BREAKING POINT: FORD
How much power can the Power Stroke handle?
ISure, some engines survive higher horsepower just fine. But then 100-percent stock engines let loose for no apparent reason once in a great while, too. So, for the purpose of this article, we’re basing our estimates off of what we’ve seen over the years throughout the diesel industry, and when (as a general rule of thumb) you can consider yourself near the breaking point.
f you’re looking to more than double the horsepower your Power Stroke brings to the table, we’ve got you covered. But first and foremost, we have to be clear: Any knowledgeable diesel enthusiast will tell you it’s not any one certain horsepower number that kills factory connecting rods, it’s the engine’s overall setup—and that is correct. Things like injection timing, EGT, oiling capability at high rpm, boost, drive pressure, and how much low-end torque you’re dealing with all dictate how long (or short) a stock rod engine will live at higher horsepower.
7.3L POWER STROKE DANGER ZONE (forged rods): 600-650 RWHP DANGER ZONE (powdered metal): 500 RWHP
When it comes to the weakest rods found in a Power Stroke engine, we have to start with the first mill offered: the 7.3L. But, it’s not nearly as bad as it used to be. Over the last decade, we’ve seen improvements in leaps and bounds in the 7.3L aftermarket. Thanks to hybrid injectors, more refined tuning, and better turbocharger technology, you no longer need a built engine to have a 500-plus-hp ’94.5-03 Ford.
However, there were two types of rods offered in the 7.3L, forged steel and powdered metal, which makes all the difference in the world when it comes to how far you should attempt to push things. If you own a ’94.5-99 7.3L Power Stroke, you’re in the clear, as all of those engines feature forged rods. However, powdered metal rods—known to be considerably weaker—began to make their way into production on late ’00 build date engines (starting with serial number 1425747). However, to run out the stock of forged rods, they were installed once more begin- ning with serial number 1440713 and going to 1498318. From there on out, powdered metal units got the nod, meaning most ’01-03 model year engines have them.
As for forged rod engines, 600-650 rwhp is the general consensus, as long as spot-on, well-tailored PCM calibrating is behind it. This means a progressive ramp-up in injection timing through the low rpm range so as not to “over-torque” the engine. Stock rods are at their most vulnerable at low engine speeds (2,500 rpm or less) when cylinder pressure and torque is high, so pouring on the fuel at higher rpm keeps the rods out of danger. The one trade-off is that the torque number won’t be huge (say 1,000 lb-ft made at 2,600 rpm on a 600rwhp setup, as opposed to 1,200 lb-ft being made at 2,000 rpm). The same tuning methodology applies with powdered metal rod engines, but enthusiasts are highly advised to draw the line at the 500rwhp mark.
“I GET ASKED ALL THE TIME HOW MY FORGED ROD 7.3L IS STILL TOGETHER, BUT PEOPLE FORGET THAT I DID (EVENTUALLY) BEND THE NUMBER 8 ROD IN THE ORIGINAL ENGINE, SO ANYTHING CAN HAPPEN.” —Matt Maier of Irate Diesel Performance
6.0L POWER STROKE DANGER ZONE: 800 RWHP
Going beyond the fact that a lack of fasteners in the block led to the inevitable head gasket failures plaguing the 6.0L engine (or the lack of head bolt diameter, or the torque-to-yield head bolts, etc.), it is one heck of a tough engine. Hard part-wise, it’s tough to beat, especially when you consider the rods found in the LB7 and LLY Duramax engines of the same era became a problem around the 600hp mark, if not sooner.
Like the late 7.3L rod, the 6.0L comes with powdered metal rods, but its rods have proven capable of handling much more power. And thanks to the utilization of a bedplate, the bottom end is extremely rigid, and the main caps don’t walk around under big horsepower and torque like they do on the 7.3L. In our opinion, the short block side of a 6.0L is a great starting point for adding horsepower.
