Duramax Diesel Performance

Duramax Performance Options & Considerations

The 6.6L Duramax diesel, introduced by GM for the 2001 model year, was the first diesel platform to arrive on scene with a common rail injection system. The common rail diesel would become a performance icon as a result of its flexibility in performance applications, posing outstanding performance potentials simply by modifying operating parameters and characteristics via inexpensive tuning devices. It was also the first engine in its class to wield aluminum cylinder heads which, despite criticism, have proved substantial even in high performance applications.

Aluminum Cylinder Head Concerns & Head Studs

The Duramax was the first mainstream diesel in its class to employ weight saving aluminum cylinder heads. At the time, the decision was relatively controversial and many questioned the strength and reliability of the system, unable to accept that aluminum could be an prospective alternative to proven cast iron designs. In the end, these concerns proved unwarranted and without foundation. The fact of the matter is that the use of aluminum cylinder heads has not provided any substantial increase in the tendency for head gasket failures than cast iron designs in similar applications.

While the aluminum cylinder head design has proven worthy (other manufacturers are even beginning to catch on to this trend in their search for weight savings) head studs remain a highly recommended modification in performance applications. A conservative rule of thumb is that head studs become necessary in the 500 to 550 rwhp range. Some engines may experience a head gasket failure at lower performance levels, as some engines may creep past this range without experiencing a problem - there are many variables at play, and we always suggest fixing problems preemptively in order to avoid compounding and/or catastrophic failures.

Tie Rod Problems on 4x4 Pickups

All Duramax equipped Chevy and GMC HD 4x4 pickups employ an independent front suspension (IFS) system. In an IFS system, the left and right tires are free to oscillate independently of each other - a tire can travel upwards or downwards without causing any subsequent movement of the opposing side. IFS provides favorable ride quality compared to a solid front axle, but is generally limited in overall strength. In Duramax equipped pickups, it's actually the steering tie rods that have a tendency to fail.

The tie rod system, which translates driver input via the steering wheel into actual movement of both tires simultaneously, is the Achilles heel of 2001 to current GMC/Chevrolet HD pickups. While a stock pickup generally will not have any issues, lifting or increasing performance may place additional stress on the factory tie rod system. In this case, the tire rods have a tendency to strip and pull out of the threaded inserts that connect the steering linkage to the ball joint assembly at either end of the bar. This most often occurs in high performance applications such as drag racing and sled pulling, where a 4 wheel drive launch places tremendous stress on the tie rod assemblies as large loads transmitted through the front tires causes them to toe in and out rapidly. The opposing inward and outward forces is what eventually causes the tie rod to fail. The greater the torque transmitted to the tires, the more likely a a failure will be realized.

This problem is even more prominent in lifted trucks. Fortunately, there are many solutions available from the aftermarket, which upgrades the tie rod sleeves with significantly stronger units. If you expect to launch your truck in 4 wheel drive, a tie rod upgrade is highly recommended.

Allison Transmission - When are upgrades needed?

The general consensus is that the Allison transmission will require supporting modifications in applications that exceed 150 rwhp over the factory ratings. The Allison 1000 transmission was offered as a five speed automatic for 2001 to 2005 model years and a six speed unit from 2006 to current model years. The max input torque rating of the Allison 1000 has been increased through the years. However, these figures provided by the manufacturer tend to align with the subsequent torque ratings of the Duramax engine and it is largely accepted that the strength of the Allison may exceed these values. The Allison 1000 is a stout machine, but taking its reputation for granted may ultimately result in decreased transmission life and a greater possibility of failure. At minimal, upgrades should include a performance valve body and torque converter.

 

Model Year(s)

Max Input Torque Rating

2001 - 2004

520 lb-ft

2005

565 lb-ft

2006 - 2007

650 lb-ft

2008 - 2009

660 lb-ft

2010 - 2016

765 lb-ft

 

Exhaust Gas Temperature (EGT) - How Hot is Too Hot?

