In the early 70s, the "92" series became popular for bus installation, with the V6-92 replacing the V8-71, at approximately the equivalent horsepower. In addition the newer engines became turbo-charged, although many of the V8-71s have been turbo charged. The newer V6-92 has better balance and better low end torque.

Along came air pollution concerns and then standards. The two stroke engine is definitely a dirty motor so the engineers at Detroit developed computer controlled fuel systems for the 92 series. This new system was known as the D-DEC, or Detroit Diesel Electronic Computer control system. More recently, Detroit has begun marketing their new series 60 engines for bus and truck power. This new engine is a four cycle engine and is much cleaner burning and has a very high efficiency rating.

However, many of us with older buses and limited budgets must be content with the older engines. Fortunately, parts and kits will continue to be made for these power plants. In addition to genuine Detroit parts, there are several after market manufacturers of parts for the older engines.

The old stand-by engine, the V8-71, is nominally rated at 318 horsepower and delivers approximately 270 horsepower to the wheels. With this motor, pulling a long tough grade tends to tax the patience of most converted bus drivers, having to be content with 25 to 30 miles per hour. So it has become popular to upgrade to the V8-92 motor. This upgrade must also include an upgrade in cooling capacity. The bigger motor produces nearly 500 horsepower and an associated increase in heat. The simple way to increase the cooling capacity is to replace your radiator with the one recommended by the factory. In some cases, the factory no longer exists, so consult with the engine manufacturer for a recommendation. In many cases, additional radiators have been added and seem to work satisfactorily.

The standard injector size for the 71 series is 60, and for the 92 series is the 90. In many cases, additional horsepower can be achieved by increasing the size of the injectors. Furthermore, even though it seems illogical, increase in fuel economy has been achieved with the larger injectors. It is common in the Northwest for logging trucks with V6-92 engines to install 105 or 110 injectors, and get not only more power, but better fuel economy. I have discussed this with Detroit representatives who have told me, "Yes, this is true, but it will shorten your engine life." So, I make no recommendation, only reporting what I have heard.

Transmissions installed in earlier buses have always been the four speed manual shift type. Many of the earlier city transit buses used a three speed automatic, but since a city transit bus is not recommended for bus conversions, this will not be discussed (I hope nobody gets their nose out of joint, sorry).

In this country the most common automatic transmission is the Allison. The Allison transmission is virtually bullet-proof. The 700 series is a heavy duty transmission and is used for semi-trucks with loads up to 80,000 pounds, and for heavy earth moving equipment. When using an V8-71 engine, the 654 Allision is a natural match. The 5 in the designated number is the number of gear ratios in the transmission. The most common automatic used against the 92 series engine is the Allison 740. This is a four speed heavy duty transmission. The reason a four speed transmission may be used on the 92 series engine is the motor has a reasonably high torque rating at low RPM. The 71 series engine does not achieve a usable torque until it reaches a high RPM , This means the RPM on the 71 must be maintained at a higher speed in order to achieve a minimum torque rating and avoid lugging the motor, hence the need for more gear splits.

The latest transmission to come on the scene is the B500 Allison World transmission. This transmission along with the new Detroit series 60 engines are now available in brand new bus shells from Eagle Coach Corporation.

For those of you on a limited budget, the ten speed manual transmission is a vast improvement over the old four speed box. Unless you have had trucker experience, the idea of ten speeds sounds formidable but actually they are quite simple. There are five gear locations in low range, then a splitter is actuated and the same five locations are repeated. The average ten speed transmission can be picked up at a truck salvage yard for about $800 to $1,000. More recently, a nine speed unit has become popular for around the same price range. Again, the Fuller Ten Speed Road Ranger is virtually bullet-proof when installed in a bus conversion. These units are made to work flawlessly in 80,000 pound semi-trucks, so a 35,000 pound bus conversion is an easy task.

One of the delightful features of the ten speed Road Ranger is the clutch is necessary to use only for starting, while idling at a stop or in moving away from a dead stop. After that, with a little practice, each gear may be shifted manually by synchronizing the move to the engine speed without the use of the clutch. In a way, it is almost like a poor-man's automatic. Another thing about the ten speed transmission is on a flat surface, it is common to move away from a dead stop in third gear. This leaves two lower gears to use when in an awkward position, such as being parked on a hill. The old four speed box was very unforgiving when the coach was parked facing up hill. It was necessary to rev up the engine to get up enough torque, then let out the clutch to get moving. This often fried the friction material of the clutch, thus wasting it. The author bought two new clutches before he had a ten speed transmission installed in his first coach. In an up hill condition, the ten speed transmission was simply put into first gear, often called granny gear, the coach would then simply crawl away from a dead stop at almost walking speed. This chapter is designed only to give an over view. No doubt, newer and better options will be available as this goes into print.