Speed Density vs MAF
- Thread starter LOLUM4D
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- maf speed density tuning
This is pretty much dead on.
The problem with large MAF intakes is that not only are they expensive but they also can cause drivability issues under closed loop (light load and cruising). Speed Density is free if you (or your tuner) are using either the Cobb Access Port or Open Source. The problem with Speed density is that it can also cause light load drivability issues if not done right/perfectly. I am not exactly sure why but I do know it has to do with how you fill out the Volumetric Efficiency, and also with fluctuating temps from heat soak and what have you (since SD uses temperature as one of the factors to figure out how much air is coming in). The fix for that is to move/place a temperature sensor as close to the throttle body as possible.
If your tuner is using Open Source you could ask him to use the MAF and SD blending, where the MAF controls closed loop and light load driving while the SD controls the Open Loop/WOT sections. This will get you the best of both worlds and not cost a penny. I am not sure if Cobb has it yet, but if they don't I am willing to bet they will soon.
HolyCrapItsFast
Drinks beer!
Of course they do for quite some time now... The difference is that cobb calls it hybrid mode and it is merely a switch that clamps the maf via set thresholds and then SD takes over fuel calculations. If it is not tuned correctly it can be quite abrupt. The OS community takes a better approach in my opinion though, They call it maf blending and it allows you to phase out the maf and phase in SD by having both calculate fuel simultaneously as you transition from one to the other and you can define the percentage between them. It is a 3D table with PSIR along the X axis and RPM along the Y and the values represented in percent of maf blending. Very simple to understand once you see it. :tup:
The issue with using only SD is the daily driving and consistency over time as temperature, atmosphere and altitude change. Since the maf is measuring the mass of the air, all of the compensations are inherently built into it's design where as with SD there are a series of tables that compensate for changes in environment. If you don't know what tables to look for you risk having a very inconsistent tune and your VE table today won't work tomorrow.
Maf blending is were it is at.
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HolyCrapItsFast
Drinks beer!
True that... For cobb the VE table is in Absolute but for OS it is Relative pressure... so it would make a difference in this case. In any case temperature is the one to really focus on. I won't explain this for your sake because you already know this but I will for the benefit of the rest who don't.
VE is based on volume and not mass and if you are not compensating for temperature you will have inconsistencies when temperature extremes occur because as the temperature of the air changes, so does its density. So colder air is denser and contains more oxygen molecules per volume and the opposite is true for warmer air. Since you are filling up the cylinder with a volume of air and basing fuel on that, you need to tell it how much fuel to trim based on temperature.
HolyCrapItsFast
Drinks beer!
Quite the opposite. Maf it is inherint in the design and operation. With speed density you have to utillize all the other temperature compensation tables which is why Edwin suggests "if tuned properly".
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Td_d
Commander In Chief
As Holy said, SD is much more sensitive to temperature swings (as IAT is part of the actual calculation used for fueling) - placement of the sensor as close as possible to the intake manifold is key, and furthermore, minimising heat soak (such as directly from the manifold itself) is important to consistent fueling. Compensation tables can then be used for the final smoothing.
Trust me, from experience, leaving the IAT stock location or CAI is not ideal, big temp swings can cause big fueling swings.
Td_d on a stock location turbo set up what where would you suggest placing a temp sensor, and also how would you route it into the ecu. Would it be possible to use the stock maf and just move it the TMIC connector pipe at the intake manifold? If not what is the solution for this, as I my self run a stock location turbo and TMIC, so would it nonviable for me to move over to the SD in open loop?
HolyCrapItsFast
Drinks beer!
I have used them in the IC tank just before and in line with the throttle body or i used the freeze out plug looking thingy on the manifold just after the throttle body on the top. Both with decent results. I haven't done it enough on subaru to be totally conclusive though but my clients are not complaining.
I was thinking of using that little silver plug on the top of the manifold for my meth spray as it looks like a good place for that since getting it in the IC would be a bitch for an even spray. Would that be viable? Also how would you get the ecu to read an external temp sensor? or would you just use the maf and replace it and use that?
The spray part was in reference to where I was thinking of putting the meth nozzle. For the IAT I think I will put it in the IC right before the TB. Good to hear you can splice in the new sensor. Since I would be using an aftermarket sensor and the maf in conjunction would it throw off the reading by having two temp sensors on the same wire both sending their signal to the ecu? or would it be more of a case of replacing the one temp sensor with another one and relocating? fuck.... I think I really need to read over that article to hopefully answer my questions. Before I ask any more stupid questions.....
