LOL...energy. Let me know when you write it up. I wanna see what your meaning. Well a buddy of mine has a built B18c hatch pushing well into the 450's on the street so I know to compete with that I need in the 5's so that seems to be my ultimate goal.

 

OK, I've got a couple clarifications. First, I was talking about a wastegate, but I was under the impression that it was controlling boost pressure by venting the intake after the turbo had compressed it, but it sounds like it is controlling boost by limiting the turbo's speed by routing the exhaust around the turbo so the turbo create additional boost. This makes sense now.

However, I still disagree with some of your explanation about the MASS AIRFLOW SENSOR. I know the difference between volume and mass, so that is not the issue. I also know that the turbo is simply pumping more air into the engine than the engine is able to suck in otherwise. The turbo itself does not actually create air. Therefore, you will have the same mass of air entering the turbo as exiting. There is no way around this unless your turbo could actually create additional air from nothing. Now, the incoming air is at a different temperature and pressure than the outgoing air, but conservation of mass will definitely apply. Therefore, if you are railing you MAF post turbo, you will still rail it pre turbo since you have the same air mass flowing through the MAF.

I do agree that the MAF could be calibrated to operate most accurately at specific temperature and pressure ranges (but I also know that like you said, it is able to compensate somewhat for changes in ambient temp, pressure, humidity, etc). I don't know enough about the specific MAF we have to know what the most accurate range is, but I can see how it could be less accurate under boost.

However, the limit problem would still exist (unless the only reason we are hitting the limit is because of the inaccuracies described above). So moving the MAF pre-turbo will let it operate more accurately, which might lessen the chance of railing the output (if it is so inaccurate under boost that it thinks it is getting far too much air), but if you then up the boost a bit more you will reach that limit again at some point.

My primary contention is that if you are trying to create obscene amounts of power, you will still need to deal with the MAF sensing limit, no matter where you place it.

What do you think?

Thanks,
D'oh!

 

There are two problems I see with the MAF in it's current location...but neither of them are terribly critical.

1) Potential damage to the MAF becuase on decel, air will be going "backwards" through the MAF...and the element is only designed to flow air one direction.

2) Regardless of MAF size, all MAF's will eventually max out at a certain point. In the case of our weak MAF, it happens very early with the TT. About 4500rpm or so. So fueling becomes a guessing game with the RPM. Even so..with proper tuning, this is not an impossible problem to solve. On the other hand...a MAP based system will never "max out". But the MAP presents its own problems, becuase on our car, there still be no provisions for air temp or barometric pressure change...so as in high-altittudes, or cold/hot weather temps. So even this solution wouldnt be perfect. With OEM MAP based systems, they have standard baro and temp sensors that make adjustments to the MAP output to ensure proper fueling.

 

I agree absolutely. However, the difference in the MAF's ability to measure air before the turbo is still greater than after the turbo. I re-read my earlier post and have to agree it was not very clear, and I think I tried to touch on too many things at once without adaquate example or explanation. Sorry for any confusion on my part, let me try again.
The MAF sensor functions by the use of a thermistor and a hot wire. A thermistor is a resistor made from a material whose electrical resistance alters with temperature in a predictable and stable fashion. The hot wire rests directly behind the thermistor in a small passage inside the MAF. As air flows through the MAF, a portion of the air flows through this small passage and across the thermistor and hot wire. The thermistor is designed to give an accurate reading of incoming air temperature, and the hot wire is used to measure the mass of air coming in. This is accomplished by running voltage through the hot wire, which is a positive temp. coefficient resister. This means that as the velocity of airflow increases, the temperature decreases via heat transfer to the air molecules, and in turn the resistance decreases. This resistance is compared to the voltage of a series of resistors it is attached to inside the MAF circuit- isolated from the airflow- and the thermistor's reading of air temp. As the resistance falls on the hot wire, the change in voltage across the resistor series are measured by the ECU and compared against a table in memory as the mass airflow rate. This allows the MAF to measure the mass of air molecules coming into the engine despite outside temperature, barometric pressure, and humidity. This also means that the MAF functions on one primary assumption; that an increase of air molecules injested by the engine can be registered by increased velocity of air to remove heat from the hot wire resistor.

