A few months ago I tuned a turbo RX-8 with the Interceptor-X. During that time I had an interesting discussion on the forums about MAF vs MAP and I admit I was a little close-minded at the time. Since then I bought my own RX-8 for a winter car and a project car as I don't like the turbo kits available on the market atm. I'm going to be doing things my own way and see how it turns out.

Anyway, I decided to give the stock PCM+MAF a shot before going full standalone+MAP since Mazdamaniac seems so adept at convincing people the stock PCM+MAF is the greatest :P. So I'm testing this for myself and going into this with a complete open mind/attitude. If I don't like how it turns out, I'll go to standalone+MAP, if I do then there would be no point of course. Either way, it will be a good learning experience in something I haven't worked with before.

Now getting the question. Is this the correct way to setup a blow-through MAF system with an open BOV? Would there be any problems in doing it this way? (I remember reading something about turbulence) Could the stock MAF sensor be used and if I use a pipe with the same dimensions as stock, would the MAF calibration still need to be calibrated? Has anyone done this before?



thewird

 

You're missing the intake on the turbo. The MAF goes in the intake tract before the turbo. The only problem when running a MAF with an atmospheric blow off valve, is that it will be counting on air that isn't there when the BOV actuates. This is because since the air is metered before it enters the turbo, and the the BOV is obviously placed after, when it opens, the MAF won't know that air is being released. One way to fix this is to recirculate your BOV back into the intake tract (after the turbo)

 

That is why it is called a blow through set-up; the MAF is after the turbo.

That is a general way to set it up yes...the real question is whether you can get the air stable when it passes the MAF sensor - and the only way to know that is to try it out.

 

Note that there is an admissible temperature range for the MAF as well. I don't remember what it is and as a pull through, it's a temp you can't hit. However, it is something you might want to check out on paper. A well designed turbo system probably doesn't have to worry about it. A greddy trying to do 300 whp might. So as a custom setup, you'd at least want that number written down.

Second, say you go to full boost, WOT and then close the throttle. When the BOV opens, could the MAF see a reverse in flow? Does the maf read in both flow directions? Would it output garbage if it saw flow reverse?

 

Yes, you will definitely need to scale the MAF and put a screen a few inches in front of the sensor. Even small percentages of correction are crucial. Scale the MAF and the injectors and you've pretty much won the war. FI on the AP isn't *that* difficult once you get the hang of it, and is much less cumbersome than microtech.

I've been contemplating trying a blow through setup myself so interested to see how you fare, or if I get to it first I'll post up my results as it should be within the next few months. Don't know what you're thinking about turbo wise, but I'm probably heading towards a 60-1.

 

That's the only thing I'm worried about. The OEM MAF reads by cooling of the sensor correct? If the flow backfeeds in reverse, it can still technically cool the sensor and as such would add fuel in?

Also, I'm assuming since the MAF works in all weather conditions, it has its temperature sensor built in to correct the reading? If not, that means its relying on the AIT correction from the PCM which would mean I might have to play with that as well to get it to work right if I do blow-through?

I'm considering running something in the range of a GT35R - GT40R. If Garrett releases the new GTX-R's in that size by the time I get working on this, I will probably get it in the 35R size. Everything will be custom made and done properly (in my eyes at least).

The only thing I'm unsure about is this MAF stuff since every time I think about it, it just seems like it adds more and more challenges to get it to run mint hehe. But I'll give it a shot nevertheless.

thewird

 

What is the main reason you want to run blow through maf ? Is it just so that you can run a vent to atmos BOV ?
No-one on here has done it yet but I don't think anyone has tried that hard either .

FWIW - I run suck through maf with the IAT sensor seperated and an alternate one just before the throttle plate .

 

Want to keep the setup as simple as possible for easier placement of things. If I put it in front of the turbo, I will need to make the MAF section and make sure its straight, have the screens, long enough etc. Also, I want to make a V-mount so I want as much flexibility with where things go. Something similar to what I did on the RX-7...

