The Cone Filter location
12-10-2009, 05:33 PM
#46
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I agree with this, but I think the ram air would really just give it more available air as compared to without the positive pressure. So if ever there was a time when the turbo was having to suck harder because the intake tract was restrictive, then positive pressure would help provide more air. Though technically speaking the intake tract before the turbo is never under positive pressure once the turbo is spooled.
But an air inlet to allow cool air in and hot out of the engine bay would help. But ram air into the motor, no.
12-10-2009, 05:42 PM
#47
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I peresonally would like to see the temp numbers under the hood at 100 mph. At that speed the air under the hood is being replaced faster than it can heat up. The only advantage of having a "cold air intake" is at lower speeds or from a standing start. Don't flame me if you don't have the numbers to back it up.
I don't have dyno numbers but surface temps and EGT were much cooler with a true cold air intake vs. air from the engine bay.
12-11-2009, 12:28 AM
#48
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Surely there is a fluids engineer in here that can back me up. The turbo inlet has a set restriction, caused by its size. Short of an external way to compress the air ahead of it, the air will be at roughly 1ATM (depending on elevation). Adding a high pressure source (aka "Ram Air" by most car guys) won't compress the inlet air, it just moves it around and causes turbulence. The air at the inlet is still going to be at 1 ATM.
If the air is at basically 1 ATM, then you are probably thinking that it doesn't matter, right? Wrong. The air flowing in is subject to laminar flow properties, which is weird science all by itself and does all sorts of things that the layperson wouldn't predict. The best way of thinking about this would be to think of air flowing around an airplane wing. Ever have a plane ride with a lot of turbulence? Same thing applies to a turbo. By adding a high pressure source, it causes disruptions to the flow (called "eddies") that increase the internal viscosity of the air and slows the flow down. Instead of flowing through the turbo in a laminar fashion so that the turbo can compress it, the kinetic energy is expending bouncing around against other air.
Theres a LOT of science that I throw at you, but ram air doesn't work with turbos and fluid dynamics will back me up on this. Pick a low pressure source with the coolest air you can find and your turbo will work as efficiently as possible, as long as the inlet source exceeds the capacity of the turbo inlet, which it will pretty much anywhere in the front end of the car.
Its interesting to note (if you are thermodynamics geek anyway) that the opposite applies to radiators and intercoolers. You want a high pressure source, as the turbulence helps with the heat transfer and mixing.
For the purposes of a 951, I think this is mostly an academic argument anyway.
Regards,
If the air is at basically 1 ATM, then you are probably thinking that it doesn't matter, right? Wrong. The air flowing in is subject to laminar flow properties, which is weird science all by itself and does all sorts of things that the layperson wouldn't predict. The best way of thinking about this would be to think of air flowing around an airplane wing. Ever have a plane ride with a lot of turbulence? Same thing applies to a turbo. By adding a high pressure source, it causes disruptions to the flow (called "eddies") that increase the internal viscosity of the air and slows the flow down. Instead of flowing through the turbo in a laminar fashion so that the turbo can compress it, the kinetic energy is expending bouncing around against other air.
Theres a LOT of science that I throw at you, but ram air doesn't work with turbos and fluid dynamics will back me up on this. Pick a low pressure source with the coolest air you can find and your turbo will work as efficiently as possible, as long as the inlet source exceeds the capacity of the turbo inlet, which it will pretty much anywhere in the front end of the car.
Its interesting to note (if you are thermodynamics geek anyway) that the opposite applies to radiators and intercoolers. You want a high pressure source, as the turbulence helps with the heat transfer and mixing.
For the purposes of a 951, I think this is mostly an academic argument anyway.
Regards,
I think you're over-complicating things, and throwing out words that are not applicable, here.
Upstream from the inducer there is tubing, an air flow meter of some kind, and an air filter; so, I don't know how the turbo is going to see any turbulence, for one thing.
All that matters is the pressure the turbo inducer "can grab", which it uses via a pressure ratio to achieve the desired boost.
For example:
Anything higher than sea level it will have less than 14.7 psi available.
On top of pikes peak there might only be 10 psi available, or whatever it is.
Any restriction in the path (which includes a smallish air filter) might cause it to "see" 13.5 or 14 psi, let's say.
Drawing air from a low pressure area around the car might also cause it to be able to "grab" only 13.7 psi of air, let's say.
