Cobb AccessPORT Discussion
#1154
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For sake of the issue at hand.....
Assume that
1 Temp is constant
2 Air consumed by the engine is the same in both models
3 No Venturi effects are occurring at the site of the MAF...constant tube diameters
This leaves us with the same mass of air being rammed into two tubes of differing cross sectional areas....the flow will be different in the different tubes....the small one will have more volume/area than the larger one
Anyone got a problem with that....
Anyone see how the flow could be smaller in the small tube..ie less per unit area???
I don't....so the MAF will read higher flow in the small tube.....ie Higher MAF Voltage
And read smaller Voltage in the larger tube
Assume that
1 Temp is constant
2 Air consumed by the engine is the same in both models
3 No Venturi effects are occurring at the site of the MAF...constant tube diameters
This leaves us with the same mass of air being rammed into two tubes of differing cross sectional areas....the flow will be different in the different tubes....the small one will have more volume/area than the larger one
Anyone got a problem with that....
Anyone see how the flow could be smaller in the small tube..ie less per unit area???
I don't....so the MAF will read higher flow in the small tube.....ie Higher MAF Voltage
And read smaller Voltage in the larger tube
#1155
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If the volume of air in going through the tubes is the same, the smaller tube will have a lower pressure and the MAF will see less air density. The larger tube will have a higher pressure and the MAF will see more air density. The amount of air going through will be the same(volume).
That's with no venturi effect, ie constant tube sizes. LOOK AT THE ANIMATION OF BERNOULLI'S PRINCIPAL. That's what we're talking about. That animation assumes constant temperature and volume of air.
That's with no venturi effect, ie constant tube sizes. LOOK AT THE ANIMATION OF BERNOULLI'S PRINCIPAL. That's what we're talking about. That animation assumes constant temperature and volume of air.
Last edited by Celronx; 03-31-2008 at 08:47 PM.
#1156
Power!!
As A goes down V will go up. The smaller tube will have a higher velocity. Density will be constant and the same for both since we're not in the compressible regime (>mach0.3) I tend to agree that Velocity will have a larger affect than stagnation pressure on the cooling of the hotwire.
#1157
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The boundary layer issue would come into play as well....as the tube size increases..the MAF filiment changes positions...and ends up in an area with slightly less flow than center ( where the flow will be maximum in a laminar model). So will skew the results to lower reading for actual flow
It still doesn't result in lower flow rates than a small tube...where the effective flow is much higher because of the area ( inv square relationship)
I think when looking at the slight differences in filament position...that the Area relationship will make the former unimportant
It still doesn't result in lower flow rates than a small tube...where the effective flow is much higher because of the area ( inv square relationship)
I think when looking at the slight differences in filament position...that the Area relationship will make the former unimportant
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Please do. I said that I hate Wikipedia, but if that's correct, then it make perfect sense why the MAF would read higher than what's actually flowing through the pipe.
#1160
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Celronx:
When you use the animation you posted, which is very cool btw, you notice that when you make a large straight tube and then a small straight tube the change in pressure is very small compared to the change in velocity ~10x less.
From this I agree with Dan that velocity will have the largest impact on the accuracy of the MAF hotwire cooling, not the pressure change. This is in line with the assumption at low velocities that gases are incompressible when looking at fluid flow in a pipe.
When you use the animation you posted, which is very cool btw, you notice that when you make a large straight tube and then a small straight tube the change in pressure is very small compared to the change in velocity ~10x less.
From this I agree with Dan that velocity will have the largest impact on the accuracy of the MAF hotwire cooling, not the pressure change. This is in line with the assumption at low velocities that gases are incompressible when looking at fluid flow in a pipe.
#1162
if anyone's still interested in talking about the accessport...
how's your cruise afr's? at light throttle, does the afr stay solid, or does it bounce around +/- 1 afr?
how's your cruise afr's? at light throttle, does the afr stay solid, or does it bounce around +/- 1 afr?
#1165
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I agree, that's 3 pages of talking about this. If someone can describe this to me other than just saying that a bigger pipe will flow more air (I know that) please PM it to me because I really do want to know. Again, I want to know why his MAF sensor would show more air entering the engine than is actually entering it.
On subject, my AP is GREAT. My LTFT is solid under light throttle, but the STFT varies pretty wildly while driving.
At idle they are solid 1 and -1 respectively
On subject, my AP is GREAT. My LTFT is solid under light throttle, but the STFT varies pretty wildly while driving.
At idle they are solid 1 and -1 respectively
Last edited by Celronx; 03-31-2008 at 09:03 PM.
#1167
how about your afr's on light throttle cruising?
I'm always second guessing if I have a dying engine or not (having had a few cases of detonation in the past), so I guess I look too deeply at many things.
Yesterday and today I noticed my afr's at light cruising was 14.5 and it bounced to 16 afr and back repeatedly - once or twice per second. So I checked the factory o2 sensor and it reported the same thing as my uego wideband. I then checked the MAF airflow and the airflow was constant. My intake is nicely setup, nothing lose. So I am basically looking to see if others see something similar or if I should keep digging. The car is very quick and feels perfectly fine. The bouncing AFR was just worrying me.
I'm always second guessing if I have a dying engine or not (having had a few cases of detonation in the past), so I guess I look too deeply at many things.
Yesterday and today I noticed my afr's at light cruising was 14.5 and it bounced to 16 afr and back repeatedly - once or twice per second. So I checked the factory o2 sensor and it reported the same thing as my uego wideband. I then checked the MAF airflow and the airflow was constant. My intake is nicely setup, nothing lose. So I am basically looking to see if others see something similar or if I should keep digging. The car is very quick and feels perfectly fine. The bouncing AFR was just worrying me.
#1168
also I have two maf sensors. So I took the other sensor, cleaned it out with electrical cleaner spray, and installed that this morning. The afr's were bouncing still. So I don't think the maf or intake is causing it. I only started noticing this yesterday, but it might have well have been happening earlier.
#1169
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That's pretty much what I'm seeing. The crusing AFR's for me are about 14.7 up to about 15.7, and they randomly change. I have seen some 16's while driving. I imagine that your's would be a little lower since you're turbo.
#1170
when i'm not in boost, my car should be pretty much the same as yours ![Smilie](https://www.rx8club.com/images/smilies/smile.gif)
So you're saying your afr bounces too? Not the range, but moving between the two values repeatedly?.
![Smilie](https://www.rx8club.com/images/smilies/smile.gif)
So you're saying your afr bounces too? Not the range, but moving between the two values repeatedly?.
#1171
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I did have one strange thing happen yesterday. I came off a cruise of about 45mph or so in 4th gear. I slowed down to about 20 or so for a 90 deg. bend in the road and hammered it in 2nd and got a weird rattle. It didn't really sound like detonation and it didn't sound like the intake rattle, but it definately came from the engine at about 4k. I didn't have time to check the AFR's before it was gone. It didn't seem to have any bad effects on the car at the time and no problems now.
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your AFR's should bounce. the PCM view the engine in standstill mode where it is so fast in making changes, that it has time to read all sensors, compare them to what a chart says based on load, TPS, and rpms, and correct the injector pulse width and ignition timing as the intake air flow fluctuates.
#1175
good. When I tested it today going into boost, the afr's quit bouncing and moved to a solid number. It's still too lean though since installing the CAI. I require a richer map.
Funny since a few days ago I was asking Jeff for a much leaner map (before the CAI)
Funny since a few days ago I was asking Jeff for a much leaner map (before the CAI)
![Smilie](https://www.rx8club.com/images/smilies/smile.gif)