Article: Making the Case for Diesel Engines

The Case For Diesel Engines

We all want diesel power - if we can afford it. "Gas or Diesel" is a common question for most of us when faced with repowering our Clippers, and it is not a slam-dunk. There are a number of factors which must be considered when making the choice. So, before we get salivating too much over diesel power, let's have a look at the following considerations:

  • Cost - Diesel engines, new or used, cost considerably more than gas engines, wherever sourced. Consequently, even when bought used at the local auto or truck wrecker, you're going to pay substantially more for a used diesel/transmission package. Similarly, if you go the DIY schoolbus wrecking route, you'll still have to pay more. Furthermore, parts are considerablymore for rebuilding - a starter alone for most diesels runs around the $1000 mark. Diesels require more expensive and harder-to-find mechanics, so unless you do your own work, this is also a consideration. If you have to change your rear end gearing (see below), this can run another couple of thousand dollars by the time the smoke clears, and is at minimum a whole lot of extra trouble.
  • Weight - Diesels are heavier - by their very nature, diesels are much more heavily built than their gas counterparts. The DD4-71, although admittedly primitive by today's diesel standards, weighed 1500 Lb more than the Buick 320. If you stand at the front of your Visicoach and look down the rivet line on either side below the windows, you may see that it takes a slight (hopefully) downturn at about the rear wheels. This is a sign that that monster engine has actually caused the body of the coach to sag - to bend the envelope. Now, _that_ is weight! The wonderful turboed 6-cylinder IHC 544 series, for example, are just too heavy - and long as well. (See below) Don't forget that the original Clipper design incorporated the 320 Buick - basically a car engine, and that the adaptation to the DD4-71 was just that - an "add-on." Flx made some chassis and bodywork mods to accomodate the heavier DD, but obviously not quite enough. And we also see that the later and much rarer Starliners incorporated even more modifications to the engine area, presumably because the extra weight of the DD was a problem with the Visicoaches. So, if you're starting with a Clipper (much more likely, since so many more were built), don't forget that your bus was essentially designed to carry the much lighter Buick gasser. Weight IS a consideration!
  • Gearing- diesels always run at lower rpm than gas engines. This means you must have suitable rear end gearing. The original Clippers used a (usually) Buick gas engine and 4-speed transmission. When they started using Detroit Diesels in the early '50's, they compensated for the lower rpm produced by the DD4-71 by adding a 5th overdrive gear to the Spicer transmission. Thus, you'll need to retain the 5-speed Spicer if you're going to replace with a diesel engine - and even this won't give you very tall gearing for today's freeways. The 3000 rpm produced by an HD gas engine at full song on the governor makes a big difference over the 2200 max produced by a diesel. 60mph at 3000 would only be 44 mph at 2200.Diesels also, naturally, have a much narrower "power band." This is the range of usable rpm - where the power is made. A typical diesel power band is 1800 - 2100 rpm. In other words, 300 rpm. For any given roadspeed, you need a gear that will show that rpm range. A typical HD big-block gas engine power band would be 2000-3000 rpm. In other words, 1000 rpm. You're much more likely to have a gear for your roadspeed with that much wider a power band. Although not much of a problem on flatland, this is a biggie in the mountains.

    If you chose to use an automatic transmission in your repower, choosing a diesel means you'll most certainly have to find an overdrive model if you are to retain your original rear gearing. The extra rpm of a gas engine means you may be able to get away with 1:1 gearing. Overdrive automatics - in heavy-duty models such as the Allison - are, just like diesels, harder to source - and always more expensive.

    If your engine is capable of producing the torque to overcome the drag of speeds over 55 or so, you'll also need the rpm to keep it up there. Choosing a diesel can make the difference between requiring re-gearing the differential (or replacing the whole axle) or not.

