Pulling is an exceptional motorsport discipline in so far as time is irrelevant. All that counts is the distance a tractor or a truck can attain in overcoming an increasing burden (see sidebar: The game). There is a bewildering array of vehicle classes (see sidebar: The rules) for each of which the burden in the form of a weighted sled is tuned to make it an exception for any competitor to complete the 320 ft off-road straight course.

The ‘Absolutely Nuts’ Super Farm tractor is the one previously campaigned by Ryan Salenbien as ‘Never Satisfied’; it is now in the hands of Brook Ferris, for whom Salenbien prepares the engine (Courtesy of MoorePhotosMN.com)

Here we consider the example of Super Farm Tractor. This is one of the largest classes in National Tractor Pulling Association (NTPA) competition. Ryan Salenbien is a former class champion, and these days he prepares engines for some of the most successful contenders in it.

A Salenbien Performance Super Farm engine is an I6 turbodiesel displacing 644 cu in (10.5 litres) and with a single turbo-supercharger creating a manifold pressure of at least 80 psi (5.5 bar absolute) offering peak power of 1700 bhp at 2400 rpm. Salenbien explains, “Maximum power will stay pretty flat above and below 2400 rpm.

“I don’t pull these engines down any further down 2300 rpm on my dyno. As you pull them down the torque just keeps rising. Who knows, they might make peak torque at 1800 rpm; they might make 3800 lb-ft [5152 Nm] before they stall.”

Before looking in detail at this engine, let’s consider its deployment in the field.

The pull

Pullers run one at a time. The 9300 lb Super Farm Tractor is shod with 24.5 x 32.5 in rear tyres and uses a modified stock chassis. Its production-derived engine drives through a Crower four-disc clutch, 12.5 in in diameter and pedal-operated. There is a four-speed gearbox simply for ratio selection according to course requirement – no gear changing is involved in a pull.

At the start of a pull, since there is no time element there is no starting signal for the driver to try to anticipate. Salenbien says, “When you’re hooked to the sled, you put tension on the chain and watch the flag men just in front of you. When they give you the green flag it’s up to you when you go.

“Basically you drive it like a farm tractor using the clutch pedal. That way, you can run really low rpm; if you had a slipper clutch, it would start falling out and slipping at 2100 rpm. Once you let your foot off this clutch it stays locked. That’s crucial for the Super Farm class.

“When I was on the seat, at the green flag I would hold the clutch pedal down and start slowly building rpm, which would slowly build boost. When I got to about 90% throttle, I would start to give it a bit of clutch [bite] so the boost would come on stronger.

A Super Farm engine developed by Salenbien Performance

“So when I hit the stop, which is wide-open throttle, I was already starting to let off on the clutch. You need to put some resistance on a diesel motor to build some boost; if you don’t it will over-rev.

“Having it wide open and slipping the clutch will give up to 70-80 lb of manifold pressure, and normally you’ll reach around 5000 rpm. Then, as soon as you let out the clutch fully, it’ll plant and the rpm will drop dramatically. If it hooks up really hard it’ll go down from 5000 to 2400 rpm [peak power speed].

“But if it doesn’t plant, if your tyres break loose, your rpm might only drop 500. That’s not so good.

“If it's a good run, you start with 5000 rpm and within a fraction of a second you’ve dropped to about 2400. Then it’ll climb back up – it’ll gain another 1000 rpm by mid-track.

“On a good run you’ve got the throttle wide open all the way. The only time you would ever want to start lifting off the throttle is if your tyres are slipping and you’re going nowhere; you’d want to back off the throttle a little bit, get it to hook up and then go again. But that’s like one out of 50 chances.”

So, we asked, throttle response is not a factor? “No. The load on the engine is what determines your power level; you have to have the throttle at 100% all the time in order for this thing to run like it’s supposed to. You can’t run down the track at 80%, it just doesn’t work.”

So, presumably the driveability of the engine is also not a factor? “Exactly.”

What sort of speed is attained? “For a Super Farm tractor the highest ground speed is typically 30-34 mph; that is moving on a track that is only 320 ft! That isn’t the wheel speed, as you have some slippage; depending on what gear you’re in, wheel speed might be 40-50 mph.”

Approximately how long would such a pull last? “Anywhere from 12 to 15 seconds. The weight of the sled is progressive, starting off from basically nothing. As you’re running down the track it gets heavier and heavier, which puts a strain on the engine.

