Head Stud Considerations - When Do I need Them?
A conservative rule of thumb is to invest in head studs and gaskets in any build beyond basic bolt-on modifications (exhaust, intake, tuner). However, the 6.0L Power Stroke is infamous for TTY head bolt bolt failures. Keep in mind that head studs are a great peace-of-mind upgrade for any engine as they are significantly favorable over stock head bolt designs. Notes on various Power Stroke cylinder head designs:
7.3L Power Strokes have (6) head bolts per cylinder from the factory, so head gasket failures does not become a problem until you get into significant fuel system modifications. The stock head gaskets will typically survive with tuning and a slightly larger injectors. If you plan on aggressive tuning with large injectors, head studs are advised.
6.0L Power Strokes have (4) 14 mm TTY (torque to yield) head bolts from the factory. It's often construed that the TTY design was a poor choice for the engine and is thus responsible for many of the engine's failures. In actuality, the TTY head bolt design is perfectly adequate for the engine at stock output. Excessive heat (created by the poor EGR/EGR cooler design) is the main killer of the TTY head bolt, allowing it to stretch under load and thus resulting in head gasket failure. That being said, 6.0L Power Stroke owners should be installing head studs before performing any performance modification. With the installation of head studs, the 6.0L Power Stroke can be capable of handling huge injectors, aggressive tuning, nitrous, and everything else you throw at it.
6.4L Power Strokes have (4) x 16 mm head bolts per cylinder. The head bolts are 2 mm larger than those used on the 6.0L Power Stroke, increasing strength significantly. 6.4L owners should comfortably get away with mild injector upgrades and aggressive tuning on factory head bolts.
6.7L Power Strokes use a (6) head bolt per cylinder design, so concerns of failure are limited with basic bolt-on modifications, even under the stress of aggressive tuning.
When are Transmission Upgrades Necessary?
Ford seems to have a pattern of producing relatively stout drivetrain combinations for their diesel applications. That being said, it's important to consider the fact that the transmission behind your Power Stroke was developed with factory power levels in mind. The laws of physics dictate that as performance levels increase, the life expectancy of the transmission decreases. More power is more heat and clutch packs will have a a tendency to slip more under greater loads. This is usually corrected and/or supplemented through tuning of the transmission, increasing line pressure as necessary and modifying shift schedules to match the demands of engine performance. This does not negate the fact more power will indefinitely lead to the occurrence of more wear. This notion does not make any claim that a transmission will in fact fail prematurely at increased power levels, but it will certainly contribute to premature wear. A transmission upgrade should include, at minimal, a performance torque converter and valve body (where applicable).
The decision to upgrade a transmission is at full discretion of the owner. Transmissions are relatively expensive items, thus it may be difficult to justify replacing an operational unit. If that is the case, be prepared to purchase an upgraded transmission or have an upgraded transmission built when the factory unit requires replacement. If you're concerned with being left stranded due to a transmission failure, drive accordingly or preventively replace the transmission with an upgraded unit. The late 5R110W and modern 6R140W TorqShift transmissions have been significantly more reliable in performance applications than the dated E4OD and 4R100, although any transmission can be built to match corresponding engine characteristics.
Manual transmissions do not typically have a tendency to experience catastrophic failure. However, factory clutches do when performance is significantly increased. Consider replacing the factory clutch with one that can meet the demands of your engine before it has a chance to give out on you.
Limitations of the HEUI Injection System
The 6.0L and 7.3L Power Stroke diesels feature an HEUI injection system. This injection type relies on a high pressure oil system (max 3,000 - 4,000 psi) to pressurize fuel in the body of the injector, as opposed to a high pressure fuel system in which fuel is pressurized in a high pressure injection pump and delivered to the injector. At the time of its development, the HEUI system was a sophisticated technology that overcame limitations of mechanical injection pumps. Since then, the HEUI system has been replaced by common rail technology, and modern injection pumps are capable of meeting and/or exceeding the pressures realized by a HEUI system.
Inherently by design, HEUI injection systems tend to be more costly and complex to upgrade. However, this is not to say that an HEUI injection system cannot compete with modern technology - it's simply a different beast altogether. Getting the most from an HEUI system requires upgrading the high pressure oil pump (HPOP). Rather than significantly increase the maximum oil pressure in the system, an upgraded HPOP builds more pressure at lower RPM, therefore providing significant improvements in performance in the low to mid RPM range and grants greater compatibility with larger injectors and aggressive tuning.
Powered Metal Rods (PMR)
2001 to 2003 model year 7.3L Power Stroke engine featured powered metal connecting rods ("PMR"). In this manufacturing process, fine grains of metal are poured into a mold and heated until the material melts and fuses into a uniform shape. The process yields a connecting rod that is significantly weaker than a forged connecting rod due to their brittle nature. The rod is perfectly adequate at stock power levels and have no greater tendency to fail than a forged rod. However, PMR equipped engines may have a higher tendency to fail in performance applications. The consensus, though widely debated, is that engines equipped with powdered metal connecting rods should not be pushed beyond a conservative 400 horsepower to the rear wheels. Beyond that, the reliability of a powdered metal rod is significantly reduced.
Powdered metal connecting rods were used in engines with sequential engine serial number 1,425,747 through 1,440,712 and 1,498,319 through final production. All other engines have more desirable forged connecting rods. There may be some discrepancies in these figures, and it's worth a visual inspection if you plan on making big power - a PMR physically appears different than a factory forged rod and can be viewed through a plug in the engine block.