MAF/MAP/AFS/AFM
03-10-2003, 06:41 PM
#16
Race Director
"The VAF/MAF is not too early, the engine is consuming the air. The MAP sensor won't register this fact until the plenum is filled while the VAF/MAF register it immediately. Again the throttle position is necessary for the MAP system."
But it's not consuming the very piece of air that's going past the VAF/MAF sensor right now! The piece of air that's being sucked in is still under vacuum. The VAF/MAF may be measuring 2.5L of flowing air at atmospheric pressure, but simultaneously, the engine is sucking in 2.5L of air at -20 inHg. Metering fuel for 2.5L of air at atmospheric presusre while the engine is only ingesting 2.5L @ -20 inHG is a mis-match. As you said, only when the plenum has been filled with the incoming air will the pressure in the manifold reach atmospheric pressure. Only then will the fuel-delivery match the air flowed into the engine. TPS position is only used for acceleration enrichment, not to compute any air-flow numbers. A lot of MAP vendors don't even have the TPS hooked up.
Another example of what Ian's talking about is the rich burst you get when venting the bypass valve directly to the atmosphere. It's obvious in this case that the metered air never makes it into the engine, yet the matching fuel is injected anyway. Again, the lag and time-delay effect.
Looking into research on stratified-charge engines and direct-injection. They have some interesting techniques on how to measure individually, only and exactly the amount of air that enters each combustion chamber on a per-stroke basis.
But it's not consuming the very piece of air that's going past the VAF/MAF sensor right now! The piece of air that's being sucked in is still under vacuum. The VAF/MAF may be measuring 2.5L of flowing air at atmospheric pressure, but simultaneously, the engine is sucking in 2.5L of air at -20 inHg. Metering fuel for 2.5L of air at atmospheric presusre while the engine is only ingesting 2.5L @ -20 inHG is a mis-match. As you said, only when the plenum has been filled with the incoming air will the pressure in the manifold reach atmospheric pressure. Only then will the fuel-delivery match the air flowed into the engine. TPS position is only used for acceleration enrichment, not to compute any air-flow numbers. A lot of MAP vendors don't even have the TPS hooked up.
Another example of what Ian's talking about is the rich burst you get when venting the bypass valve directly to the atmosphere. It's obvious in this case that the metered air never makes it into the engine, yet the matching fuel is injected anyway. Again, the lag and time-delay effect.
Looking into research on stratified-charge engines and direct-injection. They have some interesting techniques on how to measure individually, only and exactly the amount of air that enters each combustion chamber on a per-stroke basis.
03-10-2003, 11:44 PM
#17
Race Director
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by IanM:
<strong>Here's my real world experience - My MAF car with big turbo, big injectors and no cat shoots giant flames out the exhaust when upshifting when driving aggressively (ie. full throttle, letting off throttle quickly at 6500rpm+ to upshift).</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Poor tuning.
The Nissan SR20DE and SR20DET use a MAF and I have friends with cars putting down as much as 500 hp to the wheels and have never and I mean never seen one shoot flames on a shift or on overrun or ever. I myself have been driving a SR20DET for a year and a half and never had anything like you are describing.
Poor tuning.
<strong>Here's my real world experience - My MAF car with big turbo, big injectors and no cat shoots giant flames out the exhaust when upshifting when driving aggressively (ie. full throttle, letting off throttle quickly at 6500rpm+ to upshift).</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Poor tuning.
The Nissan SR20DE and SR20DET use a MAF and I have friends with cars putting down as much as 500 hp to the wheels and have never and I mean never seen one shoot flames on a shift or on overrun or ever. I myself have been driving a SR20DET for a year and a half and never had anything like you are describing.
Poor tuning.
03-10-2003, 11:47 PM
#18
Race Director
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Danno:
<strong>Another example of what Ian's talking about is the rich burst you get when venting the bypass valve directly to the atmosphere. It's obvious in this case that the metered air never makes it into the engine, yet the matching fuel is injected anyway. Again, the lag and time-delay effect. </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Don't hold up poor tuning to make your point. Venting a by-pass to atmosphere on a MAF car is just plain poor development and tuning.
<strong>Another example of what Ian's talking about is the rich burst you get when venting the bypass valve directly to the atmosphere. It's obvious in this case that the metered air never makes it into the engine, yet the matching fuel is injected anyway. Again, the lag and time-delay effect. </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Don't hold up poor tuning to make your point. Venting a by-pass to atmosphere on a MAF car is just plain poor development and tuning.
