1/2 ton factory 2.41 gears!

[LEEVON]

Somebody asked a question about gearing and it got me thinking about my last '87. It was a swb 1/2 ton with 305TBI and what I believed to be factory 2.41 rear end behind a TH375 transmission. What the heck, you ask? Well, it's a TH400 with a TH350 output shaft. Odd duck to be sure! I actually really enjoyed driving it, and with the continually low RPM's around town and on the highway it was really quiet and smooth. It did get about the same mileage in town as on the highway which was a respectable 15mpg and I never had any problems until the transmission lost reverse. Seizing an opportunity I swapped in a 700R4 and 3:73 posi. I realized later I should have just left it alone and rebuilt the TH375 if for nothing else the novelty!

Has anybody had a truck with a similar transmission / gear combo? Was this even possible, or just a funny combo of replacement parts? Keith Seymore, what say you sir?

Here's the 'ole beast:

[Ziegelsteinfaust]

I built a 77 rcsb I had years ago for pretty much nothing else then pure mpg, and those were the gears I found to get me there. Of course this was back when a OD trans was big money in the early 90's. Especially since I didn't make to much money, but luckily gas was less then $1.50 a gallon. I used a T350 with a 2000 rpm stall to make it more livable for me around town.

[Keith Seymore]

I say "nay".

2.73 was the base gearset for '87.

So - somebody must have stuck that axle and trans in there at some point during it's life.

K

http://www.gmheritagecenter.com/docs...olet-Truck.pdf

[LEEVON]

Thanks Keith, was a strange combo anyhow. From the looks of the truck, it was done very early on. I'd do it again!

[INSIDIOUS '86]

My old 305 with 2:73s and the lockup converter had to have been knocking 25+mpg on the hwy. loved it for that and that alone. Pretty sure I was at like 1300rpm at 70mph haha.

Looks like if you want factory 3:73s you have to find a v6 truck Keith?
Posted via Mobile Device

Leevon the only thing I could imagine how your truck moved is the fact the th350 has no lockup. Th375 has some th400 internals to beef it up. 273s and a 700r4 would Prob out perform the 241s in all counts

[LONGHAIR]

I had never heard of 2.41? The highest I have ever seen was 2.56 and that was in a 4x4 with a700r4. It was a real dud. Early 80s...82-83 maybe?

[clinebarger]

The TH375 could have been factory, Seen a lot of 4.3L/TH375 combos. It lists MX1 as a option on 5.0L also.

[Keith Seymore]

Originally Posted by clinebarger

The TH375 could have been factory, Seen a lot of 4.3L/TH375 combos. It lists MX1 as a option on 5.0L also.

I don't recall building any TH375s, or seeing them mixed in with the stock on the floor.

In this context I believe the MX1 refers to the TH400 (merchandised RPO for a non-overdrive automatic; MX0 being an overdrive automatic). MM3 and MM4 are Saginaw three and four speeds, respectively, and MM7 is the New Process overdrive manual MY6.

The Saginaw 3 speed was offered with two first gear ratios, depending on the application (M62 and/or M64), column shifted.

K

[Desert1957]

Originally Posted by Keith Seymore

I don't recall building any TH375s, or seeing them mixed in with the stock on the floor.

In this context I believe the MX1 refers to the TH400 (merchandised RPO for a non-overdrive automatic; MX0 being an overdrive automatic). MM3 and MM4 are Saginaw three and four speeds, respectively, and MM7 is the New Process overdrive manual MY6.

The Saginaw 3 speed was offered with two first gear ratios, depending on the application (M62 and/or M64), column shifted.

K

Interesting, My RPO code was M40. I am sure it was the original trans because I was friends with the owner. Looks identical to a Turbo 400 but has the long tail stock and shaft of a 350. 375 is stamped on the case.
Pan is a typical turbo 400 because I swapped it for a deep pan before towing.

Here is some more info I found.


Turbo Hydra-matic 400

The TH-400 or Turbo 400 was a three-speed automatic transmission first introduced at the beginning of the 1964 model year in Cadillacs and Buicks (under the name Super Turbine 400). In the following year, Oldsmobile and Pontiac and some full-sized Chevrolets received it.

The TH 400s produced in 1964-67 were equipped with a torque converter that incorporated the Switch-Pitch variable-pitch stator, which allowed variable torque application at start off and cruise. These can be identified outside the vehicle by the kick down switch on the tranny which has two prongs instead of one. His concept actually came from the Buick "Twin Turbine" Dynaflow transmission used from 1955 through 1963.

The TH 375, is simply a TH 400 with a long output shaft that mates to the smaller TH 350 drive shaft yoke. This can be identified by "375-THM" cast into the tail housing. Internally the clutch packs originally had fewer friction plates. The 375 rating was a result of the smaller yoke and fewer clutch friction plates.

