Diesel Tractor Pulling - Burnin' Rubber
Inside The World Of Smoker Tractors
Welcome to the world of diesel-powered "smoker" pulling tractors, where too much of anything, especially turbo boost and wheel-speed, is never enough. These are among the most incredible vehicles in all of motorsports, in any venue, on any continent, and yes, they began life as farm tractors.
We all know how the world of tractor-pulling got started. After a hard week in the fields, local farmers would get together with their horses, then as time progressed, their tractors, to see who had the most muscle by pulling a weighted sled. The theory and practice of this activity are still very much the same. Competitors pull that sled as far as they can before coming to a complete stop. It's the equipment that has been evolving at an ever-increasing rate.
One of the largest pulling sanctioning bodies in the U.S. is the ATPA (American Tractor Pulling Association), and its rulebook has provisions for several smoker classes. The introductory class in the smoker world is the "Super Farm Tractor" category. According to the Director of Operations, Kurt Johnson, "Super Farm tractor is the lowest horsepower category of the smoker classes. The tractors have to weigh in at 9,300 pounds and use the stock frame from the back of the engine block to the rear of the tractor. These things are making somewhere near 1,500 hp, and the torque is pushing the 2,000 lb-ft mark." It is important to note that the tractors in this class began as stockers that, according to Johnson, can weigh as much as 18,000 pounds from the factory. The fact that these guys are reducing their weight by nearly 50 percent is pretty amazing. Other restrictions in the Super Farm class include stock cylinder heads, a limit of a 3-inch inlet and outlet on the turbos, and a 24.5-32 tire. And what a tire it is.
Jim Sube is the man at Firestone when it comes to pulling tires and their development. Firestone's Puller 2000HP is the dominant tire in the world of professional tractor pulling, so we gave them a jingle to get some information on exactly how these tires survive the abuse they see during a pull. Sube told us, "These tires are built incredibly strong. We have tested them to 200 mph and not had a failure." That's right, folks, a tire that weighs in at nearly 500 pounds has been tested-we can only imagine how-to 200 mph, and it holds up. "A lot of people think that these tires are like a monster truck tire and that is not the case. A monster truck tire is a floatation tire. They do not have to deal with the centrifugal force that a pulling tire does, so they are built very differently," Sube said. The main difference is in the belts. The pulling tire has many, many more belts than the monster truck tire, because if it didn't, the incredible strain of its own weight and the wheel speed of the tractor would send pieces of it into low Earth orbit. In terms of the rubber compound, Sube said, "These tires have a compound that is designed for better chipping and tearing resistance. It is very close to our forestry compound. Just about every driver out there customizes the tires to their liking, but they could be bolted on right from the factory."
Pro Stock is the smoker tractor class that really gets every fan's blood pumping. There are many factors, but the biggest is the 80-foot plume of smoke that is belched from the exhaust stack of these machines. Pro Stocks (in ATPA trim) are allowed a 680ci limit, a single turbo, diesel fuel only, no intercoolers, and the engine can be moved to better balance the tractor. Most tractors are component chassis, meaning that they are tube chassis with a drop in style rearend. Old-style non-tube tractors have a tendency to break in half because stock bellhousings are rated for 145 hp and were being tortured with 2,500 hp. The skinny on these guys is pure overkill. We caught up with one of the biggest names in the Pro Stock class, Lance Little. He and his brother Chris have been national champs and currently run Tantrum Enterprises, one of the premier tractor-building shops in the country. Lance was able to fill us in on some of the finer points of building a Pro Stock tractor. "If the engineers at John Deere knew what we were doing with their engine blocks, they would scratch their heads and wonder how we did it," Little said. These guys are taking blocks rated in the 200hp range and pushing 2,500 horses through them and Lord knows how much torque. "There really has not been a dyno that could measure the torque, but it is astronomical."
