Product Spotlight: M20/M42 Intake Adapter July 06 2016

For the M20 and M42 engines, BMW uses a Bosch air flow meter (AFM) with a square shaped inlet. This makes it difficult to connect custom or aftermarket intake plumbing, because they almost all have round cross sections.

There are poorly designed intake adapters on the market that simply weld a round tube onto a flange with a square hole on it.

Cross section view of a poorly designed intake adapter. A cylinder is placed on a flange with a square hole in the center.

Above is a cross section view of a poorly designed intake adapter. While this is a cheaper solution, the harsh transition is detrimental to flow, which is detrimental to power. The flow visualization diagram below shows why this happens.

Flow diagram that shows how the sharp corners of poorly designed intake adapters perform poorly.

Intuitively, we can see how this abrupt transition can cause a loss in flow rate. If you imagined the molecules as cars, and the poorly designed intake adapter as a highway, you can easily see how stationary cars pulling out in front of fast moving cars will slow everything down quite substantially.

These type of transitions have been well-studied and mathematically modeled. In the engineering world, we talk about loss coefficients. These numbers describe how much pressure is lost for a particular type of plumbing feature, such as elbows, tees, inlets and nozzles. The more pressure loss, the less pressure differential is left to drive the flow. If you are familiar with electrical circuits, you can think of loss coefficients as analogous to electrical resistance. In the same way that voltage drops after passing through a resistor, pressure drops after passing through a flow restriction. Here are some textbook figures on inlets. 

Cross section of 3 different inlet types and their k values. Type 1 is a protruding cylinder that has a K value of .8. Type 2 is a flush inlet that has a K value of .4. Type 3 is a bell mouth with a K value of .2

While these loss coefficients are meaningless in themselves without knowing other parameters and doing some math, they give a good comparison between different types of inlets. The higher the number, the bigger the pressure loss. You want a smaller number. The cheap intake adapters we commonly see, likely have a loss coefficient of about 0.4, similar to the middle figure. There are definitely losses caused by this poor inlet design. Even BMW knows this and puts a small plastic trumpet on the intake of the AFM to more closely resemble the inlet on the far right with the loss coefficient of 0.2.

Unsatisfied with the offerings on the market for our M42 engine, we took it upon ourselves to create a properly engineered AFM adapter. Our AFM adapter provides a smooth transition that helps the flow to transition from a round to a square cross section smoothly. 

Our intake adapter doesn't acts less like and inlet and closer to a straight section of pipe. Pressure loss in a short section of pipe is typically negligible, assuming the flow is laminar and not turbulent. It takes extra work, equipment and know-how to form these smooth transitions, but the end result is more flow.

Red Eye Garage intake adapter.


We are proud to bring our properly engineered and properly fabricated intake to the BMW community. 

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M20/M42 Intake Manifold Adapters