iron giant
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| 03-02-2010 05:05 AM |
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Seeing as my hopes for switching to speed density have disappeared, I'm planning on switching to a blow through MAF setup on my STi. I'm also planning on running an aquamist water meth injection kit. It seems to me that putting the nozzle before the MAF wouldn't be the best idea. I'm picturing less than perfect atomization causing havoc on the hot wires accuracy. What I'm curious of is- If the nozzle is downstream of the MAF, say about 8 inches, will the MAF be segregated from the water/meth or will I see issues when the upstream BOV opens? If it matters, the aquamist is sprayed based on injector pulses width, so it should stop when I cut throttle, but a slight delay may happen.
Any experience or thoughts would be appreciated. I'm probably being a bit paranoid. |
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Big J
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| 03-02-2010 07:01 AM |
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if you put your injection 6-8 behind the MAF you should be good
you put it before the MAF and the car will run fine until the injection turns on.
An air molecule, or a given mass of air because it's a few different gasses, has a certain specific heat. Meaning it takes a certain amount of heat to raise the temp a certain amount. because that value is known, if it's taking the ECU 3v of electricity across the MAF sensor to maintain the heat in the sensor,, a calcualtion as to how much air is flowing across the sensor can be made. Water and Methanol on the MAF sensor will cause it to max the voltage pretty quick because the specific heat is WAY higher than air.
That's part of the reason density in a blow through set up isn't an issue, mass is mass regardless of density, and because AFRs are a ratio of weight of fuel to weight of air I think it's more accurate than a speed density which is dependent on a set VE at a given engine operating condition. Just my $.02 |
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iron giant
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| 03-02-2010 01:03 PM |
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Hmmmm, for some reason I had it in my head that water would make the MAF see less voltage. Maxing out voltage is a welcome alternative to lessening it (therefore going rich instead of lean).
While I'm on the subject, does anyone have a good way to calculate a good size maf housing for a specific max MAF g/s? From what I've been reading, a blow through housing has to be quite a bit bigger than a comparable g/s flowing draw through. Since there aren't a ton of blow through subies out there I don't have much experience to draw from. I'm shooting for roughly 450-500g/s max, with hopefully a bit of headroom. |
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eyesoreracing
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| 03-14-2010 08:35 AM |
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What is your objective with the blow-through setup? I don't understand why you're doing it.
-Dave |
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eric
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| 03-15-2010 09:00 AM |
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I don't understand why people blow through MAFs. If you're already talking about a larger MAF housing then why not just draw through like stock?
Assuming that you've already isolated the crankcase vapors and blow by into a separate catch can that drains back into the crankcase, there are still oil vapors that get into the system through turbo compressor seals and valve overlap (especially if you have crappy pistons and/or rings). Oil all over the hot wire isn't ideal since your engine's state of tune would be ever changing according to how much oil and dirt stays on or around the hot wire. The hot wire is extremely sensitive.
Just because its a popular mod on the forums, it doesn't mean it's the right thing to do.
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smartbomb
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| 03-15-2010 01:34 PM |
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The main reason people do so they can have a ricey BOV, Phsssh! |
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jere
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| 03-15-2010 07:56 PM |
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Posted By Eric Hsu on 03-15-2010 11:00 AM
I don't understand why people blow through MAFs. If you're already talking about a larger MAF housing then why not just draw through like stock?
Assuming that you've already isolated the crankcase vapors and blow by into a separate catch can that drains back into the crankcase, there are still oil vapors that get into the system through turbo compressor seals and valve overlap (especially if you have crappy pistons and/or rings). Oil all over the hot wire isn't ideal since your engine's state of tune would be ever changing according to how much oil and dirt stays on or around the hot wire. The hot wire is extremely sensitive.
Just because its a popular mod on the forums, it doesn't mean it's the right thing to do.
I don't know about others reasons for blow through MAFs. My goals were to get the MAF away from the air stream coming recirculating from the recirc vavle. It is a gen 2 DSM valve and it is always blowing some air that interfers with my cars MAF in draw through form (making the car run poorly) even though it is 8 or 9inches away from the recirc pipe. My blow through plans didn't end up working as I have 2in pipe and a 3in MAF. My guess was there was too much turbulance for the maf this way.
Some other common reasons without the MAF draw through one could run a shorter intake pipe or find a filter that goes on the compressor inlet. Another reason for blow through would be to lesson the influence of boost leaks if there are any or if couplers come loose.
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Big J
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| 03-16-2010 08:06 AM |
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Part of the reason on a newer Nissan is that the AIT sensor is part of the MAF. It made no sense to me to meter air temp then compress it, heat it, cool it back down, and then put it in the motor at a temp nowhere near what it was when it was metered. The metered temp can be one thing with a cool intercooler, and something else all together when it's heat soaked even if the temp at the inlet is the same. With ignition timing being some sort of fuel burn time calculation based on cylinder pressures, temps, intake cam timing etc. it was important to have the temp correct. At least that was my thinking.
Granted my car runs in closed loop and will learn to compensate for variance, but I've never seen anything that leads me to believe the metering is suffering, and I dump the crankcase directly into the turbo inlet w/o a catch can. Been doing it like that for 40K miles. You can see oil residue where the BOV vents so I know it's all over the MAF. Part of how I set my car up was simply to do things against common thinking to show it's not as detrimental as people make it out to be....... no turbo timer, no catch can, blow through, etc etc. Granted it's a street car so the margin of error is much larger.
If the car was run in open loop directly off the tables w/o closed loop feed back compensation, sure, having a dead on MAF is absolutely critical. But that's not the case. Fuel trims are monitored against metering and everything is and has been ok, the ECU will compensate up to +/-25%, but I tuned it for +/-3.
