King Tiger Engine Secrets: The Porsche-SGP Sla 16 X-16 Diesel Prototype of WWII

InSCALE Simmering Graz Pauker Sla 16 Engine. Airintake turbochargers and cooling fans absent

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Inside the advanced German tank engine — Porsche’s Type 203/212 — built for the Tiger II, E-75, and Maus, but cut short by war’s end.

When the Tiger II King Tiger rolled onto the battlefield, its sheer weight pushed Germany’s tank engines to the breaking point. Engineers at Simmering-Graz-Pauker (SGP) and Porsche responded with the Sla 16, an experimental 16-cylinder X-type diesel rated at over 700 horsepower. Known under Porsche’s Type 203 and later Type 212 designations, this compact powerhouse promised to deliver smoother torque, better cooling efficiency, and a lower profile than conventional Maybach gasoline engines. Allied intelligence documents, Porsche archives, and FKFS test reports reveal how the Sla 16 was intended for the Tiger II, Jagdtiger, E-75, and even the Maus, but never moved beyond prototype trials. Today, it stands as one of the most ambitious “what if” powerplants of WWII armored engineering.

King Tiger Tank – SGP Sla 16: 16‑Cylinder Air‑Cooled Diesel Engine (Porsche Type 203 — SGP Sla 16)

In 1943 the German Army Weapons Agency (Heereswaffenamt, HWA) tasked Simmering‑Graz‑Pauker (SGP) of Vienna with developing a new main tank engine for the Heer. The requirement was specific and demanding: an air‑cooled diesel that could be fitted as a direct replacement to the contemporary Maybach HL230 with only minor chassis or transmission changes.

At the time, the Maybach HL230 — a liquid‑cooled 23.1 L V‑12 — was rated at 690 hp at 3,000 rpm on paper but was limited in service to roughly 600 hp at 2,500 rpm because of reliability problems. Adolf Hitler favoured air‑cooled diesels for their perceived survivability and simpler battlefield maintenance; to meet this requirement SGP worked closely with Ferdinand Porsche’s bureau, which had prior experience with unusual engine layouts and air‑cooled designs.

Development and Layout

Porsche’s Paul Netzker led the preliminary design work and produced an ambitious X‑16 configuration: four banks of four cylinders around a single crankshaft. The bank spacing used an unconventional geometry — 135° between the top and bottom banks and 45° between the side banks — producing a compact but mechanically complex package. Internally, the engine combined a sheet‑steel crankcase with a single crankshaft using four master connecting rods; three articulating rods attached to each master rod to drive the remaining pistons.

Cylinders were built as finned aluminum heads screwed onto finned steel barrels to maximise air dissipation. Front‑mounted injection pumps supplied fuel to each bank, with injectors sited in the head and angled toward the 135° side. Four camshafts, placed at the base of the banks, were driven from the rear and actuated two valves per cylinder via pushrods and rocker arms. Intake and exhaust ports were arranged on the 45° flanks, the intake ports sitting at the top of each cylinder bank.

The design was issued SGP designation Sla 16, with Porsche internal type designations used in scattered documents (commonly Type 203; other sources use Type 212 or Type 220 for variants). These internal type numbers are inconsistently reported in the postwar literature and should be treated cautiously.

Induction, Turbocharging and Cooling

Induction air entered through a large filter above the engine and passed through twin turbochargers mounted at the rear. Each turbocharger delivered charge air to two separate manifolds — one feeding the upper bank, the other feeding the lower bank — keeping manifold routing compact. Boost was conservative to preserve reliability.

Cooling was a central challenge for the air‑cooled layout. Two FKFS‑designed fans (~520 mm / 20.5 in diameter) sat above the two upper cylinder banks and withdrew hot air from the narrow 45° V‑sections. The fans were mechanically driven via enclosed, bevel‑gear shafts and universal joints from the extreme rear of the engine and ran at approximately 2.05× crankshaft speed. Paired oil coolers flanked each fan and complex baffling directed airflow across the densely finned cylinders.