  • Engine Braking - diesels, by their nature, have no compression braking. This is because they have no air throttle - the engine pulls full air on each intake stroke, and the power is regulated by the timing of the injection of the fuel at the top of the compression stroke. If no fuel is introduced, the "cushion" of compressed air drives the piston back down the other side. No vacuum is produced by intake strokes with throttle closed, hence the braking effect we are all used to with gas engines, when going downhill with foot off the "gas" pedal, is not there with a diesel. Compression, or "Jake" brakes, may be added, but only at extra (and significant) cost. Thus, there is no advantage to using lower gears when going downhill; all the retardation of movement must be done with the brakes. Because of this lack of vacuum, diesel engines must also have a vacuum pump for the vacuum modulator of an automatic transmission, should you choose this option.
  • Length - Length is critical in the T-Drive configuration of the Clipper busses. Distance from Diff Flange to fan shroud must be within a certain limit. The drive shaft can only be so short, and the transmission and engine are thus limited to a total length which pretty well precludes the in-line six diesels. Thus you're limited to V-8's, which, although common in big-block gassers, do not comprise all diesels. People have been known to butcher up the shrouding and engine door arrangement of the classic Clipper design in order to accomodate the longer Cummins sixes as found in Dodge pickups, for instance, and it's not, in this writer's opinion, a pretty sight!
  • Pollution - it's finally being recognized around the world that the diesel engine is not less polluting than its gasoline counterpart. Today's efi (Electronically Fuel Injected) gas engine, with cat converters, is a very clean-running powerplant, with none of the black particulate output and smell which is coming to be associated with diesels. And its fuel efficiency is getting up there, too!
  • Noise - no contest. When one of those smoky, noisy diesel pickups goes by, the people on the sidewalk cease their conversations until it's well down the block. Of course, on the highway, where our busses belong, this is much less a problem. But, if you're planning on sneaking out of the RV park before dawn, well.....

The Good Part: Efficiency

Here's a description by one of our flx owner's members:

    "Diesel engines burn fuel relatiely slowly during the power stroke, causing the engine to produce much larger amounts of torque for a given amount of piston movement.(they are know as constant pressure engines because of this phenomenon) A gas engine on the other hand causes the fuel to actually go into a "controlled explosion" by comparison in the power stroke and the fuel is totally consumed (remember, less heat content) by the time the piston has gotten only about 25% of the way down the cylinder. At this point its cylinder pressure is exhausted. (no pressure, no push- no push, no torque) The diesel is a slower turning engine and produces most of its torque at a relatively gas engine lugging/stalling 1400-1800RPM. Thus the diesel engine will always give 30 to 50% better mileage (mpg) than a gas engine of equivalent power."

This, however, is in miles per gallon. Diesel prices relative to gas prices have almost doubled since the last energy crisis, (ie,they've both more than doubled, but diesel has doubled a lot more) and there is reason to speculate that oil companies may begin to charge considerably more for the higher-heat-content diesel fuel in future, reflecting the greater energy content in a gallon. Such a move could level the playing field for the diesel vs the gas engine. Remember, it was only a few years ago that heating your home with natural gas was half the cost of using furnace oil, but a simple stroke of the pen in some board room changed that picture!

Let's Look At Some Numbers

Everyone wants to have a big old noisy diesel clack-clack-clacking away in the back of their Clipper. And, if the price is comparable, why not? But let's remember one thing: you probably won't be driving your Clipper nearly as much as you do your other vehicles. Currently, there IS a definite per-mile cost saving with a diesel, but the pay-back for the extra cost is reached at some point.Up till then, it's all pay-out! The extra cost of powering with a modern diesel will be offset by fuel savings - although with some assumptions. But just how many miles will you have to drive to reach this point?

The Assumptions

Altogether, I think it's reasonable to assume that a Diesel conversion will run a minimum $5000, in time and trouble, more than a conversion to a big block gas engine and automatic. Let's go with that figure for the time being.

Diesel/Gas fuel costs are, also for the time being, about equal. And I think also that it's reasonable to expect about 40 to 50% greater mileage with the diesel. Flx owners members seem to be reporting 12 to 14mpg with diesel-powered Clippers (although often towing a tow'd.) Not so many results with gas powered conversions For the purpose of this discussion, let's say 8.

With these assumptions, let's take a look at two possible fuel cost scenarios:

What you do with that $5000 during the amortization time is up to you, but there just may be a better place for it than in your engine compartment - depending, of course on how much driving you do. But from here, it looks as if a yearly coast-to-coast cruise will still take over 5 years to reach payback for your diesel conversion.