Although the tractors resemble their stock brethren, below the sheetmetal, not much remains of the original mechanical assembly. "The block is a John Deere block, but everything else in that motor is billet. We are just making so much power, no stock parts will survive in that environment," Lance said. Would you believe that these tractors use steam technology, too? According to Little, "We're at 1,200-1,250 cc of fuel, and we burn about two or three gallons of fuel on a 10- to 12-second run. We are also using water injection in these tractors, and that does two things. It helps keep the motors from melting because we are making exhaust temps of 1,600-1,900 degrees. And [the] water turns into steam, a lot of steam, and that steam creates a lot of horsepower." One of the most important developments in diesel pulling history was the introduction of billet aluminum and steel cylinder heads about a decade ago. These heads are far stronger and more readily adaptable to the pulling application than the stock heads ever were. "We were blowing stock heads right off of the motors. The heat of the exhaust would make them crack, and we were just breaking them left and right. The billet stuff has allowed us to make so much horsepower because we can modify them and try new things. Our head flows 475-500 cfm, and we run a (roughly) 2-1/2-inch-diameter intake valve."
To get all of this power to the ground, a Crower-glide four-disc slider clutch is employed. This sounds familiar to you drag racing fans out there because it is the same style of clutch used in many ultra-high-horsepower drag racing applications, but unlike drag racers, once the clutch is tuned it stays where it is for most, if not all, of the season. Little said, "We start out with the clutch set to 0.085 or 0.090 of an inch, and by the end of the season we are around 0.120 of an inch. That clutch sends the power to a Pro-Fab transmission. Those guys build transmissions for all of the high-powered pulling classes." Finishing off the drivetrain, a stock rearend housing is highly modified to accept an Eaton rearend center section and differential from a semi-tractor. Little currently runs a 3.90 gear ratio to make the wheel speed needed to pull the sled.
But, the whole sled-pulling program comes together with the right turbo. The Pro Stock class has no restrictions on turbo size, so you won't be surprised to know that Little said he runs a 5-inch exhaust wheel and a 5-inch compressor wheel. Yikes!
Until just recently, the only place to get a turbo large enough and strong enough to feed one of these beasties was from one of the specialty manufacturers of the tractors equally impressive fuel systems. These turbos were built from a conglomeration of other parts and represented the best stuff pullers could get. Yes, they worked, but not for long. Then, the Calvary arrived in the form of California-based Turbonetics. Brad Lewis, the V.P. at Turbonetics filled us in, "We were approached a couple of years ago because pullers were having a hard time making good, reliable power with the turbos that were available. Primarily, those turbos were coming from specialty diesel shops and rebuilders, and they were using the best parts they could, but the turbos just couldn't hold up to the stress."
Seeing a problem and developing a solution meant that Lewis and the crew at Turbonetics had to draw on their other motorsports experience. "We took a different approach. We got with several pullers and developed a problem statement. Essentially, we got a list of issues that they were having with the turbos and began to work on something to combat each of those problems. There were guys breaking up to four turbos a weekend, and at $10,000 a piece, that's not cost effective." The major problem with the turbos that Lewis and his crew found after pulling a bunch of busted units apart, was that the turbos were failing because they could not withstand the dynamic thrust load that was being placed on them. "We developed a turbo that can withstand 1,000 pounds of thrust load. That's a huge amount by anyone's standards. We also have a no-fault, no-problem 1-year warranty on these. If you blow one up in the first year you have it, send it back and we will buy the first one for free.
The Pro Stock class is basically dominated by John Deere tractors and International tractors mainly because their factory engines are the easiest to get up to the cubic inch limit. Little told us, "We run a motor that started life as a 619ci engine from a four-wheel-drive tractor. All we do is offset grind the crank half an inch to get the stroke necessary to make the 680ci limit. The small-block-based motors [some other teams use] have to be bored and stroked, but they do not last as long, because they are working harder."
So, there's a look at smoker tractors. They really are diesel dragsters with a giant sled hooked to the rear of them. Keep your eyes peeled for the local state fair or pulling event because you don't want to miss a chance to see what are the most powerful diesel-burning, piston-powered tractors on the planet!