It’s Not Impossible!

Just because the electronically controlled Cummins mills burn cleaner and are easier to fine-tune doesn’t mean street able 12-valve applications have gone into extinction. In the age of high-pressure common-rail injection, four-digit horsepower continues to be made more and more frequently. On the dyno, on the street, and certainly at the drag strip, common-rail has overtaken the mechanical injection monsters that used to rule the roost. Rest assured, plenty of innovation has occurred within the P-pump 5.9L world over the past decade. Turbo, cylinder head, camshaft, and fuel injection technology has continued to advance, which is to say the amount of street able horsepower has increased also.

Once thought of as a lofty goal, various breakthroughs in the diesel world have made it possible to build a 1,000hp 12-valve for street use. Year-round daily driver? No. Street able? Yes. There is a difference. Much of it boils down to smoke control and driving manners—both of which can be kept in check by piecing together the right combination of parts. Spec’ing out the best pump, injectors, turbo(s), hard-parts, and supporting hardware for your needs is the key to pulling off the four-digit feat, and then being able to enjoy that power reliably once it’s all buttoned up. The following pages are dedicated to helping you build the ultimate street-worthy common-rail killer.

Machined 12 valve 5.9L Cummins engine block
Just about any competent engine shop with a reputation for building reliable diesel power plants can build a 5.9L Cummins capable of handling north of 1,000 hp. Many of these builds begin with a good used core block that’s been stripped down, hot-tanked, pressure-tested, and magnafluxed before receiving any amount of machining. Along with decking the block and boring the cylinders, a line-hone is ideal in ensuring all seven main bearing bores are perfectly round (so the crankshaft can spin as freely as possible). Many performance 12-valve builds also entail machining the block to accept cam bearings on all seven journals.
old school groove cutter for machined head
With any big horsepower goal, the trick is making things live at big boost. To keep the head secured to the block and all the immense combustion forces contained, both the block and head should be machined to accept fire-rings (an old-school groove cutter is shown here). Half the thickness of each steel fire-ring should protrude into the block, and the other half into the head. It’s good insurance to consider fire-ringing the block and head anytime 800-rwhp or more is on the table.
Keating Machine Freeze Plug for 5.9L Cummins
Excessive rpm comes with added heat and pressure in the cooling system. Because of this, the factory freeze plugs in the 5.9L’s block can blow out. Bolt-in freeze plugs—such as the three billet side freeze plugs, single rear unit, and front freeze plug (shown here from Keating Machine) eliminate that possibility. A coolant bypass system is also a must-have for stabilizing coolant pressure across all cylinders, as well as dropping coolant temp in the rear holes (especially number 6).
12 valve Cummins girdle
A girdle isn’t mandatory for a 1,000hp 12-valve build, but knowing that all the main caps are tied in with one another can provide tremendous peace of mind. Just know that if you do opt for this insurance item, it pays to know exactly which block you have. Early 5.9L blocks (’89-’97.5) left the factory utilizing 14mm diameter main bolts while later (“Encore”) blocks employed 12mm fasteners.
Industrial Injection Gorilla Girdle line, ARP head studs
Industrial Injection has offered its Gorilla Girdle line for ‘89 and later Cummins engines for years. The company’s all-inclusive kit shown here accommodates blocks that make use of 14mm main bolts (or that’ve been machined to accept them) but also comes with 14mm ARP XL main studs. Main studs are a must in any high-horsepower Cummins build, and ARP has been a leader in the diesel industry for two decades now.
Factory forged-steel Cummins 12-valve rods
Factory forged-steel 12-valve rods have been campaigned in many high-horsepower builds over the years. However, at a bare minimum, they should be shot-peened, polished, and weight-matched to give them the best chance of holding up to four-digit power and (potentially) triple-digit boost.
Wagler Competition Product’s Street Fighter rods
Best case scenario: go with a proven aftermarket connecting rod. Wagler Competition Product’s Street Fighter rods are a great budget option and are rated for 1,500 hp. They’re forged from 4340, shot-peened to prevent stress risers, come standard with ½-inch ARP rod bolts (vs. 7/16-inch), and retail for $1,850.
ARP’s 7/16-inch rod bolts
The tipping point in 12-valve performance is roughly the 800 hp mark. At this point, not only should fire-rings be employed but so should stronger rod bolts. This is precisely why ARP’s 7/16-inch rod bolts are such a common upgrade (PN 247-6303). Direct aftermarket replacements for stock rods, each of ARP’s 7/16-inch fasteners is 23-percent stronger than the OEM bolt it replaces—and the ½-inch, L19 alloy rod bolts that come with the previously mentioned Wagler Street Fighter rods are considerably stronger than that.
