Limitations, Concerns, and Supporting Mods
The limitations of the 47RE and 48RE automatic transmissions are relatively high and quickly realized when modifying the Cummins turbodiesel for greater performance. Basic add-ons like an aftermarket exhaust or air intake system are unlikely to necessitate the need for transmission upgrades, but even mild tuning can put you at risk. The level of concern depends considerably on the mileage of the transmission and if you plan on any major upgrades, don't ignore the weaknesses of the drivetrain. The Cummins has been known to snap input shafts with as little as 500 rwhp, so if your goals put you anywhere near this marker expect to stuff the transmission full of go fast goodies.
Manual transmissions hold up well but the factory clutch is not going to like your performance upgrades. You'll want to go aftermarket with a unit that is rated for your power range. Every major brand has a tiered product line, letting you choose a clutch that balances price, performance limits, and clutch pedal feel.
Head Stud Considerations - When Does My Cummins Need Head Studs?
The factory head bolt design has been known to be extremely reliable on the 5.9L and 6.7L Cummins. However, high boost levels and aggressive tuning can still blow a head gasket with ease. Some trucks may run 45 psi with factory head bolts and no problems, while others may blow head gaskets at only 35 psi of boost. The amount of boost you make before you're at risk for blowing a head gasket varies. To eliminate confusion, high boost does not directly cause heads to lift, but if you've got the fuel to match this airflow, then cylinder pressures may be high enough to cause concern. Upgrading your head gaskets and installing head studs is cheap insurance if you will be running more than 40 psi. In the 40-50 psi range (with fuel to match) on factory hardware, you're playing with fire.
Monitoring Exhaust Gas Temps
A pyrometer is highly recommended in any considerable performance build. A leading cause of catastrophic engine and turbocharger failure is excessive EGT. Trucks do not come from the factory with a pyrometer. Install a pyrometer so that you can monitor exhaust gas temperatures and avoid the danger zone. The ideal mounting location for the thermocouple (probe) is before the turbine housing inlet; the outlet temperature will be significantly lower and is not necessarily of use for the purpose of protecting the engine from damage.
Lift Pump Reliability Concerns
The factory lift pump (low pressure fuel pump) on the 24v Cummins has an expected lifespan of ~100,000 miles and is limited in terms of fuel supply potential. Aggressive tuning, bigger injectors, and other fuel system modifications can greatly decrease the life of the lift pump. Additionally, an injection pump can be damaged if fuel supply demands are not met. A high flow, performance oriented lift pump is therefore highly recommended on these models and is cheap insurance against more expensive repairs down the road.
Valve Float Concerns
Valve float has been known to occur in engines operating near the 50 psi territory in terms of turbocharger boost. This is a relatively high boost pressure for street-able trucks, but worth noting. 12v engines, which can be modified to reach revs greater than the factory design intended, can experience valve float at stock boost levels at engine speeds as low as 3,400 rpm. Heavy duty valve springs are available for all 5.9L engines (12v & 24v) that accommodate greater boost pressures and higher engine speeds.
Killer Dowel Pin (KDP)
The killer dowel pin or simply KDP is of great concern on all 12 valve Cummins engines (1989 to 1998.5 model years). On these engines, a dowel pin is pressed into the engine block that aids in the assembly process by aligning the timing gear housing to the block. The problem is that this dowel pin can dislodge over time as a result of engine vibration. When it makes its plunge from the top of the timing gear system to the bottom of the housing, a number of catastrophic events can occur should the pin become lodged between any pair of the gears in the timing system. It is believed that this risk becomes greater as power is increased. The good news is that the aftermarket has responded with dowel pin elimination kits and they are relatively inexpensive. The bad news is that the installation of such kits is rather intrusive and labor intensive. Regardless, investing in a KDP elimination kit is a no brainer for 12v owners who wish to protect their engine.
53 Block Castings
The # 53 block casting problems affects a large number of 1999 to 2001 model year 24 valve Cummins engines. These block castings are significantly weaker than earlier and later production units. They feature a particularly thin water jacket and are susceptible to cracking. These cracks can then propagate to several inches long and will leak engine coolant. At stock performance levels, problems with 53 block castings are relatively low. However, as performance increases, the potential to develop a crack becomes a greater concern. If you find yourself with a 53 block and are looking to build more power, take this into consideration.
All models of the 6.7L Cummins turbodiesel feature an exhaust aftertreatment system that includes, at minimal, a diesel particulate filter (DPF). Major fuel system modifications, including aggressive tuning, large injectors, and fuel pump modifications, may increase the risk and occurrence of DPF clogging. Any upgrade or modification that causes the engine to produce more soot will not be compatible with these engine unless the DPF system is discarded. Removal of the DPF is illegal under Federal law for vehicles driving on public roads, as is tampering with any emission control component. A DPF delete, though intended for off-road use only, will provide significant improvements in fuel economy, reduced reliability concerns, and open the door for greater performance modifications.
EGT- How hot is too hot?
For daily drivers, tow rigs, or street driven trucks, you should not exceed 1250 degrees F and should not maintain 1200 - 1250 for extended periods of time. Some will argue that the engine/turbocharger can take more than this in short bursts, but we feel there is no need to push the limits and prefer to take a conservative approach. Excessive exhaust gas temperatures do not cause "minor" damage - it'll be an expensive bill. Under load, try to keep you exhaust temps between 1000 and 1200 degrees F. If you're EGTs are unmanageable, you have too much fuel and/or not enough airflow. Look into an upgraded turbocharger, or install a water injection system to keep things cool when you decide to mash the skinny pedal. For race rigs, it all depends on how the engine is built. Some race trucks hit 1800 degrees F during a 1/4 mile run, but don't expect to do that in your daily driver.
Cummins Performance Recommendations
It is entirely possible to have a daily driver in the 750 hp range. There are so many options available to Cummins owners that it is difficult to recommend a particular route. For common rails, we'd install a tuner with multiple settings (at least an economy and a race tune), a larger ball bearing turbocharger, intercooler, air intake, high flow exhaust, and a mild increase in injector size. Our goal would be to have a truck that's powerful and doesn't break the bank when we have to put diesel in it. Additionally, we'd try to get away with the factory injection pump. For mechanically injected trucks, go with injection pump upgrades, a turbocharger, intercooler, intake/exhaust systems, and some larger injectors. Remember, for injector and injection pump upgrades, bigger is not always better. Always match these parts to the airflow you've got available.
Dedicated Tow Rigs
For dedicated tow rigs, a more reserved approach may be ideal. Don't go full bore on anything, as you want to keep EGTs under control at full load. For tuning, get a mild tune with built in safety features. If you're installing a new fuel plate in your Cummins, choose a 50 hp plate over a 150 hp plate. The goal in building a tow rig is fuel economy, torque, and low exhaust gas temperatures. You want to be able to pull a grade without losing speed and keeping your EGTs in the 1000 - 1100 F degree range when fully loaded. Look into mild fueling mods, a more efficient turbocharger, upgraded intercooler, intake/exhaust upgrades, and a water injection system (we'd stay away from methanol when you've got a load on the engine). If the truck smokes considerably or EGTs are high when towing, you need more air flow to compensate for fuel system modifications. An exhaust brake is a must have, as it will save both your truck and trailer brakes while allowing greater speed control on long descents.