The Turbo 400 power path is from the input shaft (A) through the forward drum (B) and into the high-gear drum (C). The high-gear drum spins backward at 82 percent of engine speed in low gear, stops spinning in Second gear, and must accelerate up to full engine speed in Third gear.
The Power Path
One key to making a Turbo 400 live is understanding the path the power takes through the transmission. Kilgore laid the big components out, and the information is well worth remembering. Let's assume the torque converter and the input shaft are both spinning at an engine speed of 6,000 rpm. The forward drum weighs 12 pounds and is splined to the input so it also spins at 6,000. The high-gear drum is next in line and weighs even more, at 13.5 pounds. In First gear, it spins at 82 percent of engine speed (for our example of 6,000, that means 4,920 rpm) in the opposite direction. That's a lot of weight spinning in opposite directions, which tends to absorb power. When Second gear is selected, the high-gear drum stops. When Third gear is selected, the high-gear drum must immediately accelerate to engine speed. This means the internal clutches are heavily loaded at the gear change because they must accelerate that heavy high-gear drum up to speed as quickly as possible. This is why the First-to-Second gear change generally feels harsher than the Second-to-Third upshift.
According to Kilgore, "The high-gear clutch pack is the key to making a Turbo 400 live." One key is to prevent high-impact loads from First to Second gear that shock and hurt the sprag. The sprag (also known as an overrunning clutch) is what keeps the drum from spinning in Second gear. Because the high-gear drum is so massive, higher line pressure helps the high-gear clutch pack survive the task of accelerating the high-gear drum up to speed as quickly as possible.
It seems logical that a lighter-weight high-gear drum would be a good idea-which is true when building a race-only trans. But for the street, aluminum drums will quickly wear and cause other problems. Kilgore's solution is a complete change in the Turbo 400's power path along with drastically lighter components (see the sidebar on Kilgore's SuperLite 400).
Performance Building the Turbo 400
Kilgore has been massaging the classic Turbo 400 for well over two decades and has developed several important modifications that not only improve longevity, but also reduce parasitic power losses. Here are a few things we learned watching him assemble Greg Smith's TH400 trans.
Kilgore drills a 0.060-inch-diameter hole in the high-gear drum to vent the drum of oil in low gear, when some oil may be trapped in the drum that might tend to lightly apply the clutches. This would create unnecessary drag. This vent hole is small enough that it does not affect clutch apply in Third gear.
The Kilgore SuperLite TH400
While the Turbo 400 is a great trans, it's also the heaviest of the GM three-speed automatics. As we saw in the Power Path sidebar, it suffers from extremely heavy rotating components. After years of dealing with these heavy pieces, Kilgore decided to develop what he calls the Kilgore SuperLite TH400. The key to this venture is combining the forward and direct clutch assemblies into one very light housing. The weight differential is an amazing 30-plus pounds of rotating mass that also reduces the overall weight of the trans by the same amount. Along with this reduction in weight, Kilgore offers several First-gear options ranging from 2.10 to 3.00.
While this trans can be used on the street, its real objective is trimming anywhere from 0.20 to 0.50 second off the e.t. on a serious drag car by reducing the power required to accelerate the lighter rotating components. This does come at a price, with the basic SuperLite 400 costing $4,495, but if you're into low e.t. by reducing rotating weight, here's your ticket.
Since high-line pressure in Third gear is one key to making the Turbo 400 live, Kilgore machines the exhaust portion of the boost valve (arrow) in the back of the pump to ensure that high volumes of fluid at high rpm won't drive the pressure up. This creates a stable line pressure at higher engine speeds. Kilgore says it's common for line pressure to increase in high gear, which is an unnecessary power drain. | 
In Second gear, the clutches in the high-gear drum are not applied, and the drum spins backward at 82 percent of engine speed. Kilgore uses four short lengths of 1/8-inch-diameter silicone O-ring material to lightly shim each of the five metal spacer plates apart from the adjacent clutch plates. To minimize drag in Second gear, the O-ring spacers create a tiny air gap for the clutches when they are not applied. This is a thinking man's approach to trans building. |
The Kilgore SuperLite TH400 uses the combined forward and direct clutch package (left) to trim an astounding 30-plus pounds compared with the stock layout (right). | |