4BT Cummins vs. 1.9L TDI Volkswagen

• May 15, 2020
• Story By
  Mike McGlothlin

The 4BT Cummins was designed for commercial and industrial use. Volkswagen’s 1.9L TDI was primarily intended for passenger cars and maximum fuel economy. So why in the world should these polar-opposite I4’s ever be compared in a head-to-head fashion? Believe it or not, these two NVH offenders cross paths on occasion. Usually, it’s in an off-road setting or at a diesel event, but it happens—and there’s a good reason for it. After all, both engines have 500,000-mile durability, simplistic construction and considerable performance potential on their side. This is more than enough to pique the interest of an enthusiast in search of the perfect oil-burner for his or her project.

In Jeeps, Toyotas and half-ton and smaller domestic pickups, the mechanically injected 4BT is a popular swap candidate. Unfortunately, now that the word has gotten out and demand has spiked, it isn’t exactly a cheap find anymore. For instances where swapping an engine like the 800-pound, 3.9L Cummins into place isn’t affordable (or even feasible), the ALH code 1.9L TDI parked under the hood of all those Jettas, Golfs, Passats and New Beetles is worth a look for many a gearhead. Over the years, we’ve seen 1.9L TDI’s in Isuzu Troopers, old Toyotas and even mini rod tractor pullers (more on that below).

For more on how these two clatter boxes stack up against each other in terms of strength, reliability, affordability and availability, keep scrolling.

The Industrial-Strength 4BT Cummins

If you tuned in for our last small diesel comparison, you know that the 4BT Cummins is constructed the same way the legendary 6BT is (the 5.9L). Forged-steel I-beam connecting rods with 7/16-inch rod cap bolts, 4.02-inch bore cast-aluminum and direct injection pistons with massive, 1.57-inch diameter floating piston pins, a forged-steel crankshaft with a 4.72-inch stroke and 14mm main cap bolts all speak to the robustness of the B-series line. Like its big brother, the 4BT’s tough-as-nails construction is precisely why its horsepower rating can easily be tripled without worrying about bending a rod, popping the head gasket or having to upgrade anything in the bottom end.

Cast-Iron Cylinder Head & Six Head Bolts Per Cylinder

The simplistic nature of the 4BT continues in its conventional crossflow style cylinder head. Like the block, it’s cast from gray iron. The head utilizes two overhead valves per cylinder, induction-hardened valve seats, ductile iron rockers and features exceptional head-to-block clamping force thanks to its use of six head bolts per cylinder.

Fixed Geometry Turbo & Mechanical Injection

While Volkswagen’s ALH-code 1.9L TDI boasts variable geometry turbocharging and some electronic control over the injection system, the 4BT is about as low-tech as it gets. Its Holset turbo is a fixed geometry unit and the Bosch VE distributor style, axial-piston injection pump found on most versions of the engine is completely mechanical. This means no vacuum lines running to the VGT actuator (the case with the VW) that eventually crack and leak, and no waiting around for the ECM to tell the injection pump when to fuel. While the turbo’s response may not be as quick and the fueling as precise as the VW’s, there are virtually no electronics to monkey with other than the fuel shut-off solenoid and the starter.

Gear-Driven Accessories (Pro) & No Emissions Equipment (Pro)

A major strongpoint with the 4BT, and something that once again points back to its commercial-grade roots, is the fact that the camshaft is gear-driven—along with the injection pump, oil pump and accessory drive system. And not only that, the gear train is made up of six heat-treated, ductile iron helical gears. As for emissions equipment, this is perhaps where the 4BT shows its age most (after all, it was introduced in 1983). There is no exhaust gas recirculation system to speak of, as the rather lax (by today’s standards) NOx requirements of the early 1980s were met through fine-tuning of the injection system and with proper turbocharger sizing.

Volkswagen’s 1.9L TDI (ALH)

It might’ve produced a microscopic 90 hp and only 155 lb-ft of torque when it left the factory, but it was built to take a whole lot more. The Volkswagen ALH code 1.9L TDI diesel offered in ’99.5-’03 Jettas, Passats, Golfs and New Beetles isn’t exactly a Cummins, but it is truly the little engine that could. The block is cast from gray iron, the rotating assembly entails a die-forged steel crankshaft and forged-steel connecting rods tied to cast-aluminum pistons with shallow valve pockets (a common practice on high-speed diesel engines). Horsepower can be doubled on the factory bottom end with little (if any) trade-off in durability.

