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78 k5 with th350 want th400 or 700r4???

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78K5Johnny

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I have a stock th350 bolted to a 400ci in my 1978 k5. I want better gas mileage and more usable power on the mountain passes/ highways. What are the benefits from the th400 and the 700r4 transmissions? Which is more reliable? And are they both an easy swap?? Thanks for any feedback.
 
i was going to ask the same thing. im thinking about the 700r4 for gas mileage, but dont know what it would all take to swap it over from a th350/203.
 
The TH700R4 will give you better mileage, especially if you pair it with an NP208.

With the TH400 you will lose horsepower and fuel economy.

Martin
 
The 700 has a lower first than either a th350 or th400 also. I would go with the 700.
 
Do you hammer the hell out of your truck? If so, th400. If not, 700r4. I had a fully built to the balls 700r4 and broke the hell out of it with a warmed up 350 - upgraded everything even a 5 pinion planetary out of a 4l70.

It's all how you treat it.
 
ok so if i go with a 700 is it possible to use the 203 i already have or should i look for a 208 with the trans? im currently running a 203 non converted.

will i need new drive shafts or will the ones i have work?

thanks
mike
 
You'll need new driveshafts. You can make an NP203 work, but an NP208 would be better. The easiest way would be to finds parts Blazer that has a TH700R4/NP208 combo in it for parts.

Martin
 
just did a th350/203 to a th400/208 in my 78, i would recommend just getting tranny and tcase together, bolt them up and relocate the cross member a few inches. wasnt hard at all.
 
chicken joe said:
ok so if i go with a 700 is it possible to use the 203 i already have or should i look for a 208 with the trans? im currently running a 203 non converted.

will i need new drive shafts or will the ones i have work?

thanks
mike
Click to expand...

ORD sells an output shaft for the 700 to bolt on a 203 or they offer a spacer to use the stock output shaft and th350-np203 adapter. The 700 is longer than the th350 so I would plan on driveshafts being different.

I vote for 700 though. They can be built pretty tough, have a lower first gear then the 3 spd autos and overdrive.
 
Thanks for the info. I was leaning towards the th700r4/208. Is there anything else to consider besides the drive shafts?

Sent from my LT28at using Tapatalk 2
 
78K5Johnny said:
Thanks for the info. I was leaning towards the th700r4/208. Is there anything else to consider besides the drive shafts?

Sent from my LT28at using Tapatalk 2
Click to expand...

not necessarily now, but since you will have OD you can usually gear your axles a little lower, which is nice.
 
Everyone is forgetting that you'll need the tv cable setup for it, so getting all the necessary parts from a parts truck will save you time and money. Also the torque convertor lockup electronics. At minimum you'll want the plug for the trans and you can set it up for one wire operation on a switch. That's what I did and recommend.
 
I went with the 400 when I had the choice to make. Haven't heard anything good about 700 durability wise. My dad owned the truck before me (when it had 700) and had to drive many miles in 1st gear once, just on a regular trip. I know people beef up 700's, but I'd hesitate to put a stock one in an off-roader. Having said that, the lack of OD in the 400 is a bummer on the highway. Pick your poison.
 
If purchasing the 700/208 combo you will want to get the shifter and linkage for the transfer case since the 203 parts will not work. The lockup wiring is pretty simple to do and so is the tv cable set up if you get the correct parts to work with your carb set up. While the 400 is superior in terms of strength, the 700 will shine in daily driving and economy and have a super nice low crawl ratio.
 
82355 said:
That's why I said to get a parts truck.

Martin
Click to expand...

That only makes sense if you can drive the truck, in which case it probably won't be economically feasible since a running driving truck generally isn't sold for cheap and if it is, it's probably hammered. There are those unicorns which have exactly what one would need, but everyone wants big money for their clunkers. This is my general conclusion. I was just stating what he would need for the swap, instead of just saying get a parts truck.

Yeah parts trucks are great but some of us live in apartments, residential neighborhoods and places with blight ordinances. Ask me how I know.
 
chicken joe said:
i can only have a parts truck from april 1-dec 1:D
Click to expand...

Hmm.....as long as you can find it and have room for it, a donor is an attractive option. You could get the swap done in less than a week of nights in the garage or wherever, then cut it up in a weekend and scrap whatever is junk. Might even break even or make a profit by selling whatever's good.
 
I have a TH400 in my Blazer, the 350/400/208 are from an 81 K30, with over 200,000 on them. The motor is bit tired, but the trans still seems great. The only bad thing I ever heard about TH400's was from my dad, he told me a long time ago that the TH400 could run out of fluid, and you wouldnt be able to tell until it either locked up or exploded.
Not sure if thats a testament to their duraility, or a bad thing for lack of warning of impending doom.
 
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