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Originally Posted by JRMichler
Can you tell us more about this engine? Cummins advertises a 2000 RPM redline. Is this a special engine? Or are you just running a stock engine at low RPM? Why not an X12 engine, which is almost 1000 lbs lighter?
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All modern Cummins truck engines are rated for 1800 RPM redline. Years ago they used to be 2100 RPM.
The 15L X15 Cummins that I have in the new truck is from an initial run of prototype engines, but the specs have not changed much, if at all, from the production engines. So basically, yes, we are running a stock engine at lower RPMs.
The X15 makes slightly over 200 HP at idle. So why would one have to run it at much RPMs if the vehicle that envelopes it only needs double-digit horsepower at cruise speed? The X15 makes peak torque at 1000 RPM and slightly below. It will make a lot more horsepower at 1800 RPM, 400 HP to be exact in this configuration. But when climbing a grade I'm not interested in horsepower to a degree, I'm really only interested in torque.
X12 production is not set to begin until next year. I may try one then in the StarShip. The reasoning for the current 15 liter X15 is a unique operational technique that I call "inertia displacement." When chugging along at ultra-low RPM, the inertia of the big block Cummins displacement will enable it to just keep pulling and making torque, vs. a 13, 12, or even an 11 liter engine.
Like the old saying goes, "there's no replacement for displacement."