“When you’re at around half-distance there’s still not a whole lot [of resistance] from that sled, so your rpm is high and you are going at your fastest speed. That’s when you want to have all your ducks in a row.”

Is that for when the trip on the sled activates, causing a massive increase in load? “Correct. That will drag the rpm right down and you won’t get much further unless you have enough speed and momentum.”

Salenbien reflects, “Once you engage the clutch you can’t put your foot back on it or the thing will just over-rev. Basically it’s about how and when you let your foot off the clutch. It’s all about when and how you get the tractor hooked and planted. The key is getting the rear tyres to hook up out of the hole and then you keep it locked as you run down the track.

“A lot of guys will tell you that tractor pulls are won in the first 100 ft. To let your foot off the clutch at a certain right time is very important. It is about sitting in the seat and feeling it – feeling when it locks up, feeling it when it's going. That feeling could determine whether you’re first or last.

“It's not just about getting on the seat, revving the thing up and going. You have to read the track, know where to put your tractor’s weight [distribution setting], what kind of tyre pressure [to run], and what gear to put it in.

“Having four gears gives you options for the different types of places you go to. At a really sandy track, say, you would go a higher gear, then for a real tough clay track you would definitely shift down so you can keep your rpm up.

“It is very important having the right gear to keep the rear tyres glued to the track, and you want your weight distribution front to back such you don’t get too much of a lift of the front [wheels] or not enough.

“It is all about your set-up. You could have x amount of horsepower, but a guy that has less power could beat you just on set-up. You have to have everything right. It’s about the driving and the preparation, reading the track and things like that.

“I did it for 11 years, and we won a championship that 11th year from me just knowing how the tractor feels. I’m sure a racecar driver could tell you the same thing: you drive by the seat of your pants. When you’re in that seat and you work that clutch and push that throttle, you have to become one with your vehicle.

“And once you have that feeling and you can actually make this thing connect, you’ve got a shot at winning.”

SALENBIEN PERFORMANCE

“Our family has been involved in tractor pulling since it started with both my dad and my grandpa,” explains Ryan Salenbien. “The last time I drove our Super Farm tractor was 2011. That final year of driving I won the NTPA Grand National Championship. At the same time, our Salenbien Performance business was starting to ramp up.

“After I won the championship, I decided to step down, sell our tractor and focus all of my attention on Salenbien Performance. Being in close competition for so many years had driven us to put in our own dynamometers. People took notice of the results we were getting on the track and started bringing their vehicle to our shop to test it and get it improved. 

“That first year, after I quit competing, our dyno business doubled and it has kept on growing. We had only eight dyno customers when we started our business in 2007, and now we have more than 400 dyno customers – and both dynos are running all the time.

“In 2019 we launched another division of our company, ApexTurbo and began our own turbocharger manufacturing. Since the launch the turbo business has more than doubled each year. “

Ryan Salenbien and customer Joe Batliner inspect the latter’s Super Farm engine at the Salenbien shop

The Super Farm engine

The Salenbien Performance Super Farm engine weighs “about 2500 lb as a long block”. Salenbien adds that he builds all his customers essentially the same specification engine – “basically what I would build for myself if I was still driving”.

He reflects, “It is impressive how horsepower levels jump every single year. I have had one extremely loyal customer for the past 11 years and every year before the pulling season starts he comes back to my dyno shop and every year he gains horsepower.

“In 2008, when started my dyno operation, the Super Farm Tractor engine was right at 1000 bhp. I remember that I won the points championship with 1000 bhp, whereas these days we are shooting for 1700 bhp.

“That goes to show you how much the turbo and the fuel supply have developed. As far as the diesel engine itself goes, we haven’t really changed anything [since 2008]. The head flows the same, the camshaft is the same, the pistons are the same. It’s all in the advances in airflow and fuelling.

“Obviously, if you give it more air, you’re going to need more fuel. When we first started, we were right in the 750-800 cc/minute [injector supply] range. These days we are anywhere from 1050 to 1100 cc/minute. We are adding that much fuel because of the additional air we provide with the ApexTurbo turbocharger we developed [see sidebar: ApexTurbo].”

The engine this Salenbien Performance unit is based on is the DT466 I6 turbodiesel, introduced in 1971 by International Harvester, since acquired by Navistar. Originally for agricultural applications, it has since been widely used elsewhere; for example it powers countless US school buses. Over 2 million examples have been produced.