Now, as I believe D'oh posted before, the airflow before the turbo is then higher in vacuum than after the turbo. This is important to realize. As air molecules enter the turbo, they are compressed and then pushed on to the combustion chamber. The engine's vacuum will still pull this air at the same rate if there was no turbo, but the time and volume taken by the turbo's compression action causes additional vacuum before the turbo to compensate. This also increases air velocity before the turbo compared to after it. Now, as mentioned before, the MAF primarily responds to velocity increase to measure air mass. The higher the velocity, the more molecules pass the hot wire to remove heat, the lower the resistance measured by the computer. Hence, the additional air pulled by the turbo to compress into denser air is best measured by the increase in velocity created by the turbo, rather than measured by the increase in density created after the turbo at the velocity pulled by the engine. The increased number of air molecules in the denser charge will still pull heat off the hot wire, but not as effieciently as the same number of air molecules at higher velocity. In addition, the thermister will allow the MAF to measure resistance drop compared with the incoming air temp, but the air temps caused by turbo pressure can put that measure further away from accurate as the temps rise beyond the thermistor's accuracy range. Remember, it was not designed to go after the turbo and be accurate with such temps.
So, how does this allow the MAF to measure more air before the turbo than after it? With consideration to the velocity of incoming air. The incoming air from atmo is cooler and faster and therefore more accurately measured within the MAF's design for temperature and velocity. This also means the only restriction is if the needed cfm is able to pass through at a particular speed, and a surprisingly large amount of air can be passed through the MAF opening if velocity is increased. With post turbo placement, the velocity cannot be increased as it is a fixed amount controlled by throttle, Ve, etc.. So, with the maximum velocity being that pulled by the engine at WOT, then the MAF will be more easily maxed out as the density of air increases, but not velocity. Where as the pre-turbo placement will allow the same number of air molecules to be measured at a higher rate of flow that would otherwise max out the MAF at a slower rate of flow but greater density.
The difference in the amount of air that the MAF can measure pre-turbo vs post-turbo is not all extreme, but is still higher on the pre side. This also means that since the pre-turbo MAF is more velocity capped than density capped, simply boring the MAF has a greater increase in its abilities to measure air for a pre-turbo MAF than a post-turbo placement. The same is true for using a larger MAF. With this in mind, the MAF is hardly the limiting factor in an engine build. 700hp SR20's use a MAF, not to mention the countless extreme hp RB26 that use MAF systems. It is no more the limitation than fuel injector sizing. Both can be made or replaced to flow more to meet the engine's needs. It's just that the MAF is the limiting factor for the PE and Greddy kits right now. Of course, this is what spawned the thread and led me to ask if anyone else thought this strange.
I know this thread is long, but really hope it is more clear and helps.

 

resolute:

that is some interesting reading. one possibility i was looking at for my turbo setup (non-greddy, MAF in same location, but BOV after MAF to prevent decel pressure to MAF) was this:
i use a super-AFC to fine tune my a/f ratio... the neat thing about this gizmo is the ability to swap MAF from different cars, or even add a second MAF...

the way it does this is to take the 0-5v output from 2 MAFs (both spec'd as 350Z MAFs), and add them together to 1 wire output to the ECU (spec'd as 350Z ECU). this way you could put 2 MAFs on the turbo inlet side, and get a reading of Air from where they should be located.

you could also use 2 skyline MAFs, but the problem i see is that the MAF input on the ECU can still only be 33.2 lbs/min (5.1v)... so if you run massive boost, they will be limited (but i would also suggest stand alone if running more than 10psi, anyways)...

anyone know if the emanage has this feature?

m

 

I don't believe so.

