VE table, MAP-dimension profile: Theories & Practice?
08-31-2007, 10:54 AM
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VE table, MAP-dimension profile: Theories & Practice? I'm looking at a stock '02 F-body VE table and in particular the
variationin the Y-axis (MAP). Pics & Excel sheet (.zip) attached.
I see that at higher RPM the VE-lines bend over at low MAP,
progressively more. I don't have a good idea why this would be
so, other than GM "fudging" for deceleration fueling or something.
This characteristic may be some of what makes lean decel in
non-DFCO regions. I'm not convinced it's realistic.
But what I am really wondering, is how a cam pushes on the
VE/MAP relation. I would like to get ahold of some tunes where
people believe they have done a really good job on the low end
and have a cam (especially something like 224 on 114) in there.
I would like to understand qualitatively how the "practical VE"
profile changes with overlap / duration as well as any credible
theories on "why?". Ideally I'd like to arrive at some sort of
predictive expression, of how the MAP slope (and RPM slope)
should be expected to change based on cam fundamentals.
I see what looks like a MAP -> fuel based oscillation at idle and
am wondering if the lower RPM, with cam, ought to be bending
more like the high-RPM, low-MAP lines do (Sheet 3 has only the
near-idle lines, Sheet2 has all). Wanting to get the MAP slope right.
However it seems that in steady state the VE table I have now is
pretty right. So maybe it's more in the VR table time-filters than
the values themselves?
Comments and tweaked-up VE table snippets, w/ cam info and an
idea of how clean you think the fit is, please?
Note: the Y-axis is VE value as a percentage of the table 100kPa
row, column-by-column, to make it "dimensionless".
variationin the Y-axis (MAP). Pics & Excel sheet (.zip) attached.
I see that at higher RPM the VE-lines bend over at low MAP,
progressively more. I don't have a good idea why this would be
so, other than GM "fudging" for deceleration fueling or something.
This characteristic may be some of what makes lean decel in
non-DFCO regions. I'm not convinced it's realistic.
But what I am really wondering, is how a cam pushes on the
VE/MAP relation. I would like to get ahold of some tunes where
people believe they have done a really good job on the low end
and have a cam (especially something like 224 on 114) in there.
I would like to understand qualitatively how the "practical VE"
profile changes with overlap / duration as well as any credible
theories on "why?". Ideally I'd like to arrive at some sort of
predictive expression, of how the MAP slope (and RPM slope)
should be expected to change based on cam fundamentals.
I see what looks like a MAP -> fuel based oscillation at idle and
am wondering if the lower RPM, with cam, ought to be bending
more like the high-RPM, low-MAP lines do (Sheet 3 has only the
near-idle lines, Sheet2 has all). Wanting to get the MAP slope right.
However it seems that in steady state the VE table I have now is
pretty right. So maybe it's more in the VR table time-filters than
the values themselves?
Comments and tweaked-up VE table snippets, w/ cam info and an
idea of how clean you think the fit is, please?
Note: the Y-axis is VE value as a percentage of the table 100kPa
row, column-by-column, to make it "dimensionless".
08-31-2007, 12:29 PM
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Simply put, it's an airflow table. The default units are not percentages...but (grams*Kelvin/kPa). Different cams (stock or aftermarket designs) have different valve timing events, which in turn affects vacuum and overall airflow. Because of overlap in aftermarket cams, engines don't have nearly as much vacuum at lower rpms. Therefore, airflow velocity is lower. It's like switching your vacuum cleaner from high to low - the rate at which air is sucked into the tube decreases. These changes have an effect on the entire RPM spectrum. I doubt the VE will be fudged from the factory. It may not be perfect given the vehicle is mass-produced and delivered to different parts of the world that have different driving environments. But, it should be close.
As for decel. fueling...it really doesn't matter all that much as it is only present for the first 5 seconds after you cross the DFCO threshold (even less on the earlier models). But, if you'd like to see, here's a pic of my cam'd VE table. It's still in progress as I'm dialing in the SVO30's. Cam is a 232/238 .595/.605 112.
