Performance Roadblocks of the ’89-‘93 5.9L Cummins
Thanks to its freebie fueling mods, million-mile dependability and copious amounts of low-end torque, it’s only natural that many diesel enthusiasts worship the mechanical injection 5.9L Cummins. And despite the fact that the humble, 160hp rating of the original 6BT Cummins can be doubled with relative ease (and on a shoe-string budget), there are several obstacles blocking the average ’89-’93 Dodge owner’s path to more power. The first performance barrier is the Bosch VE injection pump, which with its timid fueling from the factory leaves much to be desired in terms of performance. Luckily, with simple hand tools and $0 invested, it can be tweaked enough to add more than 70 hp to the engine’s bottom line.
The roadblocks don’t stop there, however. Not only is the VE pump stingy with its factory fueling abilities, but it’s also directly responsible for the 5.9L Cummins’ minuscule power window. Once that’s addressed in the form of a higher rpm governor spring, a new problem surfaces: the stock injectors can only fuel so much. For a few hundred bucks thrown at a set of entry-level performance injectors, you’re back in business and have another 50 hp at your disposal. But then you’ve got a factory turbocharger that’s over fueled, on the verge of overspeeding and so far out of its efficiency range that it’s moving nothing but hot air. Time for a compressor wheel upgrade…
From stock power to roughly 350rwhp, these are the performance obstacles standing in your ’89-’93 Cummins’ way.
Roadblock #1: Conservatively Set Injection Pump
With only 160 hp on tap when the 5.9L Cummins left the factory, the cam-driven, rotary style Bosch VE injection pump was set very conservatively. But despite flowing just 105cc’s of fuel in the 6BT Cummins application, this 12mm single plunger injection pump has a considerable amount of untapped fueling potential in OEM form. With a few simple hand tools, upward of 70 horsepower and 200 lb-ft of torque can be unlocked.
Not much different than the fuel screw adjustment many 6.9L and 7.3L IDI Ford owners perform on their Stanadyne DB2 pumps, the VE’s maximum fuel screw can be turned clockwise to introduce considerably more fuel into the equation. The job calls for locating and then removing the factory tamper-resistant cover on the firewall side of the VE. Once the cover is gone, you can remove the fuel screw’s locking collar and turn the screw (arrow). Gains as high as 70 hp can be realized with this mod, but just be forewarned that if you turn the screw too far the engine may try to run away on you. It’s best to start with incremental adjustments and progress from there (hint: three or four turns is plenty).
Aftermarket or Modified Fuel Pin
Further unleashing the potential of the VE pump, an aftermarket or modified factory fuel pin can provide increased fueling that comes on in parallel with a rise in boost pressure. The Denny T Performance fuel pin (there is a Stage 1 and a Stage 2) has long been a staple add-on in the ’89-’93 Cummins world and is known for providing a 30hp bump in addition to 50 more lb-ft of twist. Those on tight budgets typically modify the stocker. However, it’s best to proceed with caution when introducing the factory fuel pin to a grinder, as once key material is removed there is no getting it back.
Roadblock #2: Usable RPM
The VE pump is conservatively governed to roughly 2,500-rpm from the factory, and on many engines de-fueling begins around 2,400 rpm. This means all the added fuel from the aforementioned mods will only get you up against the governed speed quicker. Things are even worse if you’re working with an automatic transmission, where the factory torque converter’s stall speed of roughly 2,000 to 2,200 rpm shrinks your power window to a measly 300 (ish) rpm. Simply put, you can’t exploit the full horsepower gains of turning up the maximum fuel screw and adding a modified fuel pin with only 2,500 rpm worth of engine speed to work with.
3,200-RPM Governor Spring
For $20, this simple governor spring allows you to realize the full horsepower gains of turning up the maximum fuel screw and adding a modified fuel pin. It extends the rpm range out to 3,200 rpm. Making it possible for the VE to fuel to a higher rpm greases the wheels for more horsepower, better drivability and does so without making valve float an issue. Even if you don’t get carried away with cranking up the fuel screw, this is a sound upgrade for any early 5.9L Cummins.
Roadblock #3: Stock Fuel Injectors
Downwind of the VE injection pump you’ll find six pop-off style mechanical injectors, also from Bosch. Simple in design and function, they’re known to literally last forever. However, while the stock injectors can support as much as 300rwhp, eventually they have nothing left to give. To move the ball beyond 300rwhp, there is no other choice but to upgrade them.
For what seems like an eternity, first-gen Cummins owners have been more than happy running the Prince of Darkness (POD’s) injectors offered by Industrial Injection. Their price point, the return on investment they offer (hp per dollar) and fuel mileage uptick make them the quintessential entry-level injector. The Prince of Darkness injectors feature a 260 bar pop-off pressure (slightly higher than stock, and about as high as you want to go with a VE pump), are proven to add 50hp and retail for roughly $500 brand-new.
Roadblock #4: Factory Turbocharger
The factory turbocharger bolted to the ’89-’93 Cummins, the Holset H1C, was designed to see a peak boost pressure of 18 psi. After the fuel mods mentioned above have been added, the boost number can be well over 30 psi. On ’89-’91 engines, which were equipped with a 50mm compressor wheel H1C, the turbo isn’t necessarily in danger of exploding, but it is definitely out of its efficiency range and no longer able to keep exhaust gas temperature in check. The story is similar on ’91.5-’93 engines, although their version of the H1C featured a 54mm compressor, a higher flowing (21cm2) exhaust housing and a factory intercooler that helped keep EGT from getting out of hand.
Compressor Wheel Upgrade
With added fueling from a maxed out fuel screw, modified fuel pin and bigger injectors, the factory turbo is eventually going to be moving nothing more than hot air. The most affordable way to cool things off and boost horsepower is by treating the stock turbo to a larger compressor wheel. Gillett Diesel’s 60mm compressor upgrade and accompanying compressor housing for the H1C Holset takes just 30-minutes to install and frees up an additional 30 hp. The popular upgrade is known to cool EGT by 100 to 200 degrees and runs just $305.
Further Down the Road…
At this point you’re due for a larger set of injectors and a transmission upgrade, be it a better converter or clutch, if you want horsepower to keep climbing. If you pause here, your ’89-’93 Cummins-powered Dodge is capable of sending 350 hp and 700 lb-ft of torque to the wheels but is still rock-solid reliable. Another 100 hp or so can be coaxed out of the fuel-limited VE injection pump before it needs to be opened up and treated to a 14mm head, high lift cam plate and a few other things, but for many 6BT Cummins owners the drive to push further than 450rwhp often tempts them to perform a P-pump conversion. For the performance roadblocks that plague the P-pumped 5.9L Cummins (’94-’98 Dodge Rams), keep an eye out for the next installment in this series.
Interested in learning more about the First-Gen Cummins? You can discover the original 12-valve 6BT’s roots right here.