The rules dictate the use of a stock block. One characteristic of the DT466 is the use of iron sleeves (liners) despite its iron foundation. The theory is that these wet sleeves improve cylinder geometry and cooling, although Salenbien Performance does not use any coolant for its derivative.

A stock DT466 obtains its 466.16 cu in displacement from a combination of a 4.300 in bore and a 5.350 stroke. Salenbien Performance fits sleeves that accommodate a 4.875 bore and uses a 5.750 stroker crankshaft for 643.96 cu in – the Super Farm limit is 644 cu in.

Salenbien says, “There is quite a bit of work to get a block prepared. We girdle the bottom to hold the main caps straight and then line-bore it. We put a deck plate on top of the block. We typically run either a 5/8 or 1 in-thick deck plate that helps strengthen the block. Then we put in longer sleeves with longer rods.

An example of a Super Farm engine dyno result at Salenbien Performance

“The head has to be cast iron too. It has to appear stock. Previously you had to run an OEM head, but recently the NTPA has said you can run an aftermarket recast head. I use Northwoods Engineering’s recast head, and I have a port shop do port work for us.”

Salenbien reports there is no head gasket, “we run iron to iron with C-rings”. The engine remains wet sump with this specification, including a three-stage Aviaid mechanical pump, having two oil stages and a third stage to lift fuel from the tank to the high-pressure fuel pump. Salenbien recommends Lucas 20W50 lubricant; “around 100 psi oil pressure is what I shoot for”.

The billet steel crankshaft – often from Bryant – runs in seven plain main bearings. “We run a Mahle-Clevite Eight Series high-crush bearing and coat them in Teflon for added protection”. The crankshaft has an individual pin for each cylinder, driven by a steel H-beam con rod having a four-bolt cap.

Salenbien says, “I always run a 9.5 in-long con rod. I like a longer rod to keep the piston more square in a bore. If you run a shorter rod you get a lot of piston slap, especially when you are running at lower rpm.”

The full-skirt three-ring pistons are aluminium alloy with a hard-anodised coating, and each is cooled by a pair of oil squirt jets to the underside. Total Seal gapless rings are used.

In the guise eligible for Super Farm, the DT466 is a pushrod engine having two valves per cylinder, both valve stems perpendicular to the deck. The camshaft is driven by stock gears in the front of the engine, where there is an idler gear that also drives the injection pump off the crankshaft.

Stock 1.125 in-diameter factory roller lifters are used together with one-piece, tool steel aftermarket pushrods and steel aftermarket roller rockers. The 1.900 in exhaust valves are Inconel while the 2.25 in intake valves are titanium. Lift is 0.630-0.670 in off the seat.

Super Farm rules do not permit electronic engine control, and insist on the use of the classic Bosch P-Pump high-pressure mechanical injection pump. This is supplied by the Aviaid lift pump fuel at a pressure of around 50 psi. Salenbien notes, “Our injector pop-off pressure is typically 4500 psi, so the actual supply pressure to the injectors is going to be way higher than that.

“There are several companies that supply modified Bosch P-Pumps. The injectors are custom billet items.”

The fuel itself is 100% bio-fuel. Salenbien says, “You don’t have to run B100, although whatever fuel you do run has to meet the organiser’s guidelines. You cannot simply dump a bunch of additives into anything to gain power; there are qualification tests that measure key parameters such as dielectric and specific gravity.

“If you have too much cetane your dielectric number will go way too high. You are limited on how much cetane improver you can add to your regular diesel fuel to stay within the permitted specification. 

“All my customers run our own formulation, BioPower Max Performance Diesel because it provides the most horsepower. It is B100 with our own additive package to improve the cetane rating and so on. It’s specially formulated for tractor and truck pulling to avoid wet-stacking and maximise power. It is a premier bio product and we sell thousands of gallons of it.”

Salenbien Performance uses cylinder heads from Northwoods Engineering that have a distinctive D-shaped exhaust port

The compression ratio is 13:1, and Salenbien says there is no issue of detonation with this engine. “It’s not like the multi-turbo engines, which do tend to get into detonation.”

The exhaust is six into one to feed the turbine, and no wastegate is permitted. “So you have to control boost by selection of the A/R of the turbine housing,” remarks Salenbien. His own patented ApexTurbo turbocharger cleverly avoids the use of a compressor wheel nut to maximise intake area within Super Farm’s 3.0 in-diameter intake ruling.

The compressed charge air is fed through a 3.5 in-diameter ‘crossover pipe’ to the manifold, “which is like a big box that feeds all six cylinders via a custom velocity stack for each cylinder.”

