Nope, stock ls6 manifold. I really think its the size of cam/ lack of vacuum screwing things up
Nope, stock ls6 manifold. I really think its the size of cam/ lack of vacuum screwing things up
What 3 bar map sensor?
What characteristic do you have entered for it?
Where did you get them from?
It's the ZR1 map sensor. Verified characteristics from ZR1 map. It output jives with my 3 bar boost gauge. Car runs awesome at tuned altitude. I think the problem is lo vac at idle, so 75 kpa site. At altitude it will idle lower kpa, but then when I crack throttle, i have to go back through that 75 kpa idle site, which has only has enough fuel needed for idle, not enough to move a car up a mountain at 20% throttle.
I think I need to see your tune file, a log at low elevation, log at high elevation. I can share a channel config, but you probably don't need that. Just make sure the barometric pressure is in it along with the usual stuff.
Decided to map TPS vs MAP vs RPM to show the difference in TPS at altitude. I believe it shows that Alpha N, or "combo with SD" (or even MAF) will be superior for my application if intend to change elevation.
Attachment 125674
Think of an engine at idle with as a vacuum cleaner with your hand over the end of the hose... sure it's pulling a vacuum, but there is no actual airflow. In this circumstance, measuring the vacuum inside the hose doesn't really give an accurate description of the airflow through the hose (or engine)... With my cam, my vacuum cleaner is a cordless one with a dying battery, just not pulling much vacuum. So my engine idle is at 75kpa MAP at sea level. I believe the reason I'm having a problem is that my idle MAP at sea level isn't far enough below the MAP level I see at part throttle at elevation to map out an accurate VE table for both scenarios. This is why I need 150%+ more throttle to get the same MAP level at elevation
The ambient pressure at 8000' is 75 KPA. The manifold at WOT at that altitude will have 75 kpa (assuming no boost), and certainly have more airflow and need more fuel than the engine needs when at Idle 75kpa at sea level. To have SD VE table work well with change in altitude, you likely need to have at least 15-20 kpa less idle than the difference in kpa between your lowest and Highest points. So a 101kpa at sea level, and 75kpa at at 8000', a 55-60kpa idle at sea level would leave enough resolution in the part throttle area of your VE table. There's a certain crossover point where the throttle blade is open enough that MAP can be used to correlate to airflow, and you need to have idle vacuum low enough to keep that point below your intended ambient kpa range (elevations). To tune below that crossover point, alpha n would be great... then use a separate baro sensor to correct for conditions on top of all that.
The way you are thinking about it completely ignores that your manifold exist, you have ports in your heads, and valves that, all the air that makes it in to the cylinder, has to flow past. This is what ultimately determines fuel mass not only what air conditions are like in the manifold.
You also seem to be thinking that barometric pressure pushes into the cylinder and not that the piston displaces space and pulls the air into the cylinder. When the engine stops even at closed throttle position the MAP nearly instantly returns to barometric pressure. Same with the vacuum analogy, turn it off and you can't hold any vacuum in the hose with your hand, air is always flowing in the tube, just not driven by barometric pressure but by whats flowing out via the vacuums pump.
75kpa of MAP will flow into the cylinder the same, irregardless of throttle position that is keeping the MAP at a constant 75kpa. The VE value at that point in the table shouldn't change. It can be corrected for factors that change the density of the ambient air, but the Volume of the engine, the cam lobe, vavles, head ports are not changing because of the changing outside air conditions.
The throttle position doesn't make 75kpa air denser.
And your attachment isn't coming through.
So 75kpa at idle at sea level, is the same (or less considering a density correction) amount of air flowing as it would be when I'm at WOT at 8000'?
You have the RPM axis of the VE table for two reasons.Originally Posted by TimbrSS
1. The speed of the piston.
2. The actual time the valve is open. Not too need confused with it's physical duration.
Trying to add this again.
TPS_MAP_VE_compare.jpg
I will clear up my question. So 75kpa at idle (800 rpm) at sea level, is the same (or less considering a density correction) amount of air flowing as it would be when I'm at WOT (800 rpm )at 8000' ft elevation?
Or does an enge idle air requirements change significantly if a cam pulls 75kpa idle or 55kpa idle at same altitude?
I don't see this as arguing.
There are differences between steady state and dynamic conditions that make this conversation require a bit of nuance. The computer has models for both states and conditions to determine which it should use. MAP and VE is much better than MAF for determining dynamic airflows. When tuned correctly.
Ideally you need to tune your VE table while meeting steady state requirements and actually having RPM and MAP reach a steady state. Not some quick throttle change that briefly touches a cell in the table.
I thought it was the difference between 800RPM and 8000RPM but...
Now I see you want to know the difference between idle throttle angle and wide open throttle angle at 800 RPM. If the engine stayed at 800 RPM and MAP didn't change between these positions staying at 75kpa(this is unrealistic), but I would say the VE would not change. Can you achieve steady state in these conditions at different altitudes from different throttle positions? Absolutely. Will the altitude change the VE of the engine? No.
If the manifold is in a steady state at 75kpa, the throttle position that is maintaining that steady state does not effect VE for determining in cylinder air mass.
So if it is the same amount of airflow(throttle closed idle at sea level, 75kpa 800rpm and wot at 8000' elevation, 75 kpa, 800 rpm) why would the engine accelerate quickly at altitude, but stay at idle at sea level? The closed throttle is restricted the available cfm that can use to burn fuel.
Or how would the engine at altitude accelerate with only sea level idle airflow?
The throttle regulates air flow, I'm not saying it doesn't.
I'm saying fuel to be injected doesn't require throttle position for that value to be determined. This is true for MAF or SD air models. You can do it in alpha N which would then require throttle position, but OEM ecus don't do that as it's not accurate for the dynamic situations.
