air measurments

[SLIMMY240]

ive read a few post on here about what to do with the maf when going turbo, and ive got that if you put it before the turbo you dont get accurate readings because of the temperature change after the intercooler, but if you put it after the ic then it gets confussed with the change from the blow off valve letting off pressure. so is it best to go with a map sensor (mass air pressure right?), if so who make one, and what all will it consist off, or are there any other alternatives? i appreciate any input.

[piratepete]

How would the temperature change anything? It's a MASS air flow sensor. After the intercooler there would be less volume of air, but the same mass.

[ziggy682]

Most people put the MAF before the turbo because that's where it's commonly placed. It's called a draw through. With a draw through setup, if you run an open atmosphere blow off valve, the car will run rich when the bov vents.

Putting the MAF after the turbo is called a blow though. Some people are scared that the MAF will get contaminated by oil from a leaky turbo if it's put there. Every time you shut the car off, the hot wire in the MAF heats itself up to 1000 degrees to clean contaminates off. The advantages to a blow through are that the air getting metered is the same temp as the air going into the engine. Also, you can run an open atmosphere bov before the MAF with no ill effects. One of the drawbacks is your intercooler piping will be more difficult, because you need about 6" of straight pipe before and after the MAF so it gets consistent readings.

Personally, I'm going with a blow though setup in a couple of weeks. I'll let you know how it turns out.

[ziggy682]

How would the temperature change anything? It's a MASS air flow sensor. After the intercooler there would be less volume of air, but the same mass.

Your ECU adjusts timing and fuel delivery partially based on intake temp. If the MAF is before the turbo, then the engine will get hotter air than it thinks it's getting. Because of this, timing and fuel won't be optimal.

[C-Kwik]

With a draw through setup, if you run an open atmosphere blow off valve, the car will run rich when the bov vents.

Also, you can run an open atmosphere bov before the MAF with no ill effects.

You seem to contradict yourself here. With a draw through set-up, an open BOV would not have any effect if the BOV is before the MAF. Running it after the MAF would have the same effects of running an open BOV in a draw through set-up.

[C-Kwik]

Your ECU adjusts timing and fuel delivery partially based on intake temp. If the MAF is before the turbo, then the engine will get hotter air than it thinks it's getting. Because of this, timing and fuel won't be optimal.

Yes, but as long as the hotwire is at a higher temp then the air going through it, it will account for the density change from temperature differences. the heat the air takes away from the hot wire is a direct function of how many molecules of air pass around it. The more molecules there are in a given volume, the more heat it will take away. A hotwire MAF is about the single most accurate way of metering how many molcules of air are entering a motor. To be more accurate with other systems require more sensors to account for changing conditions. For this reason, there should be no difference in the voltage between a blow-through or draw through system assuming of course it is getting a proper reading.

[ziggy682]

You seem to contradict yourself here. With a draw through set-up, an open BOV would not have any effect if the BOV is before the MAF. Running it after the MAF would have the same effects of running an open BOV in a draw through set-up.

I don't think you read my post clearly. I didn't contradict myself. I was refering to a blow though setup in which you can run an open atmospere blow off valve with no ill effects as long as it's placed before the MAF.

I don't think it's possible to place a BOV before the MAF in a draw through setup. If the BOV is placed before the turbo, it will not blow off, it will suck air in, because it is in a vacuum. The blow off valve will only vent air if it's placed after the turbocharger, where the air is compressed.

[ziggy682]

Yes, but as long as the hotwire is at a higher temp then the air going through it, it will account for the density change from temperature differences. the heat the air takes away from the hot wire is a direct function of how many molecules of air pass around it. The more molecules there are in a given volume, the more heat it will take away. A hotwire MAF is about the single most accurate way of metering how many molcules of air are entering a motor. To be more accurate with other systems require more sensors to account for changing conditions. For this reason, there should be no difference in the voltage between a blow-through or draw through system assuming of course it is getting a proper reading.

Again, I don't think you understood me correctly.

In an NA car, the air that passes through the MAF will be close to the same temperature when it enters the engine.

