyenne Super: http://67-72chevytrucks.com/vboard/s...d.php?t=528308 ![]() |
|
![]() |
![]() |
![]() |
#19 | |
Senior Member
![]() ![]() Join Date: Sep 2008
Location: Harrison, Arkansas
Posts: 9,815
|
Re: 1972 K20: TH350 or 4L80?
Quote:
__________________
other Larry Build thread, Arkansas K10 https://67-72chevytrucks.com/vboard/...=755797&page=5 The ability to speak several languages is an asset, but the ability to keep your mouth shut in any language is priceless. |
|
![]() |
![]() |
![]() |
#20 |
Carpe manana
![]() ![]() Join Date: Dec 2010
Location: Eau Claire, WI
Posts: 12,379
|
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.
__________________
1972 K5 CST Highlander Blazer: http://67-72chevytrucks.com/vboard/s...d.php?t=708547 1972 K20 Cheyenne Super: http://67-72chevytrucks.com/vboard/s...d.php?t=528308 ![]() |
![]() |
![]() |
![]() |
#21 |
Senior Member
![]() Join Date: Feb 2010
Location: Falls City, Nebraska "100 Miles From Nowhere"
Posts: 2,219
|
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!"
![]()
__________________
Michael of the clan Hill, "Two Seventy Two's" 71 1-ton Dually 350 4-Speed 71 C/50 Grain Truck, 350 Split-Axle 4-Speed 02 3/4 ton Express 14 Indian Chief Vintage 1952 Ford 8N, "Only Ford Allowed On The Property" ![]() "Be American, Buy American" ![]() |
![]() |
![]() |
![]() |
#22 |
Senior Member
![]() Join Date: Dec 1999
Location: Southern California
Posts: 2,805
|
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. Richard
__________________
Richard 1972 K10 Custom Deluxe SWB Fleetside My build https://67-72chevytrucks.com/vboard/...d.php?t=800746 |
![]() |
![]() |
![]() |
#23 |
Senior Member
![]() ![]() Join Date: Sep 2008
Location: Harrison, Arkansas
Posts: 9,815
|
Re: 1972 K20: TH350 or 4L80?
It never occurred to me to check on the adapter. I bought this one for a 400 turbo in another project and switched directions.
__________________
other Larry Build thread, Arkansas K10 https://67-72chevytrucks.com/vboard/...=755797&page=5 The ability to speak several languages is an asset, but the ability to keep your mouth shut in any language is priceless. |
![]() |
![]() |
![]() |
#24 |
Post Whore
![]() Join Date: Feb 2016
Location: Portland Oregon
Posts: 11,117
|
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
__________________
Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. ![]() RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377 |
![]() |
![]() |
![]() |
#25 |
Registered User
Join Date: Jul 2012
Location: Whitehorse yukon
Posts: 1,218
|
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/ Last edited by gmc684x4; 01-20-2021 at 12:58 AM. |
![]() |
![]() |
![]() |
Bookmarks |
|
|