While the fan system enabled air cooling in tight packaging, it introduced a major tradeoff: the fans and their enclosed drives consumed substantial shaft power and added considerable mechanical complexity, maintenance burden and failure modes.

Key Specifications

  • Configuration: X‑16 (four banks of four cylinders) on a single crankshaft
  • Bore × Stroke: 135 × 160 mm (5.3 × 6.3 in)
  • Displacement: 36.6 L (2,236 cu in)
  • Compression Ratio: 14.5 : 1
  • Projected Output: ~750 hp @ 2,000 rpm
  • Prototype Test Output: up to 770 hp @ ~2,200 rpm (gross)
  • Net Output with Fans: commonly reported ≈685–700 hp
  • Weight: ~2,250 kg (4,960 lb)
  • External Size (approx): 1.68 m long × 2.50 m wide × 1.15 m tall

To mate with existing transmissions originally designed for the HL230 (3,000 rpm operating speed), the Sla 16 used helical step‑up gearing to raise the output shaft speed roughly 1.5× and lower the output shaft to the transmission axis — an elegant but loss‑bearing compromise.

Prototypes, Testing and Early Problems

Development began with single‑cylinder bench tests (the test unit is often referred to under a Porsche single‑cylinder designation). The single‑cylinder test engine — approximately 2.3 L — completed a 48‑hour endurance run and produced about 47 hp at 2,100 rpm; this result scaled to the X‑16’s target power.

Transitioning from the single cylinder to full engine bench work exposed several practical problems:

  • Lubrication and oil foaming: Early full‑scale testing revealed oil foaming and bearing distress (mains and big‑end failures). Engineers added baffles and revised crankcase ventilation/lubrication arrangements to mitigate foaming and oil aeration.
  • Fan parasitic load: Full engine runs in late 1944 recorded gross power in the 750–770 hp range; however, with the full cooling system (fans and associated drives) installed the parasitic load commonly cited is around 95 hp. That reduction pushed delivered shaft power down into the high‑600s, erasing much of the advantage over existing engines.
  • Thermal distribution: Initial vehicle‑installation trials (notably a Jagdtiger chassis installation) showed overheating in the lower cylinder banks, which required baffle and fan‑geometry changes to correct.

By January 1945 two prototype Sla 16 engines had accumulated several hundred hours of test operation on stands. Porsche’s direct involvement diminished as SGP took on greater development responsibility, though Porsche’s early design work and the single‑cylinder test units were crucial to progress.

Installation Trials and Intended Applications

The Sla 16 was targeted primarily at the Tiger II (Königstiger) and was also trial‑fitted in at least one Jagdtiger chassis for feasibility tests. A Tiger II was prepared to accept an Sla 16 but the installation was never completed prior to the end of hostilities. The engine was also proposed for other heavy German designs and experimental Porsche chassis, but production never materialised.

Production Plans and Program End

Series production was planned for mid‑1945 at Steyr‑Daimler‑Puch, which had experience building Daimler‑built aircraft engines and the industrial capacity to switch to Sla 16 work. Production proposals included simplification changes — for example, reducing the number of high‑pressure injection pumps from four in the prototype to two for production — and other manufacturability changes.

The German surrender in May 1945 halted the program. Most parts, tooling and equipment associated with the Sla 16 were captured by Allied forces, with a large portion reportedly taken by the Soviet Union. As a result, the Sla 16 remained a prototype project and never entered service.

Variants, Larger Concepts and Related Proposals

  • Scaled X‑16 (Type 212 concept): Porsche/SGP engineers explored a larger X‑16 with increased bore and stroke (projected at ~48 L displacement) intended to produce substantially more power for super‑heavy designs. A single‑cylinder test unit for the larger family was reported but no complete engine was built.
  • FKFS 24‑cylinder concept: The Research Institute for Automotive Engineering (FKFS) proposed a 24‑cylinder layout for the Panther II by combining two V‑12 sections on a common crankcase; this paper design targeted ~1,100 hp but remained on the drawing board.
  • Smaller 16‑cylinder designs: Other small 16‑cylinder concepts (reportedly developed with outside firms such as Deutz) appear in design notes and reference lists; these were lighter, lower‑power projects and the relationship (if any) to the Sla 16 is unclear.

