Provide a volume of air at the restrictor's downstream pressure for all four runners to draw from, so cylinders do not compete with each other.
A carbon fiber plenum downstream of the restrictor. Larger volume reduces the pressure drop each cylinder sees when it draws, which raises volumetric efficiency, but adds mass and slows transient response. The plenum mounts to the OEM runner boots, which isolates the runners from thermal and vibrational effects from the engine.
Volume increased from 2.5 L on CR25 to 3.5 L on CR26I. Runner length 5.35 in (135 mm), shorter than CR22, giving pressure wave tuning around 11,000 to 14,000 rpm for a VE boost in the GT Suite model. Round geometry enhances structural integrity under vacuum compared to CR25.
The 20 mm restrictor is mandatory and caps mass flow at roughly 0.0769 kg/s when choked. Packaging above the engine, under the engine cover, without trapping hot air. Differential pressure in the power band runs 6-18 kPa.
Swept in GT Suite for VE against plenum volume, then confirmed on the flow bench. CR26I flowed 122.8 CFM at 10 in-H2O against CR25's 119.5 and CR22's 109.2; at 50 in-H2O it flowed 278.2 against 272.5 and 247.2. Average increase 2.34% over CR25 and 11.15% over CR22. Testing range was extended from 50 to 70 in-H2O (17.4 kPa) to reach WOT conditions. Supporting research: Hamilton, Cowart, Lee and Amorosso, The Effects of Intake Plenum Volume on the Performance of a Small Naturally Aspirated Restricted Engine, 2009.
3D printed mold, glued, sanded, sprayed and clearcoated. Layup prep 3 hours, layup 1 hour, pull 3 hours, post-process 4 hours. The move from milled foam molds to 3D printed molds removed 1.5 hours of foam cutting and gluing, 6 hours of milling, 3 hours of spraying and 24 hours of Duratec sanding, replacing them with 0.25 hours of gluing, 2 hours of sanding, 2 hours of spraying and 3 hours of clearcoat sanding.
CFRP. Alternative considered: 3D printing the plenum directly.
Fault Vault, 2026-03-22, CR26I, Manufacturing, Complete Failure. Resin failure of the manifold. The problem did not show itself on the dyno at all. The manifold was undergoing deflection testing in the shop before shakedown when the front of it softened and sucked in. Diagnosis: the resin was so far expired that it had lowered the heat deflection temperature. Other test pieces baked to around 40 C showed similar softening, while a part made with newer hardener was baked in the oven and did not soften. The manifold was written off. The reason it passed on the dyno and failed on the car is that the dyno purge keeps ambient air cold, while on the vehicle hot air around the exhaust flows up the engine bay into the intake. The fix adopted is oven post-curing all carbon fiber parts.
Oven post-cure all carbon fiber parts, adopted after the CR26I resin failure. Manufacturing moved from milled foam molds to 3D printed molds, which cut intake manufacturing from 97 hours to 49.75 hours.
3.5 L volume. 858.56 g on CR26I against 946.47 g on CR25 and 1047.04 g on CR22. From the cost report drawings and CAD: Dimensions called out on the cost report drawings. These are the figures printed on the drawing, not a labelled specification; open the drawing itself for what each one refers to. Plenum Main Body (as drawn): 30.87, TRUE R2.74 TRUE R2.54, 10.67, R3.81, 5.49, 6.81 Source: Cost report drawings: CR22/Powertrain/Design/X - Cost Report Drawings/Manifolds/Intake/Plenum Main Body.pdf Quantity per car, as costed: 1