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Over The Network

Gas or diesel?

edited by Gary Brinck

Technical Guru Richard Mater was asked:

How does a diesel manage to do better? And while on the subject, when is the time to choose a diesel vs. gasoline? I know the diesel costs $10K or so extra and pulls better and gets better mileage and costs more to repair. Is the basic engine and chassis designed for longer life? In other words, if someone was going to buy "one last" coach for many years of use, or planned to really rack up the mileage...are they diesel candidates? I'd like to hear your thoughts on this one.

The following was Richard's response.


Diesel engines operate at a compression ratio near 22 to 1 and, as a result, produce a bigger "bang" for the buck than gasoline versions at 8 to 1. These engines are built like the proverbial brick out-house, and are able to stand these tremendous combustion chamber pressures. In racing (where gas engines rule), whenever more power is needed, most teams turn to superchargers (blowers driven mechanically off the engine), or turbo chargers (essentially the same except driven from escaping exhaust gases). The gas engine's compression is also mechanically modified up to 12 to 1 or more.

Between blowing more air (oxygen) and fuel into the cylinders on each downstroke, and increasing the squeeze (compression) put on this volatile charge, more power is produced--often at very high RPMs. It's not uncommon to find heavily modified gas engines in the 250 to 300 cubic inch range producing over 1,000 horsepower. However, they sure don't idle very well, and are intended to deliver their brute over a relatively short life span (maybe only one race). Plus, the fuel required to support the higher compression isn't even close to being available to the average consumer, and if it were, nobody would want to pay for it. In other words, gas engines can be turned into veritable fire breathers, but at a terrific cost, complexity, and reduced lifespan.

As a result, vehicle manufacturers must settle on gas engines for their "daily driver" vehicles that won't "ping" on the cheapest gas station offering, will run reliably to well over a 100,000 miles, and do it all (in all types of weather) without creating a lot of pollutants. A task and a half, to be sure.

Diesels, on the other hand, are desirable because they can produce a huge amount of power at lower engine RPMs where it can be used by large, heavy vehicles. Because of a diesel's beefiness, it can pull at max power all day long and not even work up a sweat. And since, during idle, very little fuel is injected into the cylinders, such engines can chug along on the hot day in stop-and-go traffic without a trace of overheating. Certainly, these are desirable features of a tow vehicle or motorhome engine. But, as with gas engines in specialized racing applications, there are prices to be paid for having a King Kong diesel engine sitting between the frame rails of one's RV.

These include much-increased cost and weight of the engine itself. Ditto for a motorhome's frame and drivetrain. Also to be factored in is that fact that diesel fuel does not burn as cleanly as gasoline, and as such, contaminates the engine oil at a ferocious rate. Oil and filter must be changed (in the engine group we're considering here) at least every 3,000 miles*. Alternatively, gas engines, thanks to today's much cleaner burning unleaded fuel, can easily be taken to 7,500 miles** between oil changes without threat to internal parts. Now an oil change on typical Ford, Chevy, GMC, Dodge diesel won't break anybody, but it is quite a bit more costly than a big block gas engine. If you're considering powerplants like the big Detroit Diesel, you're up against a different story. Some of these hold up to 25 quarts, and between that and the filter(s), will cost nearly $100 per oil change and lube (done every 3,000* miles).

Don't overlook the inconvenience of having to deal with special low temperature operating procedures with diesels (the fuel tends to congeal in colder climes, making starting a challenge), or the disaster of having water in the fuel reach the engine. While all credible diesels today come equipped with good water separators in the fuel line, if the owner doesn't maintain this device, catastrophe will strike. Let me spell it out--N-E-W E-N-G-I-N-E!

Also, while no engine responds well to infrequent use, diesel REALLY don't like be left to sit for months on end. The fuel often requires an additive to prevent fungus growth in the tank which, if not kept in check, can plug-up the whole fuel system. Generally, long-term storage is another area where diesel owners are confronted with maintenance costs not shared by their gas-owning contemporaries.

