Hang on a minute... don't unlearn just yet! By 'efficiency', I take this to mean heat transferred vs. area of intercooler core. My understanding is that there is a greater heat transfer coefficient from water to aluminum than from air to aluminum, which generally allows the use of a more compact core with air-to-water intercoolers. If this is correct, by this definition I think it can be correctly stated said that air-to-water intercoolers are more efficient than air-to-air intercoolers.
Normally with an air to water system, you would have a cooler that has a water bath and the air charge going into the engine passes through a coil and the water picks up the heat, and yes, there is better heat transfer characteristics as compared to an air-to-air cooler.
However, then you have to remove the heat from the water.....how does it do that?
It has another heat exchanger that is water to air......so there is a little electric pump that pumps water through the system and the hot water near the air box now gets pumped to the heat exchanger that has air flowing over it (i.e. a heat exchanger being cooled by ambient air flow).
At first, it's more efficient because you size the water volume for the time you will be under boost doing a 1/4 mile run on a drag way or the sort....the "cool" water of the system picks up all the heat from the charge. At the end of the 1/4 mile, all the volume of water in the system is now heated up. It doesn't matter, you're done with the drag.
But if you're on a race track.....you need to continuously get rid of that heat because you are boosting every time your right foot goes down.......
So now the water in the heat exchanger gets heated up significantly, it doesn't remove heat from the charge air going into the engine because the delta T is low, therefore, the effective efficiency of the cooler goes down.
An air to water cooler is really a one time thing.....then you have to drive without boost for a while, waiting for the volume of water to cool back down to outside ambient, then get back on it again.
A properly sized air-to-air cooler is a much better design when using the boost continuously, like on a road course because it has no "cool down" time.
Hi Mike.. Well, Josh seems to be doing a bit of granny-shifting these days... I'm thinking the VF 'moderate' s/c set up should be no more a strain on the engine than Dinan's version, which of course has more bells and whistles but still is based on stock internals. Dinan's S3 mule is a '00 that has piled on a lot of miles of big-time thrashing (Dan, you remember how many?) and the engine internals seem to be doing fine.
Dave
Indeed I have. Although... the tranny is stock and seems to do just fine. The stock driveshaft is on right now... and it seems to be doing good too. I think there was an issue with my previous drive shafts U-joint that really caused the failure. After some investigating.. i came up short of nothing.
Quote:
Originally Posted by CSBM5
You had it right to begin with...don't unlean it. A water-to-air intercooler system is actually an air-to-water-to-air system and is a much less efficient method to transfer heat from charge air to atmospheric air at steady state. The only gain of a water system is having the mass and specific heat of water working for you when the water mass is cooler than the charge air; however, almost all air-water-air systems are not designed for lengthy boost conditions like you would experience on the track or autobahn...hence you end up with heat soak issues unless the system is designed with a large enough mass of water coupled with large enough heat exchangers on both sides...most all systems I have ever seen for these "kit" supercharger systems are not designed as such.
As far as this is concerened... I do get crazy heat soak. And there is a HUGE difference.. in other words... you can tell when its heat soaked.....
Normally with an air to water system, you would have a cooler that has a water bath and the air charge going into the engine passes through a coil and the water picks up the heat, and yes, there is better heat transfer characteristics as compared to an air-to-air cooler.
However, then you have to remove the heat from the water.....how does it do that?
It has another heat exchanger that is water to air......so there is a little electric pump that pumps water through the system and the hot water near the air box now gets pumped to the heat exchanger that has air flowing over it (i.e. a heat exchanger being cooled by ambient air flow).
At first, it's more efficient because you size the water volume for the time you will be under boost doing a 1/4 mile run on a drag way or the sort....the "cool" water of the system picks up all the heat from the charge. At the end of the 1/4 mile, all the volume of water in the system is now heated up. It doesn't matter, you're done with the drag.
But if you're on a race track.....you need to continuously get rid of that heat because you are boosting every time your right foot goes down.......
So now the water in the heat exchanger gets heated up significantly, it doesn't remove heat from the charge air going into the engine because the delta T is low, therefore, the effective efficiency of the cooler goes down.
An air to water cooler is really a one time thing.....then you have to drive without boost for a while, waiting for the volume of water to cool back down to outside ambient, then get back on it again.
A properly sized air-to-air cooler is a much better design when using the boost continuously, like on a road course because it has no "cool down" time.
That was my understanding as well. Heat transfer coefficients would tell you that water-to-air is technically more 'efficient' than air-to-air. However, real world driving conditions would tell you there are trade-offs to both systems. A large air-to-air intercooler (ala AA) would make more sense for sustained high speed driving/track. But of course heat removal depends on air flow through the system... the car must be moving--the faster the better. Slow speed, real world stop/start operation isn't going to give you a lot of cooling power through the air-to-air core. The preferred design probably depends on one's intended application.
That was my understanding as well. Heat transfer coefficients would tell you that water-to-air is technically more 'efficient' than air-to-air. However, real world driving conditions would tell you there are trade-offs to both systems. A large air-to-air intercooler (ala AA) would make more sense for sustained high speed driving/track. But of course heat removal depends on air flow through the system... the car must be moving--the faster the better. Slow speed, real world stop/start operation isn't going to give you a lot of cooling power through the air-to-air core. The preferred design probably depends on one's intended application.