It goes without saying that a lot of factors play into piston selection, but one of the best out-of-the-box cast-aluminum options you’ll come across was actually offered in first-gen, non-intercooled 12-valve engines (’89-‘91). These pistons featured a deep top ring land, a shallow yet wide bowl, and higher compression than later pistons. The bonus with having a more shallow yet wider bowl than a later style piston is that you’re able to run more injection timing without spraying out of the bowl (which helps cut down on heat). Long story short, they’re a sound option for making good power.
Hamilton Cams 188/20 12 valve Dodge Cummins
Impressive headway has been made in camshaft design over the years, and today there are several high-horsepower cam options available for 12-valve owners. Colt Cams’ Stage 3 unit (known as the “Big Stick”) has been a mainstay for those looking for big power up top without sacrificing any mid-range. The same can be said for Colt’s Stage 4 cam, which sports a larger exhaust profile. Across town, Hamilton Cams’ 188/220 is a big hit with 12-valve owners and is a cam that performs its best work from 1,700 to 4,500 rpm.
Hamilton Cams 165 lb. valve spring, valve spring retainers, valve spring lock
Higher rpm, boost, and drive pressure means the potential for valve float and valve creep come into the fold. To combat it, valve springs with a higher seat pressure are required. Hamilton’s 165-pound valve springs—named for their 165-lb seat pressure at the proper installed height—come complete with retainers and locks, and work extremely well in engines that see as much as 4,500 rpm.
12 valve Cummins push rods
To remove flex at elevated rpm and take advantage of your cam’s added lift, a much more rigid pushrod has to be run. A common upgrade for 12-valve engines is a 3/8-inch diameter pushrod with a thicker, 0.095-inch wall, and some builders even opt for exhaust pushrods that are stronger still, with a 7/16-inch od and 0.120-inch wall.
Stock 12 valve Cummins head
Going beyond an off-the-shelf performance head, improvements can be made at your local machine shop (again, make sure it’s a reputable business that has a proven track record with the 12-valve). This includes beveling, chamfering, further opening up the valve throats by hand, and unshrouding the valves. The process of unshrouding the valves (intake and exhaust) opens up airflow in the head substantially. This is especially true at low cam lift.
Cummins 12-valve head ported, milled, fire ringed
Running a billet cylinder head on a 12-valve Cummins is nowhere near necessary for 1,000 hp, but it does pay to open it open via porting, Along with milling off the intake shelf and running a higher flow manifold, a properly ported street head with stock size intake and exhaust valves will get you where you need to be—along with allowing for lower boost pressure to achieve your horsepower goals. As previously mentioned, cutting fire-rings into the head is a must at this power level.
ARP Head stud fasteners 12-valve Cummins PN 247-4205
There is a reason ARP fasteners are endorsed by championship truck and tractor pullers as well as Top Fuel, NASCAR, and NHRA teams. The company’s products are manufactured from the highest quality 8740 chromoly steel in the world, and its head studs are an essential part of any high-horsepower diesel build. For ultimate head-to-block clamping force, ARP’s Custom Age 625+ head studs are as good as it gets (12-valve Cummins PN 247-4205).
ARP’s Ultra-Torque fastener assembly lubricant
To ensure each head stud achieves the exact same clamp load, a generous amount of ARP’s Ultra-Torque fastener assembly lubricant should be applied to the threads, nuts, and washers. In addition, ARP’s head stud torque sequence and final torque specification should be followed to a T. Over-tightening a head stud is bad news for a couple of reasons: 1. it can compromise the integrity of the fastener, and 2. the fastener can’t (or at least shouldn’t) be reused. Adhering to the correct torque spec is paramount in a fire-ring application, where a hot re-torque process has to be performed.
re-torquing head studs, 5.9L Cummins engine
A hot re-torque scenario begins with allowing the engine to reach operating temp which, although time-consuming, is best done by letting the engine idle until properly warmed up. After the re-torque procedure has been performed, valve lash should be checked. This is because re-torquing the head studs crushes the head gasket further, effectively altering valve lash. For added insurance, it pays to re-check torque one last time before the engine ever sees full load or maximum rpm.
Scheid Diesel built 12mm p-pump
P-pump selection for a high-powered 12-valve intended for street use is a loaded proposition to say the least. Some say a worked over and benched 12mm P7100 is best for street driving while others swear a 13mm unit can be set up to behave under the same circumstances. The real trick is to buy a quality pump from a reputable builder. With that said, the 13mm P7100’s offered by the likes of Industrial Injection and Northeast Diesel have proven capable of being street able, all while providing a highly beneficial, quicker injection rate. Quicker injection rates mean less timing is required, which leads to quicker turbo spool, and when properly configured (and matched to the right injector) usually leads to less smoke. Another big name in the P-pump world is Scheid Diesel, which builds everything from potent 12mm pumps to 16mm, P8600-based competition-killers.
