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MAP/Baro Tables for MegaSquirt-II

This archive document primarily applies to older V2 and V1 MicroSquirt® controllers.
For the latest V3 MicroSquirt® controller documentation, click this link: V3 MicroSquirt® Quickstart Guide

MegaSquirt-II™ code versions 2.8 and up allow you to use either the older MAP versus rpm table scheme, or a new MAP/baro versus rpm scheme.

The reason this is being put in is that it is the correct way to implement the VE table. With the baro correction equation, you end up with baro_corr(baro) * VE(map,rpm) as opposed to VE((map/baro)). The baro correction is left in, but it should now end up much closer to 100 than it is in the old scheme. This option was prompted by Ben Strader at EFI University, and after we did some digging in the literature we were able to verify why.

First, note that the default barometer correction curve originated from a 1990 Corvette ECU, obviously a measured data quantity. It may or may not have direct applications to other engines, most likely not completely. Al did add in the latest MS-II 2.6 code the ability to tweak from this default curve with a 6-element (non-linear) table of corrections.

What the MAP/BARO index generation mode does is simply adjust the pressure differential that appears in the definition of Volumetric Efficiency. Look in Taylor's "Internal Combustion Engine" text, or Heywood, and there is the (simplified) definition of VE:

n(vol) = E * ((K-1)/K) + (r - (Pb/P))/(K*(r-1))

where E is a constant to fit the measured data, K is the ratio of specific heats (K=1.4 for air), r is the compression ratio, Pb is the exhaust absolute pressure, assumed to be barometric pressure, and P is the manifold absolute pressure.

What the MAP/BARO index modification does is modify the indexing into the VE table such that to decouple the pressure part of the VE equation, leaving a constant value for VE (E, K, and R are constants). According to Ben, he has verified the use of the modified index with supercharged engines at various altitudes. It was a easy-enough code mod to implement, and it is put in as an additional option. We have not personally verified this with a real engine test, but the option is there if people want to explore this.

Note that there is also a definition of VE known as as Delivery Ratio (DR), which defines the VE values at a standard reference air density and temperature (101.325 KPa and 20 deg C). So when one determines VE it gets converted to the equivalent air mass value at the standard conditions and stored. The DR value gets back-converted for other environmental conditions.

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©2006 Bruce Bowling and Al Grippo and Lance Gardiner. All rights reserved. MegaSquirt® and MicroSquirt® are registered trademarks. This document is solely for the support of MegaSquirt® boards from Bowling and Grippo.