918NA

A no-holds-barred conversion of the Lotus Type 918 V8 from twin-turbo to naturally aspirated, paired with a comprehensive chassis upgrade. This page tracks the project's planned approach, current decisions, and open questions. Expect ongoing revisions as the build progresses.

The goal is a track-biased but street-usable Esprit V8 that maximizes the character of the Type 918 engine: throttle response, sound, and the visceral experience of a high-revving flat-plane V8 with individual throttle bodies. Power is a means to that end, not the primary target.

Power target: 400 whp baseline, with anything above as a bonus rather than a requirement. The build is spec'd so that the bottom end, cooling, and oil control comfortably handle 500+ whp, meaning the engine will be loafing at its actual output rather than straining toward it.

Design priorities, in order:

1. Reliability and longevity — the 918 block is irreplaceable
2. Engine character — throttle response, sound, powerband shape
3. Drivability — usable from 3000 rpm, clean idle, street-capable
4. Peak power — important but not the headline metric

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Bottom End

Full modern rotating assembly with all wear and stress surfaces refreshed or upgraded:

• Forged pistons, custom-spec'd to target compression, ring pack, and bore size
• Billet H-beam connecting rods with premium bolts (ARP 2000 or Custom Age 625+)
• Coated rod and main bearings for cold-start protection and oil starvation resilience
• ARP main studs and head studs throughout
• Crank inspected, polished, and reused if it passes inspection; replaced with billet only if necessary
• Balanced and weight-matched rotating assembly to tight tolerance
• Resleeved if inspection warrants, opening the option for a modest bore-out
• Cross-bolted or billet main caps under consideration if the block accepts them

Compression and Fuel

• Target compression: 12.0–12.5:1
• Flex fuel capable with dual maps (pump gas baseline, E85 for upside power)
• Pump gas mode optimized for streetability; E85 mode for track use and maximum output

Displacement

• Stock 3.5L baseline (3506cc)
• If resleeving is performed for refresh, opportunistic bore-out to 86–87mm targeting roughly 3.7–3.8L
• No stroking — custom crank not justified for the marginal gain
• Bore-out is "free" displacement only if the resleeve is happening anyway; not pursued as a standalone modification

Cylinder Heads

The most uncertain part of the build. Approach:

• Full inspection, valve job, guide work as needed
• Mild-to-moderate port work focused on flow consistency and improvement within the casting's limits
• Modest valve size increase if the casting and chamber geometry permit
• Flow bench testing before and after porting to establish actual numbers, not projected ones
• Cam profile and induction tuning to be locked after head flow numbers are known, not before

Camshafts

• Custom grinds, NA-optimized, designed against measured head flow
• Profile selected to match target rpm range and induction system tuning
• Adjustable timing via vernier gears or offset keys for fine-tuning on the dyno
• Beehive or dual valve springs rated for target rpm and lift
• Steel or titanium retainers depending on aggressiveness of final profile

Induction

• Individual throttle bodies — off-the-shelf where possible (45–48mm bore range) with custom adapter manifolds to mate to head ports
• Sand-cast aluminum plenum, F355-inspired aesthetic, designed with proper runner length and volume math first
• Velocity stacks/trumpets matched to plenum geometry
• Drive-by-wire throttle for clean integration with modern ECU
• Plenum pattern retained for future replacement parts
• Dual injector capability per cylinder under consideration for E85 flow headroom

Engine Management

• Standalone motorsport-grade ECU (MoTeC, Cosworth Pectel, or EFI Euro — final choice driven by tuner preference)
• Custom wiring harness — full fabrication, not adapted from stock
• Individual coil-on-plug ignition
• Flex fuel sensor with dual-map calibration
• Wideband O2 per bank minimum; EGT optional for initial mapping
• Per-cylinder knock detection where the ECU supports it
• Calibration on chassis dyno with possible engine dyno time outsourced for initial mapping

Oil System

• Full dry sump conversion (4-stage minimum: 3 scavenge + 1 pressure)
• Dedicated oil tank with anti-aeration baffling
• Oil cooler with thermostat, properly routed
• Engine lowered in chassis if mounts permit, for center-of-gravity benefit

Cooling

• Upsized radiator core with improved ducting
• Higher-flow water pump or supplementary electric pump
• Dedicated transmission cooler for the upgraded UN1
• Proper bleed routing for the mid-engine layout
• Cooling capacity spec'd for sustained track use, not just street duty

Exhaust

• Custom equal-length stainless or Inconel headers
• Primary length tuned to target rpm and flat-plane firing order
• Merge collectors with X or H-pipe between banks
• Modest muffler sized for streetability without killing the engine note
• Thermal management via wrapping or ceramic coating for the tight engine bay

