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Old 07-13-2009, 12:08 AM   #1
SideWay
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Default Newbie Blow thru maf questions:

I've been searching around a little bit and can't get good answers to my questions in the threads I've read.

1. What advantages do blow thru maf cars have vs the stock setup ? I can imagine it gets more accurate readings of the amount of incoming air to the motor in that position, but is it necessary ?

2. Is this something that ONLY rotated mount turbo people do ? I notice the kits come with "blow thru boost tubes" that relocate it there, but what about stock location turbos ?

Any info would be helpful. Sorry for the rookie questions.
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Old 07-13-2009, 12:23 AM   #2
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Blow through boost can be used with any FMIC where you can fit (perrin specifically) a blow through boost tube. It is harder to tune for but it has advantages where the ECU now reads the correct temperature and amount of air. With a VTA BOV it is releasing metered air, now you can run a VTA BOV without any issues. Once it is tuned for correctly it is awesome, I want to do it really bad, but my tuner doesn't like to tune it for some reason.
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Old 07-13-2009, 12:43 AM   #3
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Right I havent thought of it being exclusive for FMICs, it definitely wouldn't make sense on a TMIC.
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Old 07-13-2009, 12:46 AM   #4
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Right. It has it's benefits if it is tuned for properly but most tuners don't want to for so e reason. I don't know how it works but they say it's hard to tune for and causes a lot of problems.. Like what is what I'd like to know. I want to do it because it definitely has advantages.
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Old 07-13-2009, 12:58 AM   #5
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Yeah does anyone know what are the downsides to this setup ? I.E. exactly what tuning problems can occur
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Old 07-14-2009, 01:29 AM   #6
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as pressure changes, air density changes. As an example, the stock maf is reading velocity only; the velocity times the area of the opening gives you CFM's, or the volume of air going past.

The OAT, outside air temp is consulted to correct for the starting air density; air density and CFM gives you MAF mass air flow. Based on atmospheric conditions, the amount of oxygen is easily calculated and fuel can be metered to match. As long as the engine is still spinning airflow is almost always one way in the intake. If the bypass activates it delivers compressed air back in front of the compressor (and back to atmospheric pressure) but behind the MAF; the air was only measured once and the fueling matches the air measured.

Now with a blow through, the pressure and the temp are constantly changing with turbo boost, making density calculations based off of the outside atmospheric conditions useless. Measuring intake velocity alone no longer represents the amount of oxygen going into the cylinders, so how do you tune the right amount of fuel to inject? Somehow the density of the air needs to be calculated in order to figure out the MAF going into the cylinders. Simply measuring velocity doesn't equal MAF it only gives you CFM's at that known area.

Hope that helps you understand why it is harder to tune a blow through versus an atmospheric MAF. The velocity and Temp are all you need, the pressure is, well, atmospheric at that point.
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Old 07-14-2009, 02:07 AM   #7
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Got a good tuner? Get a blow through.... IF you need it, ie FMIC.
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Old 07-14-2009, 02:59 AM   #8
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Or go a larger MAF housing in stock location (ie. pre-turbo) and rescale your MAF table to suit. Much easier and less hassles.

Saying that, there is really no need to consider either unless you are already maxing out your MAFv or want to run a VTA BOV.

Leslie.
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Old 07-21-2009, 02:18 AM   #9
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Definitely not thinking of doing any of this soon. I just wanted to learn a little more about it. Thanks for your posts, they did inform me a lot.

I have a spare motor laying around and I was thinking of building the block, only then would I think about a rotated setup w/ a blow thru maf.
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Old 07-21-2009, 02:24 AM   #10
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Blow thru is also great for taking care of FMIC rich dips.
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Old 07-21-2009, 03:18 AM   #11
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Quote:
Originally Posted by Ty B View Post
Now with a blow through, the pressure and the temp are constantly changing with turbo boost, making density calculations based off of the outside atmospheric conditions useless.
It would, if it used outside air temp, but it doesn't. In modern Subarus, the intake air temperature sensor and MAF sensor are integrated. When you move the MAF, the IAT sensor moves with it.

It's still not clear to me why blow-through is harder to tune. I've read from a couple sources that blow-though MAF works better with a larger MAF housing than what you'd use for a draw-through MAF housing, but since I haven't tried it, I don't really know how much easier that makes it / how much more difficult that approach is, compared to draw-through.

I've heard, as Remnex noted above, that it helps with the rich dip that tends to follow sudden throttle openings with an FMIC, but I haven't seen any plots yet to support that claim. I'm not saying they don't exist, I just haven't seen the data yet, but I'm keeping an eye out for it. That claim does align nicely with the prevailing theory on why the FMIC rich dip happens - a delay in the airflow increase seen by the motor (as compared ot the airflow increase seen by a draw-through MAF sensor) due to pressure building up in the FMIC.