Imagine if you turbocharge a KA, and don't use an intercooler. If the MAF is placed before the turbocharger it will read ambient air. Let's say 75 degrees. When that air is compressed by the turbocharger, let's say it heats up to 250 degrees. In a non-intercooled system, the engine would injest 250 degree air, but the ECU would only think it's getting 75 degree air. Therefore, fuel and timing won't be optimal for combustion at that temperature.

If the MAF is placed after the turbocharger, than it will read 250 degree air, and the ECU will compensate correctly.

In an intercooled system, the temperatures would drop dramatically, but it would still make a difference.

Also, I understand that an MAF measures the mass of air, and takes temperature, density, and such into consideration. The engine will always get the same mass of air that the MAF reads(unless you have a leak), but the ECU will adjust timing parially based on intake temperature. I think, in a blow through setup, it will be more accurate because intake temperature will be read correctly.

[C-Kwik]

I don't think you read my post clearly. I didn't contradict myself. I was refering to a blow though setup in which you can run an open atmospere blow off valve with no ill effects as long as it's placed before the MAF.

I don't think it's possible to place a BOV before the MAF in a draw through setup. If the BOV is placed before the turbo, it will not blow off, it will suck air in, because it is in a vacuum. The blow off valve will only vent air if it's placed after the turbocharger, where the air is compressed.

Actually I read it very carefullly. You stated:

"Most people put the MAF before the turbo because that's where it's commonly placed. It's called a draw through. With a draw through setup, if you run an open atmosphere blow off valve, the car will run rich when the bov vents."

A vehicle that runs an open BOV anywhere after the MAF will cause it to run rich if the BOV opens. It would not make a difference if it is draw through or not. But if you placed it in front of the MAF on a draw-through set-up then it would not know any air was released. You stated above that it would run rich with a draw-through set-up and then later said it would have no ill effects. So you did contradict yourself...

[C-Kwik]

I don't think you read my post clearly. I didn't contradict myself. I was refering to a blow though setup in which you can run an open atmospere blow off valve with no ill effects as long as it's placed before the MAF.

I don't think it's possible to place a BOV before the MAF in a draw through setup. If the BOV is placed before the turbo, it will not blow off, it will suck air in, because it is in a vacuum. The blow off valve will only vent air if it's placed after the turbocharger, where the air is compressed.

Actually I read it very carefullly. You stated:

"Most people put the MAF before the turbo because that's where it's commonly placed. It's called a draw through. With a draw through setup, if you run an open atmosphere blow off valve, the car will run rich when the bov vents."

A vehicle that runs an open BOV anywhere after the MAF will cause it to run rich if the BOV opens. It would not make a difference if it is draw through or not. But if you placed it in front of the MAF on a draw-through set-up then it would not know any air was released. You stated above that it would run rich with a draw-through set-up and then later said it would have no ill effects. So you did contradict yourself...

Though I did state the wrong thing in my post. In a blow-through system, it would not have an effect on the fuel mixture. I think I do know you were trying to state this, but your post had a small contradiction. It didn't help that my correction had an error though. =):pface

[C-Kwik]

Again, I don't think you understood me correctly.

In an NA car, the air that passes through the MAF will be close to the same temperature when it enters the engine.

Imagine if you turbocharge a KA, and don't use an intercooler. If the MAF is placed before the turbocharger it will read ambient air. Let's say 75 degrees. When that air is compressed by the turbocharger, let's say it heats up to 250 degrees. In a non-intercooled system, the engine would injest 250 degree air, but the ECU would only think it's getting 75 degree air. Therefore, fuel and timing won't be optimal for combustion at that temperature.

If the MAF is placed after the turbocharger, than it will read 250 degree air, and the ECU will compensate correctly.

In an intercooled system, the temperatures would drop dramatically, but it would still make a difference.

Also, I understand that an MAF measures the mass of air, and takes temperature, density, and such into consideration. The engine will always get the same mass of air that the MAF reads(unless you have a leak), but the ECU will adjust timing parially based on intake temperature. I think, in a blow through setup, it will be more accurate because intake temperature will be read correctly.

