Pontiac Solstice Forum banner

1 - 10 of 10 Posts

·
Registered
Joined
·
340 Posts
Discussion Starter · #1 ·
Hi all,

I have Trifecta base tune for my GXP. During the cooler spring weather (under 20C), DIC shows about 23-24psi max boost @3rd or 4th gear pulls.
Yesterday we had had a bit of warm spell with the ambient temp at 27C. I noticed the DIC were showing lower max boost at around 21psi @3rd/4th gear pulls.

Seat off the pants is still pretty dang quick. Pulled away a Fiat Spyder as if parked yesterday. Wondering if higher ambient temps would cause the lower boost. Or is there something at play?
 

·
Registered
Joined
·
216 Posts
Warmer air is less dense so the same volume of cooler air will provide more pressure.

Barametric pressure will effect boost also.

When were tuning for a race we take all of that into consideration as it will determine how much fuel to add to the intake air charge and ignition timing. Basically the cooler the air the more fuel you can add to a given volume of air and thus more power.
 

·
Registered
Joined
·
340 Posts
Discussion Starter · #3 ·
So if I understood correctly, a lower peak boost in hot days is expected?

I used to think it's due to elevation changes. Good to know.
 

·
Registered
2008 Pontiac Solstice GXP
Joined
·
2,274 Posts
This is how the boost gauge in a vehicle works and why what it is telling you is misleading.

The only thing the ECM cares about is absolute manifold pressure. absolute numbers are well absolute. if the ECM wants to see 40PSI of absolute pressure then that is what it does. The boost gauge that the driver sees is relative pressure. You are probably wondering what it is relative to, that's the barometric pressure. barometric pressure is not a constant, it changes with temperature, humidity, gravitational pull and altitude just to name a couple. There is literally a plethora of variables that change what the barometric pressure is. Because the ECM deals in absolutes it could give 2 hoots what the barometric pressure is (it does care but not for what this conversation is about).

The math used for the boost gauge is this

absolute pressure - barometric pressure = boost pressure

Say you are ideal sea level barometric pressure which is 14.7psi (really 14.696 but I am rounding) and your boost gauge is showing 20psi that means that the absolute pressure is 34.7psi. Now say you drive up hill, a really steep hill and you increase your altitude to 8500 feet. No other parameters change that 14.7psi sea level pressure just turned into 10.7psi. Now remember the ECM deals in absolutes so it is still going to hit it's target of 34.7psi.. that would mean that your boost gauge now reads 24psi. The engine has no additional power because the absolute number in both locations is exactly the same.

This is the reason why I hate the boot gauge because it is misleading. The absolute pressure is the reading you really should be looking at because it is a constant and a way to really know what is happening.
 

·
Registered
2008 Pontiac Solstice GXP
Joined
·
2,274 Posts
barometric pressure is simply how much the air weighs where you are. The hotter the temperatures the further apart the molecules are so the air has less weight, colder the temperatures the molecules are closer together and the heavier the air is. Humidity is water and water has weight, the more humidity there is the more the air weighs. altitude effects the weight because well.. the closer you are to the center of the earth the more air there is above your location. The earth is not round (the teachers were liars and so was Christopher Columbus) it is more like an ellipsis. Think of what the tires do on a drag car when the car takes off. well the same thing is going on with the earth because of it's rotation. so based on where you are in the world the gravitational forces are going to be different and this has an effect on the weight of the air. The proximity to the moon also has an effect.


The cause of your lower boost pressure is not because of the temperature. hotter temperatures = molecules spread apart further = air weighs less = lower barometric pressure. I pointed out above that lower barometric pressure is going to make the boost gauge read higher not lower. Chances are your lower boost readings were due to humidity. There was probably a high humidity on that hot day and humidity is going to have way more of an effect on barometric pressure then temperature would. The moon could have also been pretty far away which would cause in increase in gravity making the air weigh more.

A donkey could have been flying above the car that day for all we know. LOL. There are so many factors it crazy. You need to look at what the barometric pressure is to know why the numbers are either up or down.
 

·
Registered
2008 Pontiac Solstice GXP
Joined
·
2,274 Posts
Barametric pressure will effect boost also.

Technically no it doesn't. Because the cars ECM deals in absolutes. 34.7psi absolute pressure is 34.7psi no matter what the barometric pressure it. That being said PSI is a weight measurement so no mater if the oxygen molecules are 2mm apart or they are 2 miles apart the pair of them still weight the same. Now.. The turbo may have to work overtime in order to collect those 2 molecules but it is going to try it's hardest to collect them as fast as possible. The mechanical limits of the turbo could be reached before collecting those 2 molecules.

