The Clipper Manual: The Power Pack

The Power Pack

This expression, of course, refers to what combination of elements have been chosen to twist the drive flange on your differential. It will start with an engine of some sort, run through a transmission or gearbox of some type, and end with a drive shaft built to link the trans output flange with its input counterpart on the differential.

The terminology here is deliberately vague, for the reason that Clippers have been fitted with a huge variety of power combinations. They are very flexible in this regard, having been built with lots of room in the engine compartment - especially the Visicoaches and Starliners, which were built with even more.

The original pre-war Clippers were powered with Buick straight 8's. There wasn't much choice in those days for higher-powered engines. In fact, the 270 cubic inch GM 6 was used well into the early '50's in medium trucks, so in this milieu the 320 cid Buick looked pretty powerful. During the period thus delineated, Flxible provided a number of engine options - the White and Fageol truck engines were supplied in some, and they even tried a Twin Engine arrangement, using two Chev truck engines in tandem. The Buick engines, having originally been designed in the early '30's for automobile use, although re-tuned for low-rpm running and greater torque, were not particularly robust. Changing engines was routine for the shops of the bus companies that ran Flxi's with Buicks. On the other hand, my '55 came to me in '99, with the Detroit welded in place, and it didn't look as if it had ever been out. It certainly hadn't been installed with easy re&re in mind!

By the time of the Visicoach - 1950 - they were beginning to standardize on the Detroit Diesel, in the 4-71 inline configuration. For gears, they used the 5-speed Spicer gearbox, giving 4 synchro road gears, and a reverse and "granny" gear for low-speed crawling. The 71-series Detroit engines were developed during WWII and saw extensive service in small boats - LST's and the like - and in tanks. It is obvious they were designed with robustness in mind, with the weight consideration being the last in line - if at all. They were, however, a godsend to those who needed diesel power in a hurry and for a low price. The power problem was finally solved for the medium-weight Sherman tanks, for instance, - after a lot of makeshift attempts - with a tandem arrangement of 6-71 Detroits. Top speed, however, was limited to 18 mph, by that time, (the Korean era) hardly fast for a tank. (The 71 Series Detroits came in a number of configurations, sharing many of the same elements, such as internal engine parts, accessory drives, etc. For a bigger engine, they just added more cylinders - inline 71's came in 2,3,4, and 6. There are also V configurations - 6V, 8V, and 12V-71.

There are a number of Visi's and Starliners still on the road with the 4-71/Spicer 5-speed combination. It is, however, at least in this writer's opinion, hardly the best option in today's market. The 4-71 is very large and heavy - it weighs 1500 Pounds more than the Buick 8, for example. It has a typically Diesel narrow power band - 18-2200 rpm at best. Thus the driver has 400 rpm to deal with - and a 4-speed transmission with which to find the right ratio. Thus, while it's satisfactory for pulling the prairies, it's limited to one of four roadspeed ranges when pulling mountain hills. Diesels, being rpm-range limited, need more gears than four to maintain a range of roadspeeds on hills. That's why trucks use such as the 16-speed Roadranger gearbox. Similarly, if you're going to use an automatic, such as an Allison, you'll need a 5 or 6-speed for satisfactory gearing with any diesel. Thus there are two considerations within which the 4-71 gets a failing grade: its high weight/power ratio, and the rpm limitations of all diesels. If you want a diesel - and the advantages are obvious, centering mostly on fuel economy - there are many much better choices in today's modern market. And a 5 or 6-speed automatic to complete the package is also a very good (and also expensive) idea.

Thus the suggested conclusion is that if you still have the DD 4-71, and it's still working, it makes sense to use it. But if you have one that's ailing, it's a good time to consider re-powering. And because of the roomy engine space, and the fact that they were built with no special commitment to any particular power package, the sky's the limit. Availability being the major consideration. It should also be borne in mind, however, that the Spicer gearbox supplied with the Detroits was in most cases an overdriveunit - meaning the top gear provided a ratio of greater than 1:1. Third gear was the direct one. Thus, if you're changing to an automatic transmission, your engine will have to produce more rpm than the DD's 21-2200 to maintain the same top road speed.

What Engine?

The choices are many. You may consider reading these discussions on GAS and DIESEL.

A better question might be: "From Where?" You can buy a factory-fresh engine from the appropriate dealer, of course, but most of us don't have the resources for that. If you buy a used engine, unless you're very familiar with its immediate history, you'll want to take it apart and replace what parts are required - this could result in anything from a general "freshening" - new rings, seals, gaskets, and bearing shells, valve refacing, etc. - to a complete rebuild, involving rebore and new pistons, crankshaft regrind, the works.

You don't want a car engine. You need a heavy-duty Truck engine, set up to develop the low-end torque required for the higher load of a bus or bus conversion. Engines from dead trucks can pretty well be counted on to have had the guts run out of them by the time they hit the wrecking yard - unless of course their career was nipped in the bud by an accident of some kind. My particular choice is school buses. They are in most cases well maintained by the school district or contractor, right up until the time they are retired due to age restrictions, often still in very nice condition. If bought directly from the source, the maintenance records are made available to prospective bidders (sometimes they are sold at auction, sometimes offered for outright sale), and you may find one with a low-mileage rebuilt or replacement engine. They often have an Allison or similar heavy-duty automatic transmission as a bonus, and if gearing is a problem, (see above) you have a rear axle assembly which can be substituted for your old one simply be being turned upside down, after of course some messing with new filler and drain plugs, brake pot mounting brackets and other such considerations.

