The Stovebolt Site Forums The Shop Area Engine Shop Chevy Turbo Hydro 400 Questions

I'll try to slip this in because of the beginning production date of the TH-400.

My wife's 1987 Suburban, 6.2 Liter Diesel, which towed both of my trucks home, has a TH-400 in it. With 300,000 miles on it, and about half a body, (rust worms got the rest) The TH-400 may become available before too long.

Three basic questions:

1. Did the model used with the 6.2 Liter Diesel have different gearing than a gasser?

2. Somewhere I read that GM started producing a TH-400 HD in 1987, for the bigger (above half ton) trucks. How would I be able to tell if this transmission is an HD model.

3. If I replace an SM-465 with a TH-400, how badly will it affect the gas mileage?

You need to be aware there are some rough spots inside the TH400 case casting that probably won't meet your quality control standards. They don't affect operation, however.

It's an extremely strong transmission, but it absorbs a lot of power internally. The power loss when adapted to a 235 would be considerable, resulting in poor performance and marginal gas mileage. I think the major difference in the transmissions used behind the diesel engines was a torque converter with a different stall speed, and possibly clutch drums that would accomodate one more steel and fiber disc for better torque capacity. The 4L80 transmission is basically a TH400 with an overdrive added, plus a lockup converter. The TH400 is good for heavy loads and towing, not so great for use with low-torque engines and/or fuel economy.
Jerry

Sorry for not making myself clear. I seem to always assume that I'm posting to my next door neighbor.

The truck I was thinking of putting in is my '73 1 ton duallie.

I'm not really a perfectionist, but, if I see something that I consider 'nekulturny', and is relatively easy to straighten out, why not do it?

One more difference in the 4L80e is that they are all computer controlled, there is not 4L80.

The TH400 was an option in your '73. I'm not aware of an HD version of the TH400, but there is a TH475 that is a little stouter and has support for an output shaft mounted emergency brake. The gearing in that TH475 is a little different is I remember correctly.

I would wonder if the valving would be different in a transmission setup for a Diesel. The torque band is lower and if the transmission was tailored for that motor it may seem to shift too soon behind a gas powered V-8.

I must admit that while I own a couple of them I don't really know a lot about them.

One more difference in the 4L80e is that they are all computer controlled, there is not 4L80.

The TH400 was an option in your '73. I'm not aware of an HD version of the TH400, but there is a TH475 that is a little stouter and has support for an output shaft mounted emergency brake. The gearing in that TH475 is a little different if I remember correctly.

I would wonder if the valving would be different in a transmission setup for a Diesel. The torque band is lower and if the transmission was tailored for that motor it may seem to shift too soon behind a gas powered V-8.

I must admit that while I own a couple of them I don't really know a lot about them.

Thanks, OldSub. Any idea if a B&M kit would have the right stuff to make it work with a 350 gas engine?

The only way to change the gear ratio would be to build a different set of planetary gears, something that's not very likely. There might be somewhat different spring tension on the shift valves, but the most likely difference would be in the calibration of the modulator and/or governor calibrating springs. Non-computerized automatic transmissions shift by balancing throttle pressure with governor pressure and allowing the shift valves to move. Throttle pressure is controlled by either mechanical linkage to the engine, or manifold vacuum applied to the modulator. Governor pressure is regulated by driveshaft speed, moving a spring-loaded valve geared to the output shaft. Since the diesel engine has no manifold vacuum, the modulator on the transmission is supplied with vacuum from an engine-mounted pump, which also generates vacuum for the brake booster, and the vacuum is metered through a mechanical valve moved by the throttle linkage.

Most modulators are adjustable, either with a setscrew and locknut on the factory-original units, or a screw inside the vacuum nipple on aftermarket replacements. Turning the screw clockwise delays the shift, while CCW adjustment makes shifts sooner, and softer. Most aftermarket adjustable modulators can be changed a maximum of 3 turns in either direction.

Stay away from shift kits! They tear up rear ends! Also, the transmission bolted to the diesel engine will not fit a Chevy engine, since the diesels have the Buick/Olds/Pontiac/Cadillac bellhousing bolt pattern. Adapter plates are available, however, and some transmissions will have dual-pattern bellhousing bolt holes.
Jerry

I don't think she's going to let me have it. She just took it out and got it up to 95 mph. Said it shifts fine.

Oh well, she's that kind of girl. It's hard for me to imagine a 6.2 Liter at 95, but she's done it before.

I ran across this thread in a search, I see It's nearly a month old... here's some info...

the 87 1/2 ton diesels have 700r4 (4l60) and the most common gear ratios I've come across are 3.41's
3/4t models have the good o'l th400 most with 4.10 gears and a few options being 3.55, 3.73, 4.10, and some 4.56

the 87 6.2'l get slightly better mileage with the sm465, I converted the other way in my p/u.

some of the th475 trannies have straight cut planetaries that give a lower 1st, 2nd, and reverse gear.

If the 73 DRW was originally automatic, It had a th400 in it.
also the th 400 bolted to the 6.2 diesel will bolt on the SBC's and BBC's. the share the same Chevrolet bell housing.

more on the above mentioned "BOP diesel" bell housing by Jerry....

none of the th400 have uni-case bell housings, some th350, th200, th2004r trannies do.

in 78 gm intro'd a 5.7 and 4.2 ltr. diesel in the form of a "look-a-like" 350 Oldsmobile, the only thing the the 350 gasaholic shares is brackets, and the water pump, all the internals are completely different and unswappable, although the diesel heads will bolt on a gas engine and vice versa. the blocks are clearly marked as 260 D, 350 D and 350 DX which was the upgraded engine for the 5.7 diesel. they in no way shape or form similar to the gm 6.2 and 6.5 diesels except for one part... the injection pump has the same internals up till 94 when they when electronic.

shift kits are often associated/misinterpreted with performance.. not all shift kits are harmful to drive train components and the tranny itself.
B&M makes performance kits and generally do some serious hammering when they shift.
Trans-go shift kits are from mild to wild and are beneficial the the life of the drive train and tranny depending on which kit ya stuff in and how it's adjusted.

the diesel converters are lower stall and have heavy duty parts
the th400 and some Allison's share converters though the Allison version is a little to low stalled for the 6.2 they tend to not want to back up when parked on a steep hill because of the low stall. the gm 6.2l and 6.5l work the best with a 1500-2200 stall speed as the torque of the factory 6.5td is up to 440 lb-ft at 1800 rpm and 215 horsepower @ 3200 rpm. latest versions are factory 300 hp. the 6.2L torques out at 257 lb-ft @ 2000 rpm and 143 hp @ 3600 rpm. up untill they get a ATS or a Banks sidewinder turbo bolted on em.
despite popular belief and what the diesel guru's preach, the gm 6.2L and 6.5L need a higher stall than than the Cummings and international diesels.

one last thing.... they'll do 95 accidentally.