Salenbien adds that charge cooling is not permitted under NTPA rules, whereas water injection is. “We inject water into the crossover pipe. The air coming out of the compressor will be upwards of 500 F [260 C] so we inject all our water at that point. That will turn into steam, which gives you additional pressure to drive the turbine. Using that water injection definitely gives you extra torque.

“Also, you need to be fully engaged on the clutch and then, as you move and that pulls the rpm down, the boost goes up even higher. Our ApexTurbo turbocharger will build over 80 lb of manifold pressure [5.5 bar absolute]. The exhaust pressure will be at the highest point at the highest rpm. So then, when you are gaining more boost but losing rpm, the exhaust pressure falls; turbochargers are very rpm-dependent.”

What would be the lowest manifold pressure you would see on a run? “If 80 is the highest, I would say 70 would be the lowest. It’s not that much of a fluctuation. As for exhaust pressure, we might see 110 psi maximum but it’ll lock down at 100 or 95, so there’s always a 10-15 lb variation throughout the pass.

“The competition is really tough these days,” concludes Salenbien. “It’s not like it was 20 or 30 years ago, when dyno shops were scarce. Basically guys were just doing it out of their garage. Nowadays, it’s serious. You need the power, you need your set-up, you need to understand everything that’s going on with the tractor and the track, in order to get out front. Because if you don’t somebody else is going to outsmart you.”

THE APEXTURBO

The ApexTurbo single-stage turbocharger has an Inconel turbine wheel, a steel shaft running on dual ceramic bearings and an aluminium compressor wheel inside an aluminium compressor housing. The turbine wheel is friction-welded to the shaft in typical turbocharger fashion. However, a key feature of the ApexTurbo is the absence of a nut or other kind of fastener on the shaft. To secure the assembly the compressor wheel itself is designed as the nut.

The ApexTurbo has no nut to hold the compressor wheel, which itself is formed as the nut

This clever approach makes for a stronger hub, while for a given wheel diameter the absence of a conventional fastener allows more air into the compressor, increasing maximum boost pressure by a surprising amount. That is very significant in Super Farm, since it is restricted to a 3 in-diameter compressor inlet area.

Ryan Salenbien says, “We developed the ApexTurbo to find an advantage in Super Farm. In bolting on an ApexTurbo you maximise the airflow within the 3 in restriction. Many customers who were running with a standard out-of-the-box turbo, who then made the switch to an ApexTurbo  would see a considerable horsepower gain while staying within the Super Farm class restricted diameter ruling.

“We are now looking at how we can apply the ApexTurbo technology to other markets – diesel pick-ups, diesel drag, diesel racecars and more. The airflow increase you can achieve with our turbo design can help significantly on other vehicles. We have been granted several patents around the apexing of turbine wheels.

“We have both foreign patents pending and several issued US patents on the turbo wheel design and technology. We continue to innovate and currently have three more inventions in the works to complement the ApexTurbo product. There are licensing opportunities – anyone interested can contact us via [email protected]”

THE GAME

Tractor Pulling – which embraces derivatives of farm tractors and highway trucks – is a form of motorsport in which competitors vie to obtain the furthest pull of a sled over a straight dirt (often clay) track that is a minimum of 30 ft wide and 320 ft long.

Mounted on the sled is a heavy box that is automatically and mechanically winched forward as it progresses along the track. A complex system of gears moves the transfer box. At the start, all its weight is over the sled’s pair of rear axles but as it moves forward it progressively transfers weight to the front where, instead of wheels a flat ‘Pan’ plate applies the force of the weight to which it is subjected to the ground, thereby creating friction.

A Trip is positioned between the frame rails along which the transfer box moves. Hitting the Trip activates a push-down system that uses hydraulic cylinders to lift the back half of the sled into the air, transferring 100% of the sled’s weight onto the Pan. In some instances the Pan will incorporate spikes that are now released into the ground.

Most aspects of the sled can be adjusted according to the desired level of frictional opposition to the puller vehicle. That opposition is such that the sled will normally halt progress before the finish line. The distance obtained from the start line is measured in thousandths of an inch to determine the winner.

In some instances a competitor will actually reach the finish line, which is known as making a ‘full pull’. If more than one competitor makes a full pull, a ‘pull-off’ is held to determine a winner.

