Relentless Automotive Development Downpipe with GESI HFC for RHO and HO Engines

RHO & High-Output Downpipes Featuring High Flow Catalytic Converter

The Relentless Automotive Development High-Flow Downpipe is designed to improve exhaust flow efficiency and reduce back pressure while maintaining durability under demanding conditions. Each unit is constructed around the GESI 85200 4.5" high-flow catalytic converter, which provides high exhaust flow while retaining emissions functionality in accordance with federal EPA guidelines for 49-state compliance.

The downpipe is built from 304 stainless steel and insulated with Heatshield Products’ Heatshield Armor, which helps retain heat inside the exhaust stream and reduce radiant temperatures within the engine bay. Each pipe is TIG-welded, fully back-purged, and equipped with 1/8" 304 stainless-steel brackets that mount directly to the OEM factory locations for a stable and precise fit.

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Key Features

49-State EPA Designation

Utilizes the GESI 85200 4.5" catalytic converter (400 CPSI) designed to meet federal EPA standards for 49-state use. This product is not CARB compliant and cannot be used in California on emissions-controlled vehicles.

Power Capability

The GESI 85200 converter supports approximately 500–850 horsepower per bank, suitable for stock and moderately modified engines.

Optimized Diameter Configuration

4" tubing immediately after the turbocharger outlet to increase exhaust flow area

4.5" catalytic converter for minimal restriction

4" tubing post-catalyst to maintain flow consistency

Gradual taper to 3" tubing for a direct connection to the OEM mid-pipe

Flow Increase

Transitioning from a 3" to 4" tube increases the internal cross-sectional area by approximately 78 percent, allowing exhaust gases to exit more freely and reducing restriction on the turbine side.

Back Pressure and Spool Characteristics

The larger diameter and smooth transitions reduce back pressure at higher flow rates, improving exhaust velocity and contributing to quicker turbocharger spool under load.

Thermal Management

Each downpipe is insulated with Heatshield Armor, composed of a heat-reflective aluminum outer layer and ceramic insulation. Based on manufacturer specifications, the material can reduce radiant heat by up to 70 percent in optimal installation conditions. This helps manage engine-bay temperatures and protects nearby components from thermal exposure.

Material and Fabrication

Constructed from 304 stainless steel for corrosion resistance and long-term structural stability. All welds are TIG-welded and argon back-purged to ensure clean internal seams and uniform strength.

Flex Section

A 3" flex joint allows for movement due to vibration and thermal expansion, reducing stress on the welds and brackets.

Sound Characteristics

When installed with the OEM mid-pipes or compatible systems, the downpipe provides a slightly deeper tone with a mild increase in turbo audibility under acceleration.

Tuning

No ECU tune is required for normal operation, daily driving, or spirited driving. Once ECU tuning becomes available for this platform, recalibration is recommended to fully optimize air-fuel ratio, boost control, and performance potential.

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Product Development

Each design begins with a baseline prototype that undergoes evaluation for fitment, flow, and temperature performance. Feedback and data from this testing are applied to subsequent versions. The production version reflects these refinements, focusing on proper geometry, reliable weld integrity, and consistent quality across each build.

Built to order — average lead time: up to 4 weeks.

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Warranty and Disclaimers

49-state legal (not CARB compliant)

No tune required for installation

Lifetime guarantee on craftsmanship (welds and structural integrity)

1-year limited warranty on the GESI catalytic converter