Speed Density Question
Thread Starter
Heel & Toe
Joined: Feb 2010
Posts: 15
Likes: 0
From: Sumter SC
Speed Density Question
Just looking for someone to clear something up for me. If I understand speed density tuning, you trick the computer to use the fuel tables and the MAP with a fake error code so it doesn't try to read from the MAF sensor that isn't there. So with that in mind, a car is either SD tuned or MAF tuned. No way to mix the two and have the car select which to use even if you reinstall a MAF sensor? I'm NOT concerned about the benefits of SD vs MAF, just is it possible to have both tunes at the same time and have the car pick automatically? I was just told this was done to my car that has been having several issues (SD tuned), and this doesn't seem possible. But then again, who am I to question people who do this everyday. Any help would really be awesome of you guys. Thanks for your time!
Last edited by Onyx_dragon; May 17, 2011 at 10:25 PM. Reason: Better wording... I think
Safety Car
Joined: May 2006
Posts: 4,016
Likes: 93
From: St. Louis, MO
Tech Contributor
You can run SD only, MAF only, or a combination of both. You can leave everything plugged in and set up the PCM to run whichever way. Setting High RPM Enable to like 1rpm will do MAF only.
Safety Car
Joined: Jun 1999
Posts: 3,570
Likes: 3
From: Austin, Singapore, Kuala Lumpur, Houston, Dallas, Hong Kong, Elgin, etc.. Texas
The PCM (or ECU) has a setting for the RPM to switch to using the MAF as the primary source of air calculation. I say primary because there are many, many other tables which modify the air calculation which is necessary for the the fuel calculation.
For the 2005 E40 PCM (i.e. ECU) that RPM is set at 4000 rpm. Below that, it uses primarily the VE table (Volumetric Efficiency). I currently have mine set at 2500 rpm but with my mods, it really does not make any difference. I just watch my LTFT% error and it seems about perfect.
The MAF reads the air density, converts it to a frequency, which the PCM (or ECU) uses in the fueling calculation. This is affected by absolute air pressure, temperature, and humidity. It usually provides very accurate information on the amount of air coming into the engine. Speed Density does not make these adjustments so some days you may be lean and others rich.
On highly modified cars, the MAFs may not provide consistent data at low RPM due to intake reversion and turbulence . That may result in the PCM (or ECU) to make unnecessary fueling adjustments causing cycling or as we call it, chasing its tail. Which is the reason some guys have better luck with just the Speed Density tune.
For the 2005 E40 PCM (i.e. ECU) that RPM is set at 4000 rpm. Below that, it uses primarily the VE table (Volumetric Efficiency). I currently have mine set at 2500 rpm but with my mods, it really does not make any difference. I just watch my LTFT% error and it seems about perfect.
The MAF reads the air density, converts it to a frequency, which the PCM (or ECU) uses in the fueling calculation. This is affected by absolute air pressure, temperature, and humidity. It usually provides very accurate information on the amount of air coming into the engine. Speed Density does not make these adjustments so some days you may be lean and others rich.
On highly modified cars, the MAFs may not provide consistent data at low RPM due to intake reversion and turbulence . That may result in the PCM (or ECU) to make unnecessary fueling adjustments causing cycling or as we call it, chasing its tail. Which is the reason some guys have better luck with just the Speed Density tune.
Last edited by Mez; May 22, 2011 at 09:31 AM.
Burning Brakes
Joined: Aug 2002
Posts: 971
Likes: 2
From: upstate sc
The PCM will use the MAF almost exclusively when you have steady throttle input. When the PCM sees a large change in throttle position is will use the VE table. So if you are somewhere from 0% throttle movement and a rapid change then the PCM uses a combo of the two and even compares the the two to decided fueling, it is called a hybrid system. Above 4000 rpm then it is 100% maf calculated.
CWB
V8muscle.net
CWB
V8muscle.net
