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The Showdown; Blow-thru MAF vs. Draw-thru

So with the plans I have for the car include Methanol. I had an SAFC2 and traded it for a MAFT. Im also kicking around the idea of goin to MegaSquirt. But for the time being im using what I have to do what I can. So anyway, my new turbo has a 4in inlet so I want to do blow-thru as I dont think I would do the turbo any favors doing draw-thru. So in BT (blow-thru) with the BOV VTA the car runs great, fuel trims are good but when coming down in RPM's such as coming to a stop, the car wants to stall out and die. Ive done all matter of clocking the MAF, adjusting Low Fuel Trims up and down, tuning the BOV, upping the idle and still nothing. I have a chip that is set for 2G MAF and the injectors I am using so the only reason I am using this is for the GM MAF and the Meth. Fuel trims are dead on. I actually have to lean out the idle in either configuration.

Well I happen to have 2 3.5 GM MAF's. So I came up with an idea....

















So I came up with intake real fast, recirculated the BOV and Im good to go. Draw-thru seems to be winning the battle of driveablity so far. But I am on the Full Throttle Speed forums talkin with Turbo Bob right now trying to get the BT situation figured out. Even the DT will stall when stopping when I first start driving and its not up to full temp. Now, this wasnt uncommon with the SAFC when I first started driving. It would do it once and that was it.

I refuse to let this problem beat me even though MegaSquirt is on the menu eventually, I want to get this sorted out. Before anyone says anything, YES the MAF is pointed the right way, YES I cleaned them, YES the MAFT is set up right. Ive tried different base settings with it. Also, the BOV is recirced now and makes no difference with BT.

Feel free to make any suggestions but Im confident I can conquer this. Turbo Bob was curious to see what my 02 Trims were when I get off throttle. I hadnt thought of that so I am goin to make some logs of it and see what happens.
 

curtis

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May 4, 2003
Messages
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Hell the blow through isn't wired up.... that's the problem. /ubbthreads/images/graemlins/rofl.gif


The fuel trims makes sense and he maybe on to something you could be dumping fuel and blowing out the spark plug at no throttle situations.


Try and log both full bore pulls with lifting to half throttle and no throttle and see if the numbers match ....they shouldn't. Also whats the wideband saying and whats the age of the narrow band O2 and have you ever ran airplane gas or race gas in the car. lead deposits screw up readings.
 

Ive never ran anything other than 92 on this. I dont have a wideband either /ubbthreads/images/graemlins/frown.gif Im strictly tuning with a logger. I could come up with a wideband if I sweet-talked my buddy into grabbing his off the Dyno on the weekend. The narrowband 02 is a year or two old.

Tomorrow I am going to do some logs like you suggested. We'll see what we get from that.

Also, when I let the car idle and warm up to 200+ degrees, the BT seems to work ok. But when it hasnt seen full temp and I drive, its not happy. When I was driving around today without warming all the way up first, the SOB wouldnt get above 158* when cruising. Im sure the 45* outside and huge FMIC and radiator didnt help.
 
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turbowop

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I don't know sh*t about tuning a MAFT setup, which is why I traded you for what I know...the SAFC. I know the MAFTpro, but that's quite a bit different than the original MAFT. With that said, in draw-thru on my car the way Lucas had originally tuned it, it worked great without a hiccup. I'm not sure which would be better for your situation, but I don't think that the 3.5" maf I gave you would be much of a restriction in front of a 4" inlet. /ubbthreads/images/graemlins/dunno.gif Some guys have 2gMaf in front of large turbos like that as well.
 
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Im sure you're right about the DT with the 52. But eventually ill make the jump to MegaSquirt and using SD. So in reality, I could go with Draw-thru and be fine. But now its become more about something that is getting the better of me and I HATE THAT. I just want to figure it out for the knowledge that can be gained and gratification Ill get when its no longer a mystery. I enjoy this stuff as well.
 

Besides, for the time being I can say "My car is so badass, it needs 2 MAF's!" /ubbthreads/images/graemlins/rofl.gif
 

G

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How much $ did all this cost you? The maft, safc, all the parts etc... everything totaled up.
 

