Think of your intercooler as a "heatsink". When you are accelerating under boost your intake air temps are higher than that of the intercooler so the intercooler drags heat out of the boosted air.
If you do this for long enough and the intercooler is too small or inefficient the temps on the I/cooler will eventually reach a point where it cannot remove any more temps out of the air, as it is heated up to a point where there are equal temps between I/cooler and boosted air.
Now when you get off boost, the intake air is now cooler than the I/cooler so it drags heat out of the I/cooler along with whatever cooling medium your intercooler uses (air/air, water/air).
This actually heats up the air from the turbo when it's not being boosted until such time as the I/cooler temps have dropped below intake temps.
A large heavy intercooler takes longer to heat up but it also takes longer to cool down so intercooler selection can be something of a "black art".
I've found the aftermarket I/cooler cores to be generally more efficient than the factory ones with some exceptions, (factory Mitsubishi Evo I/coolers are very good).
The one below on a Nissan Patrol Diesel 4x4 is using a replacement core to build a longer top mount intercooler than the factory one. It gave only a small difference in bhp (9 bhp) but an increase in torque of 10%.
The dyno runs were also done when the larger cooler was still hot from heat soak and the factory cooler was cold so the differences would have been even greater if both coolers were the same temps to start with.
The bottom graph I find interesting is the manifold pressure "M" where it is obvious the factory intercooler has "hit the wall" at 2800-2900 rpm and lost boost pressure as the wastegate opens but the new one's boost curve is as flat as Texas.
The owner tells me the new one has also picked up 1-1.5 psi boost pressure in normal running.
No temps in or out were taken but I would be very surprised if the intake temps weren't significantly better.
Regards Andrew.
