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.

http://www.subaru-svx.net/photos/fil...czko/31099.jpg


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.

http://www.subaru-svx.net/photos/fil...czko/31047.gif


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.

http://www.subaru-svx.net/photos/fil...czko/31097.gif

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

Great write-up Mychailo. The results definitely look great! :)

-Chike

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 ;)

Is the Impreza RS the only FPR that can be swapped into the SVX? Just curious...... :D

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)

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. :-)

There might be other Subaru fprs that fit, but I haven't checked because the Impreza RS fpr gets the job done.

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.

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! :D

Are you going to market this?

Are those Grey joints hard enough not to melt or change shape?

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

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

Resistance won't be an issue as the air is taken AFTER the air filter.

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.

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.

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.

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

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 :D

If they do, just get heat isolating material and cover them with it. Also the Heat isolating duct tape will work, same as I did with my copper intake... BTW i will post new pics about it very soon.

Nice
 

Very nice thread and great info Mychailo. You do realize that this info can spread like a disease to other forums for other models of Subarus right? The later modelled turbo'd Subarus of the late 80's has problems with fuel cut due to the Maf voltage reaching a certain point. If this mod could prove successful in application on these turbo'd subarus, the bypass can keep the airflow at bay by not letting too much air through the maf sensor during WOT thus preventing a fuel cut. Of course a valve can be put in line to regulate how much air can bypass. Also, you could have more than one bypass or use bigger diametered tubing, just in case not enough air bypasses.

Interesting! I posted a thread a USMB today.
http://www.ultimatesubaru.net/forum/...547#post365547

Thanks for the info.

One word of caution on the bypass mod -- everyone should notice that Mychailo took a very scientific/engineering approach to the process and used a wide band to measure exactly what the the bypass was doing to the AFR. As a result, we (the SVX community) have a baseline system that we can copy and not have to worry about burning up our engines due to overly lean AFR. The last thing you want to do in this area is adopt a "larger tube is better" approach :eek:
-Bill

I understand fully. I mentioned that it would probably be advisable to use a wideband, EGT guage or a dyno when doing this mod. My thinking is flawed though. On a boosted car, the boost pressure would push outward from the bypass instead of sucking the air in from the bypass. That is for another thread/another forum. Thanks anyways! Carry on! :)

new bypass tube...
 

Went looking for better tubing and found what looks to be ideal. Its the tubing that is actually meant to be used with the fittings. :-) The tubing is flexible yet kink resistant. All the parts are by Carlson Electric. Its 3/4" dia hose and fittings. I found that if I heated the tubing at around 150F in the oven for a few minutes, its much more flexible and easier to install.

Drove the car this evening, and its running great. The idle was a little loppy when I first started the car but after a bit of driving, it steadied out. Seems that initially, the ECU was running the afr a little lean at idle, and when it would swing way lean, the idle speed would take a dip. Under agressive driving, the afr is right around 13.2:1 from 3000-5000 rpm. I'll post a log tomorrow.

http://www.subaru-svx.net/photos/fil...czko/31158.jpg

You're over thinking it... All you have to do is bypass the MAF... even if you let the air enter 3 inches after it, you're effectively letting unmetered air in, and therefore causeing the car to go leaner. This would then be before a turbo, and it wouldn't cause any problems.

I still think the valve idea would be the best way to do it. It would effectively be an AFC and would allow you to run at whatever A/F ratio you wanted (fitting the stock maps and not knocking)

I think someone looking into a turbo setup could use this pretty effectively. maybe even use a throttlebody from the junkyard, attach it after the MAF to the intake tube. (before the turbo if its a turbo car, have a similar, or more restrictive filter) and voila! insta air fuel control. you could even get all fancy and have an electronically controlled actuator to control the TB valve. I would however suggest making sure your knock sensors are in perfect working order before you tried this. (Maybe that J&S safeguard or whatever its called)

I wonder if the H-6 cover still fits without touching the tube :rolleyes:

It's all how you decide to build it. All you need is enough hose to get around the MAF. You don't have to run all the way to the throttlebody.

What would the ~60 psi FPR out of an XT6 contribute to this??? I'll ship one out if anybody with the proper meters wants to play ;)

A 60 psi fpr will push about 30% more fuel into the motor. To get 30% of the air to go around the MAF, the bypass tube would have to be roughly 1.5" in diameter. I think that there would be some major issues with idle and timing advance with that much air going around the MAF.

I was corrected, according to the FSM's the XT6 fpr is 43.5 psi.

Mychailo,

This new version looks terrific. Did you get a chance to do that log? :)

-Chike

Haven't had a chance to do the log yet, but I can say that the car is much more peppy compared to no bypass tube, and my real-time observations of the afr readout on the LM-1 put the afr at around 13.2:1 in the 3000-5000 rpm range. As a bonus, it seems that the engine stalling issue I've had since installing my 5MT has been somewhat reduced since I installed the bypass. The only negative that I can see so far is that idle is loppy when the motor is cold. The loppy idle is caused by fairly large swings in the afr. I can watch it swing from 13.5:1 to 15.5:1 during cold idle. It doesn't stall though. Just loppy. On a side note, this leads me to conclude that the MAF does play a role in controlling idle, and going to a larger tube may result in a very uneven idle even when the engine is warm.