I have more questions than answers. I probably am going to try blow-through at some point just to see if it really is harder to tune, and to hopefully help with the rich dip I'm getting.
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Old 07-28-2009, 03:41 AM   #12
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Quote:
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It would, if it used outside air temp, but it doesn't. In modern Subarus, the intake air temperature sensor and MAF sensor are integrated. When you move the MAF, the IAT sensor moves with it.
The problem is how fast that IAT sensor can react to the change in pressure conditions. The assumption of ideal gas law is a stretch at best with compressed intake air, factor in any intake tract efficiency issues and pv=nrt for dynamic response is less than ideal. The intake air temp is skewing the tuning. Just because you moved the anemometer and temp sensor behind the turbo doesn't mean it is able to read the conditions there or keep up with the transient conditions.

Not being a tuning guru myself, I too am interested in discussing how blow through MAF are set up. Is the IAT sensor replaced to improve transient response, or are the tunes fudged around the anomometer reading just using the stock temp sensor as a rough starting point?
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Old 07-28-2009, 04:36 AM   #13
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I'd like to think that since the IAT is downstream of the intercooler, the temps aren't really going to swing around a lot more than they would with a CAI. I'm sure there will be more variation, but how much more?

Can anyone out there who is running blow-through post a datalog that shows how much IAT moves around? It would be interesting to see how far apart the extremes are, though I'm not sure how to guesstimate the actual rate of change vs. the rate of change reported by the IAT sensor.
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Old 07-28-2009, 04:42 AM   #14
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It's still not clear to me why blow-through is harder to tune.
Another thread touched on the fact that with a blow-through MAF, the stock PCV system pulls unmetered air into the intake manifold - it's basically a vacuum leak. I can't help but wonder how much that has to do with the bad reputation that blow-through has.

It's easy enough to revise the PCV system to eliminate that aspect, though. There's a thread at IWST about it...
http://www.iwsti.com/forums/2-5-lite...ns-thread.html
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Old 07-28-2009, 07:08 AM   #15
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As far as rich dips go with fmics.... I've been trying to tune mine out for months, but eventually gave up. After 167 map revisions, I just accented it as part of the characteristics of the setup.

Theoretically, placing the maf post IC in the airstream would actually cut down on temp variation. The air going into your TB shouldn't be hotter than 130F and you get that on the intake with heatsoak pretty easily on some days.

Besides rescaling the maf, there might be some other things that might need to be changed. I haven't looked through RR, but I would assume you need to keep an eye on any tables that have temperature along the axis. Like TD temp corrections.

I've heard that all some people did was scale the maf and everything was good to go. My guess is that tuners don't like blow through because they have less experience with it and there is more for the customer to messup on the install if it's a diy job.
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Old 07-29-2009, 03:38 PM   #16
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Anyone pull the manufacturer info off the stock MAF? I'm looking for the specifications from the manufacturer for the IAT probe and the anemometer.

If you have a link for these post it up here. Going out on a limb here at this point, but the goo that is on the IAT probe is going to slow down the response for temperature changes.

Despite the wishfull thinking, the IC location will not dampen the changes in the temperature as pressure changes for a blow through MAF. IC's are not dynamic, the heat exchanged remains relatively constant.

If someone wants to help search for the info on the IAT probe we might be able to shed some accurate information in regards to the validity to using the stock sensors in this fashion. If the sensors are being used in an environment that they were not intended, you can look at the data logs all day long and they will still be useless.
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Old 07-31-2009, 10:37 PM   #17
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I'm pretty sure the stock MAF sensor is a Denso part, I'll see if I can find a number on it.

After I remove my TGVs I'll have two unused 0-5v inputs, conveniently located near the compressor outlet and near the throttle body. I'd like to get one or two more MAF/IAT sensors installed... I still have TGVs on the car now, and will for another couple weeks, but after the next round of upgrades is completed and the car is tuned, this will be my next project.

Comparing inlet air temp (from the stock MAF) with compressor outlet temp should let me create a compressor map, or least figure out what the map looks like for the flow/PR that my car uses.

Comparing MAFv from the stock sensor with MAFv from a pre-throttle-body sensor should help create MAF sensor scaling for an eventual switch to blow-through.

Comparing those two MAFv plus the air temp post-IC will show whether the temperature swings are affecting the readings. If they are, then I should be able to use the information to guide changes to the MAF-compensation-for-IAT table.

Comparing MAF at the compressor outlet with MAF at the throttle body (well, a foot or two upstream of it) will also prove or disprove the theory that the "rich dip" commonly associated with FMICs is due to a time lag between air rushing past the stock MAF and air actually making it into the cylinders.

Plus there's the timing compensation for IAT table to look at. I'm sure it will need to be tweaked to accomodate the generally-higher temps in the blow-through location, but averaging the suck-through and blow-through temperature differences will give me a place to start.

I just hope I can get the sensors hooked up and make sense of the signals that come out of them.

Last edited by NSFW; 08-01-2009 at 12:20 AM.
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Old 08-03-2009, 12:21 AM   #18
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Quote:
Originally Posted by Ty B View Post
Anyone pull the manufacturer info off the stock MAF? I'm looking for the specifications from the manufacturer for the IAT probe and the anemometer.
Does this help?