03-10-2003, 11:51 PM
#19
Race Director
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Danno:
<strong>The sensor picks up extra air flow, but it's going to take 20-40 revolutions of the engine before this air fills the plenum and makes it into the engine.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">What am I missing? I cannot imagine a NA intake tract has more than about 5-10 liters volume. That would only be 2-4 revolutions unless I'm a monkey and missed something basic.
I cannot imagine a decently set up 951 having more than 20 liters intake volume. This would be 8 revolutions max. Again, unless I'm a monkey and missed something basic.
<strong>The sensor picks up extra air flow, but it's going to take 20-40 revolutions of the engine before this air fills the plenum and makes it into the engine.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">What am I missing? I cannot imagine a NA intake tract has more than about 5-10 liters volume. That would only be 2-4 revolutions unless I'm a monkey and missed something basic.
I cannot imagine a decently set up 951 having more than 20 liters intake volume. This would be 8 revolutions max. Again, unless I'm a monkey and missed something basic.
03-11-2003, 03:37 AM
#20
Race Director
Remember it requires two complete revolutions to suck in one displacement.
As for intake tract volume, add volume of J-pipe, intercooler pipes, intercooler, plenum. All of this volume contains compressed air at 2x density of atmospheric. So it would be the equivalent of double its volume in 'natural' air. I'm working on a mod to reduce that travel distance and volume significantly...
As for intake tract volume, add volume of J-pipe, intercooler pipes, intercooler, plenum. All of this volume contains compressed air at 2x density of atmospheric. So it would be the equivalent of double its volume in 'natural' air. I'm working on a mod to reduce that travel distance and volume significantly...
03-11-2003, 12:09 PM
#21
Racer
Join Date: Jun 2001
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</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"> But it's not consuming the very piece of air that's going past the VAF/MAF sensor right now! </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">There is air already in the intake tract. The equivalent would be a public water system. If you pull a gallon of water from the kitchen tap, the gallon that is metered at the curb is not the gallon leaving the tap. The meter will measure the gallon while you are drawing it. Air is in the intake before the throttle body ready to enter the engine. When you open the throttle and a liter of air enters at the throttle body, the VAF/MAF will register a liter of air coming in to replace it. Both systems rely on a closed system with no leaks to be accurate.
Bottom line getting back to what Geo said, the VAF/MAF systems are measuring actual air flow and will compensate for changes in VE due to mods or age.
Bottom line getting back to what Geo said, the VAF/MAF systems are measuring actual air flow and will compensate for changes in VE due to mods or age.
03-11-2003, 12:58 PM
#22
Poor tuning....Hmmmmm
I'll have to call my tuner, and tell him that the stand-alone Motec wideband that he installed for street-tuning in a testport directly after my turbo must be bad, and the 14.7:1 a/f ratios while cruising and flat 12.5:1 that we dialed-in under boost must also be questionable. While I'm at it, I'll give the dyno shop a call, and tell them that their wideband must also be functioning poorly, as it confirmed the results of the Motec.
The flaming isn't an obvious thing, and is only noticeable if someone's behind the car to see it happening. And it only ever happens when driving very aggressively, shifting at redline, and running without a cat. My muffler is a straight through design, so perhaps that is also a factor.
I guess I really don't know for sure why it's happened, but the theory I explained seems quite plausable and logical to me. Another poster here, "Rage2" has also experienced flaming on upshifts with his big turbo 951. But he's running an SDS system (no MAF), so perhaps there are other factors involved here.
I'll have to call my tuner, and tell him that the stand-alone Motec wideband that he installed for street-tuning in a testport directly after my turbo must be bad, and the 14.7:1 a/f ratios while cruising and flat 12.5:1 that we dialed-in under boost must also be questionable. While I'm at it, I'll give the dyno shop a call, and tell them that their wideband must also be functioning poorly, as it confirmed the results of the Motec.
The flaming isn't an obvious thing, and is only noticeable if someone's behind the car to see it happening. And it only ever happens when driving very aggressively, shifting at redline, and running without a cat. My muffler is a straight through design, so perhaps that is also a factor.
I guess I really don't know for sure why it's happened, but the theory I explained seems quite plausable and logical to me. Another poster here, "Rage2" has also experienced flaming on upshifts with his big turbo 951. But he's running an SDS system (no MAF), so perhaps there are other factors involved here.