12 valve P-7100 pump billet front cover
It’s advantageous in any extreme 12-valve build to have both an adjustable P7100 pump gear and access to it. A billet front cover such as the one pictured here from Keating Machine provides the entry point you need to advance (or retard) your pump timing. Most adjustable pump gears allow for up to 40 degrees of adjustment and the unit offered by Pure Diesel Power comes with the key needed to key the hub to the pump’s shaft.
12 valve injector
The giant can of worms that is 12-valve injector selection can overwhelm the vast majority of us. The right manufacturer, nozzle size, nozzle style (i.e. SAC vs. VCO), the amount of holes in the nozzle (5, 6 or 7), pop-off pressures, and injector line recommendation can all vary from shop to shop, and enthusiast to enthusiast. That said, injector choice boils down to which P7100 you’re running so the two systems can be perfectly matched. In the past, we’ve seen 5×20’s and 6×13’s make four-digit power in conjunction with a 13mm pump, and we’ve also seen 5×18’s combined with a 13mm P7100 (set to 550cc’s) run noticeably clean on the street.
260-gph Titanium lift pump system from FASS
As for low-pressure fuel supply, the mechanical vs. electric debate is still alive today. However, for our money we would go with an electric, 260-gph Titanium lift pump system from FASS. We would also run dual systems (in case one ever decided to fail), keep supply pressure set where our P7100 builder told us to (likely somewhere between 45-55-psi), and enjoy the finer filtering and air separation FASS’s systems are known for.
Steed Speed’s exhaust manifolds, T4 flange
Picking an exhaust manifold is part of the turbo selection process, and at this kind of power level a unit with a T4 flange is the smallest you should run. Steed Speed’s manifolds are cut from solid 1018 mild steel and are available with a straight turbo flange, an upward-facing flange for compound turbo arrangements, or with an angled flange for running a big single charger. Other selling points include its Techline coating, which is rated for 1,700 degrees F continuous use and that can also handle brief periods of 2,000 degree EGT. Steed Speed manifolds can also be built to include external wastegate provisions. Another highly prevalent exhaust manifold name you’ll come across in the diesel industry is Stainless Diesel.
Borg Werner turbo 5.9L Cummins 12 valve engine, Bluegrass Diesel valve cover
If you thought injector and P-pump choices were overflowing in the aftermarket, wait until you””” get an idea as to just how many turbocharger combinations there are. For drivability purposes, we’ll leave the big single debate for another time and focus instead on compound arrangements. Opinions vary on running the popular S300 frame BorgWarner on the manifold for a high-pressure unit or going with two S400’s, but we have seen a properly wastegated S369 SX-E live and support more than 1,200-rwhp with a T6 flange S484 also in the mix (as well as a small shot of nitrous). We’ve also seen a 68mm Garrett matched with a 94mm atmosphere charger (also a Garrett) support similar numbers. We’ve also found that an S366/S483 combination has the potential to support four-digit horsepower.
BD Diesel’s drop-in replacement intercooler for ’94-’02 Dodge
Air-to-water intercooling is brutally effective at dropping intake temps and increasing horsepower, but it’s not very practical on the street. What is practical is an aftermarket air-to-air intercooler that demolishes a factory first-gen or second-gen unit in terms of performance. For our money, it would either be spent on BD Diesel’s time-tested drop-in replacement intercooler for ’94-’02 Dodge trucks, or we would make a Banks unit intended for a Power Stroke work with our application. Either option would cool exceptionally well while also standing up to big boost.
8.3L Cummins oil pump
As horsepower rises and you’ve got an additional turbo to feed, it’s always a good idea to upgrade the 12-valve’s factory oil pump. Instead of trying to reinvent the wheel or invest in a dry sump system, many switch to the 8.3L Cummins oil pump. It can be modified to work on the 5.9L and moves 42-percent more oil volume. The one stipulation is that you have to install an oversize pickup tube when making the switch to the 8.3L pump, as the factory suction tube’s flow is questionable above 3,500 rpm.
Under extreme load and horsepower, the 12-valve Cummins’ cam gear is known to walk off the camshaft. While the cam gear can be tack-welded to the camshaft, a permanent solution is to install a cam gear retainer like the one shown here (made by Hamilton Cams and available through Pure Diesel Power for just $40). Note that your cam will need to be pre-drilled to accept the retainer, which is the case on Hamilton cams.


Colt Cams

Diesel Performance Parts, Inc. (FASS)

Industrial Injection

Keating Machine

Northeast Diesel Service

Pure Diesel Power

Scheid Diesel Service

Wagler Competition Products

Xtreme Diesel Performance


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