The department apparatus speccing committee, consisting of a battalion chief, a captain, and six other department members, was discussing the power train requirements for a new pumper and a new tractor-trailer aerial apparatus. The last department apparatus purchase was a pumper, acquired eight years ago. Only two of the officers on the present committee were on the department back then.


The committee discussed how to evaluate and select engines for new apparatus. Should it stay with the same make of engine the department has purchased since it began purchasing diesels in the early 1970s? The two models of the engine brand purchased previously were obsolete and no longer available. Should other engine brands and models be evaluated? What was the real horsepower required for each apparatus? Can the same engine be standardized on both trucks? How can these questions be answered and a decision be made?


The committee should first identify the operational application and utilization factor. In this case, the city is 10 miles at its widest part and six miles at the narrowest point and includes roughly 60 square miles. Three stations are strategically placed in the jurisdiction so one apparatus does not have to cover the entire area. All the streets are paved and are generally in fair to good condition. The apparatus will not operate off-highway or off-road. The longest hill in the city is slightly less than one mile long with a 21/2-percent grade angle. Another hill, a quarter-mile long with a four-percent grade angle, has several apartment complexes at the top. The committee determined that both new apparatus should be able to climb this grade at a minimum speed of 40 mph if possible.

Next, the committee should determine the gross vehicle weight rating (GVWR) of each apparatus to be purchased. The pumper will be a custom cab and chassis with a 40,000-pound GVWR (16,000 on rear axle, 24,000 on front axle). The three-axle tractor-trailer aerial ladder will have a 66,000-pound GVWR (18,000 on front axle, 24,000 on drive axle, 24,000 on tiller axle). Both trucks will operate at approximately full GVWR at all times. The pumper will have a 1,500-gpm, two-stage fire pump requiring a minimum of 325 horsepower to operate. The department has a formal policy of a 65-mph top speed.

Finally, the department plans to operate both trucks for 20 years in first-line service and for an additional five years on backup or reserve service. The pumper will be driven an estimated 9,000 miles annually (180,000 miles total over the first-line service life). The aerial will be driven approximately 6,000 miles annually (120,000 miles over the first-line service life). The department strongly desires that neither the engine nor the transmission components selected for each truck require an overhaul or replacement during the 20-year period.

With this information, the committtee should meet with the local Allison transmission distributor to use the Allison power train performance SCAAN (System for Computerized Application ANalysis). Allow two hours per truck at least when using SCAAN. The service is free.

The Allison SCAAN is quite accurate in projecting vehicle performance and identifies the horsepower and peak torque required to make it up a four-percent grade at a minimum speed of 40 mph. It also identifies the minimum engine horsepower and rear axle gear ratio needed to attain a top speed of 65 mph. Different engine make, model, horsepower, and peak torque variations may be substituted in the program. A printout illustrates the similarities and differences among various configurations.


In the engine manufacturing industry, the two classes of diesel engines are heavy-duty and mid-range. A heavy-duty engine generally has a minimum of 611 cubic inches (10 liters) of displacement, weighs a minimum of 2,000 pounds, has between 300 and 600 horsepower (hp), and has a peak torque of between 1,150 and 2,050 foot-pounds (ft.-lbs.). All EPA automotive-certified engines are four-cycle, regardless of size.

A mid-range engine generally has a 550-cubic-inch (nine-liter) maximum displacement, a 1,500-pound maximum weight, between 175 and 330 hp, and a peak torque of between 420 and 1,050 ft.-lbs.

Heavy-duty engines require substantially larger cooling systems, transmissions, and other components. Thus, moving up to a heavy-duty engine can raise the purchase price anywhere from between $10,000 to more than $20,000. Regardless of engine class, engine pricing is relatively simple. Generally, the larger the displacement, the heavier the weight, the higher the horsepower and peak torque rating, and the higher the price.

When evaluating engines, in addition to the horsepower, note the peak torque rating. This is a power indicator, particularly in terms of engine performance on grades. For example, frequently an engine is offered at 400 hp with a peak torque of 1,450 ft.-lbs. A second version offers 400 hp with a peak torque of 1,550 ft.-lbs. The second version with its higher peak torque rating will help get the vehicle up the grade faster. When writing a bid specification, definitely state the minimum horsepower and minimum torque rating required.

When evaluating engines today, remember how much they have changed in recent years. The diesel engines used in apparatus 20 years ago were typically rated at 175 hp for the mid-range class and between 250 and 350 hp for the heavy-duty class. Peak torque ratings were commonly 400 ft.-lbs. for mid-range and between 650 and 850 ft.-lbs. for heavy-duty engines. Today, the larger size mid-range engines have the horsepower, peak torque, and even the service life expectancy of the heavy-duty engines of a generation ago.

