View Full Version : Detroit diesel Allison Transmission Expected Life Cycle What am I Missing?

04-16-2013, 04:35 PM
I have to ask this question; because it has been annoying me.

Since Detroit diesel motors and Allison Transmissions should give a much longer service life than a comparable gas motor home. Why is it I see almost everyone with a diesel powered motor coach (Newell, Wanderlodge, Prevost, foretravel...on and on) advertising having recently rebuilt their engines with miles between 100K and 200K miles on them?

I know there are plenty of people who do not attend to their PM responsibilities and trash their units out, but it "seems" that diesel engine & power train problems are common after 100K miles even with those who do perform their annual maintenance.

I know OTR rigs can get massive miles and maybe have heavier power trains than an motor home.

What am I missing here, someone help me out on this, I need clarification if possible. What I don't want is to purchase some nice used luxury coach get it home and bam sink another 20K into it almost overnight when it should be road ready for many miles to come.

Thank you in advance,


04-16-2013, 04:58 PM
Luke, those are good questions. I think your biggest issue here is that motor home drivers drive their motor homes like they drive their cars. Probably the biggest cause of catastrophic failure is over heating. Over the road truckers are trained to pay attention to RPMs, their gauges, know when to down shift, and when to back off the throttle. All it takes is one time pushing to hard up a hill to destroy a motor. If you get an oil analysis on the engine and transmission and everything is okay there shouldn't be a problem.

04-16-2013, 06:36 PM
Excellent question Luke. That's probably the issue that concerns me the most and what keeps us from pulling the trigger on buying a coach equipped with those older detroits. It seems that most who have real experience in this arena tend to say - it all comes down to preventive maintenance . And it's not that I don't believe 'that', it's just surprising that there are so many instances of 'rebuilt' motors or transmissions on what would appear to be low mileage coaches.


04-16-2013, 07:25 PM
WhitLock, it's not limited to the older Detroits. Before I 'discovered' Newells and was looking at all types of diesel pushers, I was seeing the same issue with coaches under 100k miles with rebuilt motors. These were Cummins and Cats and anything else the manufactures supplied their coaches with.

04-29-2013, 03:11 PM
I drive up mountains with the turbo needle pegged depending on the waste gate to dump excessive pressure. Am I doing this wrong? The Allison temp never reaches the red line and the Cummins ISM500 is cool.The question is this: Is it possible to over-boost the engine?

04-29-2013, 03:40 PM
I drive up mountains with the turbo needle pegged depending on the waste gate to dump excessive pressure. Am I doing this wrong? The Allison temp never reaches the red line and the Cummins ISM500 is cool.The question is this: Is it possible to over-boost the engine?

Hi Tom,

Over-boosting an engine frequently can cause damage to the engine in a variety of ways including pre-ignition, overheating, and over-stressing the engine's internal hardware. For example, to avoid engine knocking (aka detonation) and the related physical damage to the engine, the intake manifold pressure must not get too high, thus the pressure at the intake manifold of the engine must be controlled by some means. Opening the wastegate allows the excess energy destined for the turbine to bypass it and pass directly to the exhaust pipe, thus reducing boost pressure. The wastegate can be either controlled manually (frequently seen in aircraft) or by an actuator (in automotive applications, it is often controlled by the Engine Control Unit).

Turbo lag Turbocharger applications can be categorized according to those which require changes in output power (such as automotive) and those which do not (such as marine, aircraft, commercial automotive, industrial, locomotives). While important to varying degrees, turbo lag is most problematic when rapid changes in power output are required.

Turbo lag is the time required to change power output in response to a throttle change, noticed as a hesitation or slowed throttle response when accelerating from idle as compared to a naturally aspirated engine. This is due to the time needed for the exhaust system and turbocharger to generate the required boost. Inertia, friction, and compressor load are the primary contributors to turbo lag. Superchargers do not suffer this problem, because the turbine is eliminated due to the compressor being directly powered by the engine.

Lag can be reduced in a number of ways:

lowering the rotational inertia of the turbocharger; for example by using lighter, lower radius parts to allow the spool-up to happen more quickly. Ceramic turbines are of benefit in this regard and or billet compressor wheel.
changing the aspect ratio of the turbine.
increasing the upper-deck air pressure (compressor discharge) and improving the wastegate response
reducing bearing frictional losses (such as by using a foil bearing rather than a conventional oil bearing)
using variable-nozzle or twin-scroll turbochargers (discussed below).
decreasing the volume of the upper-deck piping.
using multiple turbos sequentially or in parallel.
using an Antilag system.

Boost threshold
Lag is not to be confused with the boost threshold. The boost threshold of a turbo system describes the lower bound of the region within which the compressor will operate. Below a certain rate of flow, a compressor will not produce significant boost. This has the effect of limiting boost at particular rpm regardless of exhaust gas pressure. Newer turbocharger and engine developments have caused boost thresholds to steadily decline.

Electrical boosting ("E-boosting") is a new technology under development; it uses an electric motor to bring the turbo up to operating speed quicker than is possible using available exhaust gases. An alternative to e-boosting is to completely separate the turbine and compressor into a turbine-generator and electric-compressor as in the hybrid turbocharger. This allows the compressor speed to become independent to that of the turbine. A similar system utilising a hydraulic drive system and overspeed clutch arrangement was fitted in 1981 to accelerate the turbocharger of the MV Canadian Pioneer (Doxford 76J4CR engine).

Turbochargers start producing boost only when a certain amount of kinetic energy is present in the exhaust gasses. Without adequate exhaust gas flow to spin the turbine blades, the turbo cannot produce the necessary force needed to compress the air going into the engine. The boost threshold is determined by the engine displacement, engine rpm, throttle opening, and the size of the turbo. The operating speed (rpm) at which there is enough exhaust gas momentum to compress the air going into the engine is called the "boost threshold rpm". Reducing the "boost threshold rpm" can improve throttle response.

04-29-2013, 04:05 PM
I believe there are differences between 2-stroke and 4-stroke engines, and between electronically and mechanically controlled engines that effect longevity significantly depending on driving style. If you watch truck drivers you will notice that they keep their beasts rather wound up. Seldom do you see them lugging up a hill and frequently you see them shifting fairly frequently for passing and other speed changes. In motorhomes we tend to drive gently and we've been taught that our cars should be kept away from redline RPMs, this is bad for our big diesels since being close to or at red-line is no problem at all and in fact may help the engine's longevity. Compared to trucks we have far fewer gears which inherently means at some points we will be below optimum RPMs.

Mechanical controlled engines and 2-strokes are more vulnerable to operator "error". In particular 2-strokes really do benefit from higher RPMs. I've heard and believe its probably true that 2-strokes need to be driven like you stole them. In any case I think it likely that if we control our driving to keep RPMs up and throttle moderate our engines will do just fine.

While there are a number of coaches for sale with engines that have recently been overhauled, I've also seen others with 2-300,000 miles on them. I don't know if they are about to need significant work or not. The one thing my DD mechanic told me is that 2-strokes by design probably need an in-frame overhaul at about 250,000 miles. On my 6V92T that cost $5,000.

When looking, I would have the fluids analyzed and assess the tidiness of the mechanicals to make an assessment of how maintenance was done. If the tires are old, the engine compartment dirty and oily, or things generally look shabby I would not necessarily shy away from the coach but I would certainly build in a larger contingency fund to cover the things that can't be seen that may or may not happen but at least I've covered my butt and unlike a plumber I like to keep my butt covered.