What is the horsepower limit of a stock Power Stroke?
If you’re looking to more than double the horsepower your Power Stroke brings to the table, we’ve got you covered. If you’ve ever asked yourself much hp can a 7.3 (or a 6.0l, 6.4l, or 6.7l) Power Stroke handle? What exactly is the 6.4l Power Stroke max HP before it breaks? We’ll answer that here. 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.
Sure, 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. Stay tuned for installments on the horsepower limits of Duramax and Cummins mills in the months ahead…
7.3L Power Stroke:
When it comes to the 7.3 Power Stroke max hp limit, it’s the rods, and 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.
DANGER ZONE (forged rods): 600-650-rwhp
DANGER ZONE (powdered metal): 500-rwhp
“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
So when planning of doing a 7.3 Power Stroke horsepower increase, 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 to ’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 its stock of forged rods, they were installed once more beginning with serial number 1440713 and going to 1498318. From there on out, powdered metal units got the nod, meaning most ’01 to ’03 model year engines have them.
As for forged rod engines, 600 to 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 600-rwhp setup, as opposed to 1,200 lb-ft. being made at 2,000 lb-ft.). The same tuning methodology applies with powdered metal rod engines, but enthusiasts are highly advised to draw the line at the 500-rwhp mark.
6.0L Power Stroke:
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 600-hp mark, if not sooner.
SIDE BAR/PULL QUOTE
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 (or the lack of sufficient head bolt diameter, torque-to-yield head bolts, etc.), it is one heck of a tough engine.”
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 365-ci 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 800 hp 6.0 Power Stroke range gets you into unchartered waters with a 6.0L.
6.4L Power Stroke:
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.
DANGER ZONE: 1,000+ rwhp
“I had stock pistons in my 6.4L for 60,000 miles with five different setups—from stock turbos with a wastegate and spray to big compounds with added fuel—and never had any piston issues.” —Mike Corsilli of Maryland Performance Diesel
The weakest link on the 6.4L’s bottom end seems to be the pistons, but it’s not as cut and dry 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, is that 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 6.4L’s tend to survive higher horsepower for longer periods of time.
6.7L Power Stroke:
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).
DANGER ZONE: 700-rwhp
“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
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 Duty’s. 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 600-rwhp 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, so what’s the max hp a 6.7l Power Stroke can handle, well the stock rods can easily handle 650 to 700-rwhp.DW
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 700-hp range with your Super Duty, you may want to consider a set of Carrillo’s H-beam connecting rods for piece 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.Sources:
Irate Diesel Performance
Maryland Performance Diesel
Midwest Diesel & Auto
No Limit Diesel