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Billet Insanity—Carmack Engineering 11-inch Billet Pro Mod Housing

Carmack Engineering Billet Pro Mod Housing
Justin Carmack of Carmack Engineering set out to bring a much higher level of rearend durability to Pro Mod race cars with this billet beauty. Based on Mark Williams Enterprises 11-inch gears, the Carmack rear is fully assembled and carries a two-year warranty on ratios from 2.91 to 4.29:1 and a one-year warranty ratios from 4.57 to 5.43:1. That shows just how confident Carmack is in the durability of this part. This unit also utilizes brakes, floater end bells, hubs, spool, bearing support, yoke, and oiling system components from Mark Williams.

By Pat Bateman

Photography courtesy of Carmack Engineering

Nothing brings the NMCA fans to stands like the call for VP Racing Fuels Pro Mod to the staging lanes. With blowers, nitrous, or turbos force-feeding massive engines to produce beyond 3,000 horsepower, the track is about to get a workout. After the long burnouts, the tree lights and the hammers come down. That power travels through a rugged transmission to a rearend charged with harnessing enough thrust to 3-second elapsed times in the eighth mile.

As you might expect, the lifespan of this rearend hardware can be short. In fact, most cars only get in a handful of passes before their rearends needed meaningful maintenance. If they don’t get it, these parts can fail, which will obviously hamper the performance in a race decided by fractions of a second.

“You can only make so much power if you can’t get it to the race track efficiently and cost-effectively,” said Justin Carmack of Carmack Engineering. “If you get a gear set that’s trying to physically weld itself together because the pinion teeth are broken at 300 feet. It will slow down between three to five miles an hour…”

Typically Pro Mod teams will get about five runs on a gear set before it’s time to freshen the rearend with new parts. That can be costly, however, running $1,500 in parts, which might encourage teams to flirt with pushing the hardware for a few more runs, but that is playing with fire.

Mark Williams 11-inch Rear Gears
Thanks to a long-standing working relationship with Mark Williams Enterprises, Carmack Engineering based its billet rearend on the MW 11-inch modular ring and pinion gear sets. Built from 9310 pro-gear material, these gears fasten with 14 ring gear bolts and feature a 40-spline input. “The gear set that we’re using, and why we went to an 11-inch, which technically it’s classified as a 12-inch gear. The pitch diameter of the gear set is actually 12 inches. It could be called a 12-inch gear using the same terminology that is applied to the 9- to 10-inch Ford-style third members. Then they machine it down to 11 inches to get some weight out of it, because that is another big aspect of the Pro Mod world,” Justin Carmack, of Carmack Engineering, explained.

“Can you make 10 or 15 passes on it and have it live? Probably, but when it breaks, it typically tears the driveshaft up, tears the U-joins up, tears the transmission up, tears the torque converter up and over revs the motor and either knocks the rods out of the motor or you have to put rods in the motor,” Carmack said.

Obviously catastrophic failures like that will cost a lot more than rearend maintenance, but what if teams could reliably run a rear lap after lap? That’s what Carmack set out to deliver with his company’s latest creation — a billet 11-inch rearend housing.

“That’s our biggest goal…” he said. “…Our main focus this year is to not only fix, but completely eliminate the rearend failures in the Pro Mod industry.”

That is a bold mission, but Carmack isn’t afraid to take chances. He cut his teeth in the racing industry building engines down in Florida, until inspiration struck. A customer told him that the path to prosperity was running his own show, and he decided the business to chase was running CNC machines. His first project was building a billet intake manifold, which landed his efforts on the cover of this magazine a few years ago.

Eventually his CNC work inspired a move to North Carolina in the heart of the racing world, and the opportunity focus purely on drag racing products. Carmack’s company had already developed a center section to work with Mark Williams’ robust 11-inch gear set, and that evolved into a completely new billet unit.

“’We want to make 5,000 horsepower but put continue it through the same 1900s technology like a standard Ford 9-inch,’” Carmack said of racer’s perspective on rearend technology. “And it’s never, it’s never gotten any love. It was the, the stepchild. It just got pushed to the side and everyone’s like, ’Oh, it’s good enough, it’s good enough.’ And that’s when we decided to do something along the lines that we did where we could do something completely out of the box.”

The resulting product is so far out that Carmack even engineered its own shipping box to ensure it arrives in pristine condition. The 11-inch rearend in question is carved from huge chunk of propriety aluminum alloy that provides a robust home for Mark Williams gear and floater components, including spindles and brakes. However, this robust housing isn’t just hewn from any aluminum.

