Performance Roadblocks of the Ford 6.4L Power Stroke
Having already spotlighted the performance barriers present for the 7.3L and 6.0L Power Stroke mills found in ’94.5-’07 Ford trucks, we’re moving on to the roadblocks that face the 6.4L. And to be honest, there aren’t a lot of them. It’s what happens after you’ve added power or racked up a ton of miles with said power that most 6.4L owners encounter problems—namely in the form of blown head gaskets and cracked pistons. As far as horsepower thresholds are concerned, the factory head bolts tend to stretch around the 600rwhp mark, and sometimes sooner when prolonged abuse is present. The cracked piston phenomenon is a bit more tricky, with some living in high horsepower applications for years while others bite the dust at stock power levels.
The good news is that the 6.4L Power Stroke’s factory common-rail injection system can support as much as 700rwhp with the right turbo upgrades (remember, the 6.4L came with compound turbos from the factory and simple compressor wheel upgrades offer huge gains). Beyond that, the stock cast-iron cylinder heads flow well enough to allow 900rwhp to be made, although stiffer valve springs and stronger pushrods are highly recommended at that point. Also capable of handling 900rwhp (if not more) are the factory connecting rods, which were the stoutest units ever offered in any Power Stroke engine to date. Even though the 6.4L has earned a reputation for its catastrophic failures and extremely expensive fixes (both of which are true), it remains one of the most potent engines in the diesel industry. Mod for mod, it’s hard for any engine on the market to match what the 6.4L is capable of.
Roadblock #1: Factory Head Bolts
Like the 6.0L Power Stroke, the 6.4L utilizes just four head bolts per cylinder. However, in an effort to improve head gasket reliability the outer diameter of the 6.4L’s fasteners were increased to 16mm from 14mm. While this allows the 6.4L’s head gaskets to survive substantially higher cylinder pressure (i.e. torque), any time one of these engines has been tuned for a substantial period of time the head bolts can (and often do) stretch. Still, with sensible driving, the stock head bolts in a tuned 6.4L-powered Super Duty can keep the head gaskets alive for 80,000 to 100,000 miles—and 580rwhp accompanied by 1,100 lb-ft of torque can be pretty darn fun.
Pressurized Cooling System
In a textbook example of what happens after nearly 100,000 miles of being tuned, this 6.4L Power Stroke lost a head gasket. In fact, both head gaskets were gone when it idled into the shop. The driver side gasket let go between cylinders 6 and 8, while the passenger side gave out between number 1 and number 3. Living at an approximate power level of 550rwhp for the duration of those miles makes it a fairly good run on stock head bolts. To be sure, this type of head bolt failure is common on higher-mile Duramax-powered GM’s that’ve been laced with 500rwhp programming.
Roadblock #2: Factory Pistons
To reiterate, there is no concrete rhyme or reason as to when or why the factory pistons in a 6.4L Power Stroke crack. It would be easy to blame the failure on higher horsepower and the added stress that comes with it, but that wouldn’t explain why so many pistons crack in stock or tune-only engines. Of all the variables the 6.4L is exposed to, age seems to be the most common denominator. The cumulation of tens of thousands of heat cycles, exposure to excessive heat (EGT) and elevated cylinder pressures all play a part in the piston’s eventual demise—and when it happens there is no such thing as a simple, quick or cheap fix.
When They Crack
Though we’ve already established that cracked pistons aren’t specifically a performance roadblock, the fact that it can happen to any 6.4L makes it worth talking about. So what happens? The crack begins in the fuel bowl, where there is a sharp lip and very little material present underneath. Once the crack starts, it spreads along the center line of the wrist pin. After experiencing a cracked piston, many 6.4L owners switch to the Maxx Force 7 pistons during the rebuild. Maxx Force 7 pistons were used in Navistar’s single turbo’d version of the 6.4L (on the commercial side), and make use of a more round fuel bowl lip (no sharp edge) that’s better at coping with heat.