Dave
Dave,
Understand that in the water system you have 4 convection thermal resistances while in the air-to-air system you only have 2. Two of the four in the water system are also in the air system -- charge air to heat exchanger #1 and heat exchanger surface of exchanger #2 to ambient air. The water system has two additional thermal convection resistances compared to the air-to-air intercooler system: water to exchanger tubes in #1 and water to exchanger tubes in #2. Even in a pure steady state condition, the water system will be less efficient in net transferring heat from the charge air to ambient due to the added thermal resistances.
The main benefit, as stated above, of a water system is packaging (almost always the driving force behind kit use of water systems) and transient heat transfer performance due to the high specific heat of water. Cheap SC kits often use water systems since they do not want to properly engineer an air-to-air intercooler system which often requires relocation/mount fabrication for the AC condenser, radiator, etc. A nice professional air-to-air system is far superior...yielding huge results like this 540i: http://www.icsperformance.com/rides%20dc540%20final.htm ... putting 438HP to the rear wheels. Note PhillyM5's comment above about his system...I just hope his system is engineered enough to handle proper fuel mixtures when he goes to full heat soak.
Chuck
__________________ Current stable:
2001 M5 LeMans Blue/Silverstone
Dinan: springs, Konis, rear sway bar, monoball bushings, and exhaust
Ground Control camber plates
Stoptech front brake kit
Brake ducts opened
TC Design brackets
OE 9.5" rear wheels all around & 275/35-18 Dunlop Direzza Sport Z1 Star Spec for street
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2006 330i sport 6MT Electric Red/Black
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1996 Volvo 850R wagon
Thanks for the lesson Chuck (and Paul). I learned something new today.. it is more than just heat exchange coefficients... :p
It would be interesting to hear Bill (Shadowman's) perspective, since Discovery has also gone with air-to-water intercooling. Obviously, AA's kit (if Karl decides to go forward with it) is air-to-air. Perhaps it comes down to esthestics? It looks like quite a challenge to make an air-to-air intercooler setup look esthetically pleasing. I might be willing to take a hit in heat soak to maintain a nice, finished 'OEM-ish' look...
A few more comments. I spoke again with VF this evening and they indicated that their data indicate that their air-water intercooler design minimizes heat soak. Their intercooler features two cores with a 'two pass' configuration of water routing in which the water circulates from front to back to front again, balancing the cooling between the front and rear pistons (ie, front pistons don't see only the coldest water). Their heat exchanger is mounted in the usual location in the lower opening of the front spoiler, and is fairly innocuous looking (painted black... only the discerning eye can tell).
VF was interested in checking in on this thread. I gave them the link and invited them to comment on the heat soak question, so we may be hearing from them.
Other tidbits -
VF confirmed that the targeted kit price ($10k) includes the ECU re-flashing.
Their test mule is holding up fine without subframe reinforcement.
They are testing different exhaust/header configurations vs. ECU tuning.
I have water to air system on my S54. When the car is stuck in traffic, heat soak issue is greater with water-to-air because it takes time to cool water once it's heat up. This is why water-to-air system employs electric fans attached to heat exchangers, to fight against heat soak when the car is stationary. On the other hand, air-to-air system would immediately cool incoming air once the car starts moving.
However, once the car is moving and the water temperature reaches outside air temperature, water-to-air system is more efficient because the intake track is shorter (distance that cooled air has to travel before reaching the engine)and it has less boost drop.
For S54 system, the whole intake manifold gets too cold to touch with your hand during winter and it stays warm to touch during summer days as well. Try that with air-to-air system. Cooled air from intercooler will have to travel a lengthy pipe (and this pipe gets HOT) before it reaches intake manifold. And by the time the air reaches intake manifold, it will have to deal with even hotter intake manifold. Not to mention inherently high boost drop with air-to-air system (1-2 psi Vs .5 psi for air-to-water).
Now for S62, since the intake manifold sits on top of engine, I just can't see how the aftercooler core inside of the manifold could stay cooled even during winter. For this reason, I would think air-to-air system would be more efficient system for S62.
VF engineering has earned their respect in FI world for their VW Audi programs and it is slowly gaining market in BMW. With their solid engineering background, there is no reason to doubt that their system can be worth while. But since we are dealing with 20k engine, proven reliability will be the only ticket to sway many.
Thanks for the lesson Chuck (and Paul). I learned something new today.. it is more than just heat exchange coefficients... :p
It would be interesting to hear Bill (Shadowman's) perspective, since Discovery has also gone with air-to-water intercooling. Obviously, AA's kit (if Karl decides to go forward with it) is air-to-air. Perhaps it comes down to esthestics? It looks like quite a challenge to make an air-to-air intercooler setup look esthetically pleasing. I might be willing to take a hit in heat soak to maintain a nice, finished 'OEM-ish' look...
Dave
Here is a properly sized air to air intercooler for a 32 valve, 4.2 litre motor with a low pressure, twin turbo set up.
Great thread guy's,keep it up! ...I am debating right now which 'cooler to go with.My car is now at Turbo Hoses in Livermore and they have degreed engineers as well as a eddy-current load control unit from DynoJet.
I'm thinking the VF 'moderate' s/c set up should be no more a strain on the engine than Dinan's version, which of course has more bells and whistles but still is based on stock internals. Dinan's S3 mule is a '00 that has piled on a lot of miles of big-time thrashing (Dan, you remember how many?) and the engine internals seem to be doing fine.
...I am debating right now which 'cooler to go with.My car is now at Turbo Hoses in Livermore and they have degreed engineers as well as a eddy-current load control unit from DynoJet.
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