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Old 01-30-2012, 04:32 AM
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T-350 conversion to a T-350C

Hi there Hot Rodders, got a question. I have an 18' Open Road C30-class 1972 motor home with a 350 Chevy engine and it came stock with the TH350 heavy duty?? transmission. My current problems are the high RPM's at cruising speed of 55 miles an hour being approx. 2800 to 3000. I believe it came with 4:11 differential gears that I do not wish to currently drop down for climbing altitudes of 7,500 feet and then having to pull over to let traffic pass as required because then the "poor girl" does her best getting up an rolling again from a near dead stop.
So, given that said I am currently researching to see if I can modify this transmission to a TH350C overdrive type with a locking torque converter which I read was available and part of this transmission application. I have read the postings and felt you could answer this possible modification.
Ted, California...

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Old 01-30-2012, 05:13 AM
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some bad news, you've been miss informed..
the th350C is not an overdrive transmission.. it is a th350 with a lock up converter ONLY...
you didn't say what your budget is.. so I'll go from cheapest to $$

first up is a 2004r this willbolt in place of a th350 and gives you overdrive, a stong rebuilt one is 900-1500.oo
next up is the 7004r these where and still areused in trucks the newer ones 4l60e are computer controlled, but the 700r4 are not..
this one gives you a 308 ish first gear.. and overdrive.. but the big gear splits, can cause issues when pulling a load, but with your 4:11 gears it be fine, this will require driveshaft mods, guess'n your motorhome has a 2 piece shaft, so this might just be a yoke change. the mount is farther back so you'll be moving the crossmember.. these are about the same cost to rebuild as the 200r4.. is a little stronger and better(for you) low first gear
next up would be the 4l60e but the controller is 400+ but is the same size as the 7004r(same tranny) and in newer trucks so finding a good used take out from a total shouldn't be hard (note after 99 they used the ls based engines)
next is the 4l80e think of this as a computer controlled overdrive th400..
same here used take outs (remember after 99 the ls engines are used different block bolt pattern)
lastly, the one thats on par in cost to most above after you add the mods and rebuild and new converter.. is the gearventers over/under drive
this bolts to your th350 and is an overdrive. but it allows you to split each gear, that in your case gives you 6 gears.. they are 2200-2400 but are basicly bulletproof,
hope this helps..
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Old 01-30-2012, 05:16 AM
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Quote:
Originally Posted by tkoopen
Hi there Hot Rodders, got a question. I have an 18' Open Road C30-class 1972 motor home with a 350 Chevy engine and it came stock with the TH350 heavy duty?? transmission. My current problems are the high RPM's at cruising speed of 55 miles an hour being approx. 2800 to 3000. I believe it came with 4:11 differential gears that I do not wish to currently drop down for climbing altitudes of 7,500 feet and then having to pull over to let traffic pass as required because then the "poor girl" does her best getting up an rolling again from a near dead stop.
So, given that said I am currently researching to see if I can modify this transmission to a TH350C overdrive type with a locking torque converter which I read was available and part of this transmission application. I have read the postings and felt you could answer this possible modification.
Ted, California...

Ted,

The T-350C is not an over drive transmission. It is the same 3 speed trans as a regular T-350

The T-350C does have a lockup converter design. The fuel mileage benefits would be minimum

You can not converter your T-350 to a T-350C trans
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Old 01-30-2012, 05:23 AM
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You can convert a TH350C w/Lockup TC into a TH350 Non-Lockup TC Trans but, not vice versa.
The TH350C's Lockup TC would only drop your Engine's cruising speed about 150-200rpm.
If your Motor Home has a HD 350 Trans it's most likely a TH375B. I'm surprised GM didn't use a TH400 Trans in a C30 MH.

A better option would be a 4L80E Trans or GV/Gear Vendor OD/OverDrive unit. These would give you about 25% rpm drop in OD which would be Ok on flat/level road.

A vehicle like that should have come with a SBC400 or BBC454 Engine & TH400 Trans.
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Old 01-30-2012, 05:39 AM
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Quote:
Originally Posted by SSedan64
You can convert a TH350C w/Lockup TC into a TH350 Non-Lockup TC Trans but, not vice versa.
The TH350C's Lockup TC would only drop your Engine's cruising speed about 150-200rpm.
If your Motor Home has a HD 350 Trans it's most likely a TH375B. I'm surprised GM didn't use a TH400 Trans in a C30 MH.

A better option would be a 4L80E Trans or GV/Gear Vendor OD/OverDrive unit. These would give you about 25% rpm drop in OD which would be Ok on flat/level road.

A vehicle like that should have come with a SBC400 or BBC454 Engine & TH400 Trans.
I just pulled a (what I thought was a th350) out of a 70 motorhome.. it was a 350 c.i.d. motorhomewas based offa van nose..
how would one tell a th375 over a th350?
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Old 01-30-2012, 06:15 AM
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Quote:
Originally Posted by stich626
I just pulled a (what I thought was a th350) out of a 70 motorhome.. it was a 350 c.i.d. motorhomewas based offa van nose..
how would one tell a th375 over a th350?
There're 2 TH375 Trans, A & B.
375B is the HD 350, has TH350 Pan shape & Output Shaft, Cable Detent.
375A is a Medium duty TH400, Has TH400 shaped Pan & TH350 27-spline Output Shaft, 9" Tail, Electric Detent.
On the underside of the trans, right where the tail housing bolts up, the letters "TH-375" can usually be found cast into the trans housing. The 375B/HD350 may also have this cast in.
Some Info >> http://www.442.com/oldsfaq/oftrn.htm look in ID & Comparison links.
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Old 01-30-2012, 08:17 PM
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i think the 375 was used in buicks and olds in the 70s i figured you have a 400 if you want a over drive use a 700r4 it mounts the same as a 350 but 3 in longer so u need to shorten the drive shaft but a lower 1st gear and over dive would be great in a heavy vehical
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