In fact what I think im tring to say is, to tune either MAF or SD model correctly you need to eleminate throttle movement as much as possible to avoid dynamic influences.
Last edited by murfie; 11-05-2022 at 04:16 PM.
I'm saying it does require TPS, MAF, or some other way to model airflow in my situation, since it has no other way to determine the difference between needing idle fuel at 75kpa, or WOT fuel at 75kpa, if the only information it has is that MAP value.
VE is the ratio between the volume of air drawn the cylinder (real) and the geometric volume of the cylinder (theoretical).
When we tune, we back calculate the VE numbers by lambda target error. So we are subject to not only the atmospheric conditions available but the throttle restriction we must use as well to control engine speed.
Since we can't have the engine running wide open at all times, we close the throttle to control engine speed. This reduces the engines VE! The engine does not have the same mass of air flowing through it as it would without the restriction of the throttle. Not normally a problem, since the restriction should lead to lower vacuum as well. But if the engine is mechanically limited to the amount of vacuum it will pull, you can run into issues.
OEMs can get away with using this "restricted VE" number in the idle and low tps sites on the VE table because they are using cams that pull high vacuum (under 60kpa) and most modified engines will fall within that rage too. This makes so they can run at 8000' (75kpa ambient) but still have enough difference from the idle (closed throttle) spot that was 40kpa sea level to where the throttle is open enough to flow enough CFM to not lower the VE .
So a situation can exist with altitude changes that the manifold vacuum (75kpa idle 800rpm) area that had restricted flow at sea level(and therefore reduced VE), now has the throttle open more at altitude, where the throttle is no longer a restriction... which would raise the VE for that same site in the VE table. So it now is hitting the same 75kpa site at 800 rpm, but require more fuel since the VE is higher.
Does it make sense that this situation is possible? I believe that is what my data is telling me.
How do we let the ECU know weather we are at the closed throttle, (aka lower VE) 75kpa at 800 rpm, or the WOT (higher VE) 75kpa at 800rpm?
VE is not a ratio it is a maximum. Load is the ratio( multiplied by 100=%). in fact there are two types of load, calculated and absolute.
Calculated load ratio = in cylinder air mass / maximum in cylinder airmass as a function of RPM at the maximum manifold pressure
Absolute load ratio = in cylinder air mass / maximum in cylinder airmass at a given standard temperature and pressure
you would multiply both sides of this equation by "maximum" and get in cylinder air mass.
So depending on the load you are looking at 100% VE = maximum in cylinder airmass, just is this maximum airmass a fixed airmass of the cylinder volume at STP conditions( MAF air models use this), or a variable airmass as a function of RPM and MAP(speed-density uses this). Barometric pressure and ambient temperature of the air conditions can be used to correct this maximum in cylinder airmass of either load type. Those ambient values are not used to correct actual in cylinder airmass directly.
This is back calculating, using Fuel error isn't calculating because stoich ratio of the fuel, injector flow rates, and fuel pressure error all can come into play, just hopefully not a lot of error there.
The piston displacing space is what moves the air, not the pressure differential at the throttle body.
I think you are comparing idle/ low throttle at low elevation to like a high gear low RPM wide open throttle at high elevation and might not realise both should be a steady state.
For simplicity you can think of it as 75kpa from the intake valve only to the back of the throttle body(at low elevation idle/ light ssteady throttle) or going out to the entire atmosphere (at high elevation WOT). As long as its steady 75kpa with minimal to no differential going through the manifold, when the valve opens, piston moves, and valve closes, the amount trapped in the cylinder will be the same. The fuel mass to achieve the desired AFR will be the same.
What you are saying is some how the cam has changed the pressure differential across the manifold so much, that at 800rpm and lower elevation the manifold can achieve steady state, but at high elevations it can't. That does not make sense to me.
My car idles at at 75kpa with 95kpa baro verified with KOEO. It is a heads and aggressive cam LS7. I've shown the barometric value moves in the direction expected with a elevation change and only normal driving throttle input is needed for that. I saw 94.5kPa baro at 2500ft and 97kPa somewhere around 500ft( I wasn't there very long and baro updates were pretty slow). This is close to engineering toolbox's expected values. I have not seen any major fuel error differences in the elevations I have been logging/ tuning at (sea level to 3500ft). When I get a chance I will go up to a road that is at 6700ft elevation, shut the engine off and verify the baro with map sensor (I might bring a baro gauge with me just to be more accurate). It might be a week or two before I can do this test. I do not expect to see fueling error's like you are seeing,but maybe the baro PID will be off there, and im curious about that.
Last edited by murfie; 11-06-2022 at 01:04 AM.
I have logged from sea level to 12,000 ft in my Express van. The gas pedal felt broken at that high elevation but fueling was never an issue. Remember Baro - MAP = Vacuum. Idle vaccum is not going to change much because the air entering the throttle body is under less pressure and there is less atmospheric pressure on the exhaust opening. If you idle at 75 kpa at sea level you may idle at 55 kpa at 8,000 ft. If you make 7 in/hg idle vacuum at sea level you are still going to make 5-6 in/hg at altitude.
Last edited by Fast4.7; 11-06-2022 at 01:25 AM.
Well I think the question im tring to answer for myself is, how does a cam effect this VE correction table for barometric pressure changes? I haven't actually driven at an elevation that this table indicates it would apply a significant VE correction. I just don't go to high elevations with this car. 6000ft is only like 5%, but does a cam make that need to be 15-20%?
I'm guessing I don't have to change this at all, and if I do its because its not the original LS1 throttle and manifold, not because of the cam. Thats just my hypothesis, just need to go perform the experiment at high elevation.
Elevation vs VE.jpg
Last edited by murfie; 11-06-2022 at 02:50 AM.
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