Th400/NP208 to 700R4/NP208 Conversion V2500 Suburban

[HotRodPC]

So you have 3.73's (GT4) with Gov-Lok locking differential which is the G80.

 

[MadOgre]

That's really good to know. I was not aware of that. I usually have to look at a dimensions chart. So Th400/NP208 = 700r4/NP208 length, and the outputs on the transfer case should be the same too. Awesome.

Actually the th400, sm465, th350 and 700r4/208 all have the same length from block to tail shaft. The adapter is different lengths for all combos to make that happen.

However the pinion lengths are not the same from 10,12 and 14 bolt rear ends. So each rear end will require a different length drive shaft.

 

[MadOgre]

33s and 3.73s with a 700r4 is border line but should be ok with a good external cooler.

 

[v2500burb]

I knew it had a licking diff
As I didn't need 4wd in 18" of snow
This thing is a tank


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[v2500burb]

If a 700r4 is border
What other suggestions do y'all have?
I love the truck and want to be able to take her on road trips
But 60-65mph kills me
Not to mention she is screaming at that speed
Even with stage 4clutch packs and a trans cooler??


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[skysurfer]

This all sounds strange. I have the same transmission with 410 gears and can cruise 60-65 no problem. My tires are a bit taller (35's) but 373's shouldn't be "screaming" down the highway.

It would be interesting to see a tach reading for your true rpm's.

 

[v2500burb]

65mph I run very close to 3k


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[skysurfer]

I just ran the numbers with an online calculator. At 2500 rpm's you're doing 65.8 mph. At the same rpm's I'm doing 63.5 mph.

 

[bucket]

That's really good to know. I was not aware of that. I usually have to look at a dimensions chart. So Th400/NP208 = 700r4/NP208 length, and the outputs on the transfer case should be the same too. Awesome.

I was a happy surprise the day I figured that one out.

Actually the th400, sm465, th350 and 700r4/208 all have the same length from block to tail shaft. The adapter is different lengths for all combos to make that happen.

I just want to clarify that this only applies to the '81 and up with the np208. The overall lengths in the 70's varied depending on the trans and the t-case.

 

[HotRodPC]

You know I haven't yet spilled the beans on this, but I've pretty much figured out, and about 98% certain, if you have a later model square with an SM465/NP208, the A833/NP208 is also a direct bolt in. With the 700r4/NP208, just need to add the clutch pedal. Same driveshafts and all. As you said, the different length adapters is why this is. Now this wouldn't be true for the earlier SM465s.

 

[v2500burb]

I just ran the truck at 65mph
And it's at 2500 or close to it



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[v2500burb]

Maybe this is just the way it is
I'm so new to these trucks and am not use to them yet

With a 700r4 and 3.73
What would rpms be at 65mph??


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[350runner]

You'll be running approx 2 grand after the lockup kicks in

Sent from the dust in front of you!

 

[skysurfer]

At the same 65.8 mph you're currently running at 2500 rpm's you would be turning 1750 rpm's with the 700r4.

 

[MadOgre]

At the same 65.8 mph you're currently running at 2500 rpm's you would be turning 1750 rpm's with the 700r4.

Yes exactly right!

The reason I said border line earlier is that with 3.73s and 33s it still a bit of work to get the tires turning for a 700r4.

700r4s do not like to have to work hard to get going. They burn up easily compared to other trannies.

And as you probably guessed 1750 is kinda low for highway rpms. I would still like to see 2000ish at 65 in od.

Actually 2500 is not bad for 65 with a th400, This is due to running 33s. With stock tires you would be in to the 3000 range.