Roadblock #3: Factory Turbochargers
Thanks to the vast performance potential of the Siemens common-rail injection system on the 6.4L, the factory compound turbochargers are out of steam long before the piezo injectors or high-pressure fuel pump have given their all. Though we’ve seen the OEM BorgWarner sequential turbo arrangement support 700rwhp on nitrous, it’s not common to get much more than 600rwhp through them on fuel alone. And when attempting to push the factory chargers beyond 600rwhp, the drive pressure to boost ratio gets out of hand, with excess drive pressure being linked back to cracked pistons in many cases.
Upgraded Turbos = 700rwhp Potential
To realize the full potential of the 6.4L’s common-rail system, a slew of turbo upgrades exist. The most common consists of dropping a larger compressor wheel into each turbo. By bumping the VGT turbo from 52mm to 59-60mm and the low-pressure charger to 71-72mm vs. 65mm, an incredible amount of additional air can be matched with the engine’s immense fueling capability. In fact, with the right turbo combination in the mix as much as 700rwhp can be achieved with the stock injectors and high-pressure fuel pump.
Past 700rwhp, the Price of Horsepower Goes Up Exponentially
Moving 20-percent more fuel than a comparable Bosch CP3, the Siemens VDO K16 high pressure fuel pump is an impressive piece as far as all-out performance is concerned. Used in conjunction with quick-firing piezo injectors, it’s no wonder a 6.4L-powered Super Duty can produce 700rwhp with adequate airflow. However, for the 6.4L owner that wants to venture beyond the capability an upgraded turbo/stock fuel arrangement provides, a financial roadblock often sets in. Without question, things get expensive quickly once you move to larger injectors in a 6.4L. Dual injection pumps become necessary and a higher-flowing low-pressure fuel supply system is mandatory—not to mention you’ll be subjecting the factory pistons to even more abuse. Remember, until they’re addressed, the stock pistons are a perpetual wildcard.
The Heads Can Support 900rwhp
If you were wondering if the horsepower wall for the cylinder heads was around 700rwhp, you’d be wrong. The 6.4L’s factory 4-valve heads can support as much as 900rwhp. Of course, at this level stiffer valve springs and stronger pushrods are prerequisites, as are head studs and O-ringed heads if you want the head gaskets to live. How long they’ll support 900rwhp isn’t set in stone, as like the 6.0L Power Stroke the cast-iron heads aboard the 6.4L are prone to cracking when exposed to excessive heat.
The Rods Are Good for 900rwhp, Too
With good PCM tuning, the 6.4L’s factory connecting rods have also proven capable of handling 900rwhp. The beam is exceptionally beefy when compared to the units used in any other Power Stroke engine manufactured and they’ve even been pushed into 1,100rwhp territory in the past. On the big end, each rod clamps to a crankshaft (by way of 11/32-inch rod bolts) that’s secured via bed plate.
If The Wallet Allows, a 6.4L Is Hard to Beat
While expensive, a well-built 6.4L Power Stroke can turn out impressive numbers. To reiterate, the first roadblock you’ll encounter will be the factory head bolts around the 600rwhp mark. Shortly after that, the factory turbochargers will run out of air, if they haven’t already. Somewhere between 650 to 700rwhp, the factory injection system is maxed out and at 900rwhp you will want to start considering O-ringing the heads and upgrading connecting rods. Throughout all of the above, one or all of the factory pistons may check out on you. The 6.4L shown above has been fitted with RCD Performance billet-steel rods and fly-cut MaxxForce 7 pistons, Elite Diesel’s Stage 1X cam, Stage 2 valve springs and 3/8-inch chromoly pushrods, H-11 head studs, dual K16 pumps, 150-percent over injectors, an Elite 63mm over 88mm compound setup and made 1,100rwhp on fuel.
Curious what other failures the 6.4L Power Stroke is notorious for? Find out everything you need to know right here.