Here's the problem. The MAF voltage represents the actual amount of airflow. Thoeretically, the airflow before or after the turbo would be the same so the voltage should be the same. Because of the way the MAF works, the ECU would have no idea what the temperature of the air going through the MAF is. And since the voltage is the same, the timing will not be changed. The MAF does not measure temperature directly. It measures the effects of the temperature.

[ziggy682]

Actually I read it very carefullly. You stated:

"Most people put the MAF before the turbo because that's where it's commonly placed. It's called a draw through. With a draw through setup, if you run an open atmosphere blow off valve, the car will run rich when the bov vents."

A vehicle that runs an open BOV anywhere after the MAF will cause it to run rich if the BOV opens. It would not make a difference if it is draw through or not. But if you placed it in front of the MAF on a draw-through set-up then it would not know any air was released. You stated above that it would run rich with a draw-through set-up and then later said it would have no ill effects. So you did contradict yourself...

Though I did state the wrong thing in my post. In a blow-through system, it would not have an effect on the fuel mixture. I think I do know you were trying to state this, but your post had a small contradiction. It didn't help that my correction had an error though. =):pface

Sorry I wasn't more specific. I didn't specify that in a draw through setup, the car will run rich if the BOV is placed after the MAF. I thought it was common sense that, in order for it to work, the BOV can only be placed after the MAF in a draw though setup. If it's placed before the MAF, in a draw through setup, the air is not compressed yet, and the BOV won't blow off. Have you ever seen a BOV placed before an MAF in a draw through setup? If so, I'd love to see some pictures of it.

Also, in my first post, where I said running an open atmosphere BOV would have no ill effects, I was referring to a blow through setup. The entire paragraph was about a blow through setup, so I didn't feel the need to specify in every sentence what type of setup I was referring to.

So, I just checked my FSM about the air intake temperature sensor. According the my FSM, you can test the output of the air temp. sensor at the ECU at terminal #26. 68 degrees = 1.0-1.5 volts, 176 degrees = .3 volts. According to the FSM, the ECU does see the temperature independently of the airflow that the MAF reads.

However, I checked the ECU wiring diagram in the FSM. Guess what? There is no wire on terminal #26. I guess it's some kind of error in the FSM. So, you are right, and the ECU doesn't see intake temperature directly. I was basing my theory on my FSM, but I guess I was wrong.

By the way, no hard feelings or anything, just a little friendly disagreement.

[Turbo 510]

A BOV vented to the atomosphere will make a vehicle run richer because air is being released from the system. Since fuel in injected at the port the amount of air reaching the port will be less than if the BOV did not open. Also, why would anyone place a BOV before the turbo? it is meant to release the reversion wave that is caused by slamming the throttle plate shut. This wave, unchecked (or vented) goes back into the turbo outlet and over time will damage the compressor wheel and/or the bearings shaft,etc. The BOV should be vented back into the system after the MAF but before the turbo. This keeps the same mass of air in the system. I have never heard of anyone putting a MAF after the turbo, but it would seem to cause a bunch of problems. Maybe I'm just out of the loop on the type of set up.

[C-Kwik]

A BOV vented to the atomosphere will make a vehicle run richer because air is being released from the system. Since fuel in injected at the port the amount of air reaching the port will be less than if the BOV did not open. Also, why would anyone place a BOV before the turbo? it is meant to release the reversion wave that is caused by slamming the throttle plate shut. This wave, unchecked (or vented) goes back into the turbo outlet and over time will damage the compressor wheel and/or the bearings shaft,etc. The BOV should be vented back into the system after the MAF but before the turbo. This keeps the same mass of air in the system. I have never heard of anyone putting a MAF after the turbo, but it would seem to cause a bunch of problems. Maybe I'm just out of the loop on the type of set up.

It's been done by a few. Some with success, some with some problems. The problems do seem to stem around turbulent airflow as Ziggy mentioned. But those who get it to work say it runs just fine. The Stillen S/C kit for the Maxima uses a blow-through set-up and runs an open BOV before the MAF.

[ziggy682]

3 of the project cars on Unstable Hybrids website are running blow through MAF's.