To move air into the engine and exhaust out of it in a way that maximises volumetric efficiency in the 10,000-14,000 rpm operating band, while meeting the restrictor rule and the sound limits.
The intake path runs from a K&N RC 2500 air filter through a Bosch 40 mm electronic throttle body, into a carbon fiber restrictor that meets the mandatory 20 mm throat, into a 3.5 L carbon plenum, and out through runners onto the OEM runner boots. The restrictor's diffuser recovers pressure after the choked throat: 9.61 in long with a 4.356 deg half angle, expansion ratio 0.362 in^2/in, expansion rate 2.315. Theoretical choked mass flow is 0.0769 kg/s. Runner length is set by pressure wave tuning so the returning wave arrives at the intake valve at the target speed. The exhaust runs 4 primaries at 1.25 in diameter and 12.44 in length into a collector, then a 2 in CP1 titanium secondary carrying the Bosch LSU 4.9 lambda sensor, into a 16 in single-pass fiberglass-packed titanium muffler with a 14 in perforated tube, then out a 1.5 in tailpipe at the rear of the car.
Tune to where the car actually runs. Both intake runner length and exhaust primary length are set by pressure wave tuning at the operating speed, and CR26I shortened the primaries from 14 in to 12.44 in specifically to move the tuned point into the competition band. Mass is bought with material. Moving the secondary and tailpipe from 304 stainless to CP1 titanium gave 43% lower density (4.51 against 7.93 g/cm^3), which is most of the 9 lb the exhaust lost. The flow bench is the arbiter when the transient CFD model is not ready. CR26I used 3D printed restrictors to iterate many geometries quickly on the bench while the transient model was still in development. 1. Build the engine model in GT Suite from measured geometry: silicone-cast and 3D-scanned intake ports, dial-indicator-measured cam lobe profiles, MTS-measured valve spring curves. 2. Use GEM3D to convert manifold CAD into GT Suite flow geometry, which cut manifold setup from several hours to about 30 minutes and let subassemblies be swapped for fast comparison. 3. Sweep plenum volume, runner length and primary length in the model against volumetric efficiency. 4. Confirm on the flow bench. CR26I plenum flowed 2.34% more than CR25 and 11.15% more than CR22 on average across 10-50 in-H2O, with testing extended to 70 in-H2O (17.4 kPa) to reach WOT conditions. 5. Prototype-test the muffler on the previous car before finalising, sweeping length (14 in and 16 in), packing density, tailpipe diameter (2 in and 1.5 in) and a diffuser-style tip against the sound limits.
- Increase engine output in the 8,000-11,000 and 10,000-14,000 rpm bands depending on the year's gearing and driver habits. - Maintain serviceability and improve manufacturability and tolerances. - Reduce exhaust system weight by at least 30% (5.75 lb) without affecting reliability or sound emissions. CR26I achieved 8.94 lb. - Pass sound at idle and at 3/4 engine speed. Targets: 103 dBc idle, 110 dBc at 11,000 rpm. - Reduce backpressure as far as the sound requirement allows. - Cut manufacturing time. CR26I intake went from 97 hours to 49.75 hours, a 47.25 hour saving.
- Intake failure from expired resin hardener, which let the part deflect under temperature load. It was not noticed until on-vehicle testing because the dyno purge kept ambient air cold; on the car, hot air around the exhaust flows up the engine bay into the intake. The fix adopted is oven post-curing all carbon fiber parts. - Sanding time on foam molds. CR25 spent 24 hours sanding Duratec on the plenum and tray molds; CR26I's 3D printed molds needed 2 hours of sanding, 2 hours of spraying and 3 hours of clearcoat sanding. - Flow bench against Bosch sensor disagreement. Differential pressure readings differ by 16-25% between the bench and the Bosch sensors, and mass flow rate by 1-20%. Known and tracked rather than resolved. - Weld against bond for injector bungs. CR25 bonded them in 2 hours; CR26I welded them in 2 hours with a coping fixture in the mill.
GT Suite 2024 for the full engine model, with GEM3D for CAD-to-model conversion. ANSYS Fluent for restrictor CFD (energy equation, k-epsilon, density based, axisymmetric, ideal gas, pressure inlet 101,300 Pa at 300 K, pressure outlet 12.5 kPa gauge). Flow bench for plenum, restrictor and cylinder head port characterisation. MATLAB wave tuning script for runner and primary length. Sound meter testing against the competition limits.