Deductions from the CSL Prototype

Deductions from the CSL Prototype

The Original BMW S85B55 V10 M5 CSL design

These build journal entries are part of a special series on the development of the S85 CSL Style Carbon Plenum and written in collaboration with the design & engineering at Infinity Design. 

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The S85B55 featured a massive 1 piece carbon plenum with enlarged sealed air boxes breathing from the front bumper ducts. 

No specifications were ever released by BMW M. 

BMW M never even wanted to show off in official prototype pictures. 

It remains mythical to this day. 

What we can deduct.

There are obvious features we can deduct from looking at the V10 CSL prototype. 

The plenum has a much larger volume.

By running simulations, Infinity Design was able to estimate the CSL prototype had 10 to 20% increased volume. 

Gains are made by the reduction of approximately 4mm of wall thickness of pre-preg carbon versus ABS plastic, elimination of reinforcement beams within the plenum, and enlarged design lid design with slightly higher height and using the wasted space between the OEM 2 piece plenum design.  

The complete system allows much more air flow.

The inlet design of the plenum and air intake boxes visibly demonstrate a much larger diameter by using an oval shape inlet tube to maximize the available space of the cramped E60 engine bay. 

The air intake boxes are also noticeably larger, we can assume the internals were also flow optimized. The European cars also notably didn't make use of the charcoal filter found on US cars, a further restriction eliminated in the CSL design.

The CSL prototype used different velocity stack designs. 

This can be safely assumed when considering the 5.0 liter original displacement velocity stack design. 

It was space limited: BMW had to sub-optimally position and design the velocity stacks as the 20,000 + S85B50 engines that were mass produced needed to be service. 

Unlike the S65B40 from M3 with a flat Individual Throttle Body fitting surface, the production S85B50 were fitted at an angle. 

That angle made the 2 piece OEM design required.

BMW M had free range with the CSL and certainly optimized the intake runners position and length. 

You can also easily deduct this when looking at the OEM plenum of the S65B40 V8 found in the E9X M3. 

The S85B55 was tuned using Alpha N. 

We can deduct Alpha N was used for a simple reason: MAF sensors are not compatible with the larger, oval shaped inlet tubes of the CSL intakes and plenum. 

Unlike previous generations of BMW M DMEs like the MSS54 found in the E46 M3, the MSS65 of the V10 M5 had software and tables specifically planned and developed to safely run Alpha N tuning maps.

Comparatively, S85 uses wide band and a very fast processor to correct for fuel unlike previous generations of DMEs. 

Alpha N setup does not have an actual way of measuring since the MAF has been removed. It instead relies on two variables: throttle position and RPM.

The power delivery is often more immediate than MAF tunes and is often described as an organic, mechanical connection to the S85. 


Up next: we will explore the innovative design features of the upcoming S85 CSL Style Carbon Plenum by Infinity Design. 


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