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Is there any, how does it work? From what I understand, the ecm checks barometric pressure when the key is forward, just before the engine starts and the map sensor is used for this. At the same time, the ecm checks the ambient temps vie MAT. Then it'll offset the PW, I guess.
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Originally posted by 11sORbust Is there any, how does it work? From what I understand, the ecm checks barometric pressure when the key is forward, just before the engine starts and the map sensor is used for this. At the same time, the ecm checks the ambient temps vie MAT. Then it'll offset the PW, I guess.
There really isn't a need for a DA calculation. The PW is calculated from the VE, MAP, MAT, EGR partial pressure, cylinder displacement, and injector flow terms.
As you mentioned the baro is read at key on via the MAP sensor. There are also pseudo baro reads done while the vehicle is being driven. This allows baro to be updated during those Pikes Peak climbs
Barometric pressure is used to modify the VE term. Notice that the table at $881F increases the VE as the barometric pressure drops.
;--------------------------------------------------
; VE Mult vs BARO
;
;
; TBL = MULT * 128
;--------------------------------------------------
; MULT Kpa BARO
;----------------------------------
L881F: FCB 152 ; 1.188 65
FCB 146 ; 1.141 75
FCB 139 ; 1.086 85
FCB 133 ; 1.039 95
FCB 128 ; 1.000 105
;--------------------------------------------------
How much does the table effect fueling? More than the MAT tables? Could you explain how the calculation works? I think this stuff is starting to sink in..
Originally posted by 11sORbust How much does the table effect fueling? More than the MAT tables? Could you explain how the calculation works? I think this stuff is starting to sink in..
Lets say the VE is 78% and baro is 105 KPa (see above post, table is quoted). The calc is:
(78% * 128) / 128 = 78% VE
So no change in VE%
After a hill climb the baro is 85 KPa, the calc is:
Originally posted by 11sORbust How does that work.
As the vehicle climbs Pikes Peak the elevation increases. The barometric pressure decreases with an increase in elevation. . .
And here I thought you'd be asking about why the VE is increased with a decrease in barometric pressure. . .
That first answer wasn't the one you were looking for, was it? A pseudo baro read is done at low RPM, high TPS, with some adjustments the MAP reflects baro and is used for a new barometric term.
Sorry, I didn't make the question clear. The table at $881F is applied when the key is forward w/ engine off, right? I don't understand how the ecm can sense the change in baro while the engine is running.
Originally posted by 11sORbust Sorry, I didn't make the question clear. The table at $881F is applied when the key is forward w/ engine off, right? I don't understand how the ecm can sense the change in baro while the engine is running.
The table at $881F is in effect whenever the engine is running.
Originally posted by 11sORbust Sorry, I didn't make the question clear. The table at $881F is applied when the key is forward w/ engine off, right? I don't understand how the ecm can sense the change in baro while the engine is running.
Quote:
A pseudo baro read is done at low RPM, high TPS, with some adjustments the MAP reflects baro and is used for a new barometric term.
This is interesting, even though I don't do much driving that would cause a change in BARO.
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------------------------------
ASE Certified Master Tech
I thought BARO was higher with a higher altitude? The air is also less dense at higher altitude, therefore less efficient? So, wouldn't the VE be less at higher altitudes? (That is assuming that VE was reflecting real world differences in volumetric efficiency)
I may be way off here, but I really NEED to understand this. Not because I go to higher altitudes, but just to further my knowledge and understanding of what we're working with.
Barometric pressure decreases with an increase in elevation. Standard air at sea level has a barometric pressure of 101.3 KPa. At an elevation of 3,000 meters barometric pressure dops to 70.0 KPa.
At the mile high city of Denver (5280 feet) barometric pressure is ca. 84.0 KPa. At key on the ECM will read the barometric pressure and save it for future use. Someone in Denver with a scan tool will see the reported barometric pressure reading of 84 KPA +-. Plus/minus due to weather conditions. This can be seen as the MAP reading before the engine is started.
This same person will see a maximum MAP reading of 84 KPA even when at WOT. No 100 KPa MAP readings for this engine. As such the VE table along with the rest on the induction calculations are correct. This covers the intake side of things.
However, as previously noted, the lower BARO reading will have the affect of increasing the VE term. Something has happened that increased the volumetric efficiency of the engine system.
There is less backpressure on the exhaust portion of the system.
This same person will see a maximum MAP reading of 84 KPA even when at WOT. No 100 KPa MAP readings for this engine. As such the VE table along with the rest on the induction calculations are correct. This covers the intake side of things.
