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  #1  
Old 11-03-2005, 08:26 PM
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tune your afr and stop your MAF from maxing out for $100

Over the last year, I've been trying to figure out how I can get some control over the air-fuel ratio and stop the MAF from maxing out quite as soon. I've come up with a solution involving two items:

1) 2000 Impreza RS fpr
2) A MAF "bypass" tube.

The Impreza RS fpr, which costs about $75, raises the baseline fuel pressure from 36 psi to 43 psi. This, in turn, richens the air-fuel ratio. Comparing my wideband O2 measurements to other people's (see Chiketd's and SVXRide's dynosheets), it looks like the Impreza RS fpr richens the afr from about 14:1 to about 12.5:1 over the 3000-5000 rpm range, and above 5000 rpm, it richens from about 13.5:1 to 11.5:1. This is a bit too rich.

So, I got to thinking about how I could lean out the afr. My first thought was an SAFC-II, but the darn things are expensive and it doesn't do anything to keep the MAF from maxing out at high airflow. I got to thinking about other ways to lean out the afr, and I realized that if I could just add small intake tube that allows some air to go from the airbox straight to the intake plenum at the entrance to the throttle bodies, then some air could get to the engine without going through the MAF. So, the total amount of air getting to the engine is the same, but the MAF signal is reduced because not all the air is going through the MAF. The result is that the ECU sends less fuel to the engine, and the afr gets leaner. Seemed like a cool idea to me. So I tried it. The picture below shows the MAF bypass installed. Its 3/4" ID rubber tubing. The fittings at the airbox and the plenum were bought at Lowe's. Total cost for tubing and fittings was $10. The fittings are cool because they have nuts to hold the fittings to the airbox and to the plenum. I had to use a bit of silicon sealant at the plenum because the plenum is not completely flat. The configuration in the picture will probably not be the final configuration. The elbow significantly reduces airflow.




Next, I got out my LM-1 wideband meter and took some afr readings before and after installing the MAF bypass. The first graph below shows the baseline reading with no MAF bypass. You can see that the afr is around 12.5:1 from 3000-5000 rpm, and then goes even richer after that. You can also see that the MAF voltage is reaching an *average* peak value of about 4.4 volts (which is still less than the 4.8 volt max that it can produce). The second graph shows the result for an experimental bypass that I didn't end up using. The result is quite good though. The afr is leaned out to about 13.2:1 from 3000-5000 rpm and then sits at about 12.2:1 at peak rpm.




Here are the results for two other MAF bypass configurations that I tried. The first config works a bit better than the second. The second one is the configuration in the picture above. You can see that not as much air gets through the bypass because the afr is a little richer. Both configurations give about the same result though. Overall, I'm very pleased with the result. The engine is much more peppy when I step on the gas, and now don't have to worry quite so much about the MAF maxing out with further NA mods (ram air is next). If anyone is interested, I can post more detailed pictures and descriptions of the parts for the MAF bypass mod.

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SVX Mods: ND iridium spark plugs, Impreza RS fpr, afr tuned to 13.2:1 using a custom MAF bypass, custom exhaust, WRX 5MT w/ STi RA 1st-4th gear & stock WRX 5th gear, Exedy 13 lb flywheel & Sport Clutch, STi Group N tranny & engine mounts, urethane spacers in rear subframe, rear diff mounts, and pitch stopper, SVX Sport Strut Springs (185f/150r), custom 19 mm rear swaybar, urethane swaybar mounts, Rota Torque 17x8", 225/45-17 Proxes 4 tires, Axxis Deluxe Plus organic brake pads.
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  #2  
Old 11-03-2005, 10:49 PM
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cool... we will see how well it works on a modified engine this weekend As for me, the stage 2 is all I need. This mod definately helps out the OBDII guys until LAN finds a 97 that he likes

Tom
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  #3  
Old 11-04-2005, 07:22 AM
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Great write-up Mychailo. The results definitely look great!