That's the stock sensor (in an aftermarket housing) in my 05 Legacy GT. I get the impression that all Subarus with 32-bit ECUs used the same sensor, at least up until the 08 WRX which I'm not sure about.
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Old 08-03-2009, 12:34 AM   #19
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Quote:
Originally Posted by Bad Noodle View Post
As far as rich dips go with fmics.... I've been trying to tune mine out for months, but eventually gave up. After 167 map revisions, I just accented it as part of the characteristics of the setup.

Theoretically, placing the maf post IC in the airstream would actually cut down on temp variation. The air going into your TB shouldn't be hotter than 130F and you get that on the intake with heatsoak pretty easily on some days.

Besides rescaling the maf, there might be some other things that might need to be changed. I haven't looked through RR, but I would assume you need to keep an eye on any tables that have temperature along the axis. Like TD temp corrections.

I've heard that all some people did was scale the maf and everything was good to go. My guess is that tuners don't like blow through because they have less experience with it and there is more for the customer to messup on the install if it's a diy job.
I just pull the fuel out of primary fueling map where the rich dip tends to happen. Rescale the RPM field and you can nail it every time.
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Old 08-03-2009, 12:52 AM   #20
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How certain are you that you won't hit the same rpm/load cells without having triggered the rich dip? If that happens, you'll go lean... kinda makes me suspicious of taking that approach.

EDIT: So I just looked at last night's logs, and I find sections where I'm at around 4000 RPM and about 2.8 load, where there target AFR is 11.2, and the measured AFR is either 10.0 (the lower limit of my PLX SM-AFR) or 11.18 or 11.25. If I lean it out to fix the rich dip, I'm going to run lean when I roll on the throttle or just hold 2/3 or 3/4 throttle or whatever it takes to get the same load where the rich dip was happening.

The fuel table just doesn't seem like the right place to "fix" this.

Last edited by NSFW; 08-03-2009 at 01:11 AM.
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Old 08-03-2009, 01:06 PM   #21
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Quote:
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How certain are you that you won't hit the same rpm/load cells without having triggered the rich dip? If that happens, you'll go lean... kinda makes me suspicious of taking that approach.

EDIT: So I just looked at last night's logs, and I find sections where I'm at around 4000 RPM and about 2.8 load, where there target AFR is 11.2, and the measured AFR is either 10.0 (the lower limit of my PLX SM-AFR) or 11.18 or 11.25. If I lean it out to fix the rich dip, I'm going to run lean when I roll on the throttle or just hold 2/3 or 3/4 throttle or whatever it takes to get the same load where the rich dip was happening.

The fuel table just doesn't seem like the right place to "fix" this.
Go find a hill (or use the brakes) were you can keep the car at that same RPM and load for a bit and see if the spike goes away. If it does then you definately don't want to pull fuel there.

Reminds me of when people scale the injectors for a big maf rather than scaling the maf table like they should.
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Old 09-16-2009, 11:06 AM   #22
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I thought that since the maf is a heated wire, air density is directly measured, not cfm. You can have a fixed CFM with extremely low air density (think mt. everest), and it will hardly "cool" the hot wire cause there are not that many air molecules flowing past it for the given CFM.

Now if you take that CFM to sea level, where air molecules are much more abundant, and density much higher, a given CFM will result in much more hot wire cooling, thus the ecu will know there is more air entering the engine.

Basically i don't think a maf measures CFM, i think it measures air flow, right? If i have it wrong, maybe someone could explain.
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Old 09-16-2009, 02:01 PM   #23
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Quote:
Originally Posted by NSFW View Post
How certain are you that you won't hit the same rpm/load cells without having triggered the rich dip? If that happens, you'll go lean... kinda makes me suspicious of taking that approach.

EDIT: So I just looked at last night's logs, and I find sections where I'm at around 4000 RPM and about 2.8 load, where there target AFR is 11.2, and the measured AFR is either 10.0 (the lower limit of my PLX SM-AFR) or 11.18 or 11.25. If I lean it out to fix the rich dip, I'm going to run lean when I roll on the throttle or just hold 2/3 or 3/4 throttle or whatever it takes to get the same load where the rich dip was happening.

The fuel table just doesn't seem like the right place to "fix" this.
Unless you move to SD or some other form of placement for the MAF sensor, this is one of the best ways to handle the situation.
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Old 09-16-2009, 08:43 PM   #24
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Quote:
Originally Posted by Remnex View Post
Unless you move to SD or some other form of placement for the MAF sensor, this is one of the best ways to handle the situation.
I wouldn't feel comfortable doing this. If you have people in the car, pulling something heavy, or hit that load in anything but a WOT run, thing's won't go well. I'd rather keep the dip than blow and engine.
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Old 09-16-2009, 11:57 PM   #25
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I agree with Bad Noodle's take. I don't think it makes sense to pull fuel from RPM/load cells, it's just too easy to hit the same cells without doing the throttle-stab that creates the rich dip.

I'm going to go blow-through for sure, and in the meantime I'll tolerate the rich dip and roll on the throttle.
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