the water to air is way efficient but is more money and "lots more weight" Subaru's are lighter cars and weight is everything. if you didnt care about 75-100 more pounds then go for it. that would be my guess for an intake cooler/water/pump/mini radiator etc
intercooling sizing has a few goals, pressure drop ideally would never exceed 1 psi at desired boost level( that includes your higher E85 boost), this is dynamic test (once everything is together on the dyno) and should be checked. if air into the intercooler is 12 psi and air out of intercooler (intake pressure) is 8 psi then there its 4psi of pressure drop. no bueno. i gave seen 8psi delta before for guys who buy anything off ebay and figuers it will work cause its long--lol. that means the "charge face" is not big enough, basically the intercooler cant flow enough air. the length of the intercooler only plays the part on how much you can cool the air. its got to flow first. if the intercooler has under.5 psi then the cooler charge face is over designed and you could have extra boost lag because its basically "wasted space"
John's advice is right on -
Sizing is as important as type - the higher the boost the more pressure loss becomes a problem
I prefer water to air as well, but it is far more expensive and requires a pump, coolant and fans
CBM has a nice setup using the Whipple SC lower manifold. The Tick performance water to air Intercooler in the Holley ram is my go to as it is VERY efficient I have seen huge temperature drops with near zero pressure loss
But that being said with mild boost e85 is a far better intercooler than any air to air or air to water intercooler
Every intercooler is only as good as the turbo sizing. If the turbo is sized wrong and makes too much heat the intercooler will not be effective and pressure drop will add to lag.
On air- to air intercoolers the general rule is that it is far better to have more parallel short tubes than a few long tubes - in other words square is better than long rectangle. This is opposite of a radiator where having more time to cool the water (dwell time) is more effective. while that same thinking works for air in an intercooler, the problem is long tubes cause significant pressure drop and lag.
While many street cars have no choice but to do this (fitting intercooler in the front grille, and many have far too small intercoolers... so its a compromise
The long tubes can lower boost by 5-10 lbs on a 15lb system in severe cases and I have seen this on Camaros - so you actually end up with more heat because you are spinning the turbo harder. The long tubes due to the pressure drop "push" the output of the turbo and cause lots of backpressure. This both hard on the turbo shaft (and bearing) and more importantly will cause lag when getting on and off the throttle (especially before the boost level hits the blow off valve opening pressure.
Using the wrong sized intercooler makes you get smarter as to how ypu size tubing and where you place the Blow off to "help"
Regarding the small Manifold mounted "turnkey" style dual turbo LS intercooler and scoop. I have measured drop and pressure on those.
Pressure drop is right around 1-2Lbs depending on boost (6-7lbs at the manifold) - not too bad - but efficiency is pretty low over 7lbs or so. At 7lbs I would say it the minimum size I would ever use. For e85 its fine.
For most 15-20LB LS engines running 110 or c16 a "radiator size" intercooler is pretty much ideal 20x26 BAR type
Under 20lbs you can get away with a radiator type Intercooler - Bar types are generally more efficient but really only needed for high boost so you don't "blow them apart"
I had the opportunity to work on a 6.0 L LS car with an ebay twin turbo setup. I did not have any expectations going in... since the car came with two CHINA Gt35 turbos and 3" tubing and China Intercooler , China Waste gates (on 8Lb springs) and Blow offs. this was the typical $1700 deal on ebay and its been running 4 seasons on the car with no issues until one of the waste gates got wasted.... and surprisingly the charge stays cool and the the car is running decent timing - the intercooler is barely larger than flat manifold type. The car hauls A$$ with little noticeable lag... I was very surprised
