The Engine Cummins Never Built: A Look at the 6.4L Cummins

Jun 22, 2023

What initially started as a 5.9L engine with only 160 horsepower grew to a 6.7L in 2007. Motorheads and enthusiasts still utilize this engine today and continue to explore the limits of this powerplant. It is well known that this engine is capable of big horsepower gains with stock components while still maintaining its reliability.

When reaching the outer edge of its limitations, however, the first questionable component was the connecting rods. Early versions were forged steel while the later ones were powdered metal. The forged-steel rods were strong enough to bend without breaking, however, the powdered-metal rods had very little flexibility and could potentially snap, causing catastrophic damage. The answer to this problem was an aftermarket version of forged steel or a full billet connecting rod. There are many manufacturers that have started offering performance connecting rods for these engines, so there are several to choose from.

One component that does not need a lot of attention in this engine is the crankshaft. In stock form, it has been known to hold up with over 3,000 horsepower. Usually, a good balance job and thorough inspection is all that is needed.

After solving the connecting rod hurdle, the power could be increased enough to cause increased cylinder pressure, potentially splitting the block and ruining the entire platform. The newer 6.7L engine with a larger Siamese bore soon became the platform to use for what is now known as a 6.4L – the engine Cummins never built. By sleeving down a 6.7L block to a 4.125" bore, cylinder wall thickness is increased, resulting in greater durability to the block. This is achieved by boring out the 6.7L to accept a sleeve with a 4.125" finish bore, gaining almost .140" wall thickness on the cylinder.

Each sleeve has a flange that is installed into a counterbore and also contains a groove for the fire ring. The sleeves are typically custom-made out of ductile iron that is centrifugally cast, giving the sleeve itself more inherent strength than the block. By adding a sleeve that is stronger than the block material and increasing the cylinder wall thickness, the results begin to get bulletproof. Using a 4.125" bore and a 4.880" stroke crankshaft with a standard length rod, results in 6.4L and a very durable platform to make way for greatly increased horsepower.

The limits of this 6.4L sleeved engine is in the neighborhood of 2,000+ horsepower. For applications of even higher horsepower, a deck plate could be utilized along with longer connecting rods and longer sleeves, making it the ultimate 6.4L build. Also, by having the longer connecting rod, this reduces the rod angle that is arguably an advantage as well. At that point, it's how much fuel and air you can throw at it for whatever results you are looking for.

The deck-plated version usually comes into play with extreme cylinder pressure made by multiple turbos and nitrous. The other reason for needing a deck plate would be in a pulling truck application where the engine is pulled low into its torque range, causing extreme torque rise. The pros of this build are the reliability and strength that are added to the block, along with the ability to run a wet engine, so it can be driven on the street. Also, most 6.7L blocks, even with cylinder damage, are candidates for this build because of the addition of sleeves that would remove the damage.

The cons are the cost of sleeving the block and the addition of more power, which is typically the main intention of building a 6.4L engine. A girdle is added with bigger main studs to keep the main bearing caps from fretting and adds strength to the bottom end. Larger head studs are also added to increase the clamping force on the cylinder head. Cam bearings are installed in all positions for using a billet cam. The cost is usually between $4,000 and $5,000. A deck-plated version with the same features would cost around $6,500.

These enhancements usually go with any 6.4L build because of the goal you are trying to achieve. Another con is the cost and availability of the pistons. Being a 4.125" bore, they are usually billet or forged material, which are custom made to order at a cost between $2,200 to $2,600. These custom pistons do not have a very long life expectancy because of the absence of a steel ring land.

Another variation using the 6.7L block is sometimes sleeving it down to a 5.9L bore with the addition of a 5.9L crankshaft, resulting in a very durable 5.9L engine. The reason this is often done is because of the availability of off-the-shelf pistons that can be used. OEM cast pistons can be reliably used in this application and are effective as long as the tuning and air/fuel setup is done correctly.

Another version of the 6.4L is the 5.9L crank in a 6.7L block, which consists of connecting rods that are .080" longer, making up the difference for the compression height of the piston. The popularity of this version has faded due to the increased risk of the cylinder block splitting under extreme pressure and added horsepower as a result of not being sleeved.

Other versions of this big bore short stroke 6.4L engine consist of cutting .080" off the deck of the block. This was done in an effort to eliminate the purchase of the .080" longer connecting rod. These versions also lost popularity because of premature failures. The 6.4L platform that is very widely used now is a small bore, long stroke version. While there are many other combinations of the B-series Cummins motor in the competition and racing world, the 4.125" bore version is by far the most popular.

One disadvantage to the 4.125" bore is its inability to use a larger valve. Another version of a sleeved Cummins is a true 6.7L bore (4.213"), which relies on a deck plate to make it work. No real strength can be added by sleeving a 6.7L back to a standard bore. This is typically done just for repairing a cylinder that has a score, usually on a stock application. By removing and adding material back as a sleeve, very minimal strength is gained in this process. The advantage of a true 6.7L bore engine would be its ability to use a large intake and exhaust valve.

Cubic inches are important when it comes to driving turbochargers. While a 6.7L has the ability to drive a big charger better than a small cubic inch 5.9L, the 6.4L has enough cubic inches and strength to perform the task at hand. The availability of several billet block versions of these engines on the market today provide the ultimate strength. However, the drawbacks of these units are cost and the inability to have water jackets, therefore keeping them from being a popular choice for street or drag race applications.

The sleeved 6.4L still stands as a wise choice for building an exceptional powerplant for competition use. This is due to its affordability as well as having a cooling system and enough cubic inches to drive multiple chargers.

Being an engine builder and having the opportunity to build both pulling truck and drag race engines, the 6.4L is by far the most popular choice in both worlds due to its availability, durability and capability. With the only disadvantages being cost and piston choice, the benefits far outweigh the cons. While greatly improving the durability, it still has its limits, such as at very high cylinder pressure the block can come apart in the cam tunnel, causing catastrophic failure. This usually only happens in extreme situations such as a dyno event where a maximum limit is trying to be achieved. This is not very likely, but the full billet block has designs put into place to prevent this from happening.

At the time this article was written, if all my sources are correct, the highest horsepower ever achieved with a B-series Cummins engine was done with a cast block 6.4L. With increased technology in turbo systems as well as fuel systems, I feel that the final limits have not yet been discovered. These engines are capable of being a competitor in any event, whether it be truck pulling, drag racing, or cruising the strip. In my professional opinion, the 6.4L is the best B-series Cummins engine combination that Cummins never built. EB