Basically nothing. I had the SAFC and 2G MAF which I traded straight across to Mark for MAFT and GM MAF. I already had a second GM MAF. So im into it like $35 for silicone couplers /ubbthreads/images/graemlins/wink.gif. Everything else is all stuff I have layin around.
 
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So I took some advice from a buddy and some things that Turbo Bob over on the Full Throttle forums had said and richened up the low trim quite a bit. The car will run and drive fine with no stalls when the low trim reads 81.2 (which is the lowest it will read) up to 87 or so. Now this to me goes against everything ive ever read on tuning with a piggyback system. I always shot for 100 on the trims. So Im going to try sowly try to lean up the low and get the car to where its happiest. Now keep in mind this is the BLOW-THRU configuration. Now, wether VTA the BOV makes any difference, I havnt tried yet. Im recircing it right now. Im goin to give it a shot for all you Ricers out there.
 
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curtis

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One more think that just passed through my minds eye. Whats your fuel pressure and is it constant and not fluctuating any down low and is it climbing 1:1 under boost. It could be you have a weak pump / low voltage weak charging system and causing some of this insanity. Well thats 2 things but something to look at. I like how thisi thread is going and some good ideas and theories to be looked at can come from it.
 

gtluke

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dirty jersey
Hey guys I took the factory MAF meter off my car and replaced it with one which is the same size but off a chevy beretta. But since it uses 1950's hot wire technology I was able to rig it blow through so I can vent my BOV and impress Hondas. Drive ability has gone downhill, any suggestions?
 

curtis

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/ubbthreads/images/graemlins/rofl.gif /ubbthreads/images/graemlins/rofl.gif /ubbthreads/images/graemlins/rofl.gif Go back and check the 10 pin harness on the mustang its loose. /ubbthreads/images/graemlins/rofl.gif /ubbthreads/images/graemlins/rofl.gif
 

toybreaker

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Apr 30, 2006
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Hot wire sytle airflow meters are an excellent airflow measurement device, but they do have some issues, especially when used in a blow thru application.

Over time, the wire will begin to get a build up on it's surface, and the additional mass of the contaminates will skew the readings.

The drift isn't linear, (not even close in fact), but it is somewhat predictable.

If it's a fuzzy build up (think cotton fibers from an air filter) the fuzz will insulate the wire at low airflows.

This means a dirty hotwire airflow meter will under-report the airflow at idle/cruise.

At mid airflow levels, it's about a wash.

At higher airflow levels, it will actually over-report airflows.
The additional surface area of the fuzz will present more area to the airflow, and it will think more air is flowing past the wire than is actually happening.

This can be seen as a pattern on your fuel trims.

If the wire is contaminated with fuzzies/dirt, it will read high trim at low airflows, ~ballpark~ in the middle, and low trims up top.

Hot wire units that are contaminated with oil will also drift, but the pattern isn't as predictable. Every now and then, they will burn clean and act normal. At other times the oil forms an ash like insulating coating and then they under-report everywhere.

If you hear someone say "I had to pull a buncha fuel down low and add some up top", color them a tOOner.

They don't know what they're looking at, and are just re-acting to sensor drift, (especially if they don't check the fuel pressure.)

Your best bet is to investigate things at the airflow meter when the fuel trims get wonky or the sensor numbers seem "off"... before breaking out the jump to conclusions mat, ( /ubbthreads/images/graemlins/cool.gif deluxe tOOner edition /ubbthreads/images/graemlins/cool.gif )

Jacking with fuel pressure, injector settings, timing maps and the like only makes things worse, because you're tOOning a system that is out of calibration.

The good news is the unit can be cleaned and returned to service with good results, if some care is taken during the service.

Just spraying something on the wire can lead to disaster, as many cleaners will eat plastics, or may leave a coating behind that completely alters the characteristics of the airflow/voltage curve.

Many hot wire units allow the measurement package to be removed from the airflow meter, and that makes things easier.