“I consulted material engineers and we went over some horsepower numbers. They come up with some load, and force ratings. And we come up with a material that was very rigid, but yet had good elongation properties,” Carmack said. “So it wasn’t too stiff that it would want to stress crack. So when we first did this, I got a design done, and I reached out to Alcoa. We went back and forth for a few weeks coming up with the appropriate material. That’s why some of this stuff is expensive because the material that we use…”

Mark Williams and Carmack Engineering 11-inch Billet Housing
Carved from a proprietary aerospace aluminum alloy, the Carmack Enginnering rearend is built to deliver the strength of chrome-moly steel with the weight of aluminum. The beefier 11-inch rear is about 16 pounds heavier than a traditional setup, but its durability is promised to be far greater. One caveat with the warranty on these units is that these rears must be serviced by Carmack Engineering to retain the warranty, so the overly cautious racer might want to carry a spare center section, but the company is confident that racers shouldn’t experience issues.

The idea was to mimic the characteristics of strong, but heavier metals in a lighter aluminum form factor. This required not only a unique alloy, but some creative engineering.

“We sourced the material where we have similar properties for an elongation at 45 degrees in material grain to 4140 chrome moly, but in aluminum. Now the density is the less, so of course it’s not exactly the same because of the density, but as far as the strength of it. It’s very strong. So we designed a billet four-link bracket that is produced from 3-inch aircraft aluminum stock,” Carmack said. “We go in there and pocket-mill them to make sure it has the right honeycomb and the right areas to support the load to keep it from cracking.”

Aside from just designing a housing that would provide greater reliability, the mission was also to make a unit that could easily adapt into the popular Pro Mod chassis. That is no easy task, as there are several builders in the game, but fastidious research and the use of computer modeling allowed for a new rear that is easily adaptable to popular configurations.

Carmack Engineering billet housing
Carmack Engineering precisely sets the carrier preload by grinding the spool bearing spacers to ensure a interference of plus or minus five tenths. “We designed it on the inside where there are hardened-steel bearing spacers on the left and right,” Carmack said.” So like that’s how we set our preload is it’s a little bit of a challenge to set up, but the physical design of it, there are no threads to fail. There’s no give in anything in the case because everything is solid and fixed in the case…”

“We made the billet housing where it’ll go as narrow as 16 inches and we can offer an options for 17- or 18-inch four-link centers and custom wheel-to-wheel widths that will suit a typical Pro Mod. When we did it, we took a Jerry Bickel bracket, a Strange bracket, and a Mark Williams bracket — all of the different chassis builders’ brackets — and we put them in the computer where we could define the axle center line and all the four-link holes,” Carmack said. “We created a turn-key deal. If the customer has a Bickel car with this four-link center, we can make a plug and play where you can drop out the complete housing and you can bolt in one of these housings. It will bolt up and all of your four-link adjustments will be the same. Your shock adjustments will be the same. Your anti-roll links and everything will be in the same location, and the only modifications you’d have to make are the wheelie bars and the driveshaft.”

While billet rear is strong and ready to fit most cars, creating such a rearend is an expensive undertaking, so buying one is not for the faint of heart. The prices range from $19,995 to $29,995 based on the options, but playing in the Pro Mod world is not for those afraid to spend some money. And, installing a durable rearend like this is actually a way to save money in the long run.

“Now it takes a Pro Mod team that like races for a living, they’re counting on sponsorships, they’re counting on all of that. It’s going to get to a point where the service parts are more expensive than what it takes to race,” Carmack said. “So when you spend between $15,000 and $20,000 a year on just gear sets. That’s not having spare third members. That’s not having anything. That’s just parts, it’s easy to justify this 11-inch setup that we did.”

Considering a traditional rearend in a new chassis can run more than $17,000, this unit seems like a worthy upgrade for just a few thousand bucks more. Not only will it cut back on maintenance expenses, but it can alleviate the worry of untimely failures, which could ultimately help cars run more consistently quicker as teams don’t chase rear-gear issues.


Carmack Engineering billet 11-inch rear housing
The Carmack rearend is designed to allow for 16-, 17-, and 18-inch four-link centers, and it can bolt right into popular chassis from Bickel, Jeffers, McCoy and other builders. The base unit starts out at $19,995 with steel brakes, but you can option it up to $29,995 with options like carbon brakes.


Carmack Engineering shipping billet 11-inch billet housing
To ensure this beautiful work of billet art gets to the racer in great shape (and if necessary back to the shop for service) Carmack Engineering even created its own shipping crates for the 11-inch billet rearends.


Carmack Engineering

727 | 348 | 0000


Mark Williams Enterprises

866 | 508 | 6394