A dry sump moves the engine’s oil supply out of the pan and into an external reservoir, using multiple scavenging pumps to evacuate the crankcase and a separate pressure stage to feed the bearings. The goal is simple but critical: deliver de‑aerated oil at stable pressure regardless of lateral/longitudinal G, RPM, or vehicle attitude. By controlling oil aeration and slosh, dry sumps prevent pressure collapse in long, high‑G corners and at very high engine speeds, while reducing windage losses. The trade is weight, cost, and packaging for robust lubrication, a lower engine height, and consistent performance in racing, track-day, off‑road, and endurance duty.
The wet sump’s limits show up when oil sloshes away from the pickup, or when the crank whips the oil into foam at high RPM. Both conditions lower effective bulk modulus, the pump ingests air, and pressure drops—first in transients, then catastrophically. Even with baffles and trap doors, sustained 1.3–1.6 g lateral and hard braking can unmask these limits. A dry sump decouples oil storage from the pan.
One or more scavenge stages pull oil and blowby out of the crankcase (and often heads and timing case), sending it to a tall, baffled tank that strips air. A separate pressure stage feeds the galleries from the tank. The scavenge side is sized larger than the pressure side to maintain crankcase vacuum and keep the pan nearly dry, cutting windage and stabilizing the supply. Oil aeration: At 7000–9000 rpm, the rotating assembly shears oil into entrained air.
Aerated oil is compressible, so the pressure regulator hunts and bearing minimum film thickness drops. Dry sumps attack this two ways. First, combined scavenge capacity is typically 1.5–2.5× the pressure flow, yielding −10 to −40 kPa crankcase vacuum and fast evacuation. Second, the reservoir provides residence time and de‑aeration via tangential inlets, diffuser screens, and internal baffles—turning a 20–30% air volume fraction at the scavenge outlet into single‑digit percent at the pressure pickup.
The result is steadier 3.5–5.5 bar gallery pressure across the rev range. G‑limits and attitudes: A well‑baffled wet sump might maintain pressure to ~1.2–1.4 g lateral for a few seconds; the best road‑legal wet designs stretch to ~1.6 g with very deep sumps. Dry sumps routinely handle continuous ≥2.0 g lateral and 1.4–1.6 g braking without pressure dips, because the pickup is in a vertical tank, not in a sloshing pan. They also tolerate sustained pitch/roll—useful for off‑road vehicles that see ±30–45° inclines where wet pickups unport.
Endurance cars report pressure stability within ±0.2 bar lap‑to‑lap despite fuel load changes and tire grip variation. Scavenging stages and sizing: Small four‑cylinders often use 2–3 scavenge stages; V engines and flat engines commonly run 3–5 to cover each bay, heads, and turbo drains. As an example, a 5.0 L V8 at 8000 rpm might need 40–60 L/min pressure flow (hot, 100–120 °C oil). Total scavenge capacity would be 80–120 L/min, split across stages located low on the front cover and driven by a belt or gear.
Lines are typically −10 to −16 (5/8–1 in) to keep suction velocities low and minimize cavitation. The pan is a shallow tray with scraper and directional ramps; with the oil evacuated, wall shear and ring windage drop, often freeing 1–3% power at high RPM. Packaging compromises: The external tank (8–12 L capacity for many performance engines) adds volume and mass, and must be mounted upright, above the pump inlet, and away from crash zones. The system adds 5–12 kg including pump, tank, lines, and a larger cooler.
More oil takes longer to warm; most setups use a 90–100 °C thermostat and bypass to avoid over‑cooling. The upside is a very shallow pan—often 30–50 mm—allowing the engine to sit 20–40 mm lower for center‑of‑gravity and bonnet height targets, and more subframe or diffuser freedom. But there are more potential leak points, pump whine at idle/light load, and belt or gear drives that need inspection. When and why to choose it: Dry sumps are justified when the duty cycle includes sustained high lateral loads (>1.5–2.0 g), very high engine speeds (≥7500 rpm) where aeration becomes limiting, prolonged off‑camber operation, or when packaging demands a very low engine.
They also help emissions control by reducing oil consumption versus wet sumps under high vacuum and long sweep corners; improved air‑oil separation can cut oil carryover to the intake by 20–50%. However, crankcase vacuum alters PCV calibration; makeup air must be metered to protect lambda control. Cost and complexity are substantial compared with a gated wet sump. Implications: Reliability improves markedly—no starvation scars on bearings, more stable pressure and temperature, and better ring seal from crankcase vacuum.
Emissions can benefit from lower oil ingestion, but cold‑start HC may rise slightly if the larger oil mass warms slowly without a thermostat. Real‑world drivability is neutral if the pump is well isolated; NVH can rise at idle. For track and endurance programs, the trade pays back quickly in engine life and lap‑to‑lap consistency. For daily use, a well‑designed wet sump with baffling is lighter, cheaper, and sufficient below ~1.3–1.5 g and moderate redlines.