Everything on a diesel costs more - filters (both fuel and oil), water pumps, radiators, etc. And heaven help the person that needs an injector pump outside of warranty. Does the "National Debt" ring any bells here? I have one friend that paid nearly $1,000 just for a water pump replacement on a diesel-powered Ford pickup.

But, but despite what must appear as negative comments, diesels DO have their place IF a person is going to log a huge number of miles throughout the ownership period. However, I think you'll be shocked to learn that even the $500,000+ bus conversions have an average owner turnover rate of every two years and 20,000 miles***. I always hear people talking about how they will get 250,000 miles or more out of their rig's engine before having to have it overhauled. Though I won't do it here, I have run the numbers for the extra initial cost of a diesel rig (including tax and license), the increased maintenance required (diesel vs gas), and higher parts prices, and so on. If you do the same, I think you'll conclude that for all but a high-mile per year, liveaboard rig, gas is the way to go. Besides, the interior amenities are almost as nice on many gas motorhomes.

It has been my experience as an RV technical writer since 1980, that most folks change out their rigs well before they can even begin to hope to recapture their initial investment in diesel power. I can't tell you how many times I have heard someone suggest that THIS one would be their last rig, and they want the best, most reliable machine available. Guess what, they've got something different within five years, after a MAJOR depreciation hit.

NOW, if you or anybody else tells me that they're interested in diesels because of the way they're built, the gorgeous coachwork common to such vehicles, or because they like the power, sound, smell, or whatever, then we're talking a different ball game.

Buying a big diesel rig has a lot to do with ego, usually male ego. There's no way on earth anyone can justify using a $500,000 to $800,000 converted bus as a motorhome, or for that matter, even a $250,000, conventionally-built, rear pusher motorhome. Just run the numbers any one of these examples, and then divide by an average of usage of 45 days per year to come up with a killer, "per night" cost!!!

But justification is not, and should not, be the issue. I have done test reports on Prevosts, MCIs, Eagles, Bluebirds, as well as conventionally assembled, diesel-powered, rear pushers. I'm here to tell you there's no other experience like it! They are a dream to drive and live aboard. They're fun just to be IN. But use caution, because after you've been in the "pilot's" seat of these rigs for more than about 30 seconds, you won't want to relinquish the position.

This, I think, is the real reason people buy diesel-powered rigs. And, it's a good reason. Construction quality and luxury are valid points in any purchase, and I don't take exception with this line of thought whatsoever. I mean, really, I'd like to be trundling down the highways of America in a 45', 102-wide, 500 horsepower, stainless steel-sided bus conversion! Highline diesel rigs are, without question, among the finest built vehicles to ever see a roadway, and that alone is sufficient reason for ownership.

What I do take exception with is when the element of "cost-effectiveness" is brought up as justification for diesel ownership.**** There is simply no way that the extra few miles per gallon achievable with a more expensive (vs gas) "oil burner" powerplant can be used as a reason for purchase. Folks should just bite the bullet and say "I want it because it's NEAT". That's the real reason anyway, so I'm suggesting the potential diesel owners just say this phrase a few times like a mantra, and then write the check.

EDITOR'S NOTES:
* Since this article was written, oil changes intervals on modern RV diesels have been extended significantly, with factory recommended intervals of 5000-9000 miles or more on many models from Caterpiller and Cummins. Pick-up truck diesels, however, remain in the 3000 mile range when used for Severe Duty, which includes trailer towing.

** When gasoline engines are used for Severe Duty, including towing and motorhome usage, the manufacturer recommended interval is typically 3000 miles rather than the 7000 miles of passenger car engines.

*** According to Lazy Days (a huge Florida RV dealer), the average motorhome owner trades every 3.5 years.

**** Even with the extended oil change intervals of modern diesels, the author's conclusion is remains correct - few owners will ever save money with a diesel.