-Chike
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  #4  
Old 11-04-2005, 07:31 AM
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Without even seeing the results, i think what he did is very logical to cope with the RS AFR. Great Mike, and a bold move
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  #5  
Old 11-04-2005, 07:32 AM
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Is the Impreza RS the only FPR that can be swapped into the SVX? Just curious......
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  #6  
Old 11-04-2005, 07:48 AM
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Haha... I do remember coming up with this idea for a completely different purpose. I was thinking about adding a valve in the tube however, so that you could control the amount of air that flows through the tube.

(my purpose was because of my knock sensor... which by the way, after being replaced twice, is still giving me trouble. I haven't had the chance to bust out a multimeter and start going through the knock sensor connectors to see where my problem is.)

I'm glad it worked out for you!! If you could add larger injectors, you could allow even more "unmetered" air through. Some People do this with dsms as an effective cheap way to tune their cars. (Less now, but it used to be popular)
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  #7  
Old 11-04-2005, 08:41 AM
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Quote:
Originally Posted by TomsSVX
cool... we will see how well it works on a modified engine this weekend As for me, the stage 2 is all I need. This mod definately helps out the OBDII guys until LAN finds a 97 that he likes

Tom
Yeah, its no Stage 2, but it does nicely compliment mild NA breathing mods (exhaust, air filter, ram air). Its more like a DIY Stage 1.2. Yeah, should provide a useful bit of MAF padding for that modified engine. :-)

Quote:
Originally Posted by MidnightBlack92
Is the Impreza RS the only FPR that can be swapped into the SVX? Just curious......
There might be other Subaru fprs that fit, but I haven't checked because the Impreza RS fpr gets the job done.

Quote:
Originally Posted by drivemusicnow

...

... If you could add larger injectors, you could allow even more "unmetered" air through. Some People do this with dsms as an effective cheap way to tune their cars. (Less now, but it used to be popular)
Yeah, I thought about a larger tube, but there's not much room to add a larger tube in the path I used. If someone wanted to add a tube that entered into the driverside of the intake pipe, a larger tube could be used there and then perhaps routed into the driverside fenderwell. At some point though the reduced MAF signal from a larger tube is going to lead to too much timing advance.
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1995 Laguna Blue SVX L AWD 5MT (sold)

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SVX Mods: ND iridium spark plugs, Impreza RS fpr, afr tuned to 13.2:1 using a custom MAF bypass, custom exhaust, WRX 5MT w/ STi RA 1st-4th gear & stock WRX 5th gear, Exedy 13 lb flywheel & Sport Clutch, STi Group N tranny & engine mounts, urethane spacers in rear subframe, rear diff mounts, and pitch stopper, SVX Sport Strut Springs (185f/150r), custom 19 mm rear swaybar, urethane swaybar mounts, Rota Torque 17x8", 225/45-17 Proxes 4 tires, Axxis Deluxe Plus organic brake pads.
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  #8  
Old 11-04-2005, 12:11 PM
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Who was the first person to mention this idea? I know I mentioned it shortly after LAN realized we were maxing out the MAF. I remember discussing with Chike that the only way to fix the MAF is to either replace it with one that could read higher flow or allow a controlled amount of air to bypass the MAF.

Cool to see it works!

Are you going to market this?
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Old 11-04-2005, 01:04 PM
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Are those Grey joints hard enough not to melt or change shape?
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  #10  
Old 11-04-2005, 01:25 PM
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Quote:
Originally Posted by Earthworm
Who was the first person to mention this idea? I know I mentioned it shortly after LAN realized we were maxing out the MAF. I remember discussing with Chike that the only way to fix the MAF is to either replace it with one that could read higher flow or allow a controlled amount of air to bypass the MAF.

Cool to see it works!

Are you going to market this?
I remember that thread. However, through some PM's I've had with Mychailo, he's been tossing this idea around for quite some time. Glad to see it come to fruition and be successfull.

Maybe it'll finally prove to people what I already knew from my runs last Friday - my MAF maxed out!

