Brian Keene
- Aug 23, 2018
- 2 min read

Subaru Speed Density Explained

Updated: Aug 9, 2019

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 air flow is greater than the MAF's ability to read it and therefore the ECU needs a different way to figure out 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 allows for the MAF sensor to be removed, eliminating a potential restriction in the intake tract and allowing for more freedom in 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 still 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 new IAT sensor that you have installed, and have it tuned!

Speed Density Adapters

2002-2007 Subaru