*This picture was before the ducting was done so you can better see how its designed


This is with the ducting...


thewird

 

Considering the concerns that are "forcing" you to design a blow through set-up and the kind of turbo you're looking at maybe you should at least try if there's enough straight intake lenght before the turbo to install a draw-through (sp?) setup.
You would probably be close to the max scaling of our MAF in that way with a gt35, i don't know how it would work in a blow-through setup

 

So its basically one of those try and find out things then huh

thewird

 

Yes, you will also have to study what kind of diameters you're going to need to better calibrate the MAF for your goals in that position.
I'm not able to properly scale a MAF sensor beyond the "standard" limits but MazdaManiac could chime in setting us straight

Some people are using a split set-up with a draw-through maf and a barometer placed after the turbo in order to better adjust the maf's readings with some data after the turbo.

Playing around with a Maf based system gives a lot of opportunities tuning wise, especially when air pressure is used as a correction factor

 

it should suck thru not blow thru THIS MAF

 

Are you saying the oem maf is incapable of working in a blow through setup?

thewird

 

Should be the same MAF as on the Subies right?

 

First of all, that is a sweet drawing.

You can do a blow-through with our MAF, but I bet the plumbing restrictions are gong to be so much more difficult that it simply isn't worth the effort.
If you are going to have to make a whole bunch of fabrication puzzles, you would be better off doing them on the suck-side.

 

I was originally going to do my setup blow through. Even started fabbing up a tube for it. But once I got to looking at turbo positions and realized I'd have more than enough room for a draw through..I just stuck with what was proven.
I had trouble getting the Maf close enough to the TB for my taste anyway. The way our TB is angled aimed "across" the front of the engine bay was the problem. Had my turbo and Maf fighting over the same spot
My thought was that the closer the Maf is to the TB the more reactive it would be. Being farther away kind of puts it in the middle of a pressurized tank so it wouldnt react right away when the TB starts pulling air.
Prefering this:
|||||||||||||||||||||||MAF>-TB

To this:
||||||||MAF||||||||||>>>>-TB

Decided it was too much work to fab up for something that may not even work properly
If someone else gets a blow through setup working I may give it a go on my "rebuild" when I fab up the final version of my setup though.

 

Nah, doesn't work like that.

The pressurized air in the system is all moving at the same speed where the pipe diameter is the same (allowing for changes in temperature and, therefore, density).
Its not a tank - its a column.

As a matter of fact, the air is moving at the same speed from the filter to the TB, only speeding up for the restrictions like the volute of the turbo and perhaps the IC piping, which is usually only 2 - 2.5 inches or so.
If your filter pipe is 3.375" and your TB pipe are 3.375", the velocity is identical, regardless of how far away it is from the TB.
Additionally, since the TB is a restriction, there will be an "eddy" just in front of it that will screw with the MAF.
That is why you don't want diameter changes near the MAF.

 

Ahhh thanks for the input MM

But if my intercooler pipes are all 2.5" and I wanted my Maf tube to be 3.5" wouldnt that create issues trying to keep the air smooth within all those size transitions and curves? I know on my suck through setup I used 3.5" and 3" transitions and it takes up alot of space. More transitions would take up even more space.
I know I could put the sensor on a smaller tube, but I was of the understanding that the smaller tubes become less accurate and max out the sensor faster? Any truth to this?

 

Just make sure that wherever you mount the MAF that it is a 12" or longer straight piece of 3.375" pipe with the sensor in the middle.
Everything else doesn't matter.

 

i sure wish i had a twin clutch gearbox so i would never have to close the throttle!
OD

 

Is that the stock sizing? So if I go ahead with this going with a 3.375" (does anyone even make that specifically lol) would be the preferred diameter on the outlet of the intercooler all the way to the TB?

thewird

 

Yea that is the stock maf ID.

You probably have to reduce the size at the throttle body size which is about 3" (80 mm, I think).

You can get the right size piping by going with a 3.5 inch pipe with a 1.5 mm thickness. It would essentially be a 3.38" ID.