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C15 vs. C18: These Two CATs Are More Similar Than You Think

Written By, Mike McGlothin


When we think of big, brawny, industrial-strength, over-the-road diesel engines, we often picture the power plants that join the million-mile club in effortless fashion or that can power up a hill without breaking a sweat. In Class 8 applications, Cummins’ 855 Big Cam, Detroit Diesel’s Series 60, and Caterpillar’s 3406 come to mind as the hero engines of the past. In the modern age, the Cummins ISX, Detroit DD15, and CAT C15 are generally regarded as some of the most competent workhorses in this category. But the latter engine has a unique blood relative called the C18—an engine capable of making 1,000 hp in factory form.

Remember the Cummins KTA, the 1,150 ci (19-liter) behemoth available alongside the 855 Big Cam for OTR duty in the late 1970s and early 1980s? Although CAT’s C18 was never offered in on-highway applications, it can easily be integrated into any Freightliner, Kenworth, Western Star, or Peterbilt the C15 can. After all, these two hunks of iron share the exact same dimensions and nearly identical weight. Then come their interchangeable hard parts, the similarities in their fuel systems and, in Acert form, their carbon copy aftertreatment emissions equipment. We discuss all the components that harmonize between the two (and even a few that differ) below.


The C15

Released in 1999, CAT’s C15 effectively replaced the 3406E. And while it is perhaps bestIndustrial Diesel Cat C15 Acert Application known for its use in Class 8 applications, the C15 was also employed in dozens of other applications. Everything from gen-sets, combine harvesters, earth-movers, and loaders, to grinders, trenchers, chippers, drill rigs, and even irrigation equipment saw C15 power. Its bore and stroke check in at 5.40-inches and 6.75-inches, respectively, which brings displacement to 928 ci (or 15.2L). Just like the C18, the C15 utilizes multiple O-ring, wet liners and a common, 1-5-3-6-2-4 firing order.


The C18

The C18 primarily saw action in marine vessels, where its horsepower could really beC18 Acert Cat Diesel Engine turned up thanks to the abundance of cooling available. However, plenty of these Cats were situated in gen-sets for power generation, off-highway haul trucks, and others even made their way into firetrucks. The C18 shares the same, cast-iron block with the C15, but it features a larger, 5.71-inch bore and, thanks to its longer throw crankshaft, a torque-friendly, 7.20-inch stroke. Total displacement checks in at 1,106 ci (or 18.1L). Depending on application, compression on the C18 typically checks in between 16.0:1 and 16.6:1, like the C15.


Identical Blocks, Main Bearings, Rods & Geartrain

Front Geartrain Caterpillar C15 Diesel EngineAs mentioned, both the C15 and C18 share a common, cast-iron block. The liners and stroke of the crankshaft are the difference makers as far as displacement is concerned. However, and although it doesn’t make them directly interchangeable, the engines’ crankshafts employ the same main bearings. Both engines use the same, forged-steel connecting rods as well, with each unit’s length from center to center (rod bearing bore to piston pin bore) measuring 10.66-inches. It goes without saying that both of these heavy-duty engines are also completely gear-driven, and they share identical geartrains.



Further Similarities

Exact matches and similarities continue as you dig even deeper. For instance, both enginesCAT C15 Diesel Engine Cylinder Head Class 8 Truck use the same head bolts. Yet, for reasons other than these fasteners (and depending on the specific model engine you’re looking at), the cylinder heads are slightly different. Faint differences in fuel injector style and location, among other subtleties, make some heads non-interchangeable. As we alluded to earlier, there are distinct differences in cylinder packs between the two engines, but beneath them you’ll find duplicate, dual-nozzle piston cooling jets (for optimum cooling). And although their pistons aren’t an exact match due to wrist pin location, both the C15 Acert and C18 Acert make use of one-piece, steel pistons.


