Overdrive Transmission - Mopar Overdrive Conversion Kit

Overdrive Transmission Passion Performance Kit

Let's get down to business. Schultz always smiles when he ponders the wonders of overdrive.

Let's get down to business. Schultz always smiles when he ponders the wonders of overdrive

These are great times for Mopar musclecar owners who want to hit the highway with steep gears spinning inside the rear axle. Finally, after years of jealously watching the GM and Ford guys motor on by with the added convenience, economy, and performance of overdrive transmissions, Mopar owners can now choose from several excellent new ways to cut those engine revs while cruising on the open road. For some, the minor floor cutting required for the addition of a modern five- or six-speed manual transmission kit isn't a big deal. For those who don't want to cut, there's the possibility of those cool add-on overdrive units that bolt to the end of the transmission and effectively double the number of speeds in the transmission. But for those who want an absolutely invisible overdrive upgrade for their stick-shifted Mopars, Passon Performance offers a great conversion kit for existing Chrysler A833 manual gearboxes.

This cluster gear comparison shows how the Passon gear (bottom) is machined from 9310 steel versus the 8620 steel used in stock Chrysler gearsets. The replacement alloy is harder but has a specific heat-treatment process that adds forgiveness to prevent fractures. Jamie Passon tells us he has sold more than 60 OD conversion kits, including one to former pro wrestler Bill Goldberg, whose Hemi Dart can't break it.

This cluster gear comparison shows how the Passon gear (bottom) is machined from 9310 stee

But wait, didn't Chrysler beat Passon to the punch back in the mid-'70s with all those overdrive four-speed-equipped Darts, Dusters, Volares, Aspens, pickups, and vans? Yes and no. While it's true that Chrysler re-engineered the A833 four-speed around 1975 to yield a 0.76 overdriven top gear, those gearboxes suffer from general weakness and were never intended for high-performance use. Drawbacks include the fact that most were built with somewhat flexible aluminum cases that expand-and explode-with high loads. And while the 0.76 top gear is great at dropping engine revs by 24 percent versus a 1:1 box, the ODs (like most earlier Slant Six four-speed A833s) are stuck with super-low 3.09 First gear ratios. The problem is that when these gearboxes are used with rear-axle ratios above 3.55, the total gear multiplication zings the motor to redline much too soon and forces a premature shift into Second that wastes energy, kills the 60-foot time, and dulls the overall e.t.

The Passon overdrive kit corrects all these issues by including an 18-tooth Hemi-spec input-shaft upgrade and redesigned gearsets that yield a 2.65 First gear (same as factory stock) and a 0.80 overdrive top gear. What results is a best-of-both-worlds transmission unit that combines-for the first time ever-the economy of the Chrysler OD four-speed and the unmatched strength of the Chrysler Hemi four-speed in a totally stock-appearing package. So, if you're running a 4.10 gearset in the rear axle, pop it into Overdrive and engine speed will perform like 3.23 highway gears! Likewise, 4.56s will behave like a 3.56:1 ratio. Best of all, it's all made here in the USA, so the craftsmanship is superior.

The Hemi part of the conversion consists of this beefy, 18-spline input shaft, also known as the drive pinion. Only factory-assembled Hemi and 440 Mopars got the bigger 1.14-inch, 18-spline shaft, thus the term "Hemi box." All the rest (383s, 340s, and so on) got smaller, 0.960-inch, 23-spline inputs (top). The new pinion requires a matching 18-spline clutch disc.
The Hemi part of the conversion consists of this beefy, 18-spline input shaft, also known
Thanks to the Hemi drive pinion's added thickness, a new front bearing retainer is required. If you don't have an original Chrysler part kicking around, Passon offers the billet-steel replacement on the left for $99.00. There were three different bearing-retainer flange diameters used in production, so Passon tells customers to call with their measurements to ensure the new retainers will suit their bellhousing/blow-shield configurations. Naturally, you'll also need to change the throwout bearing to match the new, large bearing retainer.
Thanks to the Hemi drive pinion's added thickness, a new front bearing retainer is require
To affect the ratio change, the three stock main shaft gears must be replaced. They're also machined from superior steel and feature fresh bushings (foreground).
To affect the ratio change, the three stock main shaft gears must be replaced. They're als

We headed over to Icon Engineering, where proprietor Mike Schultz showed us the basics of what it takes to cram the Passon goodies into an existing A833 Chrysler four-speed. If you've never rebuilt a manual transmission, you might want to get some help from somebody who has. It can be a little tricky for the uninitiated, and a large hydraulic press is needed. In our situation, the Passon components cost $2,339.50, $1,695.00 of which paid for the basic OD Hemi conversion kit, and the rest of which paid for service parts and the modified shift handle. Icon Engineering charges about $700.00 for labor to convert a transmission (not including removal or installation in the car) for a total cost of just over three grand. That's very competitive with the other Mopar overdrive solutions on the market, but again, this is the only one that hides in plain sight and requires absolutely no vehicle modifications. Scope it out.