3% error at 320 gms/sec, isn't as bad as 3% at 550gm/sec air flow. But again I'm not running w/o feedback or compensation where sensor degradation would be as problematic. I only run open loop at 70% loads and higher, but even still, in open loop things are/have been compensated.
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eric
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| 03-17-2010 08:44 PM |
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It does make sense to get a Nissan water temp sensor which has the identical resistance range of the Nissan MAF's AIT sensor, install it in the plenum, and wire it in while leaving the MAF in front of the turbo where it belongs for superior air flow resoultion while the air temp sensor also reads temps from the most accurate place in the charge system (the plenum).
Well it works for you so that's cool. I was blowing through MAFs since 1990 (on my 87 Turbo II FC) and through the mid-early 90's on various cars, but I've discovered the resolution is much better with the MAF in front of the turbo. I'm sure today's MAFs are better though, but they still aren't designed to be run with oil and crud all over the hot wire for very long.
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Big J
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| 03-17-2010 10:35 PM |
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that's a good tip on the coolant sensor.
Another issue was metering and leaks in a draw through. If I pop a coupler, except for the last 2 in the system, the car will keep driving. If I blow a coupler in a draw through system and I can't get to it until the car cools down, I'm stranded until it does.
The new Hitachi MAFs are pretty tough, Nissan ran a hot film MAF that was garbage for a few years.
Blow through can get you in trouble if the MAF is near a bend or the piping before or after the MAF isn't a matched diameter. Clocking the MAF or even installing a pipe differently can jack up readings and your tune. The best blow though setup is one where the MAF flange is part of the piping to prevent changes that will tweak readings.
I'm sure you know most of that, I;m just putting it out there for those that don't. |
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eric
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| 03-19-2010 12:23 AM |
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Yeah give the coolant temp sensor in plenum thing a try. I think the cheapest sensor is from a 89-90 240 w/ KA24E. Most Nissan dealerships have the plug too, but you have to convince the parts guy to go to the service area and get you one. |
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Wrecked
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| 03-19-2010 09:35 AM |
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The airtemp is there as part of the mass flow calculation so the airtemp measurement should be at the same place as the MAF for correct mass flow calculation. I could see using an additional air temp sensor in the manifold as a final trim tothe timing and fueling calculations, but I don't think it would be a good idea to use it for the mass calc. |
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smartbomb
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| 03-19-2010 09:47 AM |
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On a Nissan the air temp isn't used for fuel or timing calculations. |
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Wrecked
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| 03-19-2010 08:43 PM |
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I think you misunderstood what I was saying, either that or Nissan has a crappy mass airflow system. |
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smartbomb
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| 03-19-2010 09:12 PM |
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Nissan's late model hot wire MAF's are also very sensitive to contamination. |
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Wes
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| 03-20-2010 12:12 PM |
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Derailing this tread a bit, a thought I have always had is that the air blown off through a BOV is wasted energy, and I would rather see it recirculated and directed directly at the turbo inlet. Not that this will measurably increase response, but I don't think it can hurt..... |
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Big J
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| 03-20-2010 05:15 PM |
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MAF placement and recirc vs atmosphere is a metering issue.
Venting during periods when the BOV is open conserves energy........ mainly the momentum of the turbo spinning...... the excess air is vented dropping the pressure in the charge pipes.... so basically the pressure ratio drops to 1:1 and less energy is needed to move air vs moving into a pressurized system at a higher pressure ratio......... because the turbo scrubs off less speed when the BOV closes, and high exhaust flows starts again the turbo is spinning faster than it would be had it scrubbed speed off or surged trying to pump into pressurized pipes..........
maybe if there was enough of a pressure drop at the inlet w/ a recirc setup you could get slightly better results, IDK though....... I have a flat foot shift box on my car so the throttle doesn't close between shifts if I'm beating on it.
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iron giant
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| 04-13-2010 04:45 AM |
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Wow, I forgot that I started this thread. The reason I wanted to go blow through is this- on subarus with FMICs we experience a rich dip that can't be tuned out after shifting. Supposedly, this is due to the large amount of piping leading to a delay between the MAF seeing the air and the air entering the engine. With a blow through, the MAF can be placed a lot closer to the throttle body and people are supposedly eliminating the rich dip. I personally don't have any experience with it, and I can care less about BOV sound.
Ideally I would just switch to speed density, but I want to keep the stock ECU to pass emissions. People are creating patches to get it to work via openecu flashes, but I haven't seen a patch for my specific year/model, and the patches that are out don't seem to have people too thrilled.
My build will already have a reverse mount intake manifold to shorten piping a bit, which should help, but I don't know if it will make a huge difference or not. |
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smartbomb
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| 04-13-2010 05:46 AM |
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Hopefuly Naji will respond here, he has a lot of experiance with tuning stock Subie ECU's. |
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spdracerut
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| 04-14-2010 08:57 PM |
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Posted By Iron Giant on 04-13-2010 06:45 AM
The reason I wanted to go blow through is this- on subarus with FMICs we experience a rich dip that can't be tuned out after shifting. Supposedly, this is due to the large amount of piping leading to a delay between the MAF seeing the air and the air entering the engine. With a blow through, the MAF can be placed a lot closer to the throttle body and people are supposedly eliminating the rich dip. I personally don't have any experience with it, and I can care less about BOV sound.
Hmmm..... BOV recirculated or vented? It should not go that rich if recirculated. The piping does not seem any longer than cars with the turbo mounted between the engine and firewall (Audi/VW 2.0ts, Evo X, Cobalt SS, etc). Or maybe Subaru has some crazy throttle tip-in enrichment.
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