Dimensional Twins, Fighting In The Same Weight Class

This is often the hardest similarity to grasp, perhaps because the natural inclination is toIndustrial Caterpillar C15 And C18 Engines assume that a larger engine designation always equates to a physically bigger engine, but it’s true—both the C15 and C18 are dimensionally identical. Each one measures 55.7-inches in length, 38.3-inches in width, and 49.5-inches in height. Beyond that, their overall weight is nearly identically, too, with the C15 tipping the scales at 3,673 pounds vs. a C18 weighing 3,688 pounds (gen-set comparison). In the world of Class 8 trucks, these specs help explain why so many gliders have been built with C18 power under the hood.


MEUI Injection

MEUI Injection Cat C15 Diesel Fuel InjectorAll of the parts and pieces might not be spitting images of each other, but most of the components in the C15 and C18’s injection systems are very similar. Both power plants utilize MEUI injection (mechanically actuated, electronically controlled, unit injector), a system where dedicated lobes on the camshaft actuate each injector mechanically while the ECM commands the injector solenoids to fire off. In this sense, each injector is essentially its own fuel pump. Caterpillar’s use of MEUI also means that each engine’s cylinder head features an integrated, internal fuel gallery, which stores diesel fuel for the injectors to use.


Indistinguishable Exhaust Aftertreatment Systems

In utilizing similar fuel systems and serving duty in the same era of ever-tighteningCaterpillar C15 Acert Diesel Emissions System emissions standards, it only makes sense why the C15 and C18 were equipped with identical emissions equipment. In power generation configurations, both Acert engines’ exhaust aftertreatment systems are virtually identical, with diesel particulate filter (DPF), diesel oxidation catalyst (DOC), and selective catalytic reduction (SCR) components all being shared. And not only that, both engines boasted the same, 93.7-liter diesel exhaust fluid (DEF) capacity, the same clean emission module (CEM), and called for the same, 5,000-hour DPF service interval.


What Is Different: HP And Torque Output

Factory power ratings vary for either engine, but CAT definitely turned up the heat on itsC18 Cat Marine Diesel Engine Dyno Graph marine versions of the C18. For various sea-going vessels, C18’s were rated and certified as high as 1,130 hp (and even more in some cases), with peak torque topping 3,000 lb-ft in many applications. By comparison, the highest output factory C15 turned out 625 hp and 2,050 lb-ft. Why the huge gap? For one, due to the environment they operate in, marine engines are never short on cooling. But more importantly, CAT moved the horsepower and torque curves further out in the rpm range with the C18. Whereas a C15 typically achieves its maximum horsepower by 1,800 rpm and peak torque by 1,200 rpm, a marine C18 produces maximum horsepower at 2,100 to 2,300 rpm and peak torque between 1,400 and 1,600 rpm.


Different Turbo Arrangements

Used Caterpillar Compound Turbo EngineAspiration plays a major role in an engine’s maximum horsepower, torque curve, operability, and even its emissions. With the latter item dictating the way engines were developed when particulate matter and NOx emissions standards became notably more stringent, CAT turned to multi-turbo configurations on its Acert engines. The C15 Acert would receive a compound arrangement, where a high-pressure (and wastegated) turbo gets things moving at low engine speed and a low-pressure turbo takes over at higher engine speed. The C18 Acert was graced with parallel turbos (or twins), with each (identical) turbocharger serving three of the engine’s six cylinders.


What’s Possible Wit A C15/C18 Hybrid

At the present time, a C18 swap is one of the most cost-effective ways to have a 1,000 hp,Kenworth W900L Modified Caterpillar C15 Diesel Engine “king-of-the-road” type engine in an over-the-road truck, but that’s only the tip of the iceberg. For the ultimate example of high-horsepower and that showcases the interchangeability of C15 and C18 parts, we bring you this competition-built, Kenworth sled-puller owned by Ryan DeBroux. His yellow power plant sports a factory C15 block, a modified C18 crank with an aftermarket girdle, C18 cylinder packs with modified factory pistons, custom length, billet-steel rods, and a factory-based cylinder head that’s been customized to accept common-rail fuel injectors and handle more than 3,000 rpm. The “C17” (as its final displacement suggests it is) belts out more than 4,000 hp and the big KW often dominates in the dirt.

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