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The Ultimate Diesel Commuter: More Power, Cleaner Emissions, Same-Old 49 MPG

The little Jetta that could returns this month with a batch of new parts onboard. After driving our parts-runner around—tuner-only—for several months, we decided it was time to put some more pep in the ’03 TDI’s step. Knowing that our next move would entail a set of larger injectors, our list of demands was short yet imperative. First, despite the added fueling the larger injectors would provide, we would still need the car to run as clean as a whistle. Second, the 49 mpg we were regularly seeing couldn’t be compromised. To see if we could have our cake and eat it, too, we contacted the folks at Kerma TDI.

Although the factory clutch might’ve lived with more power on tap, it might’ve failed, too. Not opting to chance it, we reached out to South Bend Clutch for a street-friendly replacement with a higher torque rating. Then to dial everything in, we re-tuned the car via our Q-Loader programmer and scheduled another appointment on the local chassis dyno. Thanks to the latest round of mods, the car runs stronger, smoke-free, and still knocks down 49 mpg. It’s proof that a bump in power can coincide with better fuel economy—and that that power can be clean.

Kerma TDI Stage 1 Injectors


With the factory injectors effectively maxed out as far as horsepower capability (by way of the Q-Loader programmer), this was the key to taking the next step: higher flowing injectors. After discussing several options with Kerma TDI, we settled on its Stage 1 injectors equipped with Bosio DLC 520 nozzles. They were flow-tested and balanced as a complete set (i.e. pop-off pressures all set the same) before being shipped to us. If you’ve got the right turbo and tuning to match them, the Stage 5 variant of these injectors can support as much as 160hp (for reference, the factory ALH 1.9L in the ’99-‘03 Jetta’s came with 90hp).

Bosio DLC 520 Nozzles


The Bosio DLC 520 nozzles are the same units used in the 110hp, European versions of the ALH engine, albeit with a Diamond Like Carbon (DLC) coating to increase longevity as much as 50 percent. Their optimized spray pattern improves injection efficiency (less smoke), provides cleaner start ups and can improve fuel economy. And thanks to providing a 61 percent improvement in flow over the factory nozzles, the Bosio DLC 520s can support as much as a 20hp and 40 lb-ft gain at the tire.

Injector Number 3


On ALH code Volkswagen TDI engines, the ECU keeps an eye on injection timing through the number three injector. This injector possesses a needle lift sensor and a square-shaped or D-shaped plug (depending on model) that connects to the injection pump. As you might’ve guessed, the number three injector carries a premium (yet necessary) price tag.

South Bend’s Stage 2 Daily Clutch


Thanks to its 325 lb-ft torque rating (which we’re told is an underrated figure), South Bend’s Stage 2 Daily clutch is a great addition to any moderately modified and daily driven Jetta. Of a VR6 clutch design, it makes use of South Bend Clutch’s organic friction material, a heavy-duty pressure plate and comes with a single mass flywheel. South Bend supplies new flywheel bolts and a clutch alignment tool with every Stage 2 Daily clutch kit it sells.

Tipping the Engine


In order to install the new clutch, the 02J five-speed transmission had to be pulled. To simplify the process, an engine hoist was used. The ability to tip the driver side of the engine downward makes a huge difference when dropping (as well as installing) the transmission out of an MK4 Jetta.

Axleshaft Once-Over


While it’s obvious that the transmission has to be pulled to install a new clutch, on the ’99-’03 Jettas it’s also a requirement to pull both front axleshafts. Once removed, it pays to take a look at the overall condition of the axleshafts. The CV boots are known to crack and tear, hence why CV joint and boot kits are available specifically for this reason.

Single Mass Flywheel


Despite its use of a single mass flywheel (vs. the factory dual mass unit), the South Bend Stage 2 Daily clutch’s operation is completely silent. In the days of old, you could deduce whether or not a diesel Jetta had been modified based on the audible rattle the aftermarket clutch emitted. That’s no longer the case. South Bend’s 22.5-pound cast-iron flywheel can only be installed one way, and during our install each 10mm bolt was hit with Loctite prior to being torqued to 65 lb-ft.

Fresh Motor Mounts


With the transmission removed for the clutch install, it was an opportune time to replace the worn factory motor mounts with 034 Motorsports' Street Density units. The Street Density line of motor mounts are approximately 25 percent stiffer than stock, which helps improve drivetrain dampening while maintaining a smooth, quiet ride.

Revised ECU Tuning


In order to get the most out of the Bosio 520 DLC nozzles, a revised tuning calibration from Kerma TDI had to be uploaded to the car’s ECU. While Kerma TDI keeps a tight seal on its tuning techniques, we can assume the new nozzles’ larger holes allow a shorter pulse width to be commanded (pulse width is also known as duration or injector on-time). As a general rule of thumb, anytime you add higher flowing injectors to a diesel less pulse width is required to make the same amount of power. In addition, in-cylinder heat (exhaust gas temperature or EGT) is almost always reduced.

Validating Our Gains


When all was said and done on the chassis dyno, the Jetta picked up another 15hp over its previous best (105hp). With 120hp and some 250 lb-ft of torque now making it to the front wheels (vs. a measly 72hp from the factory), the car is extremely fun to drive. The icing on the cake is that the little sedan still gets 49 mpg and runs just as clean as it did stock.

Mileage Log



Thinking about a TDI VW of your own? Here's what you need to know!

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