H-24 36.8L Supercharged

What it is

Bore is 127 mm, stroke is 121 mm, and the compression ratio is 7.0:1. Those figures and the 36.8 litres they multiply out to match the Napier Sabre IIB. Nothing else here was copied from one. Twenty-four cylinders sit as two flat-twelves stacked one above the other, each on its own six-throw crankshaft, and the two cranks gear together at the nose onto one four-blade propeller through a 0.2742 reduction. What that propeller absorbs is what decides where full throttle settles.

Napier phased the two cranks 180 degrees apart. Each carries six throws at 0, 120, 240, 240, 120 and 0 degrees, an opposed pair of cylinders shares every throw, and the twenty-four fires come out as twelve simultaneous pairs 60 degrees apart. An even twenty-four every 30 degrees is the Napier Dagger, which phased its cranks 30 degrees instead. Half a turn of one crank leaves the intervals alone, so what the 180 degrees settles is which two cylinders share a beat: one from each bank, and an exhaust bus takes a single pulse every 60 degrees. In step, both halves land on one side and that bus gets a doubled beat, then 120 degrees of nothing.

A single-stage centrifugal blower holds 1.76 bar absolute, the +11 psi of the IIB's sea-level military rating, and nothing cools the charge behind it because the real engine had no aftercooler. Air arrives near 366 K, heat release scales with density, and that costs a fifth of the air. Knock has room to spare: at 7.0:1 on 100/130 grade avgas the end-gas chain allows 2.126 bar, leaving the rating 0.37 bar inside the ceiling. Napier rated the IIB at 2,400 hp at 3,850 rpm and this preset lands under it. Four sleeve ports per cylinder, no poppet valve and no hot exhaust valve, are most of why the real engine made what it made, and the model has nowhere to put them but the breathing figure.

Things to try

• Intercooler, in the SUPERCHARGER block, reads 0 percent. Wind it up: charge temperature falls, density rises, and the torque curve lifts with it.
• Manifold cap holds 1.76 bar against the 2.126 printed under it. There is room above the IIB's rating, and the later Sabre marks used it.
• VE peak, in the ADVANCED drawer, sits at the 0.95 poppet-valve default. It is the one lever standing in for sleeve valves, and it scales the whole curve rather than reshaping it.