In a post at the Petty Racing site, “SpacemanSpiff” described why Petty Racing is not using the new Mopar NASCAR engine, as Penske is.
Several factors are involved. First, it appears that development was curtained by GEM for a time due to doubts about Dodge’s continued NASCAR involvement and Gillette’s commitment to Dodge.
The new engine, R6P8, is the sixth generation R-block and eighth generation polyspherical head; it is very different from the previous generation, and requires a major redesign to the car. The engine is also 100 pounds lighter, so the weight distribution in the cars needs to change, requiring more development time and making it inappropriate, again, to attempt to put the new engines into cars designed for the existing ones.
NASCAR’s Car of Tomorrow, according to this source, is less forgiving regarding handling and weight distribution, and there is a ban on testing at NASCAR tracks, which makes the situation more difficult. Penske did run the engine last year in some races, but apparently does not share their information with other Dodge teams.
The R5 was a left-bank-leading engine, while the R6 is a right-bank-leading engine, due to NASCAR’s many specifications; the distributor moved to the front of the engine, and many parts were designed for an integral cable-driven fuel pump drive (electric pumps not being allowed). The R5 was retrofitted to allow this, but the R6 was designed for it. The R6 also has a shorter block, despite slightly wider bore spacing, and has a narrower head casting; some external oil lines were replaced by internal passages. There is now more coolant flow through the engine, and it can be directed by engine builders as needed via orifices (as can oil flow; there is room for oil restrictors as well). This engine therefore has more consistent horsepower for longer periods.
(Thanks to “Hemi Magnum” and “Mopar Bob.”)