Gear Vendor's Overdrive - Mopar How-To! 3+1=6

Convert Your 727 To A Six-Speed Overdrive With Gear Vendors' Tailshaft-Mounted Overdrive Unit.

From the June, 2000 issue of Mopar Muscle

King of the Road? Well past the muscle car days and into the '80s, there was no question about it. The performance edge on the open road was there with almost any sensibly-geared muscle car. For the first two decades after their production years, with the 55mph speed limit and pathetic Detroit offerings, muscle cars reigned supreme. In the '90s, the picture began to turn, speed limits went up, and new cars picked up in performance, but more importantly, in gearing.

Originally incorporated to increase fuel economy, overdrive transmissions in both standard and automatic form have become the norm for any new car or truck. With the overdrives came the additional benefits of reduced engine wear, noise, and effortless high-speed cruising. Crank any new vehicle up to 90 mph, and it's a Sunday drive. With the high-speed gearing provided by overdrive ratios, under the hood, the engine is quietly spinning along at a lazy rpm. On the open road, drivers of newer vehicles unconsciously use the capabilities afforded by modern gearing, and average road speeds have steadily risen well beyond today's higher speed limits.

Driving a muscle car on the open highway out West can be downright humbling today. Hit the fast lane in our 440-powered '71 Charger R/T, crank up to just shy of 4,000 rpm on the tach, and watch the traffic back up behind you. A never ending stream of minivans, late Euro-cruisers, and Hondas duck to the inside to make the move past. You know the chassis is there, you have 500 lb/ft on hand, but it just sucks as that gal in the Geo Metro effortlessly blows by. And that's with those 3.55 gears that once seemed so sensible. Sure, a hot 440 will spin 6,500 rpm+, and will rocket to insane speeds, devouring all comers-but it's fleeting. You really just can't, and don't want to, hold it at a screaming rpm just cruising along.

The Alternatives
If your muscle car activities are confined to the track, showfield, or occasional burger-joint cruise, even 4.56s won't be a problem. Otherwise, the benefits of higher-speed gearing are clear, even if it's just to reduce wear on that fresh mill. There are a number of alternatives to give older Mopars the open road gearing advantages of a modern vehicle, some with more compromises than others. The easiest and cheapest thing to do is ditch the drag gears for a higher rear ratio. A set of 2.76s out back will make for a great freeway flier, but it comes at a steep cost in off-the-line oomph. Those low gears you started with were in there for a reason: more torque multiplication off-the-line and through the gears. A high-stall converter will help the launch in an automatic car, but the ratio spread will turn the machine into a two speed at the drag strip.

Another choice is to drop in a late-model four-speed 518/618 overdrive auto. The late auto overdrive swaps are popular with some makes; however, the fact is it's not a great swap in early Mopars. A big-block version of the Mopar four-speed auto was never produced, requiring the use of an adapter plate and the related mods and expense. Even in a small block application, the bulk and positioning of the fat overdrive tailhousing on the Mopar overdrive auto causes problems. Serious surgery is required to the critical torsion bar crossmember in the area of the trans tunnel. It's just not a good fit down there. Most of these trannies were built with lock-up torque converters, poorly matched to the power curve of a performance V8; and nowhere near as capable of handling serious output as a conventional converter. Finally, the swap means pulling out and tossing that reliable 727 TorqueFlite. The late-overdrive trans swap can be successfully pulled off, but weigh the downside.

The Gear Vendors
What if you could take the 727 that's already there and add some gears? It would be nice. With the Gear Vendors' overdrive conversion it's not only possible, but it's a simple bolt-in, without even having to remove the tranny. The Gear Vendor is a self-contained, two-speed, auxiliary transmission with direct drive and a .78:1 overdrive ratio, taking its operating principles from the LayCock/DÈNormanville overdrive. The mechanical basis of it is a planetary gear set, similar to those in any automatic tranny. The main difference is that the planetaries are activated by a cone clutch, engaged by two hydraulic servos. The servos and the unit's lubrication are provided by a mainshaft-driven, cam-operated pump drawing oil from the unit's own sump. A 12-volt electrical solenoid valves oil to the servos to engage the cone clutch, setting the planetaries and overdrive into motion. The hydraulic actuation system, operating at 650+psi, makes for near instant shifts.

The cone clutch and planetaries provide for extremely efficient transfer of torque and unbelievable power handling capabilities-the manufacturer claims it takes less than 1 horsepower per 400 to operate, and in standard form can handle 1,200 horsepower, or modified up to 2,000-plus horsepower. Owing to the clutch/planetary design, the overdrive can be shifted at that power level-at full throttle. Obviously, we haven't tested these rates, but we do know that the Gear Vendor is the overdrive unit of choice by builders in extreme horsepower applications, from blown street rods, drag cars, and land speed cars, to OE applications such as the Mopar-powered Jensen Interceptors and, we hate to say it, the Callaway Twin Turbo 'Vettes (the GM overdrive auto was too weak).

Since the Gear Vendor overdrive can be found working reliably behind 8-second drag cars, we knew that its power handling capabilities were far beyond what our modified 440 would deliver. Power handling we weren't worried about, and the two-year unlimited mileage warranty told us the manufacturer wasn't worried about reliability. Looking at what the unit had to offer in our application, it seemed like the most logical solution to our gearing problems. Plus,we'd get to keep our excellent B&M-equipped 727. With the Gear Vendors overdrive, our 3.55 ratio would drop down to 2.77 in overdrive top, more than enough to get the revs down at a high speed cruise. Finally, and often overlooked, the Gear Vendor would give us gearing in Third over, which is compatible with our Holeshot B&M converter.

Converter stall speed has to be taken into account when considering overall gearing. Our converter stalled at 2,900 rpm. Running at the anticipated cruise speed, the converter shouldn't be too far below the stall range, or the converter will slip excessively, costing efficiency, heating the tranny, and negating a portion of the gearing potential of the overdrive. That's why manufacturers universally use lock-up converters with their overdrive autos-to work with the extremely tall overdrive ratio of their automatics. The 22 percent overdrive of the Gear Vendor would give us an almost ideal ratio for a high-speed cruise, while not being so drastic that we would need to reduce the stall speed or require a lock-up converter to operate efficiently. The Gear Vendors seemed to be the perfect choice for our R/T, so we went out to get one.