Or, how about a complete front axle assembly, complete with fresh kingpins, brake pots, lines, brake shoes and drums, and even wheels and tires? If the measurements are right, this can be a one-day swap, resulting in a "rebuild" of any and all these parts in one fell swoop! Furthermore, you will find many fittings on a "wreck-it-yourself" schoolie which will make it a worthwhile investment indeed. Worth much more in pieces than together!

Newly-acquired old Clipper with donor schoolie in background

Thus, if you have space for it, purchasing a complete "district-fresh" schoolbus can be a great investment. Once you're through with it, your local scrapyard can often be persuaded to take the shell away; their usefulness as excellent weatherproof and secure parts sheds is legendary. They can be had in diesel or gas configurations, with air brakes (good) or hydraulics (useless), and with the same wheel (and therefore tires) pattern as your Clipper. 10-stud Budds - you want the drop-center tubeless types.


The "T-Drive" configuration of the Flx Clipper series dictates that engine length is important - the total power pack, consisting of drive shaft, transmission, and engine, must fit between the rear-facing axle drive flange and the fan shroud just ahead of the rear engine door. A short drive shaft, flange to flange, is about 14 inches. The total remaining distance is about------ inches, for the engine and transmission.


If you decide to use an automatic transmission, you'll need to have a custom linkage cable made to link a front shifter to the transmission in the rear.

The "Cradle" Concept

The Buick-engined Clippers most commonly had their engines mounted in a quickly-detachable "cradle" which could be slid in and out of the engine compartment on a specially-built trolley. The DD 4-71 in my bus took up so much room there was no space for any cradle, but I really wish I'd known about it when I put in the much smaller big-block Ford and Allison combo I used. This is obviously a great idea for power train swaps for a number of reasons. For example, a cradle can easily be cut-and-fit built in the compartment space, then brought out to the shop floor, where the engine and gearbox mounts may be fitted much more easily than in place in the compartment. Then, once it's all together, it can be slid in and bolted up as a unit.


A 320 Buick Engine And Cradle picture from a Flxible sales brochure

Cooling Considerations

Proper cooling is of course dependent on proper airflow through the radiator. This airflow is provided in two ways: first, at road speeds, from a "ram" effect through the airscoop at the rear of the roofline. This air is ducted through the radiator, across the closed space above the engine and in front of the engine door, and forward through the fan shroud. This airflow must then deflect downward and be picked up by the airflow under the bus in a rearward direction. "Air stall" - from the meeting of rear-flowing air into the engine compartment meeting the forward flow through the fan - was prevented by a full belly pan under the engine, with rear-facing louvers to allow flow down and back. This bellypan formed the bottom of a completely enclosed engine compartment, baffled at the front, just behind the rear axle, to prevent the entry of rear-flowing air from under the bus.

This principle seems to have escaped many who have chosen to re-power their Clippers, with resultant problems in cooling due to the Airstall mentioned above. Many of them failed accurately to diagnose resultant cooling problems, instead attacking them with such bodges as auxilliary cooling fans, special water pumps, misters, larger radiators, etc., thinking it to be associated with the switching to a larger engine. Although the usual cooling considerations are still relevant, providing for proper cooling system airflow is of major importance.

The Mud-Flap Bodge

There is one popular bodge with which some owners report success. Since the belly pan is usually missing, and usually awkward to duplicate, some owners have elected to build an "air dam" - a full-width mudflap behind the rear wheels.This mudflap is mounted at the bottom of a full baffle designed to prevent airflow from floor to mudflap. Thus the rearward underbus airflow is directed to the sides, hopefully even aiding in the creation of a low-pressure area behind it, which will help the proper flow through the scoop, above-engine duct, and fan/shroud.

Low-Speed Cooling

The second airflow provider is of course the fan itself. At speeds beow those which provide for "ram" effect from the airscoop, the fan comes into play, drawing air via the duct work and radiator through the fan shroud, and dumping it into the space under the engine. This route should be as airtight as possible - right up to the point at which it exits the shroud. If cooling difficulty is experienced at slow speeds, such as in heavy bumper-to-bumper traffic, it can often be improved by attention to the engine door seals - any air drawn through the door joint by the fan is air not drawn through the radiator. The cure, of course, is to get to work on those door seals!

"Mr. Mister"

Another popular bodge is to add a "mister" to your cooling system. A spray nozzle or two ahead of the radiator, which distributes a fine water spray and can be activated from the driving position. Water can come from a separate tank, or from a dedicated tank located near the radiator tunnel. The latter is best, since distilled (rain) water is best used, to prevent scale buildup between the radiator fins. Pump can be a second RV water pump, or a high-output windshield washer pump. This is an old gag used by the travel trailer and stock-car crowds, and just may help get you over the hump in marginal situations.

Click HERE to return to the Clipper Manual page.

Skip to toolbar