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th350 to 700R4?

Cowracer

Cowracer

Cognito Ergo Moo
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My 89 winnebago has a 454 with plenty of grunt, and I presume a TH350 tranny. Definalty just a 3 speed. It dont have a tach, but the big block gasser is really churning along at highway speeds. I guess upwards of 3500-3700 Rpm at 65.

I really think that a 4-speed OD tranny would help alot on flat highways. I am thinking that a 700R4 would just bolt right in place of the TH350 with a minimum of fuss.

Anyone ever swap a Th350 for a 700R4? whats involved?

Tim
 
I'd be more inclined to go to a TH400. The 700R4 was notoriously weak with any serious torque put to it. The local bone yard should have plenty of rebuildable ones.
 
btfarm said:
I'd be more inclined to go to a TH400. The 700R4 was notoriously weak with any serious torque put to it. The local bone yard should have plenty of rebuildable ones.
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A 400 is also a 3 spd, he wants a 4 speed. A 700r4 is not a bad trans if it's treated properly. Their weak point is the mere existance of OD while under a load. Stiffen it up and use a decent tow convertor and you'll be set. It's an easy swap too.
 
A 454 will have a THM-400 in it, not a THM-350. The 350/700 are not strong enough to handle a 454's torque. They barely held up behind the 6.2 naturals. You would need a 4L80E to go behind a 454 if you want it to last. I'm rebuilding a 4L60E right now with a whopping 9000 miles on it. I built it up for him all stock, and then he had me build a torquey 350 and it proceeded to self distruct in short order. A 700R can be built to handle up to 450 foot pounds of torque and tow 6000 pounds in OD without much issue. A motorhome with a 454 would be overweight all the time rendering the weak overdrive useless all the time, not to mention a mild 454 can push 450 torque. A gear vendor or US GEAR add on would be the easiest option, or just slow down some.
 
I'm sure you have a THM-400 in that motorhome, if it's a 3-spd. The THM-350 and the 700R4 cannot handle the torque from a 454 big block.

I would think that adding a heavy duty aftermarket add-on overdrive unit would probably be easier and less expensive to do in the long run.

If you can post your rear axle gear ratio and tire size, I can get you a fairly close RPM vs Speed chart.

Does your Motorhome have a locking torque converter? --That can make a huge difference in RPM and fuel economy, especially for a heavy vehicle.
 
SORRY FOR BEING OT - But what is the code designation for the new 6-speed auto trans that GM is now using behind the 6.0L / 6.2L gas engines on the 2500HD and 3500?

Does anyone know how well it's holding up in those application?
 
RayMich said:
SORRY FOR BEING OT - But what is the code designation for the new 6-speed auto trans that GM is now using behind the 6.0L / 6.2L gas engines on the 2500HD and 3500?

Does anyone know how well it's holding up in those application?
Click to expand...

6L80E and 6L90E are the 2 new 6 speeds out. Don't know about durability though.
 
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