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TH400 Replacing TH350 Question

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Old 11-16-2012, 10:37 AM   #1
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Default TH400 Replacing TH350 Question

I'm thinking of replacing my TH350C with a TH400 in my 1980 to handle the 550+ HP 400ci small block performance engine. Correct me if I'm wrong, but I believe a new shorter driveshaft with stronger U-joints will be needed, as well as stronger half shafts. All I have now is a dual trans crossmember. What do I need to know to make the change? A 700-R4 was considered, but a stock 400 is probably as strong as a moderately built 700. I also was told that the 700 is a little too wide to fit between the C3 dual exhaust pipes. Any suggestions or other help on any of these points would be appreciated.
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Old 11-16-2012, 11:24 AM   #2
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The pan on a 700R4 is wider. so for clearance i custom built a crossmember for the exhaust to pass under (Not through) the crossmember. BT overdrives sells a pass under Cross-X

A 400 tranny can be built to withstand over 1000 hp.

Unless you are drag racing or only have one leg, I would question why anybody would not want a manual tranny
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Old 11-16-2012, 11:56 AM   #3
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Hi gkull!
I like the stick in my 61,and wished my 72 had a stick. I HATED the TH400 at first, the shifts were all WRONG.
But, with the governor recalibrated, it now shifts at 5700 in Drive. and of course it NEVER shifts late at WOT(i've been late sometimes in my 61 and hit 7000 instead of 6k). And with the adjustable vacuum modulator it kicks down instantly to 2nd.
and the "paddle" switch on the console lets me go back to 1st electronically coming around a tight corner. Plus the GREEN light on TOP of the dash lets me paddle shift at an earlier rpm(with the gas pedal) at WOT. So i really like it, with the mods.

Last edited by Matt Gruber; 11-16-2012 at 12:08 PM.
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Old 11-16-2012, 06:26 PM   #4
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If you are only gonna race it or only drive in short hops I would stick with a TH400,, if you plan on driving it a fair amount including distances I would not use anything less then an O/D, and in my opinion if streetability is important your hp of 550 plus may be to much with a carberated engine to have street manners. ( Idle and vac issues should be quite fun) and the stall converter you will need to keep it happy will make all standing starts feel like a standard with a slipping clutch.

Alsoi with that kind of power you also should look at changing the rear end totally including half shafts and joints . I am pretty sure an aluminum differential will go off like a bomb at some point during hard acceleration

And the 700R4 will fit like a glove in the tunnel , if you go that way get a cross member from bto and google shifter detents for the company who makes your 3spd stock shifter a 4spd shifter
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Old 11-17-2012, 12:50 PM   #5
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if u built the 400sb for say ~450hp the TH400 would fry the tires w/3.08 stock stall 2100, or 2500 for sticky tires.. no need for OD. less breaking the driveline and more driving fun. Then if u get the urge to break parts, add 100 shot of nitrous.
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