Comparison of a Subaru Speed Density and Hybrid Speed Density Harness

Subaru Speed Density Explained

Apr 15, '21
Our friends at Flatirons did an excellent in-depth look at the Speed Density Tuning below.

Speed Density? MAF Plugs? Intake air temperature sensors? It all seems a little confusing, but no fear, iWire is here! Before we get started, a quick glossary of acronyms will help: 

SD - Speed Density

IAT - Intake Air Temperature

MAF - Mass Air Flow

MAP - Manifold Absolute Pressure

ECU - Engine Control Unit


What is Speed Density? 

Speed Density is a tuning method where the ECU uses information based on manifold pressure and temperature using the MAP sensor and an added IAT sensor to calculate how much air is coming into the engine. In comparison, MAF-based tuning (standard on most Subaru applications) uses a MAF sensor built into the intake tube that reads how much airflow is coming directly through the sensor to determine the amount of air going into the engine.


Why Would I Switch to Speed Density? 

For some applications, usually due to a larger turbo, the intake airflow is greater than the MAF's ability to read it. Therefore the ECU needs a different way to determine how much air is coming into the engine. Cobb Tuning has summarized some specific situations pretty well: 

  • SD can eliminate the noisy airflow calculation sometimes seen when using a MAF sensor-based configuration with heavily modified cars.

  • SD removes the MAF sensor, eliminating a potential restriction in the intake tract and allowing for more freedom in the intake piping design.

  • In a special hybrid mode, SD can be used on the high end to overcome a maxed-out MAF sensor while still retaining MAF sensor operation on the low end. Or SD can be used on the low end to improve idle/cruise characteristics when a big MAF is used while retaining MAF sensor operation on the high end.


How Do I Switch to Speed Density?

First, call your tuner (many of them are iWire dealers) and see whether they recommend a Speed Density or Hybrid MAF tune based on your setup. Then purchase an iWire SD Adapter or Hybrid MAF adapter, plug it into your existing MAF sensor connector and the new IAT sensor you have installed, and have it tuned!



Speed Density Adapters - Click on the pictures for links to the product.


2002-2007 Subaru




2008+ Subaru (Excludes DIT models like 15+ WRX)





Hybrid MAF Adapters


2002-2007 Subaru



2008+ Subaru (Excludes DIT models like 15+ WRX)

IAT wiring plug and play speed density tuning
2015+ WRX and other Direct Injected Platforms
These models do not need a harness because they have a sensor called a TMAP built into the intake manifold. This sensor handles pressure and temperature. So with a Cobb AP tuning update, it can run Speed Density. Below is what the TMAP looks like.