As for decel. fueling...it really doesn't matter all that much as it is only present for the first 5 seconds after you cross the DFCO threshold (even less on the earlier models). But, if you'd like to see, here's a pic of my cam'd VE table. It's still in progress as I'm dialing in the SVO30's. Cam is a 232/238 .595/.605 112.
08-31-2007, 12:53 PM
#3
On the stock 2002 F-body VE table. On your high RPM low MAP region. I argee that the values appear low.
When I ran that region in OL the exhuast sounded exactly like when my car is in DFCO. Since I "tuned" that region it no longer made that sound unless the car was in DFCO.
This is an inductive conclusion so I might be wrong. I would think that the region is lean, though. MY WB shows this.
Any similar experiences?
When I ran that region in OL the exhuast sounded exactly like when my car is in DFCO. Since I "tuned" that region it no longer made that sound unless the car was in DFCO.
This is an inductive conclusion so I might be wrong. I would think that the region is lean, though. MY WB shows this.
Any similar experiences?
08-31-2007, 08:30 PM
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The Y-axis in the charts is not the actual VE value, it is all
relative to the 100kPa row. I did that to look better at the
slope. If I receive other VE tables I will do the same thing,
my intention was to be able to look at the MAP-slope and
see what happens with cam etc, without having to mentally
account for the actual VE differences vehicle-to-vehicle.
I want to know if there is a consistent MAP dependence or
not, or if you can predict it given cam details, etc.
What I see is that generally there is a small MAP dependence
of VE but at higher RPM there is that peculiar "breakpoint"
into a high MAP dependence. Such nonlinearities always make
me suspicious. If it models a real effect then I wonder what,
and about how a cam will move the inflection point around. If
it's hokeythen I wonder what happens when you put it straight.
relative to the 100kPa row. I did that to look better at the
slope. If I receive other VE tables I will do the same thing,
my intention was to be able to look at the MAP-slope and
see what happens with cam etc, without having to mentally
account for the actual VE differences vehicle-to-vehicle.
I want to know if there is a consistent MAP dependence or
not, or if you can predict it given cam details, etc.
What I see is that generally there is a small MAP dependence
of VE but at higher RPM there is that peculiar "breakpoint"
into a high MAP dependence. Such nonlinearities always make
me suspicious. If it models a real effect then I wonder what,
and about how a cam will move the inflection point around. If
it's hokeythen I wonder what happens when you put it straight.
09-01-2007, 09:29 AM
#6
This phenomenon becomes really apparent when comparing an 00 VE table to an 02. On the 00, the VE is steady from from 5200 on up in all cells, whereas on the 02, there is a huge drop-off in the lower MAP regions.
This has always puzzled me. I can't believe that the operating characteristics of the engines are that different. GM engineers apparently adopted new VE modeling.
This has always puzzled me. I can't believe that the operating characteristics of the engines are that different. GM engineers apparently adopted new VE modeling.
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09-01-2007, 04:26 PM
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Only in steady-MAP and I don't know how much of the street
driving envelope that is. Whether or not the MAF covers it up
"most of the time" I still want to know the speed density side
is sensible because it will get exposed in transitional-throttle
driving.
And "artifacts" aside I am still interested in understanding how
the MAP slope of the VE table ought to look on a well tuned
vehicle, for various mods. Particularly at lower RPM where I
seem to see the MAP and fueling chasing each other around
the tree.
driving envelope that is. Whether or not the MAF covers it up
"most of the time" I still want to know the speed density side
is sensible because it will get exposed in transitional-throttle
driving.
And "artifacts" aside I am still interested in understanding how
the MAP slope of the VE table ought to look on a well tuned
vehicle, for various mods. Particularly at lower RPM where I
seem to see the MAP and fueling chasing each other around
the tree.
09-02-2007, 12:47 AM
#9
Keep in mind that the VE table "may" be fudged to account for the non-vac/boost referenced fuel pressure. The changing IFR table may cause "irregularities" in the VE table as a result of GM corporate saving a dollar on a return fuel line. On some of the later models, the VE table is definitely BS'd.
