The Pro Pulling League (PPL), headquartered in Charlestown, Indiana, calls its Super Modified Tractor class "an engineer's dream." The goal is the same as other pulling classes: Drag the sled down the 300-plus-foot track as quickly as possible while a weight on the sled moves forward, making progress increasingly difficult.

But the variables in this class are unique. Pullers can choose engine types, from automotive-style internal combustion to aircraft engines, turbine engines, even industrial engines. They can also choose how many of those engines to use on the tractor.

The biggest limiting factor in the class is weight. "They're all limited to a 7,700-pound maximum weight, full of fuel, driver on the seat, ready to go," said Larry Richwine of PPL.

"There's no real set chassis design or engine location," Richwine continued. "It's pretty much whatever you can think up, get it on there, get it within weight, and get it to where you can control and balance it. Years ago, it used to be a combination of two, three, or four [engines], and they got as high as six. Now the average in the Super Modified class is four or three."

Innovation in the class is "a matter of, 'what can we think of? What did somebody else already try that didn't work? What can we do better?'" Richwine said.

It's a popular class among other organizations as well. The National Tractor Pullers Association (NTPA), based in Marion, Ohio, calls them Modified Tractors. Its rules are similar to the PPL, though the NTPA limits weight to 7,500 pounds.

Full Pull Productions of Scottdale, Pennsylvania, added the class, using PPL rules, to four of its events this year. "We had a need for it. We had promoters who were interested in it. A lot of people expressed interest in running for my company," said Richard Love.

Pulling in the Super Modified Tractor class "is more of a drag race anymore," Love said. "We have a sled in that class that weighs about 60,000 pounds. We go from 0 to 30 or 35 miles per hour and come to a stop within about 350 feet. Everything happens in basically less than 10 seconds. That's a lot to happen in that period of time."

10,000 Horsepower

While the class is open to many engine choices, the most popular are methanol-burning supercharged V8s, based on either Hemi or Chevrolet big block configurations. One of the few restrictions in the class concerns supercharger size. To maintain parity, four-engine tractors are limited to 8-71 superchargers, while three-engine tractors use 14-71 superchargers. Additionally, the three-engine tractors "can have unlimited overdrive on their superchargers, while the 8-71s must run at 45% overdrive," said Adam Koester of JAK Racing Engines, Wadesville, Indiana.

Koester builds engines developed by Bob Miner of Miner Brothers Racing Engines, which Miner adapted from a design Nick Arias created years ago. They are built using a solid billet block with "big block Chevy bore spacing, cam height, crankshaft dimensions, and stuff like that. There's a Ford head-bolt pattern on it, and it uses hemispherical heads like a Hemi. They basically took the best ideas from different engines and put them all together," Koester explained.

The two most popular engines Koester builds for pulling displace either 526 or 557 cubic inches. Their output varies depending on blower size. "A 14-71 engine, for a three-engine vehicle, makes roughly 3,000 horsepower. So if you're running good, you're around 9,000 hp for the three engines. The engines in a four-engine tractor make about 2,500 hp apiece, so they're around 10,000 hp for the entire vehicle."

Koester likened the engines to what you'd find in a Pro Mod drag racing car, though "drag racers tend to have shorter stroke engines, where the pulling engines have bigger stroke, higher torque engines. You need torque for pulling." Fuel is another differentiator. "In Pro Mod they run an 18-gallon-per-minute pump. We run a 20-gpm pump with 8-71 engines and a 23-gpm pump with 14-71 engines. We have more fuel going to the engine because it runs for a longer period of time." And while pulling a 60,000-pound sled puts demands on a tractor not seen in drag racing, "our objective when we build the engine for the customer is to see them run the entire year without rebuilding the engine. I try to go an entire year without changing rods. A Top Alcohol Funny Car could be changing rods every 16 runs. We're changing rods every 30 to 35 runs."

