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ITB Sizing

AN or SD Only

Blended AN/SD

ITB Mode

Sizing_the_throttle_bodies | SD_or_AN_only_tuning | Dual_table_blended_tuning | Single_table_ITB_mode_tuning

Introduction

I have broken up my original ITB tuning page into a collection of multiple pages.  With the addition of a new tuning mode designed specifically for ITBs, the original page just got too large.  All the original content is still provided, it is just distributed across multiple pages linked to at the top and left.

This collection of pages is going to go through the issues I have had with tuning ITBs and the solutions I have used to resolve each problem.  My intended use for my car is as a daily driver so low-RPM tuning and performance is very important.  This is also the operating area where ITBs are traditionally the hardest to tune well.

For the purposes of this discussion on ITB tuning, Manifold Absolute Pressure (MAP), will be expressed as a percentage of barometric pressure.  For example, 50% is a vacuum equal to half barometric and 100% indicates no vacuum.  Alpha-n tuning will be abbreviated 'AN' and speed-density tuning will be 'SD'.

I have had the opportunity to tune two different BMW M10 engines that use aftermarket ITBs, and try two different ITB sizes on one of these engines.  Tuning information from these two engines will be presented within these pages. 

The BMW M10 is a 4 cylinder, SOHC, 2 valve per cylinder engine that was manufactured from 1961 until 1987 in several different displacements.  This engine's design is fairly old and it is not nearly as efficient and today's modern DOHC 4 cylinder engines.  The most powerful normally aspirated version of this engine appeared in the BMW 2002tii and was rated at 130HP.  Most versions had considerably less power.  The M10 engine in my 1977 320i was 2.0L and rated at 110HP.

The examples given on this section are using aftermarket ITBs.  These throttles do not have a separate idle air circuit and require the throttles to be slightly open for idle.  This design effects some aspects of tuning which will be described in more detail below.

I believe the data from these two engines is representative of ITB tuning in general when using aftermarket ITBs.  Much of the data and the tuning concepts I used should apply to most normally aspirated engines running ITBs.

The two engines I have tuned are:

I have divided the discussion on tuning into several sections dedicated to my experiences in the following areas

• Selecting an appropriately sized set of throttle bodies
• My attempts at using either SD only or AN only tuning with ITBs
• My experiences with the Megasquirt tuning method that blends two separate tuning tables to combine SD and AN tuning.
• My experiences with using a new Megasquirt tuning mode that combines SD and AN tuning within a single fuel table, called "ITB" tuning mode.

There is a link on the left side of this page and at the top for each of these sections.