hans14914

One of the reasons that people like water to air is that it can offer below ambient temp cooling. The air-air will never cool the charge to a point below ambient without assistance of an outside source (c02/n02 or water misting.) Since water has a much better thermal conductivity, it can transfer more heat, and also, retain more heat. Air is considered an insulator, becaus its heat transfer isnt the best. With a properly sized air-water unit, with a good sized front radiator, you will see the best performance results. Also, you can dump a bag of ice in your washer tank for some added perfomarmance. As far as complexitivty consider this. What is harder to bend, 2.5" tubing, or .5" vinyl tubing? Most of the "complicated" parts of the liquid install will be running water lines to the pump, and radiator. Those are all done with flexible lines. It is much easier, especialy for the ammount of space in the 928 to run a liquid system.

John..

Yes, but how often do you have the opportunity to add ice on say a 80 degree day on a 100+ mile trip. It can only offer below ambient if the water is below ambient.

For drag racing I would agree because ice can be added each time, but steady state all day performance I think my money would still be on the air to air. Still, I am tempted by the packaging of the air to water.

Consider this: At steady state conditions, the water is taking the heat from the charge air and transporting it to the front of the vehicle to exchange the heat again with the atmosphere, so the total efficiency becomes linked to the two efficiencies of the heat exchangers. In this model, the water is nothing more than a carrier of the heat to the front of the vehicle for exchange to outside air. The air to air is a straight shot, one step exchange. For short bursts it is different because the mass of water in the system can heat soak and then exchange its heat while off boost by running the pump continuously, which is probably fine for street driving considering most boosted runs are well under 30 seconds and the duration between boost is at least a few minutes.

I agree with the routing of the hoses and for short bursts on the street I beleive it can offer very good performance. I still like the look of a massive air to air exchanger up front.

I agree it probably is easier and quite honestly I have not made my mind up just yet. For me, the ice arguement is not a valid one because I don't carry fresh ice with me when I drive. Still, others have used an AC system to chill the water with excellent results, but again that takes power to move that heat out.

Tony

http://www.aquamist.co.uk/

Off topic perhaps, but another solution.

ViribusUnits

Well, what would it take to do it eighter way? Then figure out how to do it both ways.

Then when you get around to offering your kit, it's a simple matter of which set of parts you put together.

Or the expence may be prohibitive, and a no go. But then again, maybe...

hans14914

You can still use a mister in conjunction with the front mounted radiator, many people forget that the water spray has equal if not better bennefits on the water-air radiator. I agree that ice is only a part time solution, but it is a plus. Another plus is that the water is still removing heat from the intake when the car is at a stop. While an air-air is almost fuctionless when at standstill, the water is still absorbing heat. Water, if i remember correctly, has about 4x greater specific heat when compared to air. Even if the pump is not running, the water will still absorb heat, until it is of equal heat to the compressor discharge. Just helping support the liquid side, as everyone is so fast to name air-air king.

hans14914

Another almost never talked about intercooler is the wax filled core cooler. Not for high performance cars, but most dont even know its existance. The wax will absorb heat until it melts, say under wot, then when crusing and the intake charge heat disapates, it resolidifies, and is ready for another go. Interesting concept. I will see if i can find any more in depth info. Not applicable to your project, but does exersize the mind, and give a better understanding for cooling technology.

blau928

I did not notice anyone mention pressure drop, thermal load, and many other things. However, putting a good fan on the radiator for the water to air cooling system will increase the standstill operational efficiency. If this is linked to a speed control sensor, that turns the fan on at say below 50mph, then the moving issue is solved, coupled with the water's superior thermal absorbtion capacity.. (4X greater than Air as Hans stated)

Other options include refrigerating the water with the A/C to further subcool the water below ambient... my point is if you design the system to cool the amount of airflow based on the application, they will both work fine.. However, as most of us drive on roads in the USA, IMHO water to air offers significant advantages to air to air on a 928 engine.......

Ask MBZ & Mc Laren & Ford & others who have much bigger and deeper pockets, and see what they are doing with boosted street motors with over 500hp...

I am in process of building a Water to Air cooler. It is systematically designed to cool 1000 HP of airflow at 90% efficiency, and will fit in my 928, and no I am not selling kits or parts for the design. I will load pixx when complete. And yes, the science is already there...

hans14914

I am working on an audi turbo project car, and am using water-air. A small frontmount radiator, from a small car, big bike, ot even an ac condesor, with fan and water misting will give the best results. There are several smart intercooler systems out there, the cheapest i believe is available through autospeed. It will allow you to control an auxilery fan and water misting system.
Hans

bshaw

John...

