Why less boost in hot weather?
06-11-2004, 07:58 PM
#46
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I'm still not convinved it's a wastegate issue. I tried to combat this by upgrading to a deltagate as well as an AVC-R. I would ALWAYS run less boost when it's hot, regardless of settings. Frustrating as hell.
I upgraded the turbo, injectors, standalone next, and the problem went away. It'd ALWAYS run the same amount of boost regardless of temperature, humidity, pressure, whatever. It'd feel faster on cold days, slower on hot days (air density), but boost pressure would show the same.
So my theory is still gonna be somethign to do with the turbo's efficiency range, and not enough time to spool to high enough boost on warm days before running out of steam
.
I upgraded the turbo, injectors, standalone next, and the problem went away. It'd ALWAYS run the same amount of boost regardless of temperature, humidity, pressure, whatever. It'd feel faster on cold days, slower on hot days (air density), but boost pressure would show the same.
So my theory is still gonna be somethign to do with the turbo's efficiency range, and not enough time to spool to high enough boost on warm days before running out of steam
.
06-11-2004, 08:31 PM
#47
Nordschleife Master
Okay. here's my take:
The reason we show less boost in hot weather/high alt is because our Boost Gauges are based on Absolute Pressure, which means they add the current atmospheric pressure to your current manifold pressure.
If your boost gauge was calibrated, it would always show just over 1bar at 15psi. But since our boost gauges are in absolute, it shows just over 2 bar at 15psi, at sea level. But as the weather gets hotter, and the air gets thinner, that 2 bar drops to 1.9 or 1.8 bar.
make sense?
The reason we show less boost in hot weather/high alt is because our Boost Gauges are based on Absolute Pressure, which means they add the current atmospheric pressure to your current manifold pressure.
If your boost gauge was calibrated, it would always show just over 1bar at 15psi. But since our boost gauges are in absolute, it shows just over 2 bar at 15psi, at sea level. But as the weather gets hotter, and the air gets thinner, that 2 bar drops to 1.9 or 1.8 bar.
make sense?
06-11-2004, 09:00 PM
#48
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JustinL:
The temp rise over the turbo is the same on a hot day as a cold day unless the turbo get very much outside it's efficieny range. This means that if you on a day with 10°C ambient have a temp after the turbo of 80°C, then you will have 100°C on a day with 30°C ambient temp.
This also means that the cooling effect in the ic will be the same. Say that when you try to cool the 80°C charge with the 10°C ambient air you end up with 40°C out of the IC. Then you will get 60°C when you cool the 100°C air with the 30°C air.
You cant use the gas law on an open system as the IC is. It only applies on closed volumes. The pressure drop over the ic must be calculated with fluid flow formulas.
Tomas
The temp rise over the turbo is the same on a hot day as a cold day unless the turbo get very much outside it's efficieny range. This means that if you on a day with 10°C ambient have a temp after the turbo of 80°C, then you will have 100°C on a day with 30°C ambient temp.
This also means that the cooling effect in the ic will be the same. Say that when you try to cool the 80°C charge with the 10°C ambient air you end up with 40°C out of the IC. Then you will get 60°C when you cool the 100°C air with the 30°C air.
You cant use the gas law on an open system as the IC is. It only applies on closed volumes. The pressure drop over the ic must be calculated with fluid flow formulas.
Tomas
06-11-2004, 09:04 PM
#49
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Rich:
Just because the ambient temperature is higher or lower doesn't necessarily mean that the air pressure is higher or lower. What you are describing is the effect of changes in barometric pressure, not air temp.
Tomas
Just because the ambient temperature is higher or lower doesn't necessarily mean that the air pressure is higher or lower. What you are describing is the effect of changes in barometric pressure, not air temp.
Tomas
06-12-2004, 04:25 AM
#51
Drifting
I gotcha Thomas. I was trying to point out the assumptions that I was making as I went along, and you are right... assuming temp difference before and after the intercooler is linear from hot days to cool days. And not addressing the heat soak issue. But yes I think the effect of changing intercooler efficiency is fairly small, but if there was any effect from it, it would lead toward the observation of less bost on hot days.
Still theorizing,
Justin
Still theorizing,
Justin
06-12-2004, 12:08 PM
#52
Rennlist Member
I will state it again; the answer to the orginal question, which has nothing to do with IC eff. at different temps, altitude , or any other speculative theories.
The root of all this is a turbocharger phenomenon. The turbo rotating assembly has to spin faster and faster to make the same amount of boost out of proportionally higher and higher ambient temps. That's it; that's the reason.
Now, this will manifest itself in different ways. With the majority of our 951's, having a relative high sensitivity to back pressure (the way the wastegate is setup), it will show itself in a slightly earlier opening of the wastegate, because there is a little bit more back pressure because the turbo wants to spin faster to achieve the desired boost with the higher ambient temperature.
It doesn't matter what boost controller you use; if any wastegate is even a little bit affected by extra heat and/or flow this phenomenon will occur to a greater or lesser extent, depending on the setup.
Rage,
a bigger turbo that flows alot more air isn't even phased by all this because the back pressure might only go up a very small amount relative to the extra work it needs to do in the warmer ambient temps; therefore no loss of boost.
It used to be that some compressor maps were even published with correction factors according to ambient temps (if the ambient temperature varied from a certain standard, turbo RPM would go up and down to achieve x amount of boost).
The root of all this is a turbocharger phenomenon. The turbo rotating assembly has to spin faster and faster to make the same amount of boost out of proportionally higher and higher ambient temps. That's it; that's the reason.
Now, this will manifest itself in different ways. With the majority of our 951's, having a relative high sensitivity to back pressure (the way the wastegate is setup), it will show itself in a slightly earlier opening of the wastegate, because there is a little bit more back pressure because the turbo wants to spin faster to achieve the desired boost with the higher ambient temperature.
It doesn't matter what boost controller you use; if any wastegate is even a little bit affected by extra heat and/or flow this phenomenon will occur to a greater or lesser extent, depending on the setup.
Rage,
a bigger turbo that flows alot more air isn't even phased by all this because the back pressure might only go up a very small amount relative to the extra work it needs to do in the warmer ambient temps; therefore no loss of boost.
It used to be that some compressor maps were even published with correction factors according to ambient temps (if the ambient temperature varied from a certain standard, turbo RPM would go up and down to achieve x amount of boost).
06-12-2004, 12:36 PM
#53
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"With the majority of our 951's, having a relative high sensitivity to back pressure (the way the wastegate is setup), it will show itself in a slightly earlier opening of the wastegate, because there is a little bit more back pressure because the turbo wants to spin faster to achieve the desired boost with the higher ambient temperature."
I can attest to this point as last summer I had stock exhaust and would see 1.5-2psi loss on hot days. Now, with a 3"cat back (no cat/flowmaster) I have only seen about 1 psi drop in 2nd and 3rd 4th I have not noticed any less boost, or maybe .5 psi.
I can attest to this point as last summer I had stock exhaust and would see 1.5-2psi loss on hot days. Now, with a 3"cat back (no cat/flowmaster) I have only seen about 1 psi drop in 2nd and 3rd 4th I have not noticed any less boost, or maybe .5 psi.
