The 911-sourced engine is a Dean Polopolus “Polo 4,” a 2.5-liter twin-plug four-cylinder with 911 S cams. Polopolus, owner of Advanced Performance Engineering, San Juan Capistrano, California, designed this special engine taking into account what the Porsche factory had itself come up with many years earlier.
In 1962 Porsche asked Paul Hensler to investigate a four-cylinder motor based on the architecture of the 911’s six. He found that the internal dimensions limited the stroke, and the head stud pattern limited the bore, so the displacement at that time ended up around 1320 cc. Porsche scrapped the program, and the flat six went on to make history.
In the mid ’80s the air-cooled 911 had reached 3.3L, and it became apparent to Dean that a four-cylinder 911 of just 2.1 liter could be more powerful than the original six. It also became apparent to Dean, after building numerous high-performance 356 and 912 race motors, that a durable motor for the early Porsches was something to be desired.
To the question of building a four-cylinder from a 911 engine, he answers, “The four-cylinder brings some of the early 356 into the 21st century. Almost everyone would like to have a few more cc and another gear to drive on the freeways of today or even the Autobahn. Here again, you see benefits. The four-cylinder outlay in expenses is rewarded in durability and power; there are no trade-offs.”
The value can be seen in the uncompromised German engineering and the quality of parts to assemble. Whether it is a four or a six makes no difference; the premise is the same. Ferry Porsche himself said, “If we knew it had so much potential, we would have made it smaller.”
The Polo 4, which can be built with displacements from 1500 cc to 2800 cc, is typically very reliable and boasts plenty of horsepower and torque, but a few components must be changed and added, including removal of the rear engine shelf. Since the 911 engine is dry sump, an oil tank had to be built, patterned after a 356 Carrera tank, and it was modified to fit in the right rear fender. Some internal baffling was added to avoid starvation in hard cornering.
One of the prime advantages of the Polo 4 is it weighs 100 lb less than the six. It is also shorter (16.25 in.) from mating surface to the nut on the rear pulley, has twice the power of the original 356 engine, and fits properly into a 356’s engine bay.