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The Ultimate Heavy Haul Engine

The holy grail of engines is the C15 Acert MXS lower end with a single turbo 6NZ head. When the goal is to assemble the most reliable diesel engine possible — of course you don’t prioritize putting the biggest power numbers to the ground. Ultimately this reliability results in the most profit.

At Select Reman Exchange (SRX), we believe an appropriately powerful yet reliable engine can be in service for a million miles. When it does have issues, it is easy to work on because it doesn’t have EGR, DPF, or twin turbos.

Our build details

Our vast experience as diesel engine remanufacturers at SRX informs our opinions and recommendations for the ultimate heavy haul engine configuration. Using the MXS lower end you have a stronger, better quality casted cylinder block.

The crankshafts are the same between the engines, but You have a stronger connecting rod with 4-rod bolts instead of 2 connecting rod bolts like the 6NZ has. When building the MXS lower you can use the industrial piston pack out of a JRE C15 engine to achieve the same 16:1 compression ratio that the 6NZ has. This lower compression is better for bigger power and reliability.

One-piece MXS steel piston

They hold up to a lot more power and are more reliable than that two-piece 6NZ piston with the steel crown and aluminum piston skirt. The MXS lower end also has a higher-flowing cooling system. The water pump is larger and flows more keeping the engine cooler under higher load scenarios. 

Use new casted cylinder heads

Don’t use a remanufactured caterpillar head. They are prone to cracking. We offer an aftermarket casted cylinder head that has a better, thicker casting between the valves that eliminates the cracking issue that many people have run into. Our head will work for both the MXS and 6nz engines. 

For injectors, we like to use the 10R8502 injectors. We flow match the injectors giving you optimal performance and a smooth Idle. We typically have these injectors on the shelf. Call us and we can save you a lot over Caterpillar pricing. 


We do not recommend using an aftermarket program. We have seen countless issues on head gaskets that are directly caused by programs created by people that don’t really know what they are doing. We like to use the 6NZ 550 HP program. This has proven to be a great, reliable and powerful program. 

The 550 HP program does seem to build higher than desired EGTs. The best way to get rid of that heat is by installing a bigger turbocharger on the truck. We offer a new turbocharger that is perfect. It is reliable and builds awesome power but is not going to break the bank like a lot of the other unnecessarily expensive turbochargers. Another thing you can add to keep the EGTs down is a ceramic-coated aftermarket exhaust. We do not have one specific manifold to recommend for performance-related reasons, But we do thoroughly appreciate PAI’s Ceramic coated manifold as it seems to be a very quality product and does not break the bank. 

An added touch

Use the bigger 10R7155 camshaft for more reliability. Again, don’t go to Caterpillar to buy this camshaft. We at SRX can supply it to you at a fraction of the cost. 

In summary – a reliable powerhouse

With the bigger turbocharger, flow-matched injectors and the bigger camshaft we have seen many numbers in the 630 HP range.

If you are wanting to build this engine on your own we are here every step of the way to help! We can sell you all of the same parts we use in our own engines and save you thousands of dollars over going to Caterpillar. 

Always take into consideration the emissions laws and the states you are wanting to haul in. We always use an existing 6NZ engine to build from and would recommend you do the same. We have quite a few 6NZ engines left and would love to assist you in your core engine needs as well. 

Are you looking to buy a Caterpillar C15 like this?

We often have completely remanufactured and used Caterpillar C15 diesel engines in stock. An extensive and competitively-priced inventory of parts is on our shelves! Give our Sales or Parts Teams a call or visit our Contact Us page.