Ford AOD Transmissions
LenTech Automatics Earn the AOD Some New Respect
From the July, 2000 issue of 5.0 Mustang & Super Fords
By Dale Amy
Photography by Dale Amy
LenTechs quartet of...
LenTechs quartet of beefy AODs includes the Street Terminator Lockup that well take a close look at here (shown in the foreground with 10-inch converter), and the Pro 5.0L (in back, wearing its SFI-approved bellhousing and an 8-inch converter). Note the race-oriented, three-speed Pro 5.0L versions stubby tailhousing, which hides a short, fat 31-spline output shaftwith no speedo drive provisioninstead of the AODs usual 28-spline output.
Instead of the factorys...
Instead of the factorys 1-D-OD pattern, all LenTech transmissions benefit from a 1-2-D/OD shift pattern for when you want to shift manually. In an upcoming issue, well take a closer look at how each of LenTechs four distinct valvebodies function. Unless requested otherwise by the customer, LenTech builds these transmissions with a full-pressure valvebody with preset operating pressure based on vehicle combination and proposed use, not a pressure-modulated version. This explains the empty holes in the case side of the body, where the 2-3 and 3-4 accumulators used to reside. Incidentally, this integral valvebody structure makes the AODs case far more rigid than the cases of others.
In order to alter power routing...
In order to alter power routing through the tranny and thereby dramatically increase its power handling capability, LenTech teaches the AODs valvebody new tricks, some of which are even patented. The weak part of the factory powerflow is that in Third gear, power mostly goes through the AODs small inner input shaftwhich is permanently locked up to the converter front cover at one endand the 3/4 direct clutch drum at the back. In Third, this direct clutch is engaged by the valvebody to lock the planetary gear carrier to the forward-driven sun gear. In a planetary gearset, if you lock any two of the sun gear, planet carrier, or ring gear together, then you get direct 1:1 drive. In its Street Terminator Lockup AOD, LenTech still uses the pair of concentric input shafts, but reroutes Third-gear power to the stronger, outer shaft, driven off the converter turbine (i.e., not locked up) by using the valvebody to lock the reverse sun gear and shell via the Reverse clutch, to the forward-driven sun gear. Overdrive remains the responsibility of the inner, locked-up shaft.
In case you should have any...
In case you should have any doubt about the basic strength of the AOD, heres a comparison of the FMX-based overdrives internals (on the right), with those of little-brother C4. This is not to knock the C4, which certainly has its place, but Bertrand feels the AOD has a much higher ultimate power-handling capabilitymathematically as much as 2,000 hpwhile offering the undeniable street benefit of a 0.67:1 Overdrive.
Some AODs came with a three-disc...
Some AODs came with a three-disc reverse clutch drum (right), while others had a four-disc drum. Since this clutch pack is also used in the Street Terminator Lockups Third-gear powerflow, the four-disc version is always substituted. This clutch drum is then machined to accept a Torrington bearing in place of a factory thrust washer, and drilled to better evacuate tranny fluid and a 3-4 upshift or 3-2 downshift. It should be noted that four discs in the reverse drum provide 15 percent more clutch area than six in the direct clutch. Finally, to eliminate sprag failure, the intermediate sprag retainer snap ring gets tossed in favor of a Shift Technology Products spiral ring.
The Overdrive band servo (on...
The Overdrive band servo (on right) is available in three different configurationsA, B, and C, with A being the strongest and controlling the clamping force on the OD band. Len Bertrand finds the B servo (as installed here, and formerly fitted in stock 351 truck applications) more than adequate for the Street Terminator Lockup, where Overdrive will typically be not much more than a cruising gear.
The factory fits five clutch...
The factory fits five clutch discs and plates in the AODs forward and 3/4 direct clutch drums. In the Street Terminator Lockup, LenTech removes some material from the back face of the forward- and direct-clutch pistons in order to fit a full half-dozen plates and Raybestos high-performance discs. The forward-clutch piston is drilled with a small hole, both to promote quick drum drainage for immediate disengagement on a 3-4 upshift and to cushion clutch engagement as the tranny is shifted from Park into gear. Even though the direct clutch is used only for Overdriveand not Thirdin the Street Terminator Lockup powerflow, it also receives six clutch discs/plates.
Normally the weak link because...
Normally the weak link because its asked in the factory powerflow to handle not just Overdrive but Third as well, the small inner input shaft Bertrand is shown slipping inside the hollow outer shaft does nothing but OD duty in the Street Terminator Lockup. This inner shaft is directly driven off the converter front cover, and so spins at engine rpm in permanent lockup. The outer shaft is turned by the converter turbine and can benefit from converter torque multiplication. LenTechs revised powerflow means the characteristic bogging and big rpm dropcommon to the factory AODs 2-3 shift going from torque-converted Second into lockup Thirdis eliminated. LenTechs patented Reverse clutch Third-gear powerflow bumps the Street Terminator Lockups power handling capability up to 700 hp (or 800 if you opt for LenTechs new stronger, chromoly outer input shaft).
With the AODs internal...