CRUISING RPM
FOURTH GEAR	TIRE SIZE	REAREND RATIO	RPM	SPEED
1.00:1	28 inches	4.10:1	3,248	65 mph
80:1	28 inches	4.10:1	2,580	65 mph

A833 GEAR RATIOS
	STOCK	OVERDRIVE	STOCK REAR RATIO 1:1 FOURTH	ADJUSTED REAR RATIO WITH OVERDRIVE
First	2.65:1	2.66:1	3.23:1	2.58:1
Second	1.93:1	1.59:1	3.54:1	2.83:1
Third	1.39:1	1.00:1	3.55:1	2.84:1
Fourth	1.00:1	0.80:1	3.91:1	3.13:1
			4.10:1	3.28:1
			4.56:1	3.65:1

Schultz suggests replacing wear items in the transmission as a matter of preventive maintenance. Here, a new rear main-shaft bearing is pressed on.
Schultz suggests replacing wear items in the transmission as a matter of preventive mainte
Likewise, a fresh input-shaft bearing is pressed into position.
Schultz poses with the boxes of Passon Performance overdrive conversion goodies. The surgery takes about one full day, and everything Passon supplies is brand-new; nothing is remanufactured.
Schultz poses with the boxes of Passon Performance overdrive conversion goodies. The surg

The pen points to the key wear areas on the shift forks. The light marks shown here are acceptable for reuse, but Passon sells brand-new replacements if yours are shot. If you see iron forks in your transmission, they're factory original. Bronze forks are a typical replacement part and offer greater lubricity for extended service life.
The pen points to the key wear areas on the shift forks. The light marks shown here are ac
The custom shifter (left) is a $289.00 Passon option that accepts a stock, Inland-style, reverse-lockout shift handle. This tranny is going into a sleeper-themed '66 Dart, and the customer wants stock appearances inside and out. The pen points to the stock Hurst bolt-on shift-handle configuration; notice the difference.
The custom shifter (left) is a $289.00 Passon option that accepts a stock, Inland-style, r
Here's a look at an original, aluminum-case Chrysler OD four-speed. Note that the bolt-on shift arm for the 3-4 shift rod points down (pen) versus the upward orientation of the two other shift arms. Chrysler's original overdrive tactic employed this inverted shift arm, a redesigned 3-4 shift rod, and a new 0.76 Third gearset (from 1.39) to trick the transmission into OD status. The original Fourth gear remained at 1:1 but now was positioned in the shift quadrant where Third gear would normally be. Unfortunately, Chrysler's budget strategy included an undercut main shaft to suit the new Third gearset that limits reliability to the 300hp level. Passon's custom gearsets retain fullsize main-shaft diameters and sidestep this issue.
Here's a look at an original, aluminum-case Chrysler OD four-speed. Note that the bolt-on

With all the custom Passon overdrive gears in their proper places, the parts are ready to be assembled into the spotlessly clean transmission case. For long life, it's always best to give the old transmission parts a thorough scrubbing to prevent contamination of the new parts.
With all the custom Passon overdrive gears in their proper places, the parts are ready to
The front bearing retainer must be indexed so the drain hole in the case and gasket is aligned with the matching relief in the retainer. Get this wrong and your transmission may leak gear lube and contaminate the clutch.
The front bearing retainer must be indexed so the drain hole in the case and gasket is ali
Countershaft wear is a classic issue with any four-speed. The pen points to visible wear on the old countershaft from years of bearing contact. It's easily solved with a brand-new replacement from Passon for a whopping 29 bucks (background).
Countershaft wear is a classic issue with any four-speed. The pen points to visible wear o

The main shaft installs from the rear of the case. There are several important bushings, thrust washers, and snap rings we can't show you due to space limitations. If you're unsure, have an expert shop such as Icon Engineering handle the swap for you. Do-it-yourself types will benefit greatly from the illustrations and hints contained in factory service manuals. Get 'em on eBay.
The main shaft installs from the rear of the case. There are several important bushings, t
One of the most painstaking aspects of the rebuild is the installation of the roller bearings on the countershaft. Schultz uses thick wheel-bearing grease to stick them in place as shown. To ease the process of loading them into the case, he uses a cut-down countershaft that fits into the side-cover opening. Then he drives the actual countershaft (too long to go in from the side) into the case from the back to the front. As it passes through the case and gearsets, it pushes the temporary-use short countershaft out the front. All you four-speed rebuilders should dig this trick.
One of the most painstaking aspects of the rebuild is the installation of the roller beari
Passon's gearsets are machined with greater accuracy than factory stuff for precise mesh and maximum load-bearing capacity. The gearset shown here is the heart of the OD feature. All engine power flows through this gear junction when you're running full throttle in Overdrive.
Passon's gearsets are machined with greater accuracy than factory stuff for precise mesh a

Reassembly starts by driving the new input shaft into the case. The slight interference fit requires a drift pin and light hammer blows.
Reassembly starts by driving the new input shaft into the case. The slight interference fi
The main-shaft extension housing bolts to the case using a fresh gasket.
Side-cover installation is a tricky affair for those with only 10 fingers. As always, a fresh gasket (included) prevents leaks.
Side-cover installation is a tricky affair for those with only 10 fingers. As always, a fr

All buttoned up, it looks like a factory stocker but hides an overdrive surprise. As with the original Chrysler OD four-speed strategy, Passon uses the same inverted 3-4 shift-rod tactic except the Passon rod is a beefy 11/42-inch diameter versus 0.370 for stock parts. Though the custom 3-4 shift rod is included with the overdrive conversion kit, tell Passon if yours is an A-, B-, E-, F-, or C-Body so you get the right one.
All buttoned up, it looks like a factory stocker but hides an overdrive surprise. As with
Icon Engineering is a Mopar-specific shop that's ready for any and all restoration or high-performance work. Here's a view of just one corner of the shop.
Icon Engineering is a Mopar-specific shop that's ready for any and all restoration or high