Another point not often considered in selecting an engine today is the application for which it is designed. The heavy-duty engines are primarily designed to haul an 80,000-pound GVW, five-axle highway truck and trailer at up to 80 mph across the country for hours. This involves a high GVWR, high speeds, and fierce wind resistance. In addition, these engines are designed to provide a 700,000-mile-plus service life for a highway truck application. Using one of these engines to power fire apparatus is child’s play.

To achieve maximum engine service life and fuel mileage, highway truck engines today are governed at 1,800 rpm. However, engine manufacturers also offer these engines in 2,100-rpm versions. When using a heavy-duty engine with an Allison transmission, Allison recommends a 2,100-rpm version to widen the operating range available to the automatic transmission and thus improve performance.

When Detroit Diesel was designing the Series 60 engine in the early 1980s, it designed an in-line six-cylinder engine with two different displacements in the same basic engine block, 667 cubic inches (11 liters) and 778 cubic inches (12.7 liters), by changing the length of the stroke. Today the Navistar DT 466E and 530E mid-range engines use the same design concept, as do the Caterpillar C 10 and C 12 heavy-duty diesel engines.

The in-line, six-cylinder diesel engine is marketed to the fire service today as the Detroit Diesel Series 40E and also as the Navistar DT 466E and 530E engines. Navistar has a marketing agreement with Detroit Diesel to sell these engines for fire service applications. The Navistar engine, however, is not equipped with the Detroit Diesel DDEC IV electronic control and fuel injection system.

When evaluating any of these engines, remember that in going from a smaller displacement to a larger one, there is an increase in price. If your Allison SCAAN indicates that you really don’t need the additional horsepower the larger engine block provides, why spend the additional money?


One last item to discuss is engine electronic control systems. These systems now control and manage all engine functions, and all diesel engines are equipped with them to reduce exhaust gas emissions. However, they are not all the same; some are definitely better in design and reliability than others. Moreover, they are all frequently and continually changing in design.

Detroit Diesel, on its Series 50 (four-cylinder) and Series 60 model engines, offers an optional pressure sensor governor (PSG) that is integrated into its electronic control system. This is of interest when speccing a pumper. The PSG allows automatic fire pump sensing and pressure control that continually senses the fire pump output pressure and continually adjusts the engine rpm accordingly to keep the water pressure constant. The PSG must be specified in the engine portion of your bid specification.

However, just because this feature is integrated into the engine electronics doesn’t mean you are automatically going to receive it when purchasing these engines. The apparatus manufacturer must wire the Detroit Diesel Electronic Fire Commander control system into the pump panel. In other words, it must be fully specced in detail in your bid specification.

You can also get automatic fire pump sensing and pressure control with Caterpillar, Cummins, Detroit Diesel Series 40, and Navistar International engines. The Class 1 Captain Pressure Governor and the Class 1 Engine Status Center (ESC) are two different panels mounted on the pump panel. They provide all of the monitoring and control functions as the single Detroit Diesel Electronic Fire Commander product.

Should you desire only electronic engine monitoring on the pump panel without pump pressure control, there is the Fire Research Corporation Tachplus all-in-one instrument panel. This product gives the engineer all of the engine operating information and data he needs in one compact instrument. Tachplus is designed to accommodate all electronically controlled engines.


Allison Transmission, a division of General Motors, produces the World automatic transmissions, which are electronically interfaced to the electronically controlled engines. This allows these two major power train components to continually make operating decisions together and ensure maximum vehicle performance at all times.

The fire service uses the MD series (primarily used with mid-range engines) and the HD series (primarily used with heavy-duty engines). Each series is available in two operating gear ranges: wide-ratio for vehicles requiring lower gearing at the start for vehicles used in both on- and off-highway applications and a close-ratio for vehicles operated on-highway. The more expensive wide-ratio models are designated MD 3560 and HD 4560.

The close-ratio models are MD 3060 and HD 4060. The MD 3066 is available only in close-ratio gearing.

In emergency vehicle applications, the MD 3060 and 3560 have 300 gross input horsepower and 950 ft.-lbs. maximum input torque ratings. The HD 4060 and 4560 have 525 gross input horsepower and 1,550 ft.-lbs. maximum input torque ratings.

The MD and HD series transmissions are all manufactured with six forward gears in the case. The fourth gear is direct, the fifth gear is overdrive, and the sixth gear is double overdrive. If desired, the fifth and sixth gears can be electonically locked out when speccing. In the fire service, it is common to lock out the sixth gear double overdrive, which significantly reduces driveline speed. Controlling and limiting driveline speed are most important. The five-speed version is definitely the most popular version used in fire service applications.