-Chike
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  #11  
Old 11-04-2005, 02:27 PM
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Quote:
Originally Posted by mbtoloczko
Yeah, its no Stage 2, but it does nicely compliment mild NA breathing mods (exhaust, air filter, ram air). Its more like a DIY Stage 1.2. Yeah, should provide a useful bit of MAF padding for that modified engine. :-)


Yeah, I thought about a larger tube, but there's not much room to add a larger tube in the path I used. If someone wanted to add a tube that entered into the driverside of the intake pipe, a larger tube could be used there and then perhaps routed into the driverside fenderwell. At some point though the reduced MAF signal from a larger tube is going to lead to too much timing advance.
You might be able to use a seperate filter like those used as crankcase vents. The biggest problem you need to avoid is if the "unmetered" air has less resistance than the metered air.. this would result in possibly not idling
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  #12  
Old 11-04-2005, 02:37 PM
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Resistance won't be an issue as the air is taken AFTER the air filter.
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  #13  
Old 11-04-2005, 04:21 PM
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Quote:
Originally Posted by Earthworm
...

Are you going to market this?
Since the parts are so easy to find, and its so easy to make, I'm not sure that anyone would want to buy a kit. The hardest part was drilling the airbox and the intake pipe, and that was actually more time consuming than it was difficult because I didn't have a large enough drill bit on hand and used a dremel tool to open up the holes.

With that said, if anyone wants a MAF bypass "kit", I'd be happy to gather up the parts and mail them out to whoever wants it. Should probably cost about $20/kit including postage. The Impreza RS fpr would have to be bought at your favorite Subie dealer. I can post the PN if anyone is interested.

Quote:
Originally Posted by SilverSpear
Are those Grey joints hard enough not to melt or change shape?
I was thinking about that last night. Those gray plastic joints are made of a thermoplastic material, so I can heat one up and reshape it. I'll give it a try on Saturday morning.

Quote:
Originally Posted by Chiketkd
I remember that thread. However, through some PM's I've had with Mychailo, he's been tossing this idea around for quite some time. Glad to see it come to fruition and be successfull. ...
The first person I know of to realize that the MAF is maxing out was Porter back in August, 2003.

http://www.subaru-svx.net/forum/show...12611&p=139768

I think that the first time I saw anything on the SVX board about a bypass was something written by drivemusicnow.
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Mychailo
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1995 Laguna Blue SVX L AWD 5MT (sold)

Visit my locker

SVX Mods: ND iridium spark plugs, Impreza RS fpr, afr tuned to 13.2:1 using a custom MAF bypass, custom exhaust, WRX 5MT w/ STi RA 1st-4th gear & stock WRX 5th gear, Exedy 13 lb flywheel & Sport Clutch, STi Group N tranny & engine mounts, urethane spacers in rear subframe, rear diff mounts, and pitch stopper, SVX Sport Strut Springs (185f/150r), custom 19 mm rear swaybar, urethane swaybar mounts, Rota Torque 17x8", 225/45-17 Proxes 4 tires, Axxis Deluxe Plus organic brake pads.
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  #14  
Old 11-04-2005, 07:56 PM
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Quote:
Originally Posted by drivemusicnow
You might be able to use a seperate filter like those used as crankcase vents. The biggest problem you need to avoid is if the "unmetered" air has less resistance than the metered air.. this would result in possibly not idling
idling won't be an issue. The MAF is next to unused at idle as the air intake is controlled by the IAC and the SIAC. The SAIC is the cold start idle controller and the IAC is just the idle air controlled. It is a small rotating valve on the bottom of the throttle body. Any unmetered air at idle would need to come into the engine after the butterflies...

Tom
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  #15  
Old 11-04-2005, 11:50 PM
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AppStateSVX AppStateSVX is offline
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I just bought a 2002 WRX FPR today on ebay, I got it for pretty cheap. Does anyone know if this will work? I hope so, haha
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