Back to the diesel proponent:

      "But who said that a lower fuel cost was all there was to a diesel repower? Does smoother and quieter performance mean anything? Does the freedom of not having to be concerned about the many and mundane interrelated pieces of a gasoline engine have any value?

Diesels are inherently built stronger because of the internal stresses they must put up with due to their design. This strength and dependability has been legendary for 65 years. You are buying dependability with a diesel. With a gasoline engine you are buying a newer model every 3-5 years.

Now I know people are going to tell me that they get 100-200K on their Cheby 350 or their Ford 351 gas job. But they don't get it while pulling 18-20,000 lbs. of iron up and down the mountains of the western United States. It just is not in the cards."

Smoother? Maybe. Quieter? I don't think so! Stronger? Definitely - but a short-block rebuilt big-block gas engine comes in at the $1-2K range - and 50-75,000 miles is a very realistic expectation - even for a gas big-block in a 10-ton gravel truck!

Now, let's get back to the diesel proponent's recommendations:

"Now, what is the BESTEST and MOSTEST COMBO out there today?

1. The Cummins 5.9/Allison 4-Speed

#1 is the Cummins 5.9L Diesel with the Bosch "P" injection pump (Inline injection pump) rated at 225 HP or better. With an Allison 4 speed automatic and a high stepping (low numerical ratio rear axle). Your top speed at max engine RPM should be around 86 MPH (Don't worry, I ain't advocating going that fast on a 55 year old Clipper) If your calculated road speed is that fast then your "Rocking Chair" speed, or cruising speed should be around 62-66 mph. This will put your engine operating speed in its' "sweet spot" or maximum fuel economy range of 1400-1600 RPM depending on the make of engine. Engine noise, wear, and stress will be at its minimum at this speed while fuel economy and torque (that's the part that gets you up a hill) will be at their maximum.

#2 is the 1116 Caterpillar with the same combo.

3. Ford Powerstroke V-8

#3 is the 95 and up V-8 Power Stroke Ford with the Allison or the E4OD overdrive tranny.

NOTE: Ford (IHC) has a new kid out on the block with a V-6 and a V-8 variation of less CID. It features NO camshaft.Thats right. Both the valves AND the injectors are operated by hydraulic pressure controled by an engine computer. What's next...NO DRIVER? Hmmm!!!

4. 6.6/7.8L Ford Diesel

#4 is the 6.6L 225 HP Ford Diesel and its bigger brother the 7.8L This is a Tractor Engine that started life in Dearborn and moved to Daganham, England and then onto Brazil. It is a bullet proof engine made from the very best materials and components but it is a bit heavy-not unmanagable but bulky. It is a turbo'ed engine and can be had with or without an air compressor mount.

Navistar 366 Diesel

#5 is the Navistar 366 Diesel. A bit of a long and heavy engine like the Ford but it has a simpler and more modern design and is based on the world famous Navistar 466.

None of these engines is a ready fit into the Clipper engine compartment, but a custom engine rail system will make the job much easier. This is where the profesional installation come in. Mounting, angles of inclination, proper rear axle ratios, engine aeration piping, linkages, fluid piping, electrical cabling routing, all leave room for error and possible catastrophic consequences if a small but important point is overlooked. Even when done correctly, things can go wrong. I Know. My shop is located on Murphy Avenue in Atlanta and Murphy's Law Offices are just down the street."

Back To Your Author

I started this article as one espousing the virtues of the Diesel power choice. I sincerely hope this didn't turn into a "sour grapes" sort of thing - I solved my personal dilemma by choosing a '73 Ford 391 and Allison AT534 combination that I happened to have in stock. But I don't think so. I've always liked that clack-clak-clack sound, and the smell of diesel smoke, and most importantly, the low-rpm pulling that only a diesel really does. But a big-block tuned-for-trucks gasser pulls down fairly low, too, and has a much wider power band (fewer gears needed to stay in the wider "sweet spot." So I hope this article won't fall on deaf ears! And whichever way you go, best of luck with it, and I hope to see you on the road one day!

-Pete Snidal, FOI 415

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