Air to air Intercooler vs M4 air to water

[Boosted_N54]

Why doesn't the new m4 use a air to air intercooler? like the N54

[Kgolf31]

Because you reject more heat with air to water. Thus being more efficient.

[Boosted_N54]

Originally Posted by Kgolf31

Because you reject more heat with air to water. Thus being more efficient.

Yea but isn't more true you can get heat soak more with air to water and it's more likely get leaks?

[PeteA]

Smaller, water has a much higher specific heat capacity than air so doesn't 'heat soak' anywhere near as quickly (but they still can heat soak if they are not big enough for the application), you can mount the cooling radiator anywhere you like along the charge air piping and still yield fantastic results with minimal airflow.

Negatives are it's more complex, needs a water pump to circulate cooling fluid etc etc.

Chuck a search in google - this is what I found

http://www.enginebasics.com/Advanced...tercooler.html

[Freon]

Water ultimately exchanges heat out to air, so saying you can reject more heat with a water intercooler is a bit silly. There's probably more heat mass in a water intercooler, so initially it might perform better, but under a sustained load they can actually do worse. Heat exchangers only transfer heat energy with a temperature delta, and the rate of that transfer is in proportion to the temperature delta. In a direct air-to-air intercooler you only have one heat exchange and one required temperature delta. In water to air you have two.

Discussing heat mass is just as valid for heavy bar and plate air intercoolers that people put on their cars to replace the lightweight factory tube and fin unit. Even a really poorly designed aftermarket bar and plate unit will perform great the first gear while you're just heating up the aluminum mass, for instance. Looking at sustained load is a lot more interesting.

Water intercoolers reduce the volume of air you need to compress, reducing lag. There's a lot less piping between the turbo and intake manifold. I assume that's the reasoning here for BMW's choice. They're now trying to add ~120hp over an N54/N55 without increasing displacement. This means larger, slower turbos that lag more. Water intercooler allows them to fight back on lag. This is really the only reason I can see for the change.

Water intercoolers add complexity (extra pump for water in particular, more moving parts, more stuff to pop a leak).
Water intercooler systems generally weigh more due to the water and extra heat exchangers. The longer pipes in air-to-air intercoolers don't really weigh much. Water is pretty dense, so all the water in both heat exchangers, pump, and lines adds up.

Either can heat soak if the heat rejection rate less than the heat addition rate. It's a bit complex, again, because the heat rejection rate depends on temperature delta. So there is a negative feedback loop of a intercooler not rejected heat fast enough, heating up, and rejecting heat faster due to that higher temperature (heat soaking). It's very hard to say one intercooler heat soaks while another does not. It's more meaningful to compare the stable temperature of two under the same conditions. *ALL* intercoolers could be said to heat soak. They don't work if they don't heat up!

edit: Water can be easier to package. You can run thin water lines to the front heat exchanger rather than large pipes that are carrying charge air. Bumpers, fenders, engine accessories only leave so much space to run lines or pipes. This could be a good reason as well.

[Boosted_N54]

Air to Air VS Air to Water