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Old 01-17-2012, 11:01 AM   #6
Cobb Tuning
Member#: 4803
Join Date: Mar 2001
Location: Austin, TX

Originally Posted by WhatTurboLag? View Post
YES, finally!!! time to call up clark turner

only downside i can find so far of straight SD.

Any SD calculation, including COBB SD, requires an input for cylinder charge temperature, which is critical to the
determination of accurate airflow via SD. The estimation of cylinder change temperature is accomplished for COBB SD
via the IAT sensor input. Generally, when the IAT sensor is in the recommended location (post-IC), the vehicle is
moving and the driver is on the throttle, the IAT input can be a fairly reliable representation of actual cylinder charge
temp. However, when the vehicle is sitting still (or at low speeds) and the driver is off the throttle (or low throttle), or
the vehicle has been sitting with the engine off and a hot engine bay for a period of time, there is the potential for the
IAT sensor to become heat soaked. That is, the sensor now reads higher than the actual intake air temp. When SD is
active, this would cause the calculated SD airflow (as well as load) to be lower than it should be, causing the car to run
lean (and with generally more timing advance). This effect may subside after the vehicle gets moving and throttle (as
well as MAP) increases, but it will generally not be an instantaneous improvement. As such, it is critical that the
owner/driver of the car understands the specific scenarios in which a heat soaked IAT sensor can potentially occur and
to avoid putting the car under high load when these scenarios are present (and for a period shortly after). This is
another reason why a wideband o2 sensor and gauge should be installed in the car and that the driver instructed on
how to determine when fueling is incorrect.

The owner of the car also needs to understand that practically any engine mod that impacts airflow in any way
may require a re-tune or tweaking of the VE table for cars running SD. This is important as even seemingly
minor mods that a MAF sensor-based tune would have no problem accounting for, might cause a significant
enough change in VE that there could be fueling and load issues for SD.
You are correct - that would be the two things to consider when going with any SD solution. We have found that placing the IAT sensor post-intercooler (in the case of front-mount) is the most accurate means of estimating charge temp and, generally speaking, the best location to minimize IAT heat soak. The factory IAT sensor is housed in the MAF-sensor assembly, so if the MAF sensor is removed, you'll need to go with an aftermarket sensor.

All cars using SD should be running a wideband o2 sensor with in-car gauge. That is really a no-brainer and even cars still running MAF at power levels that would benefit from SD should have this set-up anyway.

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