Design

The Nissan GT-R features one of the most potent turbo powertrains of any current production car with a huge amount of careful engineering going into its original design.

The issue with the standard car, when looking to significantly improve performance, is the restrictive design of the factory manifold fitted to each bank of the 3.8 V6 twin turbo engine. The turbine housing and the exhaust manifold that connect it to the cylinder head are formed in one piece. As a result, in order to upgrade the turbo, the combined manifold and turbine housing must be modified together.

We offer a halfway house to get around this problem, achieved by machining and welding each manifold and turbine housing. This allows it to accommodate different turbo cores allowing the user to select from a wider range of larger turbochargers.

This approach is restricted by the size of the internal volume (the inner scroll of the turbo) and the available thickness of material in the standard turbine housing. This therefore limits the size of exhaust turbine wheel that can be fitted.

With so much compromise we decided to start again from scratch and design our perfect manifold for the Nissan GT-R. Not an easy task!

Log Vs Equal Length Designs

The first decision to be made was to determine the style of manifold that would best suit the GT-R.

Conventional wisdom and math says that an equal length manifold is better at producing top end power but then a log manifold should have an advantage in low end torque. However this is in an ideal world scenario, rather than in the space available to us in the GTR engine bay. Mizuno San originally designed the GT-R to have a log manifold. He believed it offered increased torque and improved heat retention (to help with response and emissions).

The current F1 world champions looked at both equal length and log manifolds when designing their championship winning car. They found that in a similar perfect-world scenario they could get a tiny extra amount from the equal length design (barely 10bhp). However that performance benefit was at the expense of low-end response and increased the amount of space needed to allow for the correct type of bends. Continuing with an equal length manifold would mean either compromising the shape of the car to allow for the larger design or a compromise in the shape of the manifold, neither of which was an acceptable approach.

Similarly in the GT-R, pursuing a ‘perfect’ equal length manifold would ultimately be an impossible task given the tight packaging constraints found in the R35 engine bay. As our testing would later prove, even the slightest change in internal angles has a profound effect on manifold efficiency and packaging constraints would therefore surely compromise the equal length design.

We needed the car to still be driveable in the real world, meaning low end torque and quick turbo response.
This and packaging were our most important considerations, just as they were for Nissan.

We chose to design an asymmetrical log manifold and enlisted the help of some of the UK's top motorsport firms and leading F1 engineers to make it a reality.

The Design Process

We began by designing and fabricating a pair of custom Inconel exhaust manifolds (above) using our combined knowledge and experience. These performed brilliantly but the cost and sheer amount of time needed to produce each kit (over four weeks) made them impractical for the majority of projects.

We were also sure we could improve them further.

These manifolds were scanned into a computer in three dimensions along with the space limitations of the tight GT-R engine bay. We even made moulds of the cylinder head exhaust ports all the way back to the exhaust valves to fully understand the production of the exhaust energy.

Once we had defined the limits and constraints, we engaged another F1 consultancy firm for their expertise in Computational Fluid Dynamics (CFD) analysis.

The CFD analysis allowed us to make several key design modifications within the CAD software. Although often small, these modifications combine to help improve the flow characteristics throughout the manifold and increase the energy received by the turbo. These subtle improvements included the development of our unique conical design in just the right shape. This aids flow uniformity and scavenging from each cylinder as well as reducing areas of flow interference. Careful attention to detail was given to each of the internal curves and surfaces to further improve the efficiency.

This increase in gas efficiency maximises the energy to the turbo’s turbine wheel to improve response and reduce turbo lag. As the flow increases the same design improvements reduce the restrictions to peak engine power.

At the end of the process we had improved the manifold efficiency by over 18%. When we could no longer make meaningful improvements we knew we had arrived at our final, Patent Pending, conical log manifold design.

Below are samples of the uniformity tests completed on different manifold design configurations. Please click to enlarge this small selection:

The final manifold design was then 3D printed to allow all the components to be physically installed and checked. Once happy with the fitment we were able to progress onto making the multiple, and integrate, casting moulds. These then allowed our first samples to be completed ready for real world testing.

The manifolds are cast in the same British foundry that produces components for the UK’s premier Supercar manufacturers.

Uncompromised Quality

Whilst the typical approach to modifying is to make things faster and lighter, our priority has always been to make things better. With this in mind the Litchfield Manifold has deliberately been designed to be more substantial than the OEM part it replaces.

We use the highest grade Ni-Resist D5S, which is an outstanding cast iron alloy in regard to coping with long term heat cycling and extreme temperatures.

There are only a few foundries in the UK capable of forming this best-of-the-best iron alloy. The material choice and thickness ensures it retains heat, vital for optimum spool, and won’t crack regardless of the power in even the most extreme of engine builds.

The manifold kit also consists of extra thick cylinder head plates with integrated EGT boss plates for high end engine calibration requirements.

Each manifold is CNC machined and then hand-finished for a smooth gas flow.

Our complete engineered solution also means that no gaskets are required for the turbochargers themselves. This avoids the likely risk of important, and harmful, exhaust gases escaping from a failed gasket in such a harsh environment.

Sealing is provided by CNC 321 steel V-band clamps, and unique in this application, custom-made high temperature Wills Ring seals which are typically only reserved for extreme cylinder head sealing applications.

Fastenings are in the form of custom made M10 Inconel bolts and Super Alloy K-Nut fixings with FEA infinite cycle testing to ensure a lifetime of durability. These were specified for us by a leading F1 expect who specialises solely in the technology of fastenings.

The confidence we have in our build quality and design is backed by a 5-year warranty on the manifold itself, separate from the manufacturer's warranty for the Turbo and wastegate components.