That rebuild schedule has a lot to do with driving style. "What drag racers don't understand about tractor pullers is, we don't swap feet at the starting line. We don't have that instant-load type scenario," said Craig Sage of SCS Gearbox, Bellevue, Ohio. "When you leave the starting line in tractor pulling, the sled is next to zero load on the tractor, except for a little bit you have from the draw bar, so you use your hand to actually drive, hook the tractor into the track, and drive out."

When talking to tractor pullers about their lubrication needs, "they really want something that's reliable," said Josh Steinmetz of Hot Shot's Secret, Mt. Gilead, Ohio. The company is PPL's title sponsor and NTPA sponsor, "and we sponsor a few drivers," Steinmetz said. "They'd like to get about 35–40 passes out of an engine without having to tear it down and get through their season without doing any major rebuilds--minus catastrophic failure."

Hot Shot's Secret isn't formulating pulling-specific lubrication--yet, said Steinmetz. "The fluids we make work really well in anything we've ever thrown at them, and tractor pulling has been no different. We do see pulling using some heavier weight oils and lubricants sometimes, so we have some things in development to help meet those needs. Sometimes there's a need for a thicker lubricant to help with these extreme stresses."

Steinmetz said when Hot Shot's Secret is providing oil to a puller, "we tell them we can get you a couple of extra horsepower. But horsepower isn't everything in this world. You do need horsepower to be competitive, but you need to be able to put it to the ground efficiently and be able to do it repetitively and do it without a whole lot of issues that are going to keep you on the sidelines."

In the Harness

The job of sending the engines' power to the tall rear tires falls to two key components: the gearbox and rearend.

"Everybody goes, 'They've got four or five engines. How do you make them run together?'" said Richwine. "Hooking the engines together is really not that big a deal. They're not running together because they've all got their own clutch. They're independently putting power into that gearbox with a clutch between them and the gearbox."

When it comes to designing the gearbox for a tractor, "they first have to start with what kind of engine setup they want to do--whether they want to put the motors side-by-side or inline," said Sage.

The inline "crank-to-crank setup ties the engines together with a flange on the rear of the crankshaft and a driveshaft hub on the front of the back engine," Koester said. "The engines run together, and there's only one clutch at the back of two engines." That makes it a lighter setup than having a clutch for each engine, "but it's harder on the crankshaft of the front engines."

Gearbox ratios are another important consideration, Sage said, which has more to do with the rearend than the gearbox itself. "The ring-and-pinions that these tractors run were designed for use in a semi-truck. The ring gears in these things are, like, 16 inches. They're made for big torque applications. But if you try and take four motors and put them right to the pinion, typically the pinion bearings or the pinion itself is just going to get very argumentative with you."

The solution for a four-engine tractor "is to design the gearbox with overdrive," Sage continued. "We'll put somewhere in the neighborhood of 40–50% overdrive in there, and that cancels the torque of the drive of the four motors. The driveline only thinks it's hanging on to the two motors. So we accelerate the driveline up, and then we reduce it back down with the ring-and-pinion and the wheel planetaries."

"The ring-and-pinion is called a 21-145 Rockwell," Koester said. "It still fits in the factory Rockwell housing, but the ring-and-pinion is a little bit bigger. Then they utilize either Caterpillar or Franklin planetaries. Franklin planetaries are lighter, where the Caterpillar planetaries are heavier and stronger." By using planetaries, "gear reduction is done in the wheels," he added. "It takes stress off the ring-and-pinion and the axles."

Custom Carved

"Tire technology has come a long way in the last 10 years," said Love. "Many years ago, they started out with ag tires and just cut the tread down. Now there are companies specific to the pulling industry that have developed their own tires and molds." Love said the tires preferred among Modified tractors are from the overseas company Mitas. "They are molded basically as a racing slick, so you can do anything with them in terms of lug angle, lug count, and lug depth. Back in the day, when tires where being used from the ag industry, they would come in with set lug count and angles. Now you see tractors with various lug counts, angles, and depth."