 

"The increased number of air molecules in the denser charge will still pull heat off the hot wire, but not as effieciently as the same number of air molecules at higher velocity"

I'm assuming this is because the dense air has been compressed and heated to a temperature closer to the wire's base state. Obviously the closer they are in temperature, the less accurately the wire will be able to measure mass. When they are equal, the sensor will be oblivious to airflow as the air will not alter the wire's temperature. Correct?

 

Close, but not quite. The engineers realize that air temp would affect the hot wire's readings, so they included the thermistor before the hot wire to allow accurate reading despite air temperature. The thermistor is wired to the resistor series to form a balanced bridge of resistors, so the hot wire's voltage is always in comparison to the voltage of the resistor series sealed from airflow, which in turn is balanced with the thermistor reading. Of course, for most MAF's, the heated charge of air is outside the thermistors accuracy range, so this affects the reading of the hot wire.
Consider that faster moving air will allow the heat to dissipate to more molecules than denser, slower air by allowing the heat saturated molecules to be replaced with fresh, cool air molecules faster. This is simple thermodynamics. Imagine your car in dense air trying to cool off with no breeze. Now, imagine dry thinner air, but with a breeze. This over-simplified, but should give an dea of what is happening with the hot wire. However, the MAF is designed to measure an increase in air mass as a function of velocity, not measure air velocity alone. As the throttle opens, more air is needed to fill the resulting vacuum. This vacuum increases velocity through the MAF as more air is ingested. So, if you could take a snapshot of the MAF at part throttle you might see, say.. 100 cf of air ingested in the MAF in that instant. At WOT, the snapshot might show 500 cf of air in the MAF at that instant. This huge difference in air mass is not so critical on the MAF's capacity because the 500 cf of air is now traveling faster through the MAF than the 100 cf was at part throttle. This is exactly how it was designed to register extra mass. Now add a turbo behind the MAF and the needed air mass increases even more, but so does velocity so the extra mass pours through the MAF at a higher velocity that is essentially along the same linear rate as before. As velocity increases, so does the mass of air in the sensor. The relationship is that simple, and that is the basic understanding upon which the MAF is designed. Now, put the MAF after the turbo and things change. The mass of the air in the MAF is the same, but at a slower velocity. This negates the MAF's ability to measure extra air mass with coresponding extra velocity. So now, the MAF measures the air mass by leaning more on the density of the air than the velocity of it to carry heat off the hot wire, when the MAF is designed to use both density and a corresponding velocity to carry heat off the hot wire.

 

Resolute...very impressive. How the hell do you know some much about this stuff. I think you need a girlfriend or something.

Skidazzle, to answer your Q, the emanage can only support 1 MAF sensor, but has provisions for "upgraded" MAF sensors. Not sure if there could be a hack to join 2 MAF's together and create one final voltage signal that the eManage/ECU could use.

 

I am not sure about this as I have not tuned with the SAFC personally. I have seen some really well tuned SR20's with this on sr20forum.com, but I prefer the E-manage as it has the ability to adjust timing. In any case, if the SAFC can run two MAF's in parallel that would be ideal. As far as being the limitation, it is no different for the MAF than it is with injectors, as the SAFC can only "compensate" injector size increases of 50% over stock.

 

Interesting, informative discussion....and I thought eight years of "higher" education was sufficient...back to the books.

 

To elaborate further on Resolutes comments. The eManage has two...maybe three big advantages over a typical AFC.

1) The eManage can control upgraded injectors up to 150% bigger than stock, and can scale them down so the car will idle and cruise correctly.

2) An AFC enriches fuel by modifying MAF signals, where the eManage enriches fuel by increasing injector duty cycle directly. Over-agressive MAF conditioning can causes its own problems...such as limp modes and unintended timing changes. Also, AFC's are limited to a much smaller scale of fuel enrichment, where the eManage really allows you to let loose with the fuel!

3) eManage can do timing retard, and the AFC does not.

I have become a pseudo-advocate of the eManage not just on F/I applications, but it is a terrific and cost effective device for N/A cars as well, that are looking for fuel enrichment or MAF conditioning.