New Doorslammer Differentials: Engineered to Handle The Horsepower

Pro Modifieds, along with a bevy of extreme doorslammer class racers, are continuously developing more power to put to the pavement. Those power levels come with a price: increased strain on drivetrain hardware. Each individual component between the crankshaft and the dragstrip now faces the scrutiny of re-engineering.

Racers and chassis builders have made recent strides to improve upon the competition differential. The days are counting down for a well-built 9-inch Ford rearend to continue to have the ability to handle today’s load dynamics.

Insert the now typical 3,200- to 3,500-horsepower in front of a standard 9-inch gear design, and you are effectively welding the differential gears together under the extreme tooth pressures during a 5-second dragstrip pass. Many manufacturers have developed various ring gears ranging from 9.5- to 11-inches in diameter — these larger gears offer additional strength to handle the extreme loads.

To house these beefiest of 10.5- and 11-inch gear sets has required a fresh approach to the entire differential housing. It’s nothing new for nitro and alcohol racers to utilize various “clean slate” engineered rearend housings of modular design. These designs are now also being applied to the doorslammer. In many cases, these door cars are rivaling the power that nitro and alcohol classes produced in the 1980s and early ’90s.

Justin Carmack has developed his own offering in the new realm of extreme horsepower differentials with his unit combining his Carmack Engineering manufactured components with Mark Williams Enterprises hardware.

The term “live axle,” as explained by Steve Chrisman from Chrisman Driveline Components, is a misnomer. “I define what many call a live axle as rather a solid, one-piece axle shaft,” Chrisman says. “The NHRA description of a ‘live axle’ was actually derived by them back in the 1960s. It has stuck to this day. If you look in a dictionary, a live axle is spelled out as ‘any mechanical differential design that powers a vehicle.’ ”

So, NHRA once asked me what I would call a rearend with a solid, one-piece 3-inch axle shaft in it if not a live axle? I answered, simply a rearend with a solid, one-piece 3-inch axle (chuckles.) – Steve Chrisman

The engineering challenge requires a larger and stronger overall housing to contain these beefier components.  The assembly must also be mated to the existing 4-link suspension mounts and suspension geometry created by today’s chassis gurus.

Steven Ham, racer and owner of TSP Racing, has supplied a new differential design that has become quite popular with the Supercharged and torque converter racers in his Australian homeland. His design is now being introduced in the US through Jerry Bickel Race Cars and G-Force Race Cars.

“We’re running screw blown 2700-pound cars with lockup torque converters down here in our Australian Top Doorslammer class,” says Ham. “The blower/torque converter combination is extremely hard on a smaller Ford-style third-member. They were only lasting one- to two-passes. We knew we had to do something because we couldn’t properly tune the car because you spent more time underneath working on the rearends.”

One of the new differential designs grabbing a lot of attention at the 2019 Performance Racing Industry show was the TSP Racing rearend on display at the Jerry Bickel Race Cars booths. It features another modular design rearend to house the larger ring and pinions.

“Our overall differential is a direct bolt-in for many different professional-built cars such as an RJ Race Cars chassis,” explains Justin Carmack, whose Carmack Engineering has been a leader in large doorslammer housings. “We have 4-link brackets ranging from 16- to 18-inches in center-to-center width for popular chassis, so this housing bolts right in.”

“Because of its physical size and structural design, the Mark Williams 11-inch ring and pinion is heavier than a traditional Pro Mod Ford unit we are replacing,” Carmack says. “Our goal was a strong but lighter overall differential that factored out the heavier weight of the drive components.”

The Chrisman housing design has been proven in nitromethane and supercharged alcohol competition for years. Its new housing variation utilizes those same components for the growing load parameters that doorslammers are now facing.

When Ham designed the TSP Racing rearend, he wanted racers to be able to use a variety of components they already had. “We began with a design using the 10.5-inch Chrisman gear sets and then adapted modular sections so you could continue to use Strange Engineering or Mark Williams components. A racer can even adapt their existing brake/floater components to save the money.”

All these craftsmen note that since the pinion gear location is different in relation to the axle centerline, the mounting brackets factor that difference as not to change driveshaft-linkage-axle geometry when it comes to the bolt-in design efforts.

Carmack said those weight savings are the reason his differential design utilizes two separate floater-style axles compared to a one-piece, through-axle design. “We think this design will still handle up to 5,200-horsepower without issue.”

The advent of computer-aided design and machining processes are the underlying technologies that make rapid development of many new components in all of motorsports.

Ham also chimes-in on the weight of his differential. “We removed the chrome-moly rear from my own Camaro. It weighed in at 212-pounds, loaded ready to go. Our first unit weighed in at 212-pounds. Our second-generation rear comes in at 206-pounds.”

The expense of adapting this “bigger is better” theory can be put into context by examining a season of overall expenditures to maintain a “first-gen” Pro Mod differential. A typical racer can funnel through up to $20,000 in gear sets, bearings, spools, and more if they are campaigning hard over a season-long competition series.

