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Old 04-29-2012, 12:36 PM   #1
toneh
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Default Speed density question

Hi there sorry for a newbie type question
But I'm not new to cars at all
But new to Subaru
I've got a 2001 edm wrx ( im in the uk)
I'm just about to start modding as my latest project
I know pretty much about modding the mechanical side , turbo , injectors , exhaust ,ect
But I want to get more into the os tuning Side
Got the cable /software and getting my head down reading up , comparing maps and generally trying to get my head round it all
I was just going to start out with straight logging , edit and flashing but been reading more about sd , on my old BMW m3 when you fitted a carbon airbox /inlet you ditched the maf and had an alpha n chip (the car went into alpha n mode ) then took readings from the Map sensor Not the maf
Am I correct in thinking this is the same as Subaru sd , only you can access your ecu to manually adjust , we're as the BMW was a chip replacement
I know going sd will still require the same time and input as the normal os route but I'm not sure wether to start the sd route from the start
Will it give me more flexibility /add ons at a later date
Sorry for what may seem a basic question but just checking I'm on the right track and understanding correctly
Cheers tony
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Old 04-29-2012, 01:39 PM   #2
quazimoto
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Old 04-29-2012, 01:51 PM   #3
toneh
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Yes spent hours on romraider forum , just wanted to know is it the equivalent to BMW alpha n
So I can know I'm on the right track
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Old 04-29-2012, 09:24 PM   #4
eminehart
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Location: Tweakin your ECU-Northglenn CO
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Quote:
Originally Posted by toneh View Post
Yes spent hours on romraider forum , just wanted to know is it the equivalent to BMW alpha n
So I can know I'm on the right track
You can't use alpha n on a turbo car. Alpha N uses throttle angle to determine airflow. Map based tuning is called speed density.
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Old 04-30-2012, 05:52 AM   #5
toneh
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Thanks so not the same thing then , better keep reading and studying
Cheers tony
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Old 05-01-2012, 01:08 AM   #6
NSFW
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Alpha-N works on NA cars because at any given RPM and throttle opening angle, you'll get a consistent manifold pressure. The throttle angle is basically a very good predictor of manifold pressure (for a given RPM).

But for turbo cars, there's a pretty wide range of manifold pressure that you might get from a given Alpha and a given N. It all depends on how much the turbo has spooled up so far - maybe not at all, maybe full boost, it depends on what's been happening for the past couple/few seconds.

So SD is kinda like Alpha-N, in that both AN and SD are ways of predicting airflow, and RPM is one of the things the ECU bases its predictions on.

For turbo cars, manifold pressure is the other key ingredient. For NA cars, throttle angle is a good enough predictor of manifold density (for a known RPM), so a pressure sensor isn't really needed. And in both cases IAT has to be factored in to get a good prediction.
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Old 05-02-2012, 03:06 PM   #7
toneh
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Thanks chaps , gonna spend a lot of time reading and researching into o/s tuning , it's all very new to me and opened a whole new dimension to working on my car , I,m just resisting temptation to plug in and start flashing until I'm 100% sure of what I'm doing ,
Think I'm going to start with some very small steps first and leave the s/d until I'm more confident Because from what I understand there is a larger potential of a major **** up with s/d
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