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Tuning the C20LET - Chargecoolers
When engine space is tight, one may choose to go for a chargecooler. Fitting instructions can be found in Andy Kirwan's classic 'How-to' pages
A chargecooler setup will always be more complicated (and probably heavier, too) than a FMIC.
However, the weight disadvantage is offset by it's
- Location, most of it being closer to the car's centre of gravity, aiding weight distribution.
- Overall size: the heat exchanger volume doesn't need to be anywhere near as that of the inside of an intercooler. This is because the heat transfer properties of water are so much better than those of air. Moreover, the water rad (that should be mounted frontally) can be far smaller than an intercooler core of similar heat exchange capabilities. This is because the specific heat of water is 4 times that of air (4.2 kJ/kg/degreeC instead of 1). Bearing also in mind the volume of 1Kg of air (1 kg of water is only 1 litre) there is no comparison really for a better material than water when it comes to absorbing energy without raising its temperature. Aluminium, for example is 0.94 and copper 0.4
- Flexibility: The smaller heat exchanger can be easily mounted without disturbing the thick compressed air pipes. It could even be integrated with the inlet manifold. The smaller/thinner water rad can be easier located up front, more like a large oil cooler. The pipes to/from the pump are thin and easily routed without drama.
Keep in mind that the chargecoolers mentioned above are commercial attempts at 'special' designs for the LET. They probably originate from the same tuners who fit double head gaskets and boost stock KKKs up to 22psi with stock fuelling and intercooling. Upon closer inspection they look small, almost out of the set of 'Thunderbirds' or 'Captain Scarlet'. You wouldn't know by the price though, as they ask for £500-600 for them. Plus fitting. They do offer some improvement over the stock LET intercooler setup, but that's it. Their pathetic tiny water rads cannot cope with serious heatloads, so after a couple of full-boost runs they are heatsoaked completely. With the car left parked, it could take hours for their coolant to come down to ambient temperature. The heat exchanger itself is also a bit on the small side. Considering the speed of the airflow during full boost conditions, there's no time for the charge to get rid of much heat.
Note that there is no need for the heat exchanger to be as big as a FMIC, because the water jacket around it leads to much better efficiency (roughly four times better). Therefore I wouldn't want my chargecooler to have an inner core volume less than one fourth of that of a good FMIC.
Here is Richie's page relating to his LET chargecooler/intercooler comparison. (local copy here)
Below are his three dyno graphs:
with standar intercooler:
With toy chargecooler:
With EDS intercooler:
The superiority of the FMIC is obvious
Here is chargecooler custom-made for an American 8-cyl turbocharged beast. For the full story go to toohighpsi (local copy here and here)
View of the inlet manifold side: Fitted in the bay:
This plot shows just how efficient a chargecooler/ice combo can be on the strip. The blue line (cooler output) is totally disconnected from the red line (cooler input) even at 20psi boost!
This whole run lasted about half a minute. If it were to last another minute or two, saturation would have pushed the blue line sky-high. But for a dragstrip it works a treat!
There are definite advantages in a chargecooler setup. Packaging is the main one. The smaller heat exchanger can be easier positioned in the bay without resorting to strange angles with the thick airpipes. It doesn't need to be exposed to the airflow, either. The water rad doesn't need to be huge, either, but it certainly has to be bigger than the ones that go with these kits. Note that I use the terms 'water' and 'coolant' interchangeably. Of course it's always a mixture of distilled water and coolant (30% minimum), or distilled water and WaterWetter (only in the summer)
The good news is that you can use almost any oilcooler rad for this purpose. An aircon condenser can also be used. Just take a look at the condenser of a stock Calibra, and imagine it as a rad for the chargecooler. There will be no heatsoak there! It will also eliminate the need for a big swirl tank, as the large rad and (longer) pipes will hold enough coolant. This is useful, because this tank will have to be up high in the engine bay, higher than any other part of the chargecooler (easy bleeding, the pump won't appreciate air pockets, neither will the heat exchanger)
But how much coolant do you need? And what sort of speed should the (chargecooler) water pump run at? That depends on the expected use of the car. If you intend to hold full throttle for a maximum of 10 seconds, you really don't want re-used, pre-heated coolant coming back for a second helping. So the pump should (as a minimum) need more than 10seconds to circulate the lot through the whole circuit. If you intend to change gear and boot it up again, factor this too.
Once the coolant has been through the heat exchanger, it will be warm (or very hot if you're giving it some). You want that heat out of there, ASAP. That's where the large rad comes in. Not only will the water be reusable after just one pass, but the larger volume of coolant will ensure that it's average temperature is lower, too. A fan is needed behind the rad, thermostatically controlled (NOT boost controlled). Keep in mind that most of the rad's work is after the boost party is over. You can't expect it to cope with the extreme heatloads during boost, it doesn't stand a chance in hell to do it in real-time.
The water pump will be electric, and probably not very silent. If it's two-speed, then that could be boost-controled: high speed during boost, low speed during vacuum.
....So to recoup, the chargecooler is like a heatsink - it absorbs the temp spikes when they do appear and gets rid of the load once the boost is no more. If you're on a race circuit full throttle all the time, remember that there may be no chance for it to shed the load. Not much use after the first lap, dead weight mostly!
The 'heatsink' factor is less prominent on intercoolers, but it's still there. The large surface of a FMIC ensures that a lot of the charge's heat is being shed on the fly, but the boost temp spikes (very dangerous) are being absorbed(and shared) by the mass of the whole assembly, including the endcaps (and part of the car's frame, if the installation is such). A heavy intercooler is a good thing, just be careful that it's not too far at the front upsetting the car's handling.
Back to Intercooler
Phase-change materials
Nasa cryogenic thermal storage unit
Material Specific Heat (kJ/kg/degree C)
Water Aluminium Copper Air Concrete