If your new apparatus requires a power take-off (PTO), add the letter “P” after the model selected, which indicates the transmission will be manufactured with a provision for a PTO assembly. If Allison’s integral output-type retarder is desired, add the letter “R” after the model selected.

If speccing the World transmission integral output retarder, be aware that it requires an external oil-to-water heat exchanger to be installed by the chassis OEM. Additional shared oil is added to the transmission to help cool the retarder. The truck chassis cooling system has to adequately cool the retarder in addition to the engine and transmission.

Allison has strongly recommended the Stewart Warner Southwind product line of external oil-to-water heat exchangers for use with its transmissions. When speccing a new apparatus with a World transmission that includes an integral output retarder, specifically require that “the heat exchanger furnished be of a make and model approved by the transmission manufacturer for the specific make and model of transmission furnished.” This wording could help you should the transmission fail during the warranty period.

In May 1999, Allison introduced its new TranSynd full synthetic automatic transmission fluid. Castrol and Allison jointly developed the TranSynd product for more than two years, and Allison endorses and recommends TranSynd as the only full synthetic automatic transmission fluid for use in its transmissions. With standard petroleum-based automatic transmission fluid, the Allison recommended change interval is now 12 months/25,000 miles for transmissions not equipped with retarders and six months/12,000 miles for transmissions that are retarder-equipped. With the new TranSynd synthetic oil, the recommended change intervals are increased to 48 months/100,000 miles for nonretarder- equipped transmissions and 24 months/ 50,000 miles for transmissions that are retarder-equipped-a 300 percent increase in oil change intervals.

According to Allison, other benefits of TranSynd are superior wear resistance, improved oil viscosity stability, and potentially longer transmission service life. Over the past several years, synthetic-based gear lubricants have generally established a performance history of working better over a wider thermal spectrum (in both low- and high-heat operating conditions) and generally degrade at a much lower rate when compared with petroleum-based products. You should definitely investigate specifying TranSynd in your new World Transmission.

Automatic oil level sensing is one low-cost option (less than $200) that should definitely be included. When the apparatus is on flat ground, this option accurately indicates on the in-cab gear selector if the transmission oil level is adequate, low, or overfilled. The option guards against an expensive transmission overhaul. All of these options, including the retarder if desired, should be listed at the time the apparatus is specced, because they become very expensive to install later.

The World transmission includes a choice of a T-lever or pushbutton shift control. Pushbutton is by far the most popular choice, since it can be dash-mounted easily.

When speccing a new pumper apparatus equipped with a heavy-duty engine, consider speccing the smaller displacement version with the Allison HD 4560 transmission. This permits low-end power and acceleration performance when leaving the station and maximum dollar value by saving money on the engine and spending it on the wide-ratio transmission. Additionally, it maximizes transmission service life since the transmission will not be stressed to its design limits. The transmission life should exceed that of the apparatus itself. The MD 3560 and HD 4560 transmissions have a harder downshift from second to first gear, compared with the MD 3060 and HD 4060.

Aerial apparatus can also be equipped with the HD 4560 to achieve maximum acceleration at lower speeds, but the three axles and high GVWR definitely require a larger displacement engine with a minimum 425 hp.

In addition to recommending the 2,100-rpm heavy-duty engine, Allison recommends that transmission electronics for all World transmission models be programmed to upshift at 150 rpm below the engine-governed rpm. This helps control the driveline rpm and may be done when the transmission is manufactured or later by an Allison dealer.

If apparatus being purchased will travel a significant number of highway miles at highway speed in fifth gear (overdrive), use the Allison SCAAN to select a rear-axle ratio that will set the top speed desired when the engine is operating between 1,500 and 1,600 rpm. This gearing method is commonly used in highway trucks, providing increased fuel mileage and maximum component life by reducing operating rpm and limiting maximum driveline rpm to an acceptable level.

Engine and transmission components have changed in recent years. Gone are the Detroit Diesel 71 and 92 series two-cycle diesel engines that were popular in the fire service for many years. Because of federal exhaust gas emission requirements, all diesels today are four-cycle engines. Recently, the Caterpillar C 10 and C 12 engines have joined Detroit Diesel as popular engine models offering features specifically for the fire service.

Speccing electronically controlled engine and transmission components isn’t like it was in the past. The new components are much better and far more complex in design and construction. To make correct selection decisions, use tools such as the Allison SCAAN program to evaluate and select the combination of engine and transmission you need. Once you have made these decisions, spec them in great detail in the bid specification. A specification such as “Transmission: Allison HD 4060” is inadequate today.


JIM WILKINS is a California-based fire apparatus specification and cost control consultant with a background in chassis and power train engineering. He has a bachelor of science degree from California Polytechnic State University in San Luis Obispo.

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