Conversely, if at sea level, with a big air filter, drawing from a high pressure area, you'll get the full 14.7 psi and probably more if you draw from an area where the "rammed air" is forced into stagnation.
When a turbo has to work less (less pressure ratio) because it's starting out by "grabbing" denser air (as opposed to air that has been "stretched apart" because of restrictions), it pays big dividends in the way of less discharge heat and less back pressure at the turbine.
There's no such thing as "any more air and the turbo can't use it anyway", or "it's not going to get any more efficient". (I'm not saying these are exact quotes from anybody)
Another example of the "ram" effect is directly at the engine; a later closing of the intake valve utilizes the velocity of the incoming air to pack it tighter. Again, it's not like there's doubling of the pressure or anything, but they are getting volumetric efficiencies of 110% nowadays. Engineers are trying to get the last bit of power from their efforts. So, why would anybody just be indifferent to the possible merits of ram air for a turbo.
12-11-2009, 05:00 AM
#49
Then feel free to try a ram air set up if you think it will help you. My physics is correct. 
Last edited by User 41221; 12-11-2009 at 11:25 AM.
12-11-2009, 03:34 PM
#51
Really? lol
Feel free to post your results of your ram air. I can't wait.
Regards,
Feel free to post your results of your ram air. I can't wait.
Regards,
12-11-2009, 03:41 PM
#52
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Effects of ram air
Hi Gents,
This article isn't about turbo Porsches so technically it's off topic but it is interesting and presents some science and testing results about ram air.
http://www.sportrider.com/tech/146_9910_ram/index.html
Cheers
This article isn't about turbo Porsches so technically it's off topic but it is interesting and presents some science and testing results about ram air.
http://www.sportrider.com/tech/146_9910_ram/index.html
Cheers
12-11-2009, 04:54 PM
#53
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Here are my thoughts on Ram Air for a turbo system.
1) The boost control system will adjust the wastegate activity so as to maintain boost pressure at the set level, e.g. 15 psig.
2) If the Ram Air effect is real, then the boost control will open the wastegate more aggressively as to slow down the turbo rpm and maintain boost pressure at the set level of 15 psig (i.e., the turbo will operate at a lower pressure ratio),
3) If there is no Ram Air, or if there is a low pressure zone, the boost control will lessen the amount of wastegate activity so as to restore turbo rpm (increase turbo pressure ratio) and maintain boost at the set level of 15 psig.
So increasing or decreasing the pressure at the air filter will only cause the boost control system to speed up or slowdown the turbo in order to maintain the set boost pressure.
However, the boost controller has no way of detecting air density, so one can make more more power at the set boost level, by feeding the turbo colder air.
1) The boost control system will adjust the wastegate activity so as to maintain boost pressure at the set level, e.g. 15 psig.
2) If the Ram Air effect is real, then the boost control will open the wastegate more aggressively as to slow down the turbo rpm and maintain boost pressure at the set level of 15 psig (i.e., the turbo will operate at a lower pressure ratio),
3) If there is no Ram Air, or if there is a low pressure zone, the boost control will lessen the amount of wastegate activity so as to restore turbo rpm (increase turbo pressure ratio) and maintain boost at the set level of 15 psig.
So increasing or decreasing the pressure at the air filter will only cause the boost control system to speed up or slowdown the turbo in order to maintain the set boost pressure.
However, the boost controller has no way of detecting air density, so one can make more more power at the set boost level, by feeding the turbo colder air.
03-11-2011, 07:00 AM
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03-11-2011, 11:05 AM
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So is that a blow through design?
I currently run the maf wrapped with screen mesh so it is right at the fender hole and I notice a major difference, at least in cooler temps.
In the colder weather even after numerous wot runs, the IC out pipe is as cold as the sheet metal and the intake is only luke warm. Also, my spool up time reduced a good 200-300rpm.
Even with my big laggy 75, in 4th I see 1 bar by 3,600 in the colder months.
I currently run the maf wrapped with screen mesh so it is right at the fender hole and I notice a major difference, at least in cooler temps.
In the colder weather even after numerous wot runs, the IC out pipe is as cold as the sheet metal and the intake is only luke warm. Also, my spool up time reduced a good 200-300rpm.
Even with my big laggy 75, in 4th I see 1 bar by 3,600 in the colder months.