The premier governing body of this extreme sport is the National Tractor Pullers Association (NTPA), which was established in 1969 by representatives from eight US states keen to have uniform rules. Those rules are now respected by most US and many overseas pulling competition associations. Governed by a board of directors, the NTPA has been managed for more than 30 years by World Pulling International (WPI), an independently owned private company.

Before the formation of the NTPA, pulling competition was primarily at state level. Now it also embraces regional and national-level competition, the latter incorporating both Grand National and Super National events. Two or three NTPA Super Nationals are at the top of the pyramid, headed by the annual National Tractor Pulling Championships event at Bowling Green, Ohio.

THE RULES

Under NTPA rules there is a bewildering list of classes embracing vehicles recognisable as derived from a regular farm tractor through to tractors purpose-designed for pulling competitions, some with multiple engines. There are also classes for competition versions of pick-up trucks through to modified semi-trailer tractor units.

There are two- and four-wheel-drive truck classes, while across all vehicle types engine options embrace naturally aspirated and blown spark ignition running on methanol, turbodiesel and even compression ignition engines converted to spark ignition. Some classes also cater for turbine engines.

Within a given class the rules restricting engine modification tend to be few, aside from the imposition of some basic parameters according to the characteristics of the class. Weight and tyre size are other key class parameters.

The most radical vehicles run in the Unlimited division of the (30.5 x 32 in shod) Modified Tractor class. This division allows free choice of power plant within an overall vehicle weight of 8000 lb. Then there is Light Unlimited running at 6000 lb and excluding turbines. Two other Modified divisions run at 7500 lb, and while incorporating turbines limit the number of engines according to type and also restrict blower sizes.

Further down the performance ladder are many other classes, some more restrictive in respect of powerplant than others. For example, running on 24.5 x 32 tyres at 9300 lb, the Super Farm Tractor class is diesel-only and overhead camshaft engines are not permitted.

By contrast, running on the same-size tyres at 10,000 lb the likewise diesel-only, single-turbo Pro Stock Tractor class does allow overhead camshafts and charge-air cooling, and it imposes a 680 cu in limit. Then there is a Light version of this class, at 8500 lb, having a 540 cu in limit, banning charge cooling and outlawing overhead camshafts while imposing the use of an OEM cylinder head.

And so it goes, through Limited Pro Stock, which is diesel-only; Super Stock, which is spark ignition or diesel; and Modified Minis, which allows turbines. Then there are the various truck classes, starting at 6200 lb, 18.4 x 16.1-shod Two Wheel Drive Trucks having a 575 cu in limit and allowing blown engines and alcohol fuel, and going through to 20,000 lb stock-appearing Super Semis. Running on any DOT-approved tyres, those are limited to production-based diesel engines with two-stage turbocharging permitted.

DATASHEET

Salenbien DT 466

NTPA Super Farm

I6

4.875 x 5.75 in = 643.96 cu in (10,553 cc)

Turbodiesel with single turbo

Biodiesel fuel

Iron block and head

Cast iron liners

Seven main bearings, plain

Steel crankshaft, six pins

Steel con rods

Light alloy pistons; three rings

Pushrod; gear drive single camshaft

Two valves/cylinder

Upright valves

2.25 in intake valve, 1.00 in exhaust

Mechanical fuel injection

No electronic engine control

13:1 compression ratio

Maximum rpm, 5000

SOME KEY SUPPLIERS TO THIS ENGINE

Head: Northwoods Engineering

Block/crankcase: Navistar International

Sleeves: Powerbore

Oil pan: in-house

Crankshaft: Bryant

Crankshaft: Southern Illinois

Camshaft grinding: Vogel Manufacturing

Timing drive: Navistar International

Lifters: Navistar International

Pushrods: Trend

Rockers: Hypermax

Pistons: Diamond

Rings: Total Seal

Piston pins: Trend

Con rods: Lemke Manufacturing

Con rods: Riverside Engines

Big-end bearings: Mahle-Clevite

Main bearings: Mahle-Clevite

Camshaft bearings: Riverside Engines

Valves: Northwoods Engineering

Valve seats: Northwoods Engineering

Valve guides: Northwoods Engineering

Valve springs: Northwoods Engineering

Fuel injectors: various Custom Billet

Oil pump: Aviaid

Oil filter: System One

Turbocharger: ApexTurbo

Exhaust: Navistar International

Fluid lines: Aeroquip

Fuel: in-house [BioPower Max]

Oil: Lucas

Clutch: Crower

Gearbox: ProFab

Differential: Engler Machine

Dynos: Land and Sea