To confuse the matter, some heavy duty TH 350s were also designated THM375-B. The key to telling the difference is the shape of the pan. Later, there was a 3L80HD, which is sometimes referred to as a "Turbo 475". This 3L80HD has a straight-cut planetary gear set and there is no externally visible way to determine whether the transmission contains the straight-cut planetary gear set. Some 3L80HD units had an extra deep oil pan, but it is not a reliable indicator. The TH 425 front wheel drive transmission of the Toronado and Eldorado shares almost all its internal parts with the TH 400.

Your best bet for identification is the transmission oil pan.

By 1980, usage of the TH 400 in GM passenger cars started to decline. The demand for better gas mileage convinced GM to design a 4-speed auto, with 4th gear being overdrive. By 1984 GM passenger cars no longer used the TH 400. From 1984 through 1986 the C- and K-series full size Chevrolet/GMC pickups and G-series vans. Thereafter it was renamed to the 3L80 and used more in heavy duty applications. The United States Army HMMWV is the only vehicle using the 3L80. The civilian Hummer H1 originally used the 3L80, but since the mid 1990s the 4L80E was used.

Here is the full article link

http://wildaboutcarsonline.com/cgi-b...=9990364942279

Desert

[Keith Seymore]

Originally Posted by Desert1957

Interesting, My RPO code was M40.

Correct. M40 is the "non merchandised" RPO.

MX1 is the "merchandised" RPO.

The merchandised RPO is the one that is selected by the customer, and then the non merchandised RPO is computer selected based on GVW, powertrain, rear axle, etc - to choose the correct trans.

Sometimes it doesn't matter, but in the case of the two 3 speed manuals the computer will select the correct trans based on the powertrain configuration.

Said differently, the purchaser says "I'd like a three on the tree, please", so MM3 is chosen. The computer then selects M62 or M64 based on the other variables.

Originally Posted by Desert1957

Turbo Hydra-matic 400

The TH-400 or Turbo 400 was a three-speed automatic transmission first introduced at the beginning of the 1964 model year in Cadillacs and Buicks (under the name Super Turbine 400). In the following year, Oldsmobile and Pontiac and some full-sized Chevrolets received it.

The TH 400s produced in 1964-67 were equipped with a torque converter that incorporated the Switch-Pitch variable-pitch stator, which allowed variable torque application at start off and cruise. These can be identified outside the vehicle by the kick down switch on the tranny which has two prongs instead of one. His concept actually came from the Buick "Twin Turbine" Dynaflow transmission used from 1955 through 1963.

The TH 375, is simply a TH 400 with a long output shaft that mates to the smaller TH 350 drive shaft yoke. This can be identified by "375-THM" cast into the tail housing. Internally the clutch packs originally had fewer friction plates. The 375 rating was a result of the smaller yoke and fewer clutch friction plates.

That may be why I didn't notice. The TH400 had historically come in a long tail and short tail configurations (I have a long tail TH400 retrofit into my '63 Grand Prix. The TH400 in my Chevelle is short tail).



K

[Keith Seymore]

Originally Posted by INSIDIOUS '86

Looks like if you want factory 3:73s you have to find a v6 truck Keith?

So it would seem.

For the purposes of this discussion I grabbed the powertrain chart specific to R10s, so it could be different as you scroll down to the other model/powertrain combinations.

K

[KQQL IT]

Had 2:73's in my 77.
Also 3:08's in my 73.

3:82 in a 9" now + 10" converter. 60's great 65 mph not so much.
Its next life it'll be geared tall..
Posted via Mobile Device

[LEEVON]

I suppose I could have been mistaken and the rear end was really 2.73. That would make more more sense according to the available options. On the other hand, there were some El Caminos and b-bodies I think that you could get this gear, so it may have been swapped at some point.

I couldn't get anybody to buy the TH375 or even come pick it up for almost nothing so I gave it to the guys at the shop down the road.

[clinebarger]

I had a '79 T/A 403/TH350 with a 8.5" 2:41 Posi

[bigd65]

Originally Posted by clinebarger

I had a '79 T/A 403/TH350 with a 8.5" 2:41 Posi

My 79 T/A was a 403 (6.6) with a FACTORY 4 spd manual tranny and 3.23 posi in the rear. I sure wish I still had that car....
Posted via Mobile Device

[Desert1957]

Originally Posted by LEEVON

I suppose I could have been mistaken and the rear end was really 2.73. That would make more more sense according to the available options. On the other hand, there were some El Caminos and b-bodies I think that you could get this gear, so it may have been swapped at some point.

I couldn't get anybody to buy the TH375 or even come pick it up for almost nothing so I gave it to the guys at the shop down the road.

My truck is the same as yours. 305 TBI , 375 with 2.73's.

Shame to give away your trans, when I rebuilt mine all the internals were turbo 400 except the rear output shaft seal due to smaller dia.



Desert

[68longhorn]

my truck has the 2.73 rear and my caprice has a th350 and 2.41 gear. Sucks for launching, but bonnevile would be a great place to test them. lol