For cleaning, I've used sterile, medical grade q-tips, dipped in pure isopropyl alcohol with very good results for years.

Just pour some pure isopropyl alcohol in a shot glass, dip a q-tip, and then >gently< roll the q-tip down the wire.

>DO NOT SCRUB
 

Quoting curtis:
One more think that just passed through my minds eye. Whats your fuel pressure and is it constant and not fluctuating any down low and is it climbing 1:1 under boost. It could be you have a weak pump / low voltage weak charging system and causing some of this insanity. Well thats 2 things but something to look at. I like how thisi thread is going and some good ideas and theories to be looked at can come from it.



Fuel pressure is set at 45 psi. Aeromotive afpr with -6 lines ALL the way through. Fuel delivery shouldnt be a problem. Its stable at idle, I can only view this in the engine bay. Fuel pump is a Denso that I wired with 6 or 8 gauge wire. The charging system is definately on option. The alternator is a Bosch thats only a few months old, but we all know that doesnt mean sh*t. At idle its got 14.1 volts and cruising its got 13.8 to 14. Ive got a half-dozen grounds in the engine bay.

On the second day of driving it around, everything seems ok with the low trim down at 82. Today I will lean it out until it acts up and see where the trims sit at then. I really really need a wideband to know what the afr's are. To do this solely with a logger doesnt provide enough feedback.

Toybreaker: Prior to install of both sensors, I cleaned them with an aerosol MAF cleaner, inspected for any debris stuck to the wires and then let dry and installed. I do know that in Blow-Thru the sensors can be temperamental. But so far it seems to drive and go thru the rpms clean at the moment.
 

gtluke

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dirty jersey
Quoting curtis:
/ubbthreads/images/graemlins/rofl.gif /ubbthreads/images/graemlins/rofl.gif /ubbthreads/images/graemlins/rofl.gif Go back and check the 10 pin harness on the mustang its loose. /ubbthreads/images/graemlins/rofl.gif /ubbthreads/images/graemlins/rofl.gif


I'm so elite that my harness doesn't have the 10 pin connectors /ubbthreads/images/graemlins/smile.gif
 

So I kinda dropped the ball on this investigation. I got busy with life and havnt even been online in a loooonnggg time. Anyway, Ive been working on my new build which I will post some pics of in a few.

Shortly after my last posts I got a wideband in the car. I had tuned the MAFT so that my low fuel trim was at 82. The WB02 reads 14.7- 15. Cool. I can only imagine what it reads when the the LFT is at 100. I also found that with everything else on the MAFT zeroed out that my WOT was RICH AS HELL. At least 10 or lower. Fixed. Now im at 11.5 -12.

The intial problem ive been having is still there, but without a working ISC, I dont think I can get any real answers. I have tried to find or borrow a working ISC with no luck and im not goin to buy one since I wont be needing it here soon.

That is all.
 

Also, I killed my datalogger shortly after too. I left it sitting on my lap when I got out of the car. So I cant use that to cross-reference the WB02. /ubbthreads/images/graemlins/dunno.gif
 

raptorWagon

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I believe I have a palm sitting around with MMcd 1.8g loaded on it I could ship to you if you want? /ubbthreads/images/graemlins/dunno.gif old m130 color.
 
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Brianawd

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Apr 18, 2005
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Portland OR,
Quote:
Hey guys I took the factory MAF meter off my car and replaced it with one which is the same size but off a chevy beretta. But since it uses 1950's hot wire technology I was able to rig it blow through so I can vent my BOV and impress Hondas. Drive ability has gone downhill, any suggestions?



My car has been Blow through 3in gm maf for over 3 years. Car runs perfect and still gets 23mpg on E85. Drive ability is perfect. I have even tried it in draw through with no change in how the car runs. It works and works well and has for years.
 

turbowop

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Messages
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Location
Yakima, WA
Luke also thinks that intercooler piping should be no larger than the compressor outlet of a given turbo. Fail. /ubbthreads/images/graemlins/keke.gif
 
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