THM350C alternatives or keep it?

[Geoduck]

Recently bought an 85 long bed thinking it had a 700R4 with top two gears out. Turns out its a THM350C a lock-up type with two working gears.
I was unaware there was a Lock-Up THM350C.
What are your thoughts on this transmission? Should I have it rebuilt or go with a standard TH350 if that's possible. If I have it rebuilt plan on a mild +200-+400RPM torque converter.
Am rebuilding the 305 with vortec 059 heads dual plane intake, headers and a mild cam. All in a maybe 250HP fairly nice torque topping out under 4500RPM. Rear gears are 3.2 best I can tell. I don't plan long trips on the interstate just putting around with no loads.
Sorry if this has been asked before. I couldn't find much using the search function.

 

[Strick]

Seems like the input shaft is different on the "C" trans. It has a pressure sensor in the valve body & the large 4-pin plug too. I would definitely use a standard TH350 case for rebuilding.

HS

 

[NickTransmissions]

Recently bought an 85 long bed thinking it had a 700R4 with top two gears out. Turns out its a THM350C a lock-up type with two working gears.
I was unaware there was a Lock-Up THM350C.
What are your thoughts on this transmission? Should I have it rebuilt or go with a standard TH350 if that's possible. If I have it rebuilt plan on a mild +200-+400RPM torque converter.
Am rebuilding the 305 with vortec 059 heads dual plane intake, headers and a mild cam. All in a maybe 250HP fairly nice torque topping out under 4500RPM. Rear gears are 3.2 best I can tell. I don't plan long trips on the interstate just putting around with no loads.
Sorry if this has been asked before. I couldn't find much using the search function.

The TH350C, if built appropriately, is fine for your application. Having TCC enabled will reduce heat somewhat and give a small boost in hwy gas mileage.

Main things to do:

1. Sonnax HD intermediate outer sprag race
2. 4L60E center support (drops right in, no mods needed)
3. Sonnax boost valve - replaces worn out factory boost valve
4. High Energy direct clutch module

If any high perf or heavy duty: dual feed the direc lt clutch

 

[Turbo4whl]

When cruising, the TH350C the converter should lock, dropping the RPM's about 300. Better fuel economy than the TH350. Otherwise the two trans's are the same. A 700R4 is a bolt in, but with your 3.23:1 ratio axle you would probably want to switch the axle ratio lower.

 

[NickTransmissions]

When cruising, the TH350C the converter should lock, dropping the RPM's about 300. Better fuel economy than the TH350. Otherwise the two trans's are the same. A 700R4 is a bolt in, but with your 3.23:1 ratio axle you would probably want to switch the axle ratio lower.

3:23 is fine for a 700 trans, though the motor would wake up a bit with 3.73s, esp when paired with the 700R4's 3.08:1 first and .67:1 overdrive. Put a 2000-2200 stall tc (700R4) and it would make for a nice cruiser that can still get up and go when needed.

All he'd need to do is move the x member back and have the drive line trimmed and re-balanced, swap is an easy deal.

 

[Geoduck]

Nick, I read much of what you had to share in the sticky area.
Thanks for your useful answer, much appreciated. Will follow your suggestions.

"1. Sonnax HD intermediate outer sprag race
2. 4L60E center support (drops right in, no mods needed)
3. Sonnax boost valve - replaces worn out factory boost valve
4. High Energy direct clutch module"

 

[NickTransmissions]

Nick, I read much of what you had to share in the sticky area.
Thanks for your useful answer, much appreciated. Will follow your suggestions.

"1. Sonnax HD intermediate outer sprag race
2. 4L60E center support (drops right in, no mods needed)
3. Sonnax boost valve - replaces worn out factory boost valve
4. High Energy direct clutch module"

You're welcome, Geoduck. Also, dont be afraid to machine your direct clutch piston to add one more clutch and flat steel in an otherwise four-clutch stack up. You can do the same with the forward but it's usually not necessary outside serious HD or high performance.

 

[BadBowtie]

Never heard of 3:20 gears.

3.08, 3.42 are around that....

 

[Turbo4whl]

Never heard of 3:20 gears.

3.08, 3.42 are around that....

Not 3.2, but actuality 3.23:1. Hard to see that .03 when your counting axle or driveshaft turns.

Some GM RPO codes for non standard axle ratios.

GQ1 -- AXLE REAR, STD RATIO
GT4 -- AXLE REAR, 3.73 RATIO
GT5 -- AXLE REAR, 4.10 RATIO
GU4 -- AXLE REAR, 3.08 RATIO
GU5 -- AXLE REAR, 3.23 RATIO
GU6 -- AXLE REAR, 3.42 RATIO
G80 -- AXLE REAR, LIMITED SLIP (POSITRACTION)

GM8 -- 2.56 ratio
GU2 -- 2.73 ratio

These codes only show in the sticker if a different ratio was optioned out from as built.


Here are some axle stamp codes:


HB,LL,RA.TR,TT,TU are 3.07s

HA,HJ,HQ,MF,TQ,MI,MK are 3.73s

HK,HL,HR,ME,MG,TS are 4.11s

HC,HN,MU are 3.73 Positraction

HO,HP,JQ, are 4.11 Positraction


The four numbers are the Julian date.

Example: 0426 is the 42nd day of 1976