That explains a lot. Someone told me the map sensor wouldn't hit 100kpa if there is an air restriction somewhere on the engine. I can't hit 100kpa because of my elevation.
Originally posted by RBob However, as previously noted, the lower BARO reading will have the affect of increasing the VE term. Something has happened that increased the volumetric efficiency of the engine system.
There is less backpressure on the exhaust portion of the system.
This increases the VE of the engine.
RBob.
Now I get that.
Although, I still don't see the total picture. Not doubting you, just not understanding. Even though there's less backpressure on the exhaust, the air going into the intake is less dense, and under less pressure (without a power-adder) and I would think that would still offset the gains in exhaust flow. If higher altitude made more power, why are sea level corrections higher than high altitude?
Sorry to be so antagonistic, but I'm sure this will help some other people, too.
Although, I still don't see the total picture. Not doubting you, just not understanding. Even though there's less backpressure on the exhaust, the air going into the intake is less dense, and under less pressure (without a power-adder) and I would think that would still offset the gains in exhaust flow. If higher altitude made more power, why are sea level corrections higher than high altitude?
Sorry to be so antagonistic, but I'm sure this will help some other people, too.
At a greater elevation the VE will be higher for any given point of MAP & RPM.
Lets take that Denver car and put it on a dyno. Lets say at 5,000 RPM and WOT (84 KPa MAP) it is making 300HP.
Drive to sea level dragging that same dyno along (eliminating all other variables here).
Strapped to the dyno at sea level with the engine throttled to produce 84 KPa of manifold pressure (MAP) that engine will be making less power.
It is less efficient due to higher exhaust back pressure. It will not make the same 300 HP as it did in Denver, it will be less.
According to the VE baro adjustment table GM has provided (see post above) the HP will be down ca 8.5%.
Originally posted by RBob At a greater elevation the VE will be higher for any given point of MAP & RPM.
Lets take that Denver car and put it on a dyno. Lets say at 5,000 RPM and WOT (84 KPa MAP) it is making 300HP.
Drive to sea level dragging that same dyno along (eliminating all other variables here).
Strapped to the dyno at sea level with the engine throttled to produce 84 KPa of manifold pressure (MAP) that engine will be making less power.
It is less efficient due to higher exhaust back pressure. It will not make the same 300 HP as it did in Denver, it will be less.
According to the VE baro adjustment table GM has provided (see post above) the HP will be down ca 8.5%.
RBob.
So at a given KPa of MAP, it will be more efficient? At the lower altitude, it will take a higher KPa to make the same horsepower? Will the peak horsepower be the same (as long as the proper corrections are made for the altitude compensation)?
Originally posted by 11sORbust Sorry to keep bothering you on this but could you explain exactly how they work?
Would basically how they work be close enough? The TPS & MAP need to be in a steady state. The TPS% needs to be above a certain level, while the RPM needs to be below a certain level.
The current RPM & TPS is used to lookup a correction term. This correction is applied to the current MAP value and is used as the new barometric value.
The correction term is based on N-alpha principles.
On modified vehicles I'll first disable the pseudo baro read. Once the tune is in better shape I'll re-enable it and tune in the pseudo baro read. A lot of work here can simply be done from data logs. Engine changes such as heads, cam, TB, intake manifold, will change the pseudo baro read.
Along with less exhaust pressure (which won't change VE significantly), the angle of the throttle DOES make a difference so long as the pressure ratio (downstream of throttle to upstream of throttle) is ~.5 or higher, meaning 50-100 at 100kPa baro, 40-80 at 80, 30-60 at 60. Less than that, and the sonic flow through the throttle will not change or give a darn what's happening upstream, and only knows what's on the manifold side (meaning the angle of the throttle still controls the flow, but the pressure on the intake side is what determines the flow directly).
At pressure ratios greater than .5, the ambient pressure makes a difference. In order to get the intake to 75 kPa in an ambient of 100 will take 30% throttle on car X. At 80 kPa BARO, it takes 50% throttle = more open throttle = less drag. This small amount makes a difference on VE (that's VE -- not maximum airflow, but airflow vs MAP).
Here's another mind blower: Why does idle MAP get lower at higher BARO? It should still need the same power to overcome accessories right?? Well, I'm thinking it may have something to do with the pressure ratio again, but not the same way - not at the throttle. I'm thinking of the crank case pressure, on the underside of the pistons. Less pressure to push against = higher efficiency. This one just popped into my head while typing, so could be seriously flawed.
