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Low Gears for Auto Trannies

Posted in How To: Transmission Drivetrain on June 3, 2005
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Contributors: Rick Péwé

Low gears are cool. Lower gears are even cooler. That's what makes the difference in the wheeling and towing world: getting the right combination of gearing to go slow and fast--and anything in between. Of course, most production vehicles are a compromise of sorts, with fuel economy driving that compromise. To this end, high axle gears and double overdrive transmissions are the rule these days in production vehicles, but fortunately we know how to make things lower, at least for older rigs. Sure, you can always change axle gears to get those stump-pulling numbers like 5.89 and such, but highway rpm will be in the gas-sucking range--and your engine will probably be way out of its powerband. A manual truck tranny can solve the problem with a low First gear and an Overdrive Fifth, but if you want an automatic tranny in your rig, the options get much slimmer.

The heart of our TH400 build is the low 2.75:1 First gear planetary assembly from John Kilgore Transmissions. The stock ratio is only 2.48, so the deeper gear is a decided advantage for towing and 4x4 use. The stepped sun gear (center) fits into the output carrier assembly with the special planetaries inside (left). It then slides onto the rear internal gear attached to the mainshaft (right).

Most non-overdrive-equipped slush boxes have a relatively high First gear, around 2.50:1. This isn't anywhere close to what we need for heavy-duty towing or ultimate creepability, regardless of the torque converters ability to multiply torque. That's one reason why many wheelers build their rigs with a Chevy TH350 instead of a TH400. While the TH400 is far stouter, it is longer and heavier than a TH350 and has a 2.48 First gear, while the TH350 has a slightly better 2.52 First. Overdrive trannies sometimes have 3.0 or better, but never have the reliability of a TH400. Off road this lower gear ratio can make a big difference, and when towing a heavy load the lower First is a definite plus. That's why we called John Kilgore Transmissions in Sun Valley, California. John Kilgore has been building race transmissions for years, and offers a lower 2.75 First gear ratio planetary for TH400 trannies, and they have proven to be the missing link for specialized applications.

Our base vehicle for this project is an '86 GMC 2500, originally equipped with the TH400 and NP208 transfer case and 4.10 gears. As a wheeler it's a bit big, and for towing it needs a tad more go from the 350ci engine. Sure, we could hop up the engine or lower the axle gears, but since the case was cracked on the tranny, we figured a rebuild with a lower First gear would be just right. This way we don't alter top gear, and 4.10 axle gears will be fine with 35- to 37-inch tires. Not only that, if we drop in the NP205 transfer case from Performance Gear and Axle ("Building a Bulletproof 205", May '05) and the Off Road Design Doubler we plan to build, we should have a tight little tow rig that can still creep around off road.

Performance Gear and Axle in Picayune, Mississippi, finished our NP205 transfer case last month, and they also have a division for building bulletproof automatics, as well as the manual transmissions they are known for. Owner Ed Hotard agreed to beef a TH400 for us along with the lower-ratio Kilgore gears, and also make a few other mods to make sure this tranny would be the last one we'd have to be concerned with. Joe Palmisano of Transmission Depot Inc. showed us how to put together the slush box, and make it part of our super drivetrain combo.

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A Case for the Right Case
The proper TH400 case to use on your 4x4 is not the same as your everyday car. Look for an "HD" or "K" on the case, which indicates the 4x4 truck style, and then look for the all-important tabs and tapped holes on the ears (left). The standard car-style tranny uses a flimsy plastic or sheet steel cover on the bottom, which is simply screwed into place. The 4x4 truck style uses a cast-aluminum flywheel cover, which bolts on and allows for the reinforcing struts, which run up to the engine mounts. Using the car-style tranny case without the cast cover and struts can lead to a cracked case and a lunched tranny after that.

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The truck TH400 also comes factory with a deep pan for more fluid capacity. This also requires the factory adapter to drop the internal filter down into the pan, and also helps cool the fluid. Heat is the major destruction device of any automatic tranny, so adding the biggest capacity front-mounted cooler is paramount for long transmission life.

Handling the Torque Converter
Fins, Lugs, and Pilots

The torque converter is a critical part of any auto, as it acts as a fluid coupler as well as a torque multiplier. These units are generally rated by diameter and stall speed. Generally a smaller-diameter racing converter has a high stall speed, which is the rpm at which the engine stops or stalls with the brakes fully on--around 1,800 or more. Stock converters stall around 1,500, but we want a lower stall for better off-roading and towing. An extreme, heavy-capacity converter stalls around 1,100 rpm, and is heavily reinforced for good torque-handling capability. Notice the double-welded flexplate attachment lugs compared to a stock unit, and the beefy pilot hub, which prevents ballooning of the pilot. The extra beefy fins are also furnace-brazed and reinforced for maximum durability.

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