Exhaust gas temperature (EGT) is a primary failure metric in diesel engines. Understanding and controlling EGTs is therefore of utmost importance in performance applications, especially those with considerably modified fuel systems. A pyrometer not only issues feedback as to the temperature the turbocharger turbine is subjected to, but also provides insight into temperatures within the combustion chamber. Excessively high EGTs do not cause minor damage; most outcomes will be at or near catastrophic.

That being said, we always recommend taking a conservative approach in controlling EGTs and not exceeding 1250° F in short bursts and not maintaining 1200° F - 1250° F under load for extended periods of time. A high EGT condition is a sign of poorly matched airflow and fueling characteristics. If EGTs are an issue, consider increasing available airflow or invest in a water injection system to bring the EGTs down under full power.

Duramax RPM Limits

It is not uncommon to increase the rev limit of the Duramax diesel in performance applications, allowing drivers to tap into greater horsepower and broaden the available speed range in each gear. The general consensus is that the Duramax will hold together up to 4,000 RPM, but the engine will experience increased wear when operating at these engine speeds. Whether or not the additional rate of wear is significant remains debatable. In reality, the safest operating range is the one selected by the manufacturer, as that is what the engine was designed for. From the factory, the Duramax diesel will operate at a maximum ~3,200 to 3,400 RPM (the max engine speeds at which shifts are commanded and fueling is limited differ).

Without supporting modifications, rail pressure and turbocharger airflow limitations will be realized operating well beyond the factory intended limits. In addition, valve float becomes a primary concern in addition to greater strain produced by the centrifugal force of the rotating assembly. Case in point, unless these issues are properly addressed, there is little benefit to pushing the factory determined maximum engine speed. After all, this is a 4 cycle diesel engine, not a two stroke, which produces peak torque at low engine speeds.

Diesel Particulate Filters (DPF) and Performance

2007.5 model year and newer engines, beginning with the LMM, feature a diesel particulate filter (DPF) in addition to other emission control systems. A DPF is not compatible with many performance modifications available for earlier engines. Therefore, it is necessary to delete the DPF in performance applications. Keep in mind that tampering with an emissions control device is illegal under Federal emission laws. Therefore, deleting the DPF is a feasible option for off-road applications only.

There are many performance enhancing products available for DPF equipped trucks - tuning, intake systems, upgraded turbochargers, etc. However, DPF equipped trucks are not compatible with significant fuel modifications as any increase in soot production will lead to filter clogging. For this reason, trucks that feature such exhaust aftertreatment systems will be limited in the amount of power that can be added via upgrades unless the DPF is first deleted.

Supporting Modifications and Considerations

Do it once and do it right - if you're in the market for more power, supporting upgrades are recommended in order to reduce or eliminate reliability concerns. The laws of physics dictate that an increase in power will consequently reduce longevity. However, there are many things you can do to minimize this risk.

A pyrometer is recommended in any performance build, even if the most substantial modifications performed is tuning. A pyro will allow you to monitor exhaust gas temperature in real time and therefore keep the temperature out of the danger zone. On that note, pyrometer probes should be installed before the turbocharger turbine inlet, not after. The outlet temperature of the turbine will always be less than the turbine inlet, and is therefore not an ideal mounting location. A pyrometer is an inexpensive investment in an otherwise expensive powertrain system.

Head studs may not be necessary for mild performance builds, but they are insurance against head gasket failures and will allow for greater performance potentials with limited reliability concerns. The install is relatively invasive and head stud/gasket kits are not particularly inexpensive, but the result is a bulletproof top end. There are a variety of options available for all generations of the Duramax engine platform. Also consider investing in a performance oriented lift (low pressure) pump. Aggressive tuning can cause a drop in rail pressure at high engine speeds and may starve the injection (high pressure) pump. These pumps are relatively expensive to replace, and you'll have greater overall performance with an upgraded lift pump.

If your primary objective is towing, don't go overboard on your modifications. Big injectors and aggressive tuning are not healthy for trucks that tow considerably as the engine load realized in these scenarios may compromise engine reliability and longevity. EGTs are a concern in towing applications as a result of greater loads, and therefore your modifications should be centered around airflow and air cooling. Intercooler upgrades, free flowing intake/exhaust systems, turbocharger upgrades, and mild tuning (specifically designed for towing) are the makings of a perfect tow rig build.