The Nettuno specs list a 90-degree angle for the engine, a match for the F154. A cursory glance of the rear also shows a layout that is very similar to Ferrari's V-8. The 88mm bore and 82mm stroke are a direct match for the F154 as found in the SF90 Stradale. The rev limit of 8,000 RPM is also a match for the SF90, which leads us to believe that development of the Nettuno may have started by removing two cylinders from that engine. The firing order of 1-6-3-4-2-5 for the Nettuno is also the same as the firing order of the F154-based V-6 found in Alfa Romeo vehicles.
The most apparent connection to the F154 can be seen from the rear. The bottom right corner of the rear cover uses the same bolt pattern and flanges on the side of the block, similarly placed to the Ferrari engine. The same crankshaft position sensor and mounting can also be seen in the valley of the engine.
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The Nettuno does run an 11:1 compression ratio which does not match any Ferrari engine, and that is likely due to the head design. The heads are unique and appear to be a completely new development by Maserati. They feature dual spark plugs and a “pre-chamber,” which is a small combustion chamber that is connected to the traditional combustion chamber by a series of uniquely designed holes. This “pre-chamber” system is based on Formula One technology.
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While the cylinder head design is unique and the patent shows that Maserati has done large scale development on its own, the engine has a large basis on the F154 V-8. The big difference now is that this new engine will be assembled by Fiat Chrysler Automobiles, not by Ferrari. That also means that other bits around the engine have been borrowed from the FCA stable. One of these items clearly visible is the A/C compressor, part number 50537067, a direct match for the unit found on the Alfa Romeo Quadrifoglio models. There's even an Alfa Romeo logo visible on it. The alternator is also visible as Denso part number 670050552, another part directly from the Alfa Romeo vehicles.
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While Maserati has done significant development in-house, particularly with the novel head design, it's clear that the engine has roots in the Ferrari F154. While parts of this engine are unique and certainly "100% Maserati," the entire engine is not.