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Old 01-17-2020, 10:46 AM   #1
mattshea125
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K20 with TH350/NP205

Looking for some input from anyone with a 3/4 ton 4x4 that is running a TH350 / NP205 transmission.

I have a 1978 K20 with this transmission / transfer case combo and will be putting a stock 454 big block in place of the tired 350 this year.

The transmission currently functions perfectly, but I am wondering if I should swap it to a TH400 / NP208 or just rebuild what I have so I don't have to change my whole driveline around.

The truck is used for weekend driving, picking up mulch and some towing, nothing too crazy. I rarely if ever take it off road or beat on it.
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Old 01-17-2020, 11:50 AM   #2
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Re: K20 with TH350/NP205

T350 transmissions were not really designed for the torque of a big block. With that being said....my tree guy has a square body 3/4 ton 4x that somebody hacked a 454 into. Still has the 350/205 combo. He's been driving it that way for years. He just putts along, but he does occasionally tow a small chipper with it.
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Old 01-18-2020, 12:32 PM   #3
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Re: K20 with TH350/NP205

friend of mine has been running them for years built with ood parts and biggest clutch pack options .

he did one for me and my built 400sbc with 4.10 gears and 38" swampers on 1ton axles i run for years .

heat is the #1 killer with dirt #2 just behind it .
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Old 01-18-2020, 01:17 PM   #4
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Re: K20 with TH350/NP205

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Originally Posted by mattshea125 View Post
Looking for some input from anyone with a 3/4 ton 4x4 that is running a TH350 / NP205 transmission.

I have a 1978 K20 with this transmission / transfer case combo and will be putting a stock 454 big block in place of the tired 350 this year.

The transmission currently functions perfectly, but I am wondering if I should swap it to a TH400 / NP208 or just rebuild what I have so I don't have to change my whole driveline around.

The truck is used for weekend driving, picking up mulch and some towing, nothing too crazy. I rarely if ever take it off road or beat on it.
Well, I used to have one a long time ago.

The TH350 was the weak point of the whole pickup, and I traded it off for a 4 speed.

The NP205 that you have is the strongest transfer case, though not the lowest geared.


Why don't you just rebuild the 350 V8 ???????
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Old 01-20-2020, 12:54 PM   #5
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Re: K20 with TH350/NP205

I think if you are wondering whether a big block 454/turbo-350/205 combo is strong enough you should contact Livrat here on the board. He ran that combo for years in his 68 GMC/Chev K20 and used the truck to haul heavy loads and pull a heavy trailer all the time.
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Old 01-31-2020, 01:55 PM   #6
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Re: K20 with TH350/NP205

If you havent all ready bought the 454, I'd do a serious checklist of all its gonna take to swap in the BB. motor , exhaust, accessories, labor to move the trans/TC back, drive shaft changes etc everything.

then price up re-build on your 350, maybe turn it into a 383 and put it right back in.
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Old 02-01-2020, 06:23 PM   #7
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Re: K20 with TH350/NP205

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If you havent all ready bought the 454, I'd do a serious checklist of all its gonna take to swap in the BB. motor , exhaust, accessories, labor to move the trans/TC back, drive shaft changes etc everything.

then price up re-build on your 350, maybe turn it into a 383 and put it right back in.
That's exactly what I'm going to do with my 72 K10. I originally bought a 69 gmc with a 396 with power steering, A/C ect and was going to put that in ,but then decided to sell it and go with a 383. Here in the Mid west there are lots of machine shops building 383 for around 2700 with new parts.
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Old 02-05-2020, 03:28 AM   #8
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Re: K20 with TH350/NP205

I have a TH350/205 in my LS swapped 72 K10. The transmission shifts good and truck doesn't vibrate at 70 on the freeway. The 6.0 is around 300hp..
What most people don't realize is the TH400 takes about 15HP more to drive then the TH350..
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Old 02-05-2020, 12:40 PM   #9
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Re: K20 with TH350/NP205

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What most people don't realize is the TH400 takes about 15HP more to drive then the TH350..
Is there some sort of established data for that sort of thing?

Reason I ask is that my observation was that a TH350 sucks off about 35% of your power compared to a manual transmission.
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Old 02-06-2020, 07:50 PM   #10
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Re: K20 with TH350/NP205

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Originally Posted by 1976gmc20 View Post
Is there some sort of established data for that sort of thing?

Reason I ask is that my observation was that a TH350 sucks off about 35% of your power compared to a manual transmission.
auto does eat more than man trans .
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Old 02-07-2020, 10:02 AM   #11
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Re: K20 with TH350/NP205

if your not driving it hard, I'd think you will be fine. I had a 79 Malibu that was a factory 4-speed Saginaw. I've heard and read everywhere that they were the weakest 4-speed GM built. I had a healthy 454 that I put in, guessing 400 horse. I can't drive a stick very good, more used to truck 4-speed and I have a CDL, so big truck tranny's. Well, I beat that tranny pretty good, and it would sorta hook, posi with 295/50s, but really romp in it will would spin. Tranny never even sniffled. A TH350 is definitely stronger than a Saginaw, tho it is in a heavier vehicle. But as mentioned, get a good cooler, maybe an inline filter, I think you will be fine.
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Old 02-08-2020, 04:08 PM   #12
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Re: K20 with TH350/NP205

My truck has been running a fairly strong 402, turbo 350/205 for years.....
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Old 02-08-2020, 09:58 PM   #13
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Re: K20 with TH350/NP205

Gmc20..heres some info...https://www.carsdirect.com/car-maint...-transmissions
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Old 02-09-2020, 12:44 PM   #14
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Re: K20 with TH350/NP205

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Thanks, that makes sense. I always thought it was the slippage of the TC. The 700r4 with lock-up doesn't seem as inefficient as the old TH350. But my 5 speed pickup definitely seems to have more power than the Suburban, with basically the same engine.
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