While the technology in those tires has been "tremendous," Love said, "that has actually started to hurt the rearend situation. Without this new tire technology, the tractors weren't able to get as much power to the ground, therefore not creating as much load and force on the drivetrain. Now, with the tires being able to grip the track so much better, it's put a lot more strain on the driveline components."

"There are three or four manufacturers who make custom tires out of a mold," Richwine said. "The pullers still modify them--sharpen them and grind them to get their own personal designer cut on the tire. But they're all chevron-type ag tires. The newest ones have lugs that aren't any taller than a half inch. And most of the guys that run Super Modified are running a quarter-of-an-inch-tall lug. They would probably run slicks if we would let them."

Slicks on dirt? "It's the friction on the ground that they're going for," he explained. "The shorter the lug, the less dirt they're throwing in front of the sled and the skid pan that's directly behind them. If a guy has an older school design on his tires with maybe a half-inch lug, which sounds very little, but he'll put a lot more dirt between him and the thing he's pulling and drag it down the track with him."

Balance

Tying it all together is the tractor's chassis. "The chassis is limited in overall length to a maximum of 15 feet forward from the center of the rear axle," Richwine said. Engler Machine and Tool of Princeton, Indiana, and Modern Machine and Tool of Van Buren, Indiana, are builders of "the chassis at the top level of both the Pro Pulling League and the NTPA," Richwine said. "They figured out ways to have a strong and durable chassis that's lightweight and adaptable to multiple engine configurations," added Koester.

Koester, who used to work for Tim Engler, said chassis tend to fall into one of two types. There's the wedge, "which is like sheet metal bent into a C channel, tapered from the rearend where it's wider, to the front end where it's narrower. It's a little bit heavier than a tube chassis, a little bit more flexible, and they hook a little bit better. But a lot of people are starting to utilize the chromoly tube frame, which is a little bit more rigid and a little bit lighter, so it allows for that extra movable weight."

Koester was referring to what Richwine described as "100-pound, suitcase-looking weights that they can hang on the front or the back" of the tractor to optimize its balance.

"They also utilize roll cage position," Koester added. "On some of the tractors they try to push the roll cage as far forward as they can, to try to keep as much weight off the rear of the tractor as possible. The further you move it forward, the more rigid it makes the chassis, and it kind of loosens up the rearend. So chassis designers have made ways to stiffen the rearend up while they're moving the roll cage forward. The weight seems to be more beneficial than the rigidity of the chassis."

"Weight" and "balance" came up a lot in our conversations, especially with Richwine, who started pulling competitively in 1974 and spent 29 years as the NTPA's head tech director before moving to the PPL.

"The challenge is the balance," he said, "to get that 7,700-pound total weight to the ground. You could have all the tire speed and power in the world, but if you can't get it to hook to the ground and go on down the track, it doesn't matter. That's why they went away from the six-engine and five-engine combinations so much, because they had too much power-to-weight ratio, and they couldn't get it hooked up."

Richwine described his idea of a perfect, balanced run: "You pick that front axle up maybe 6, 8 inches, a foot max, and you'll get the maximum weight transfer to the back of the tractor. [Steering is done via rear-wheel brakes.] If your front end's too light, it's going to go in the air higher. The guy who does the looks-amazing wheelie isn't going to go as far, because the draw bar on the other end of the tractor is going to go down. And that's going to reduce the draft, and it won't be as efficient to the ground."

There are a lot of variables to get the balance right, from power-to-weight ratios to tire air pressure, Richwine said. "It's just an engineering nightmare. And a dream," he added, recalling the company's website verbiage.

Sources

Full Pull Productions

fullpullproductions.com

Hot Shot's Secret

hotshotsecret.com

JAK Racing Engines

National Tractor Pullers Association

ntpapull.com

Pro Pulling League

propulling.com

SCS Gearbox

scsgearbox.com

Walsh Bros. Motorsports

facebook.com/WalshBrosPullingTeam/

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