Transmission

• Renault UN1 transaxle with full upgrade kit (gears, bearings, LSD)
• Installation and setup by Lotus PBC
• Spec'd to handle 500+ hp comfortably, well above target output
• Dedicated cooler given the abuse profile of an NA high-rpm motor

Suspension

• MCS 3-way adjustable dampers front and rear
• Spring rates and geometry to be set up around modern tire grip and target use case
• Corner balancing after all weight changes (dry sump tank, plenum, headers) are finalized

Tires

• Modern performance compound (Cup 2, Trofeo R, or equivalent)
• Staggered sizing to suit the chassis and power output
• Sized for grip first, not aesthetics

Power and Torque

These are realistic ranges, not maximum theoretical numbers. The lower end represents the conservative outcome if head flow caps below projections; the upper end represents the favorable outcome if porting and calibration come together well.

Powerband Shape

• Below 3000 rpm: softer than the stock turbo car (180–220 lb-ft) but clean and tractable
• 3000–4500 rpm: building, usable for normal driving
• 4500–6000 rpm: powerband alive, real acceleration available
• 6000–8200 rpm: peak power zone, where the engine wants to live on track
• Throttle response: essentially instantaneous, no turbo lag or boost threshold

Vehicle Performance Estimates

• Curb weight: ~1380–1420 kg (depending on final spec)
• Power-to-weight: roughly 290–340 hp/ton at the crank, depending on fuel
• Performance class: comparable to current 991/992 GT3 in straight-line metrics, with a more distinctive engine character

Highest Risk: Head Flow Ceiling

The 918 head was designed in the early 1990s as a turbocharged production component, prioritizing castability and pulse energy retention over peak NA flow. No established porting program exists for this casting. The realistic flow improvement from porting is uncertain and bounded by:

• Limited valve size growth potential (chamber geometry and valve seat constraints)
• Unknown port shape responsiveness without prior R&D on this casting
• Risk of breaking through to water jackets or oil passages on an irreplaceable core

The head flow result will largely determine whether the build lands in the 400–430 whp range (likely) or the 430–460 whp range (possible with favorable porting outcomes). This is the dominant variable in the project's power outcome.

Block Irreplaceability

The Type 918 block has no known spares in circulation. Any inspection finding (cracks, water jacket corrosion, main saddle damage) could end the project. Build philosophy compensates by over-engineering protection: ARP studs, generous oil control, conservative compression in pump gas mode, knock detection that's trusted, and a tuner who knows when to back off.

Calibration Complexity

A one-off ITB flat-plane V8 with flex fuel has no base map. All calibration is calculated from first principles and refined empirically. Realistic expectation is 40–80 hours of dyno work spread over 2–4 months, with ongoing refinement through the first 1000–2000 miles after the car returns to service. This is normal for a bespoke build and should not be treated as a failure of the initial tune.

Coupled Design Variables

Cam profile, head flow, piston compression, and plenum geometry are mutually dependent. Optimal sequencing is:

1. Port heads, flow bench, establish actual numbers
2. Spec cam profile against measured flow
3. Calculate piston-to-valve clearance, finalize piston spec
4. Design plenum runner length around cam timing and rpm target
5. Final assembly and calibration

Deviating from this sequence risks leaving meaningful performance on the table or requiring expensive rework.

Timeline

Realistic estimate: 18–30 months from contract to shakedown complete. Plan for 24 months. Long-pole items include pattern-making and plenum casting (4–6 months), custom rotating assembly parts, header fabrication (waits on engine mock-up), and calibration phase.

Excludes MCS suspension, UN1 upgrade and Lotus PBC labor, tires, brakes, bodywork, and consumables.

If the build delivers below expectations after first calibration, available paths in increasing order of cost and risk:

1. Accept and tune for character — 400 whp in this chassis is already an exceptional car
2. Cam and induction revision — $8–15k to re-optimize within existing parts
3. Second-pass head program with larger valves — $8–15k, requires engine removal, valve seat risk
4. Pivot to forced induction — $40–60k, leverages existing bottom end, yields 550–650 whp on E85
5. GT1/GT2 spec heads if a damaged set surfaces — long-shot parts hunt

None of the current decisions foreclose these options. The bottom end, cooling, fuel system, and chassis work are all power-agnostic and carry over to alternative paths.

To be updated as the project progresses.

• [ ] Contract signed with builder
• [ ] Engine removed and disassembled
• [ ] Block inspection complete
• [ ] Head flow benchmarking complete
• [ ] Rotating assembly machined
• [ ] Cam profile finalized
• [ ] Plenum pattern complete
• [ ] Plenum cast and machined
• [ ] Engine assembled
• [ ] Engine mock-up in chassis
• [ ] Initial calibration on dyno
• [ ] Shakedown miles begun
• [ ] Build considered complete

Last updated: May 29, 2026