With the AODs internal lubrication architecture, Bertrand has found that one of the most important criteria in delivering lubricant to the planetary gearset is to have endplay of the rotating assembly as little as 0.002 inch, but not exceeding 0.005 inch. Endplay is determined with a thrust washer (available in varying thicknesses) on the back face of the pump stator, as Bertrand is shown installing here. Internal lubrication is also improved by removing a check ball from the pump stator and drilling a 1/16-inch hole in one of the valvebody lands to assure adequate flow through the converter and lubrication circuit at low-engine rpm.
LenTechs converters are made-to-order by the Canadian firm Torque Converter Specialties, and feature needle thrust bearings, forged front covers, and fully furnace-brazed fins. A 10-inch converter is standard on the $2,395 Street Terminator Lockup transmission. The factory AOD had 1,500-stall 12-inch converters. LenTech staff consults with the purchaser on vehicle use and engine combination to determine the most appropriate stall converter properties, whichon the 10-inch unitcan range from 2,400 to 4,000 rpm.
Like an automotive Rodney Dangerfield, the AOD automatic gets no respect, and perhaps not without some justification. Take its decidedly nonperformance-oriented First-Drive-OD shift detent (please!), which makes finding Second gearwhile manual shiftingan indirect and indelicate affair involving a slam from First to Drive, and then back down to First. This type of arrangement does not command an enthusiasts respect, nor does it make for long tranny life. Neither is respect earned by the AODs engine-bogging leap from Second to Third, nor the early versions constant shuffling between Drive and OD.
Then again, at least it has Overdrivewhich for any civilized form of street use puts the AOD miles ahead of slush boxes having only a direct-driven top gear. And if someone could engineer fixes to its known shortcomings, the strong-by-design AOD would lend itself to performance applications in a huge array of FoMoCos, ranging from street or race late-model Mustangs to street rods to 60s musclecars updated for the new century. Enter LenTech Automatics, a firm dedicated to earning the AOD some serious respect. LenTechs Len Bertrand saw opportunity in the culprit responsible for some of the AODs poor reputationits unique dual input shaft arrangement.
Before going any further, a quick look at an AODs stock power input is in order. As delivered in thousands of Fox-bodied Mustangs, older Crown Vics, light-duty pickups and vans, and all manners of pre-electronic, rear-drive Fords, the ubiquitous AOD went without a typical torque converter lockup clutch. Instead, it used a concentric pair of input shafts. The hollow outer input shaft was driven off the torque converter turbine and powered First, Second, and Reverse gears, while the inner shaft took engine rpm directly off the converter front covernot via the turbineto engage Third and OD. Thus, First and Second could benefit from the converters torque multiplication, while Third and OD were effectively driven straight off the crank, always in lockupa prime cause of bogging or stumbling on the 2-3 upshift. To be precise, Third gear does receive some power via the converter and the outer shaft, but the brunt is borne by the little flexplate-driven inner shaft.
This small-diameter inner shaft is rightfully viewed as a weak link, not so much in Overdrivewhich will never see much track use or even the peaks of the horsepower curvebut more so in Third, where a high-horsepower engines full-power upshift from Second can wreak internal havoc.
Ironically, with that three-dimensional visualization that is the hallmark of the mechanically gifted, Bertrand was able to see some potential advantage in this dual input flow. It offered flexibility in the means by which power could be channeled through the AODs compound planetary gearsetso named because it uses two sets of spiders mounted on a common planetary carrier instead of two separate carriers. If youre familiar with the tough, old FMX, then youll feel right at home inside an AODwhich shares similar planetary architecture. Bertrand tells us the AOD in stock form can handle up to about 450 hpwith Third gears actuation by the small shaft being the limiting factor in an otherwise quite strong gearbox. By comparison, LenTechs range of modified AODs have power-handling capabilities from 600 to 1,200 hp. Most of the increased power-handling capability of LenTechs various AODs comes from modifying the route taken by engine power to and through the planetary geartrain. In fact, by specific modifications to the valvebody (the hydraulic brain of the tranny responsible for clutch and band operation) and, in one case, the use of a unique converter, LenTech is able to offer four different AOD models that use three distinct powerflows (see sidebar).
Well take a more detailed look here at one of the LenTech quartetthe Street Terminator Lockup. This stout and streetable AOD sells for $2,395 and offers lockup (properly, just in Overdrive) for maximum efficiency yet shifts with authorityeither manually or automatically. It has a proper 1-2-¾ pattern for manual shifting, and has components and internal power flow modified to deal with up to 700 hpand up to 800 hp with an optional revised outer input shaft.
Though the model line consists of four transmissions, no two AODs leaving the shop are likely to be identical, as LenTech has a converter, line pressure, and other options that can be tailored to suit most any car-engine combination. Bellhousing patterns are available for small-block and 429/460 applications, and will be for FEs, too, by the time you read this. The company backs all its AODsstreet or racewith a rather remarkable full one-year parts and labor warranty, three years on the valvebody, and two on the torque converter. The purchaser is responsible only for shipping in the event of problems. This confidence alone should reap LenTech AODs some much deserved respect.
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