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Lentech AOD Valvebodies: Body Building
Lentech Valvebodies Can Muscle-Up Your AOD
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In our July issue ("Shifting Priorities," p. 60), we looked at Lentech Automatics' robust quartet of high- performance AOD transmissions. In comparison to stock, these over-achieving overdrives offer greatly improved manual- and automatic-shift characteristics, along with varying levels of increased power handling capability (up to 1,200 or more horsepower). A lot of credit for this shifty character building must go to Lentech's carefully reengineered valvebodies, which are also available separately for bolting onto your own AOD, and which we'll look at here. For those who, like the author, are already at or near the limit on all of their credit cards, this can be an affordable option-as long as the recipient AOD's internals are up to the task.
Despite appearing to be an unintelligible aluminum maze, a valvebody is really a hydromechanical computer with fluid programming that dictates nearly all aspects of the AOD's mechanical power flow and shift behavior. As such, changing or modifying the valvebody can have enormous impact on transmission behavior.
One of the most annoying and destructive traits of an AOD's factory behavior is its 1-3-OD manual-shift pattern. This apparently cost-saving arrangement forces those wanting to upshift for themselves into a kamikaze routine of whacking the lever up from First to Third, and then immediately back down to Firstall in search of the elusive Second gear that is AWOL from the shift quadrant. Repeated use of this decidedly imprecise tactic is virtually guaranteed to bring an AOD to a premature demise.
Every one of Lentech's four available valvebodies fixes this stupid arrangement, allowing you to manually upshift accurately and directly, with a proper 1-2-3/OD pattern. In another nice touch, Lentech's valvebodies deal with overdrive in the same fashion as current electronically controlled overdrive auto- matic--with a switch to electrically delete OD anytime you don't want it. Unlike some other products, Lentech's valvebodies allow a normal automatic upshift into Overdrive, unless you lock it out with the switch. In other words, you don't have to manually toggle a switch every time you want the tranny to go into OD.
The other glaring weakness of the production AOD is a peculiar internal architecture and power flow that drives First and Second gears (and Reverse) via the fluid coupling of the torque converter turbine, yet uses a second, small-diameter shaft to drive both Third and OD directly off the converter front face, at crankshaft speed. In other words, both Third and Fourth are in permanent lockup. This power flow makes the AOD's Third gear a weak link, particularly when horsepower levels go up (as they inevitably will, won't they?).
Each of Lentech's quartet of valvebodies deals with this power flow in its own specific fashion to improve transmission longevity. Three of them actually revise this power flow and, with no internal modifications to the gearbox, are claimed to increase the AOD's Third gear power-handling capability substantially over its absolute factory maximum of 450 hp.
We've seen the AOD's weaknesse--now let's see how each of the Lentech valvebodies deals with them. By the way, should you invest in one of the company's valvebodies and decide within 30 days you don't like it, you can return it for a full refund. In all likelihood you'll keep it, in which case it's covered by a generous three-year warranty.
As with all the Lentech valvebodies, the $349 Street Terminator immediately fixes the annoying and destructive factory 1-3-OD manual-shift quadrant snafu with a proper 1-2-3/OD pattern, having electric OD delete. This alone is worth its price. But plugging a Street Terminator into your AOD should also extend its clutch-pack life expectancy. This is principally the result of eliminating the factory's 33-percent operating pressure cutback in Third and Fourth, for improved clamping force, though the Street Terminator does remain a pressure-modulated valvebody as is the factory version (meaning line pressures vary with throttle position, as determined by the throttle-valve cable).
The Street Terminator does not alter factory power flow, so if your AOD still has its dual concentric input shafts, both Third and OD will be in permanent lockup. Substituting a one-piece input shaft, or a special non-lockup torque converter, will raise Third-gear power-handling capacity to around 600 hp, but will eliminate lockup altogether.
Street Terminator Lock-Up
By itself, the $499 Street Terminator Lock-Up will allow your AOD to deal with up to 800 hp, while offering the efficiency of converter lockup in OD only (as it should be). It accomplishes both by Lentech's patented process of rerouting your AOD's Third-gear power flow through its turbine-driven outer input shaft, and Reverse/Forward clutches, instead of via the direct-driven inner shaft and direct clutch. Only Overdrive receives power through the latter route, and therefore retains full lockup operation for maximum cruise efficiency. We told you the valvebody was a brain.
Unlike the basic Street Terminator, the Lock-Up is a full-pressure valvebody, beneficial to clutch-pack engagement and longevity, with relatively firm part-throttle upshifts as the only penalty. Actually, its target audience is the dual-purpose, street/strip car, so line pressures in the Street Terminator Lock-Up are normally kept in the 150-160 psi rangeplenty for positive clutch actuation without loosening dentures on every shift. Lentech will fine-tune the line pressure depending on your car's combination and use.
The Strip Terminator ($599 with transbrake or $499 without) requires use of a one-piece input shaft ($249), but rewards the expense with the ability to deal with up to 1,200 hp. Via the pair of output splines found on the one-piece shaft, this valvebody engages Third gear through both the direct clutch pack and the reverse clutch pack; the latter tricked into functioning similar to the High/ Reverse clutch in the old FMX, which shares similar planetary-gear configuration. A one-piece input shaft eliminates lockup operation in any gear, so this combination is aimed primarily at strip cars or powerful dual-purpose cars where lockup efficiency is not high on the wish list. With this decided dragstrip orientation, most buyers will likely opt for the extra-cost transbrake.
Strip Terminator Lock-Up
With full-time line pressure dialed in at somewhere between 180 and 240 psi (again, depending on your particular combination), the Lock-Up version of Strip Terminator ($649 including transbrake) could possibly endanger your orthodontic work, but that just means the resulting grin will be a toothless one. This valvebody induces the same Third-gear power flow as the Street Terminator Lock-Up version, and is therefore subject to the same 800hp limit, but it can use the stock dual-input shaft arrangement. In fact, the Lock-Up versions of the Street and Strip Terminator valve-bodies are identical except for the latter's higher line pressure and its included transbrake.
Benefiting from Lentech's unique AOD power flow, the company's AOD transbrake is unlike other products because, in manual First, only the Reverse/High clutch need be applied, as the Low/Reverse band is already engaged. Other AOD transbrakes must simulta-neously apply (and subsequently, release) both the clutch and band, a process that Lentech says is less instantaneous. Another of Lentech's transbrake advantages is that it does not impede normal reverse operation-you don't have to hold the transbrake button to back up.
Though this array of valvebodies seems rather prolific, it's really only the beginning, as Lentech will essentially custom build one to suit your particular application. If you can't decide which is for you, Lentech owner Len Bertrand is a drag racer himself, so he can talk the talk, walk the walk, and recommend the best valvebody for your needs. You can reach him on Lentech's tech line.
Because of its internal design, no matter what you do, an AOD's Fourth gear will not take much more than 400 hp. However, even if you own an 800hp, blown, dual- purpose terror, this is really no limitation at all since OD is just a cruising gear anyway. There's something wrong if you get into Fourth on the dragstrip.