What is a modular rear end housing? As demonstrated by the TSP Racing components, the term modular is described as “an object composed of standardized units or sections for easy construction or flexible arrangement.” A modular differential contains multiple sections that can be exchanged or replaced to alter overall width, mounting points, or internal components.

Looking at those maintenance expenses can reasonably justify the up-front costs of upgrading to these new differentials. “Obviously, large-scale rearend failures can add up to even greater costs overall,” Carmack adds. “Especially when a failure like that can take out everything from driveshafts, transmissions, and even damage the lower-end of your engine.”

“The Top Alcohol Funny Cars probably put out 3,000- to 3500-horsepower on a weekly basis with our 11-inch ring gear and rearend,” Chrisman expresses. “We use that same ring and pinion in our Pro Modified housing. We currently have 4:29- and 4:56-ratio gears of our own design.”

Justin Carmack describes the material used to machine his new housing design as a "proprietary aerospace aluminum alloy." With no more specification than that, he describes the aluminum as offering the overall strength of chrome-moly steel combined with the weight of aluminum.

The investment by these companies to create a totally new ring and pinion unit is no small expenditure. Chrisman demonstrates, “We may make some 5:14 gears for the 1/8-mile guys down the road, but to engineer, tool, and manufacture an entirely new ring and pinion set can cost as much as $100,000 for every gear ratio you create.”

These comparatively massive ring and pinion gears that these rearends now house have another strength advantage surpassing just the increased girth: the larger size offers better contact angles along with more significant tooth contact between the two gears.

When we saw some social media posts by Larry Jeffers Race Cars concerning the announcement of new differential technology, it was an eye-catcher. The posts noted the "game changer" with the use of the Chrisman rear end in some of its customers' cars.

Chrisman described his theory of the advantages of using the one-piece “live” axle and housing design.

“The number of splines doesn’t make any difference — it’s the diameter of the shaft,” he says. “A 1.710-inch diameter, 40-spline unit is pretty much the standard of the racing axle industry. Our one-piece axle may also be a 40-spline design, but it is 2.562-inches in diameter, which makes a big difference in overall strength.”

“Our spline teeth are much larger and wider compared to a 1-inch axle,” Chrisman continues. “Plus, the contact tolerances between the spool and axle make a big difference in strength. There is a plumbers’ analogy that doubling the size of a pipe allows 4-times the volume of fluid through it. Though that is a rough comparison, the overall increased diameters of axles, spools, and splines can increase strength on a similarly exponential basis.”

“We are developing more modular components and spools for the TSP rearend,” says Ham. “It will have a one-piece axle based around 40-splines. We have also made a relationship with a foundry here in Australia that is going to produce many different ratios of 10.5-to 11-inch ring and pinions for us. We will have many ratios for 1/8- and 1/4-mile racing.”

The goal at Carmack Engineering was to create a housing that contains the heavier 11-inch ring and pinion but will still have an overall differential weight similar to a standard chrome-moly fabricated rearend.

Both companies agree that the specific materials chosen to construct their overall rear ends assemblies are critical.

“Calculating various load and force ratings, we chose materials with rigidity, yet had good elongation characteristics,” Carmack said. “You can’t use material so hard that it will tend to stress-crack. We went back and forth in discussions with Alcoa Aluminum concerning the best material choices. Unfortunately, these materials aren’t cheap, either.”

One interesting aspect to the Carmack Engineering rearend that is unheard-of in extreme motorsports is the offered warranty. Its rearend assembly carries a one- or two-year warranty, depending on the gear ratio used. This protection for the customer illustrates Carmack’s certainty concerning the reliability of his product.

The Carmack, Chrisman, and TSP Racing rear ends offer all suspension, shock, and wheelie bar mounting points to incorporate the housings into a current doorslammer tube chassis without modifications to the cars themselves.

Steve Chrisman is also pleased with his rearend when it comes to doorslammer racing.

“Our first rears were an effort between Larry Jeffers Race Cars and us. The first unit was installed in Randy Merick’s Pro Modified Corvette. He was throwing gear sets away after just two to four passes. With our 11-inch differential in there, they have 15 passes on it already and haven’t even looked at it yet. Similar to the typical Top Alcohol Funny Car, the Pro Modifieds can see 25-30 runs on them without a problem.”

Another testimonial to the new differential designs, Steve Ham describes how one of Australia’s quickest Pro Modifieds, the Moits Racing Mustang has put over 80 passes on his original gear set and bearings.

The old saying that “the only thing constant is change” definitely applies to all of motorsports, especially with the ever-increasing horsepower numbers coming from supercharged, nitrous, and turbo-boosted doorslammer. With that increase in power, count on the world of hardware engineered for use in everything from nitromethane to extreme door cars to continually evolve. As quickly as companies can design and machine stronger hardware, the war will continue between horsepower and the sticky dragstrip surface.

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About the author

Todd Silvey

Todd has been a hardcore drag racing journalist since 1987. He is constantly on both sides of the guardwall from racing photography and editorship to drag racing cars of every shape and class.
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