Anyhow, I'll try to get a log tonight.

After a few more days of driving with the MAF bypass, I have found that when the engine is not up to full temperature, there is some tendency for the engine to stall, at least with my 5MT. No stalling problems when the engine is stone cold, and no problems when the temperature needle is in the full warm spot, but about halfway between stone cold and full warm, there is some tendency. Putting a slight restriction in the bypass line should fix it.

the nature of bypassing the meter will most likely bring a condition like this at some point in the range of function.
phil

I see a use for a temp controlled valve... seems like it would work... Or RPM controlled, however that would be harder.

or just buy a stage 2 and do it the right way without messing around!!!
phil

the whole point behind this is for those of us who can't afford Stage 2, or, in the case of the OBD-II guys it isn't available for :)

and i thought you used to be a dsmer :( :( :(

Just kidding haha. Seriously though, just for having your MAF not max out, running a say, 100*F temp switch (20$) that opens and closes a valve (20$) in a tube that bypasses the MAF (10$) seems cheap.

Although I'd kinda like to know why subie would use the MAF to control idle...

Anywho. its all good.

I don't believe the MAF is used during idle... Mychailo you have the ability to monitor MAF voltage...what is the reading at cold idle and warm idle?? also some in between. The Idle Air Controllers should handle that as the engine runs in open loop when warming up and closed loop when fully warm and shutting off the SIAC and using only the IAC to function for breathing at idle. This is only an assumption so don't chew me out for it...

I agree that the Stage 2 is the proper thing to do for anyone with OBD I but the OBD II guys might be able to utylize this.

Tom

i like this idea. so i was reading the charts and is it better with the hose spliced or just curving around everything. i still need to figure out where to drill into my cold air intake that runs in the fender. how big of a hose is to big to use?

I'll get some data logs tomorrow. I'll do one with the engine stone cold, then one when its lukewarm, and then when its full warm.

My general observations are that the engine switches to closed-loop fairly quickly after the car is started. It generally takes only two minutes before its running in closed-loop. When the car is lukewarm, there is a tendency when the car is idling for the afr to swing from ~13:1 to as high as 17.5:1, and the stalls occur when it swings super lean.

KC, I'd recommend first trying 3/4" ID. That's the largest you'll want to use with an Impreza RS fpr. If you install an adjustable fpr and go with a higher fuel pressure, then you can try larger diameter hoses as you increase the fuel pressure, but you'll have to be very careful about building in too much ignition advance.

i see i see ty

One thing that has not been discussed is that the smaller bypass tube has fluid-dynamic (aerodynamic) differences other than just being smaller than the main intake tube. If you were to plot the flow resistance over airspeed, you'd notice that the resistance will increase on a curve. If you compared the curve of the main intake tube to the bypass tube, the curves will be different. (The small tube will have a much steeper curve.)

Here's another way of saying it: Let's presume the total area of both tubes is 10 square inches. Let's say the small tube is one square inch and the large tube is 9 square inches - 10% and 90% respectively. At idle, each tube will handle almost exactly their respective percentage of air flow. At higher air speeds, the small tube will handle a lower percentage, say 8% while the large tube will handle more, 92%. At the extremes, the difference could be so large that the small tube might as well not even be there. Honestly, I don't have any precise values. It might be negligible, but then again, it might not. Those joints and right angles in the small tube won't have much effect at low flow rates, but at high rates it'll really add up. The smaller diameter alone will make a difference simply because the aerodynamics are different on a smaller scale. It sounds like the primary goal is to fix a problem that occurs at higher speeds. Unfortunately the bypass tube will function best at lower speeds. Perhaps you could install a vacuum-actuated valve in the bypass tube that opens when the engine is under load, or maybe a vacuum-actuated electrical switch that changes the output value of the MAF.

Just something to think about.

That's absolutely true, UberRoo. I figure that at 6000 rpm the engine is drawing about 350 cfm of air (the volume of air at standard temperature and pressure would be a little less because of reduced pressure in the intake tube). The inside diameter of the intake tube is about 3", giving it a cross sectional area of about 0.05 sqft. That means the average velocity in the intake tube is about 7000 fpm. Pressure drop across the intake tube at that flow is about 0.03 psi, including the bend, but not including the corrugations (I can't figure that until I do the CFD). An equal pressure drop through a 1 inch tube would yield between 16 and 18 cfm (only about 3200 fpm average velocity). So, even though the cross sectional area of the 1" tube is 11% of the 3" stock intake tube, the flow rate through it would be only 5% of the amount flowing through the main tube at 6000 rpm. That's because the wall/flow area is so much greater in the smaller tube, so that frictional losses rise much more quickly in the smaller tube as flow and velocity increase.