The 6.0L Power Stroke’s small bore, four-valve design lends itself to the higher rpm range, which means peak torque is achieved higher as well. This keeps the rev-happy 365ci mill relatively clear of excessive cylinder pressure and big torque down low—provided appropriate custom tuning is being employed. Even a decade ago, before PCM tuning was anywhere near as refined as it is now, several 6.0L owners proved the 6.0L’s bottom end could withstand 700 rwhp. Today, a lot of serious enthusiasts conclude that the 800hp range gets you into unchartered waters with a 6.0L. 6.4L POWER STROKE DANGER ZONE: 1,000+ RWHP
In the early days of the 6.4L’s production run, no one knew how well this sequential turbocharged mill would respond to power adders, nor how durable it would be at elevated horsepower and torque levels. But once enthusiasts discovered how far you could push them, high horsepower builds sprang up everywhere. To this day, the bottom end on the 6.4L continues to impress us with its ability to handle four-digit horsepower. We’ve seen a slew of ‘08-10 Fords make north of 900 rwhp on the factory rotating assembly. We’ve also witnessed our fair share of trucks churn out well over 1,000 rwhp, run 10s in the quarter-mile, and live to fight another day.
“GOING BEYOND THE FACT THAT A LACK OF FASTENERS IN THE BLOCK LED TO THE INEVITABLE HEAD GASKET FAILURES PLAGUING THE 6.0L (OR THE LACK OF SUFFICIENT HEAD BOLT DIAMETER, TORQUE-TO-YIELD HEAD BOLTS, ETC.), IT IS ONE HECK OF A TOUGH ENGINE.”
The weakest link on the 6.4L’s bottom end seems to be the pistons, but it’s not as cut and dried as you might think. While some crack under extreme power conditions, most failures occur due to age, and it can even happen on stock or “tune-only” trucks. From what we’ve gathered after speaking to multiple high-end 6.4L shops across the country, it’s a toss-up on whether a stock piston will let go due to high horsepower or simply due to age or abuse.
Some gurus believe that piston failures happen more often on trucks that still utilize the factory compound turbo arrangement. The multiplication of quick spool-up (lots of low-end torque), aggressive tuning, high drive pressure, and the factory piston’s design can all contribute to its failure. To some extent, we agree with this theory based on what we’ve seen, and the fact that large single-turbo 6.4Ls tend to survive higher horsepower for longer periods of time.
“I HAD STOCK PISTONS IN MY 6.4L FOR 60,000 MILES WITH FIVE DIFFERENTSETUPS—FROMSTOCKTURBOS WITH A WASTEGATE AND SPRAY TO BIG COMPOUNDSWITHADDEDFUEL—AND NEVER HAD ANY PISTON ISSUES.” —Mike Corsilli of Maryland Performance
6.7L POWER STROKE DANGER ZONE: 700 RWHP
While Ford’s initial plans for the 6.7L Power Stroke entailed the engine getting forged connecting rods, powdered metal units ended up getting the nod (most likely to keep production costs down). It’s clear that lightening up the rotating assembly was one of Ford’s goals with this engine, as the rods are considerably smaller than what was used in the 6.4L (not to mention also being smaller than 6.0L rods).
The add-a-turbo craze, which is an affordable way to have compound turbos under the hood on Duramax and Cummins-powered trucks, was not lost on the 6.7L-powered Ford crowd. However, this is where rod failures began to surface on ’11-14 Super Duties. The single sequential, dual-inducer Garrett turbo proved to be a major restriction in making more power. Soon after S400 framed turbos were being installed in front of the stock charger in the valley, extreme drive pressure was present and a host of engines bent rods in or around the 600rwhp mark.
This problem was alleviated once a freer flowing valley charger was utilized and tuning became more and more refined. Now, with the right custom tuning, a second high-pressure fuel pump, and a turbo(s) that doesn’t produce a ton of excess drive pressure, the stock rods can easily handle 650-700 rwhp. UDBG
“We advise 6.7L customers going over 700 hp that they are in the danger zone, as we’ve seen two motors let go between 725-750 hp.” —Mike Dillehay of No Limit Diesel
A telltale sign you’re looking at: a powdered metal vs. forged steel 7.3L connecting rod lies in the rod bolts. Forged rods utilize studs and nuts to secure the cap, while powdered metal units use bolts.