The Cheap(er) Option

As for price point, the TDI is generally more attractive than the 4BT, with running take-out VW mills going for anywhere from $700 to $1,100 and a 4BT costing between $1,500 to $3,500 depending on its condition and where it’s located. Complete, rebuilt Volkswagen long-blocks range from $2,000 to $3,500 depending on the builder, while rebuilt short-blocks range from $1,500 to 2,500. By comparison, having a 4BT long-block rebuilt by a quality shop can crest $3,500 in no time.

Cast-Aluminum Head & Four Head Bolts Per Cylinder

The TDI’s cast-aluminum cylinder head houses the engine’s single overhead cam (SOHC), which actuates one intake valve and one exhaust valve per cylinder through the use of bucket tappets. Unlike the 4BT Cummins’ head, the VW’s is of a non-crossflow design. Fresh air enters the head through a cast-aluminum intake manifold and exhaust leaves it through a cast-iron exhaust manifold—both of which are located on the same side of the engine. Also unlike the 4BT, which employs six fasteners to attach the head to the block, the TDI relies on four head bolts to perform the same job.

Electronically-Controlled Injection Pump, Mechanical Injectors

In what essentially amounts to an electronic version of the VE found on the 4BT Cummins, the Bosch VP37 injection pump used on the 1.9L TDI is in charge of pressurizing low-pressure fuel and sending it to the injectors. Unfortunately, the VP37 isn’t aided by a factory lift pump in Volkswagen applications, but it doesn’t seem to hinder performance—even when enthusiasts double the ALH engine’s power output. The job of distributing high-pressure diesel into each direct injection piston’s fuel bowl is left to pop-off style injectors equipped with 5-hole nozzles.

Variable Geometry Turbo

It’s primitive by today’s electronically controlled and actuated variable geometry turbochargers, but the Garrett VNT-15 aboard the 1.9L TDI is still light-years ahead of the fixed geometry unit bolted to the 4BT Cummins. Its VGT functionality is vacuum-actuated, which can lead to failed operation in the event of a vacuum leak, but on a positive note the VNT-15 can support as much as 170 hp. And unlike many variable geometry turbochargers on the OEM market, it’s known to be very reliable and durable—so long as it’s exercised on occasion (so the turbine vanes don’t stick) and kept in the 20-psi of boost range.

Cons: Timing Belt & EGR

Of the few downsides the Volkswagen 1.9L TDI has, its overhead cam, injection pump, water pump and tensioner are all belt-driven rather than gear-driven. Obviously, the timing belt carries an inevitable service interval, which varies quite a bit between model years (40,000 miles on early engines with automatic transmissions on up to 100,000 miles on ’03 model years). On the emissions side of things, the TDI is equipped with exhaust gas recirculation (EGR). And while its EGR system isn’t prone to failure, the eventual buildup of soot, oil and carbon will cause enough of an airflow restriction into the cylinder head that performance will suffer. Sticking EGR valve and cracked EGR cooler issues also tend to surface at some point, but usually on higher mileage engines.

Durable and Simplistic—But Best Known For Its Fuel Economy

One of the reasons the ALH is so highly sought after, other than its relative mechanical simplicity and long-term durability, is its uncanny ability to sip fuel. Tales of hyper-miling to 55-60-mpg are common and daily commuters are known to see anywhere from 45 to 50-mpg on the regular. From a fuel economy standpoint, many believe Volkswagen had its diesel-powered cars perfected during the ’99.5-’03 time frame. While the 1.9L TDI definitely had a lot to do with it, part of this platform’s fuel efficiency magic was the fact that the cars themselves were featherweights, the Jetta shown here only weighing 3,000 pounds, along with measuring just 57 inches tall.

Performance Applications

Similar to the 4BT Cummins, the ALH TDI engine is heavily supported by a thriving aftermarket. Injector nozzle, turbocharger and ECM tuning upgrades can boost the 1.9L beyond 150hp with relative ease, and 200hp or more has been pulled off on countless occasions while retaining the factory rotating assembly. With more extensive work (rods, better crank, cam, valve relieved pistons, larger injection pump), 300 hp is achievable, but for one of the most extreme examples of what a 1.9L TDI can do, look no further than the 350 to 400hp version pictured above. Pulling Crew Ostfriesland of northern Germany built its TDI to power a mini rod tractor puller and it’s an absolute beast. A big single turbo, a P-pump (ironically enough, it might even be a Bosch P7100 from a 4BT application) and custom injectors have made it a contender in one of tractor pulling’s neatest classes.

For a little Cummins on Cummins crime, check out this comparison between the 4BT and the R2.8 crate engine.