I'm looking at air to air for mine. How and where does the Callaway 928 TT intercooler mount? Do you have a pic of it available? I asked the guys at Callaway but they couldn't come up with any documentation on the 928, just the 944 (they've changed personnel since the 928 project was done). They owe me a favor and are going to help with the a-a intercooler.

The Callaway 944 turbo kit has the a-a intercooler laying flat, parallel to the road, between the engine and the radiator. I'm considering a similar approach for my 928, with a scoop built into the front spoiler to feed it air. This helps by keeping the piping behind the radiator, making it a little easire to plumb. The other approach I'm considering is to mount it way up front in place of the grill. It looks like there's room for a 10" x 28" cooler there. Getting the 3" pipes over (around?) the radiator will be a bit of a trick though.

Bill
'84s 5 sp. (FrankenPorsche)
http://mywebpages.comcast.net/p-928/749.html

Old & New

It's only a matter of time before someone builds a hideous custom nose over an air-air...

2V4V

John & Bill,

You might want to check out one of the Bell books on supercharging or turbocharging. I think you'll find some useful science regarding placement and airflow if you choose to go the A/A (or A/W) route. And they're just good informative basic text.

There has been a LOT learned since the early days when Callaway was slappin' stuff together and hopin' that it worked - he was doing (semi)experimental stuff. Mind you, for the time, it worked much better than other stuff, but his early work would be judged to be primitive at best by contemporary standards. Once again, it was great for the time - I drove an old Callaway GTV6 Twin Turbo not that long ago, great fun - but the stuff would be much different, and better, if done today.

I agree that A/A is the (mechanically) simpler transaction, but you can't beat the ability of liquid to transfer heat compared to gas (air). This is why the 928 engine is liquid cooled - it simply works better than air cooling.

One can surely have an interesting +/- equation regarding other dimensions of the A/A v. A/W topic, but as has been pointed out many times, the superiority of liquid heat transfer is really beyond doubt.

As long as you size it correctly, and route as cleanly as you can, you should be able to get pretty close to the efficiency of A/W with an A/A setup.

And good enough is (for many scenarios) just fine.

Greg

John..

I still don't buy the greater efficiency of the Air to Water over the long haul in steady state or high use applications. There are two boundaries to heat transfer there....first you transfer the heat from the charge to the water, but you then have to transfer it back to the air at the front of the vehicle. At steady state the water becomes nothing more than a transporter of heat that you have to re-exchange at the front of the car at a loss of efficiency due to the 2 step process . Again, we are talking steady state here after running lots of boost, not a one shot deal after you drove 10 miles with no boost to cool off the water. In that case the air to water is probably equal in efficiency until the water heats up to a certain temp.

If the sizing of the cores in either scenario is equal (heat transfer wise) I still vote at steady state the air to air will win out every time.....i.e. 80% efficient air to air is 80% efficient, take two liquid to air exchangers at 80% and at steady state you get 64% efficiency (0.8*0.8=0.64).

I agree for a one shot deal the air to liquid can probably have a very good efficiency, but I don't think it is more efficient unless the water is chilled and at that it only lasts for one shot or short bursts. Assumuming you don't dip into the boost too much you have a good solution with the air to water.

No, I still have not made up my mind on which way to go, but that is about to change in another couple of weeks.

Greg, the only things I changed from Callaway's work was the intercooler and the air mass metering system, then I turned the screw down 5 lbs and the car made 435HP. Let's give credit where credit is due, some of Callaway's early work was some of his best. My car is refined, idles smoothly and runs like a freight train.

John..

Here is some interesting reading on the subject, though it is somewhat swayed towards air to air. Nonetheless the write-up makes sense.

http://www.panhandleperformance.com/intercooler4.html

Old & New

John..,

I'm not thoroughly convinced that air-fluid is substantially less efficient just because it is a two step process. What it comes down to (with either type of system) is whether there is a bottleneck in the heat transfer. I'm not really a thermodymamics guy, but you need to realize that the fluid coming out of the intercooler going into the radiator returns to ambient as long as the radiator is large enough. If the radiator is not quite large enough to shed all the heat in a single pass, the four gallon liquid volume buffers the system to an enormous degree, and the radiator catches up promptly.

I want to re-emphasize that my intercooler radiator has kept the core fluid cool no matter how much racing around I have done. I guess the bottom line is that it works well regardless of what all the theory and calculations indicate.

...and I know you are only on a fact-finding mission, but I am emphasizing the case for the air-fluid since you are naturally biased, and I want to balance that off so that you can remain objective.

Tim Murphy

http://autos.groups.yahoo.com/group/...-Intercoolers/

Join this Yahoo Group. This is where I got most of my information from when I was developing the intercooler for my kit. There is a lot of good information there.