Matt Maier has been at the forefront of pushing the limits of forged rod 7.3L engines over the last few years. His 1997 F-250 has cleared well over 600 rwhp on a chassis dyno (fuel only), and a whopping 926 rwhp on nitrous—not to mention running bottom 11s in the quarter mile. To be sure, he has had his share of failures, as the previous (and original) engine was pushed to the brink and bent the number 8 rod.
Here you get an idea what happens to 7.3L powdered metal rods at high horsepower. While they can be made to live in the 500rwhp range with adequate PCM tuning, this image actually speaks to how important custom tuning is on an engine equipped with bigger injectors. The owner was running a set of 238/100 hybrids, which is fine, but an off-the-shelf programmer was being used for tuning. The added timing and pulse width that’s ideal (and needed to make power) on trucks equipped with stock injectors is way overkill when the injector size has more than doubled.
We’ve all seen this image play out plenty of times at the local diesel shop: the cab being removed from the frame of an ’03-07 Super Duty. But don’t let the 6.0L engine’s top-end problems fool you; the rotating assembly is quite stout, with a bedplate, rods capable of handling up to 800 rwhp, and cast-aluminum pistons that are known to hang in there just as long.
While cracked pistons aren’t a commonality with 6.0L mills, nothing is off the table when added horsepower is in play. Believe it or not, aggressive tuning cracked the piston in this stock-injector Super Duty after just 15,000 miles of use.
This is the hard way to find out how much power the 6.0L’s bottom end will take. Shortly after clearing a little over 800-rwhp on the dyno, this engine ejected one of the factory rods at the track.
It took plenty of fuel, two stages of nitrous (a 0.136 jet and a .093 jet), and more than 1,100 rwhp to bend this 6.4L connecting rod. And upon teardown, the owner of the truck discovered the rod wasn’t even the problem. He told us that too much spray and not enough wastegate took out a valve, which was the actual culprit that led to all of this mayhem.
As is the case with any engine, unexpected failures can occur with a leadfooted driver, a big load in tow, and the programmer set on kill. This 6.4L came in with a dead cylinder due to the aforementioned scenario. Also, like most other piston failures, notice the crack formed along the centerline of the wrist pin.
Because the 6.4L’s rods have proven so capable of handling big horsepower, a lot of enthusiasts focus their attention elsewhere when building an engine. To rule out piston failure, a large sum of folks settle on Maxxforce 7 pistons from International that feature a more desirable style lip that’s better suited for high performance. The Maxxforce 7 is the sister engine to the 6.4L, so these pistons are easy to come by and can be had for a very affordable price tag—hence their popularity. The Maxxforce 7 piston shown here came from River City Diesel and features the company’s thermal barrier coating. The valve reliefs are an added option for those putting all-out competition style engines together.
The 6.4L rod and piston assembly is bigger than the 6.0L hardware in almost every imaginable way—which is one reason it lends itself to a bit more durability and being able to survive higher horsepower and torque levels. Notice the larger diameter wrist pin here, not to mention the beefier connecting rod used in the 6.4L.
Thanks to great R&D work among the premier tuning companies in the industry, the 6.7L’s stock rods are holding up well currently. However, if you’re thinking about bridging the 700hp range with your Super Duty, you may want to consider a set of Carrillo’s H-beam connecting rods for peace of mind. These rods were released very early on after the ’11 trucks debuted. We think Carrillo probably knew the lighter, smaller rods in the 6.7L might not be enough for power hungry owners.
Back when the 6.7L was equipped with the single sequential turbo (’11-14 model years), rod failures were definitely more prevalent than they are today. The 43mm dual compressor wheeled Garrett was great for low-end, towing grunt, but when left in the valley with a low-pressure charger feeding it, way too much torque was produced down low to keep the factory rods safe. Once out of its map, the tiny charger produced crazy high drive pressure as well—a recipe for disaster just waiting to happen.
By and large, the general consensus is a maximum of roughly 700 rwhp on the 6.7L Power Stroke. While it’s not as high as what you can potentially get out of the 6.4L it replaced, it’s a major improvement over the early years of this mill’s tenure—when a host of stock rod failures surfaced with aggressive tuning and an atmospheric turbo added to the mix.