The Caterpillar D9G was introduced in 1961 and nearly 15,000 of they were produced through 1974 when it was superseded by the D9H which was a similar machine with the same D353 engine. However, aftercooling in the D9H supported a horsepower increase from the 385 standard in the D9G to 420 provided in the D9H.
This machine has what cat calls the D353 engine. It has 1480 or CU in. (24.2 liters) turns at a relatively low RPM, likely 1200 but I'm not sure of that. Early models had pony motors to start them but by 1970 electric start was standard, and our machines though older have been retrofit with electric start.
The D9G is the the classic large dozer that helped sustain Caterpillar as a legend. It is a well balanced machine, and effectively a D8 of the day on steroids.
The big problem with a D9 is moving them from job to job because they are heavy and have a 16 foot wide blade. making transport difficult, but on the job the D9 has always been a good performer. It has modestly less horsepower than the Fiat Allis 31, but is vastly more nimble. It's quick and responsive compared to the 31.
Cat has a way of evolving their models. They have been making D9 dozers since the 1950's but not all D9's are equal. As they have changed them the letter following the D9 has been incremented. The current model is up to about a D9R, but this 1988 model will give you a feeling for what the the current dozers look like. Horsepower creep means that every new model is a little bigger and a little more powerful than the previous model. Cat has been using this high drive design for about 20 years now, but no one else has emulated it.