The original 6 series vehicle by BMW is appreciated today as one of the most beautiful examples of BMW design of that era. In Europe the M635 version was also the first series production model that the
Motorsport division prepared and produced using the a refined version of the M1 supercar engine. This fantastic engine used the early version of the Bosch Motronic engine management system. Later versions like
the US model M6 also incorporated catalytic converters. As the engine management systems continued to evolve, the airflow meter (AFM), the ubiqutious "barn door", was replaced by the less restrictive mass airflow
meter (MAF). With proper design and tuning, this improvement can be made on AFM equipped engines resulting in improved driveability and power without affecting emissions performance.
Phase 1
On my M6 I started the conversion by using a compact MAF unit by Siemens. I made a special inlet tube and air horn to properly mount the MAF on the air cleaner box. To develop the transfer function to match the
output signal of the AFM, a signal conditioner was used which was programmable via my laptop computer. I installed a wideband oxygen sensor to monitor the running condition of the engine, and used road tests
to tune the system to obtain the proper performance.
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For the initial MAF tests I installed the MAF in series with the AFM and monitored both the signal outputs.
Baseline tests were run with the original AFM system to study the tuning prior to installing the MAF.
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A laptop computer was used to tune the system by making adjustments to the signal conditioner.
The wideband oxygen sensor and signal conditioning equipment for monitoring and recording other inputs is from Innovate Motorsports.
The programmable signal conditioner is from Split Second.
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Here's the inlet to the original "barn door" AFM - "RESTRICTED ENTRY!"
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Here's the inlet to the Siemens MAF - "COME ON IN!"
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Here's the final installation with the Siemens MAF during testing.
The original airbox is used with the adaptor for the MAF unit installed.
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Phase 2
In testing I found that the capacity of the Siemens MAF was too small for the engine. Although a proper running tuning was achieved throughout the rpm and load range, the unit flow maximum was
reached before the highest flow developed by the engine was reached. Consequently I prepared a second installation using a larger capacity MAF.
A further refinement incorporated in this installation was using a cone air filter and cold air box for the inlet. I used a cone filter in some of the development tests with the Siemens MAF and frankly speaking
I fell in love with the intake sound of the engine which was released by removing the original airbox. For maximum performance it is necessary to isolate the intake from injesting hot engine room air, so I developed
the cold air intake to compliment the filter installation. The geometry was maintained for a straight flow path into the engine plenum.
Once the installation was complete I again tuned the system using road tests. The final system runs fantastic and driveability is greatly improved.
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The cone filter is contained inside the cold air box.
Heat insulation is applied to the inside walls of the air box.
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Some wrinkle paint and a custom made logo provide an OE look.
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