If you compared the turbine speeds when the barometric pressure is 14.7 vs 10.7 you will find that the RPM's of the turbine are higher with the lower barometric pressure. So there is a limit that can be reached and this would cause the engine to see a lower absolute pressure with a lower barometric pressure. This will only happen if the turbo is not the correct size so the lower absolute pressure the engine has is really not an effect of the barometric pressure but is instead caused by an incorrect selection of hardware.
 

·
Super Moderator
Joined
·
16,870 Posts
(3) GM Stage II Kit review | Pontiac Solstice Forum

The tune provides boost based on conditions. Humidity, air density (altitude), air temperature they all play a part in determinng the amount of boost provided.

We found very early on that 12 pounds of boost at sea level equated to 18 pounds of boost at 5000 feet. Flash's Owner actually has a thread that asks for data from various locations around the country that he translates into air density. Its an interesting thread.

The ECM is torque managed. the HP is a product of the torque produced. The tune establishes a torque curve then sets paramaters to achieve that torque level. Thus the "learn down" of the stock tune. With the Stage 2 GM tune, the torque target was raised from 260 to 320? or there abouts. But the Stage 2 tune also increased the boundary for torque produced to a higher level such that if you added hardware that produced more than the target 320 or 340 torque target, it "allowed" the engine to achieve that higher level of torque. My 2.0 when last dyno'd was at about 400 torque.

This is the thread on boost and atmospheric conditions

(3) Boost levels and Atmospheric Conditions thread | Pontiac Solstice Forum

From the above thread
I am especially interested in stock vehicles. I think a typical LNF car is not going to be too far away from this:

-2000' (cold day below freezing) 14-15 PSI
-1000' (15-16)
0' (16-17)
+1000' (17~18)
+2000' 19 PSI (close to SAE J1349 standard)
+4000' 20 PSI
+6000' 20~21 PSI
 

·
Registered
2008 Pontiac Solstice GXP
Joined
·
2,274 Posts
I can tell you that 18psi of boost is what is seen on a bone stock LNF at around 8000 foot elevation with a RH of 15% and an ambient temperature of around 70°F

This falls right into line with what I was saying with respect to the absolute manifold pressure, barometric pressure and what is seen on the boost gauge. The barometric pressure is right around 10.7psi where I live, almost exactly 4 PSI lower then at sea level. At sea level the boost gauge would read 14PSI exactly 4 PSI lower then what is seen at 8000 foot elevation. All the ECM does for a calculation for the boost gauge is it takes the absolute manifold pressure and subtracts the barometric from it. that's it. The end result is the engine is still seeing the exact same amount of air whether you are at 8000 foot with a boost of 18psi or at sea level with a boost of 14 psi. In both cases the absolute manifold pressure is the same (or close enough to being the same) so the same number of oxygen molecules are entering the engine thus the weight of the air is going to be the same. a MAF sensor measures the weight of the incoming air not the volume because volume is technically irrelevant when needing to know how much fuel to inject into the cylinders. The turbine on the turbo is going to have to spin at a much higher RPM in order to move a higher volume of air when there is a lower barometric pressure. This is because the engine still wants the same weight of air going into the engine but with the lower barometric pressure the volume of the air is going to be more then a higher barometric pressure for the same weight of air.
 

·
Registered
Joined
·
48 Posts
barometric pressure is simply how much the air weighs where you are. The hotter the temperatures the further apart the molecules are so the air has less weight, colder the temperatures the molecules are closer together and the heavier the air is. Humidity is water and water has weight, the more humidity there is the more the air weighs. altitude effects the weight because well.. the closer you are to the center of the earth the more air there is above your location. The earth is not round (the teachers were liars and so was Christopher Columbus) it is more like an ellipsis. Think of what the tires do on a drag car when the car takes off. well the same thing is going on with the earth because of it's rotation. so based on where you are in the world the gravitational forces are going to be different and this has an effect on the weight of the air. The proximity to the moon also has an effect.


The cause of your lower boost pressure is not because of the temperature. hotter temperatures = molecules spread apart further = air weighs less = lower barometric pressure. I pointed out above that lower barometric pressure is going to make the boost gauge read higher not lower. Chances are your lower boost readings were due to humidity. There was probably a high humidity on that hot day and humidity is going to have way more of an effect on barometric pressure then temperature would. The moon could have also been pretty far away which would cause in increase in gravity making the air weigh more.

A donkey could have been flying above the car that day for all we know. LOL. There are so many factors it crazy. You need to look at what the barometric pressure is to know why the numbers are either up or down.
actually the world is FLAT
 

·
Registered
2008 Pontiac Solstice GXP
Joined
·
2,274 Posts
LMAO
 
1 - 10 of 10 Posts
Top