Notes on Sources and Uncertainties

This engine program is unusually well documented in technical illustrations and postwar summaries, but wartime record fragmentation has left a few persistent uncertainties. Key areas of disagreement in the literature include:

  • Porsche Type designations: Documents and secondary sources vary in calling the Sla 16 Type 203, Type 212, Type 220, and other numbers. Some designations clearly refer to single‑cylinder test units or scaled variants rather than the production prototype; nonetheless the inconsistency is widespread and unavoidable.
  • Exact production counts: The number of fully completed Sla 16 full engines built is not consistently stated across sources; claims range from a single prototype to multiple test engines. The clear evidence supports at least one single‑cylinder test unit and at least one full‑size prototype engine run on stands.
  • Small numeric variances: Net vs gross power figures, fan horsepower absorption and a few dimensional details vary slightly by source — typical for late‑war prototypes documented unevenly.

Because of these ambiguities, wherever you present a specific Type number, production count or single‑figure claim it is safest to add a short caveat (e.g., “sources vary on the Porsche Type number” or “net delivered power after fan drive losses was commonly reported in the high‑600s”).

Prototype Problems & Program End

Prototype problems & program end. Early bench tests revealed troublesome oil foaming and bearing failures, prompting lubrication‑system baffles and crankcase changes before the full‑size engine runs. A single‑cylinder Type 192 test unit (~2.3 L) passed a 48‑hour test (47 hp at 2,100 rpm), which scaled to the X‑16’s target output. Full‑engine test stands in late 1944 reported ~750–770 hp gross, but the heavily‑driven cooling fans consumed a large proportion of that (~95 hp), reducing net delivered power to the high‑600s. Production was planned at Steyr‑Daimler‑Puch with some simplified changes proposed, but tooling and parts were captured as the war ended and the Sla‑16 never reached series production.

Conclusion

The SGP Sla 16 stands out as one of WWII Germany’s most ambitious tank‑engine projects: an advanced air‑cooled X‑16 that attempted to reconcile compact packaging, air cooling and interchangeability with existing drivetrain arrangements. Prototype testing demonstrated that the design could reach its gross power targets, but practical issues — lubrication foaming, parasitic fan losses, thermal distribution and wartime disruption — ultimately prevented the engine from reaching series production. The Sla 16 remains a compelling demonstration of late‑war German engineering ambition and its practical limits.

Sources: Online & Secondary References

“Maus Engine” by CaptianNemo — For the Record blog (2014): http://ftr.wot-news.com/2014/11/25/maus-engine-by-captiannemo/

Strengths: Concise synthesis of German- and English-language material, with some period illustrations. Useful as a gateway for hobbyists.
Caveats: Not an academic source; citations are limited and some technical data is secondhand. Should be cross-checked against Spielberger and archival documents.

Maybach engine overview — Alan Hamby’s Tiger Tank site: http://www.alanhamby.com/maybach.shtml

Strengths: Long-established reference site, widely cited in the armor modeling community. Offers clear explanations of Maybach HL series engines.
Caveats: Hobbyist-maintained; lacks formal citations and has not been updated in recent years. Good background but not primary research.

VK.com archival material on Porsche’s tank projects (Russian-language): https://vk.com/page-39215368_53036748

Strengths: Hosts scans of rare documents, diagrams, and Russian-language secondary commentary that are not always accessible elsewhere.
Caveats: Provenance of documents is not always verifiable; some scans may be incomplete or taken from tertiary compilations. Recommended only as a supplementary resource, not as a stand-alone citation.


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